Phenotypic correlates of the lianescent growth form: a review

Effects of lianas on forest biogeochemistry during their lives and afterlives

Climate change and other anthropogenic disturbances are increasing liana abundance and biomass in many tropical and subtropical forests. While the effects of living lianas on species diversity, ecosystem carbon, and nutrient dynamics are receiving increasing attention, the role of dead lianas in forest ecosystems has been little studied and is poorly understood. Trees and lianas coexist as the major woody components of forests worldwide, but they have very different ecological strategies, with lianas relying on trees for mechanical support. Consequently, trees and lianas have evolved highly divergent stem, leaf, and root traits. Here we show that this trait divergence is likely to persist after death, into the afterlives of these organs, leading to divergent effects on forest biogeochemistry. We introduce a conceptual framework combining horizontal, vertical, and time dimensions for the effects of liana proliferation and liana tissue decomposition on ecosystem carbon and nutrient cycling. We propose a series of empirical studies comparing traits between lianas and trees to answer questions concerning the influence of trait afterlives on the decomposability of liana and tree organs. Such studies will increase our understanding of the contribution of lianas to terrestrial biogeochemical cycling, and help predict the effects of their increasing abundance.

Climbing mechanisms and the diversification of neotropical climbing plants across time and space

Climbers germinate on the ground but need external support to sustain their stems, which are maintained attached to supports through modified organs, that is, climbing mechanisms. Specialized climbing mechanisms have been linked to higher diversification rates. Also, different mechanisms may have different support diameter restrictions, which might influence climbers' spatial distribution. We test these assumptions by linking climbing mechanisms to the spatiotemporal diversification of neotropical climbers. A dataset of climbing mechanisms is presented for 9071 species. WCVP was used to standardize species names, map geographical distributions, and estimate diversification rates of lineages with different mechanisms. Twiners appear concentrated in the Dry Diagonal of South America and climbers with adhesive roots in the Chocó region and Central America. However, climbing mechanisms do not significantly influence the distribution of neotropical climbers. Also, we found no strong support for correlations between specialized climbing mechanisms and higher diversification rates. Climbing mechanisms do not strongly impact the spatiotemporal diversification of neotropical climbers on a macroevolutionary scale. We argue that the climbing habit is a synnovation, meaning the spatiotemporal diversification it promotes is due to the sum effect of all the habit's traits rather than isolated traits, such as climbing mechanisms.

Maximum stem diameter predicts liana population demography

Determining population demographic rates is fundamental to understanding differences in species' life-history strategies and their capacity to coexist. Calculating demographic rates, however, is challenging and requires long-term, large-scale censuses. Body size may serve as a simple predictor of demographic rate; can it act as a proxy for demographic rate when those data are unavailable? We tested the hypothesis that maximum body size predicts species' demographic rate using repeated censuses of the 77 most common liana species on the Barro Colorado Island, Panama (BCI) 50-ha plot. We found that maximum stem diameter does predict species' population turnover and demography. We also found that lianas on BCI can grow to the enormous diameter of 635 mm, indicating that they can store large amounts of carbon and compete intensely with tropical canopy trees. This study is the first to show that maximum stem diameter can predict plant species' demographic rates and that lianas can attain extremely large diameters. Understanding liana demography is particularly timely because lianas are increasing rapidly in many tropical forests, yet their species-level population dynamics remain chronically understudied. Determining per-species maximum liana diameters in additional forests will enable systematic comparative analyses of liana demography and potential influence across forest types.

Vessel dimorphism and wood traits in lianas and trees among three contrasting environments

PREMISE

Determining how xylem vessel diameters vary among plants and across environments gives insights into different water-use strategies among species and ultimately their distributions. Here, we tested the vessel dimorphism hypothesis that the simultaneous occurrence of many narrow and a few wide vessels gives lianas an advantage over trees in seasonally dry environments.

METHODS

We measured the diameters of 13,958 vessels from 15 liana species and 10,430 vessels from 16 tree species in a tropical seasonal rainforest, savanna, and subtropical evergreen broadleaved forest. We compared differences in mean and hydraulically weighted vessel diameter (MVD and D_h ), vessel density (VD), theoretical hydraulic conductivity (K_t ), vessel area fraction (VAF), and wood density (WD) between lianas and trees and among three sites.

RESULTS

Nine liana species and four tree species had dimorphic vessels. From the tropical seasonal rainforest to the savanna, liana MVD, D_h and K_t decreased, and VD and WD increased, while only tree WD increased. From the tropical seasonal rainforest to the subtropical forest, six wood traits remained unchanged for lianas, while tree MVD, D_h and K_t decreased and VD increased. Trait space for lianas and trees were more similar in the savanna and more divergent in the subtropical forest compared to the tropical seasonal rainforest.

CONCLUSIONS

These results suggest that lianas tend to possess greater vessel dimorphism, which may explain how lianas grow well during seasonal drought, influencing their unique distribution across tropical rainfall gradients.

The climbing flora of India: A comprehensive checklist

The climbing plants in India are listed in detail in this data note. This comprehensive list of climbers was compiled using more than 100 published and unpublished sources that span more than a century. It includes a total of 2,608 species representing 585 genera and 104 spermatophyte plant families. Each species listed in the dataset is categorised according to its degree of woodiness and climbing strategies. The dataset also includes information on The International Union for Conservation of Nature (IUCN) status of all climber species from India. The botanical nomenclature used in the data has been updated to reflect Angiosperm Phylogeny Group (APG) IV classification. Researchers who are conducting ecological, taxonomic, phylogenetic, and evolutionary studies on climbers will be particularly interested in this dataset.

Diameters of phloem sieve elements can predict stem growth rates of woody plants

Understanding forest dynamics is crucial to addressing climate change and reforestation challenges. Plant anatomy can help predict growth rates of woody plants, contributing key information on forest dynamics. Although features of the water-transport system (xylem) have long been used to predict plant growth, the potential contribution of carbon-transporting tissue (phloem) remains virtually unexplored. Here, we use data from 347 woody plant species to investigate whether species-specific stem diameter growth rates can be predicted by the diameter of both the xylem and phloem conducting cells when corrected for phylogenetic relatedness. We found positive correlations between growth rate, phloem sieve element diameter and xylem vessel diameter in liana species sampled in the field. Moreover, we obtained similar results for data extracted from the Xylem Database, an online repository of functional, anatomical and image data for woody plant species. Information from this database confirmed the correlation of sieve element diameter and growth rate across woody plants of various growth forms. Furthermore, we used data subsets to explore potential influences of biomes, growth forms and botanical family association. Subsequently, we combined anatomical and geoclimatic data to train an artificial neural network to predict growth rates. Our results demonstrate that sugar transport architecture is associated with growth rate to a similar degree as water-transport architecture. Furthermore, our results illustrate the potential value of artificial neural networks for modeling plant growth under future climatic scenarios.

Sieve tube structural variation in Austrobaileya scandens and its significance for lianescence

Lianas combine large leaf areas with slender stems, features that require an efficient vascular system. The only extant member of the Austrobaileyaceae is an endemic twining liana of the tropical Australian forests with well-known xylem hydraulics, but the vascular phloem continuum aboveground remains understudied. Microscopy analysis across leaf vein orders and stems of Austrobaileya scandens revealed a low foliar xylem:phloem ratio, with isodiametric vascular elements along the midrib, but tapered across vein orders. Sieve plate pore radii increased from 0.08 µm in minor veins to 0.12 µm in the petiole, but only to 0.20 µm at the stem base, tens of metres away. In easily bent searcher branches, phloem conduits have pectin-rich walls and simple plates, whereas in twining stems, conduits were connected through highly angled and densely porated sieve plates. The hydraulic resistance of phloem conduits in the twisted and elongated stems of A. scandens is large compared with trees of similar stature; phloem hydraulic resistance decreases from leaves to stems, consistent with the efficient delivery of photoassimilates from sources under Münch predictions. Sink strength of a continuously growing canopy might be stronger than in self-supporting understory plants, favoring resource allocation to aerial organs and the attainment of vertical stature.

Comparative transcriptomics of tropical woody plants supports fast and furious strategy along the leaf economics spectrum in lianas

Lianas, climbing woody plants, influence the structure and function of tropical forests. Climbing traits have evolved multiple times, including ancestral groups such as gymnosperms and pteridophytes, but the genetic basis of the liana strategy is largely unknown. Here, we use a comparative transcriptomic approach for 47 tropical plant species, including ten lianas of diverse taxonomic origins, to identify genes that are consistently expressed or downregulated only in lianas. Our comparative analysis of full-length transcripts enabled the identification of a core interactomic network common to lianas. Sets of transcripts identified from our analysis reveal features related to functional traits pertinent to leaf economics spectrum in lianas, include upregulation of genes controlling epidermal cuticular properties, cell wall remodeling, carbon concentrating mechanism, cell cycle progression, DNA repair and a large suit of downregulated transcription factors and enzymes involved in ABA-mediated stress response as well as lignin and suberin synthesis. All together, these genes are known to be significant in shaping plant morphologies through responses such as gravitropism, phyllotaxy and shade avoidance.

Summary: The full-length fraction of liana transcriptomes mapped on a protein–protein interactome revealed the nature of their convergence through distinct sets of expressed and downregulated genes not observed in free-standing plants.

Vegetative phenologies of lianas and trees in two Neotropical forests with contrasting rainfall regimes

Among tropical forests, lianas are predicted to have a growth advantage over trees during seasonal drought, with substantial implications for tree and forest dynamics. We tested the hypotheses that lianas maintain higher water status than trees during seasonal drought and that lianas maximize leaf cover to match high, dry-season light conditions, while trees are more limited by moisture availability during the dry season. We monitored the seasonal dynamics of predawn and midday leaf water potentials and leaf phenology for branches of 16 liana and 16 tree species in the canopies of two lowland tropical forests with contrasting rainfall regimes in Panama. In a wet, weakly seasonal forest, lianas maintained higher water balance than trees and maximized their leaf cover during dry-season conditions, when light availability was high, while trees experienced drought stress. In a drier, strongly seasonal forest, lianas and trees displayed similar dry season reductions in leaf cover following strong decreases in soil water availability. Greater soil moisture availability and a higher capacity to maintain water status allow lianas to maintain the turgor potentials that are critical for plant growth in a wet and weakly seasonal forest but not in a dry and strongly seasonal forest.

Climate and hydraulic traits interact to set thresholds for liana viability

Lianas, or woody vines, and trees dominate the canopy of tropical forests and comprise the majority of tropical aboveground carbon storage. These growth forms respond differently to contemporary variation in climate and resource availability, but their responses to future climate change are poorly understood because there are very few predictive ecosystem models representing lianas. We compile a database of liana functional traits (846 species) and use it to parameterize a mechanistic model of liana-tree competition. The substantial difference between liana and tree hydraulic conductivity represents a critical source of inter-growth form variation. Here, we show that lianas are many times more sensitive to drying atmospheric conditions than trees as a result of this trait difference. Further, we use our competition model and projections of tropical hydroclimate based on Representative Concentration Pathway 4.5 to show that lianas are more susceptible to reaching a hydraulic threshold for viability by 2100.

JA, Sanchez‐Azofeifa GA, De Deurwaerder HPT, Krishna Moorthy SM, Schnitzer SA, Marvin DC, Longo M, Liu C, Broadbent EN, Almeyda Zambrano AM, Muller‐Landau HC, Detto M, Verbeeck H. Liana optical traits increase tropical forest albedo and reduce ecosystem productivity

Lianas are a key growth form in tropical forests. Their lack of self-supporting tissues and their vertical position on top of the canopy make them strong competitors of resources. A few pioneer studies have shown that liana optical traits differ on average from those of colocated trees. Those trait discrepancies were hypothesized to be responsible for the competitive advantage of lianas over trees. Yet, in the absence of reliable modelling tools, it is impossible to unravel their impact on the forest energy balance, light competition, and on the liana success in Neotropical forests. To bridge this gap, we performed a meta-analysis of the literature to gather all published liana leaf optical spectra, as well as all canopy spectra measured over different levels of liana infestation. We then used a Bayesian data assimilation framework applied to two radiative transfer models (RTMs) covering the leaf and canopy scales to derive tropical tree and liana trait distributions, which finally informed a full dynamic vegetation model. According to the RTMs inversion, lianas grew thinner, more horizontal leaves with lower pigment concentrations. Those traits made the lianas very efficient at light interception and significantly modified the forest energy balance and its carbon cycle. While forest albedo increased by 14% in the shortwave, light availability was reduced in the understorey (-30% of the PAR radiation) and soil temperature decreased by 0.5°C. Those liana-specific traits were also responsible for a significant reduction of tree (-19%) and ecosystem (-7%) gross primary productivity (GPP) while lianas benefited from them (their GPP increased by +27%). This study provides a novel mechanistic explanation to the increase in liana abundance, new evidence of the impact of lianas on forest functioning, and paves the way for the evaluation of the large-scale impacts of lianas on forest biogeochemical cycles.

Differentiation in stem and leaf traits among sympatric lianas, scandent shrubs and trees in a subalpine cold temperate forest

The scandent shrub plant form is a variant of liana that has upright and self-supporting stems when young but later becomes a climber. We aimed to explore the associations of stem and leaf traits among sympatric lianas, scandent shrubs and trees, and the effects of growth form and leaf habit on variation in stem or leaf traits. We measured 16 functional traits related to stem xylem anatomy, leaf morphology and nutrient stoichiometry in eight liana, eight scandent shrub and 21 tree species co-occurring in a subalpine cold temperate forest at an elevation of 2600-3200 m in Southwest China. Overall, lianas, scandent shrubs and trees were ordered along a fast-slow continuum of stem and leaf functional traits, with some traits overlapping. We found a consistent pattern of lianas > scandent shrubs > trees for hydraulically weighted vessel diameter, maximum vessel diameter and theoretical hydraulic conductivity. Vessel density and sapwood density showed a pattern of lianas = scandent shrubs < trees, and lianas < scandent shrubs = trees, respectively. Lianas had significantly higher specific leaf area and lower carbon concentration than co-occurring trees, with scandent shrubs showing intermediate values that overlapped with lianas and trees. The differentiation among lianas, scandent shrubs and trees was mainly explained by variation in stem traits. Additionally, deciduous lianas were positioned at the fast end of the trait spectrum, and evergreen trees at the slow end of the spectrum. Our results showed for the first time clear differentiation in stem and leaf traits among sympatric liana, scandent shrub and tree species in a subalpine cold temperate forest. This work will contribute to understanding the mechanisms responsible for variation in ecological strategies of different growth forms of woody plants.

Unraveling the relative role of light and water competition between lianas and trees in tropical forests: A vegetation model analysis

Despite their low contribution to forest carbon stocks, lianas (woody vines) play an important role in the carbon dynamics of tropical forests. As structural parasites, they hinder tree survival, growth and fecundity; hence, they negatively impact net ecosystem productivity and long-term carbon sequestration.Competition (for water and light) drives various forest processes and depends on the local abundance of resources over time. However, evaluating the relative role of resource availability on the interactions between lianas and trees from empirical observations is particularly challenging. Previous approaches have used labour-intensive and ecosystem-scale manipulation experiments, which are infeasible in most situations.We propose to circumvent this challenge by evaluating the uncertainty of water and light capture processes of a process-based vegetation model (ED2) including the liana growth form. We further developed the liana plant functional type in ED2 to mechanistically simulate water uptake and transport from roots to leaves, and start the model from prescribed initial conditions. We then used the PEcAn bioinformatics platform to constrain liana parameters and run uncertainty analyses.Baseline runs successfully reproduced ecosystem gas exchange fluxes (gross primary productivity and latent heat) and forest structural features (leaf area index, aboveground biomass) in two sites (Barro Colorado Island, Panama and Paracou, French Guiana) characterized by different rainfall regimes and levels of liana abundance.Model uncertainty analyses revealed that water limitation was the factor driving the competition between trees and lianas at the drier site (BCI), and during the relatively short dry season of the wetter site (Paracou). In young patches, light competition dominated in Paracou but alternated with water competition between the wet and the dry season on BCI according to the model simulations.The modelling workflow also identified key liana traits (photosynthetic quantum efficiency, stomatal regulation parameters, allometric relationships) and processes (water use, respiration, climbing) driving the model uncertainty. They should be considered as priorities for future data acquisition and model development to improve predictions of the carbon dynamics of liana-infested forests. Synthesis. Competition for water plays a larger role in the interaction between lianas and trees than previously hypothesized, as demonstrated by simulations from a process-based vegetation model.

The response of lianas to 20 yr of nutrient addition in a Panamanian forest

Over the past two decades, liana density and basal area have been increasing in many tropical forests, which has profound consequences for forest diversity and functioning. One hypothesis to explain increasing lianas is elevated nutrient deposition in tropical forests resulting from fossil fuels, agricultural fertilizer, and biomass burning. We tested this hypothesis by surveying all lianas ≥1 cm in diameter (n = 3,967) in 32 plots in a fully factorial nitrogen (N), phosphorus (P), and potassium (K) addition experiment in a mature tropical forest in central Panama. We conducted the nutrient-addition experiment from 1998 until present and we first censused lianas in 2013 and then again in 2018. After 20 yr of nutrient addition (1998-2018), liana density, basal area, and rarefied species richness did not differ significantly among any of the nutrient-addition and control treatments. Moreover, nutrient addition in the most recent 5 yr of the experiment did not affect liana relative growth, recruitment, or mortality rates. From 2013 until 2018, liana density, basal area, and species richness increased annually by 1.6%, 1.4%, and 2.4%, respectively. Nutrient addition did not influence these increases. Our findings indicate that nutrient deposition does not explain increasing lianas in this tropical forest. Instead, increases in tree mortality and disturbance, atmospheric carbon dioxide, drought frequency and severity, and hunting pressure may be more likely explanations for the increase in lianas in tropical forests.

Lianas in silico, ecological insights from a model of structural parasitism

Tropical forests are a critical component of the Earth system, storing half of the global forest carbon stocks and accounting for a third of terrestrial photosynthesis. Lianas are structural parasites that can substantially reduce the carbon sequestration capacity of these forests. Simulations of this peculiar growth form have only recently started and a single vegetation model included lianas so far. In this work we present a new liana implementation within the individual based model Formind. Initial tests indicate high structural realism both horizontal and vertical. In particular, we benchmarked the model against empirical observations of size distribution, mean liana cluster size and vertical leaf distribution for the Paracou site in French Guiana. Our model predicted a reduction of above-ground biomass between 10% for mature stands to 45% for secondary plots upon inclusion of lianas in the simulations. The reduced biomass was the result of a lower productivity due to a combination of lower tree photosynthesis and high liana respiration. We evaluated structural metrics (LAI, basal area, mean tree-height) and carbon fluxes (GPP, respiration) by comparing simulations with and without lianas. At the equilibrium, liana productivity was 1.9t_C ha−1 y−1, or 23% of the total GPP and the forest carbon stocks were between 5% and 11% lower in simulations with lianas. We also highlight the main strengths and limitations of this new approach and propose new field measurements to further the understanding of liana ecology in a modelling framework.

Costs and benefits of gas inside wood and its relationship with anatomical traits: a contrast between trees and lianas

Gas inside wood plays an important role in plant functioning, but there has been no study examining the adaptive nature of gas inside wood across plants differing in biomechanical demands. Using a comparative approach, we measured gas volumetric content, xylem's anatomical traits and wood density of 15 tree and 16 liana species, to test whether gas content varies between these plant types strongly differing in their biomechanical demands. We asked (i) whether trees and lianas differ in gas content and (ii) how anatomical traits and wood density are related to gas content. Lianas had significantly less gas content in their branches compared with tree species. In tree species, gas content scaled positively with fiber, vessel and xylem cross-sectional area and fiber and vessel diameter, and negatively with dry-mass density. When pooling trees and lianas together, fiber cross-sectional area was the strongest predictor of gas content, with higher xylem cross-sectional area of fiber associated with higher gas content. In addition, we showed, through a simple analytical model, that gas inside wood increases the minimum branch diameter needed to prevent rupture, and this effect was stronger on trees compared with lianas. Our results support the view that gas inside wood plays an important role in the evolution of biomechanical functioning in different plant forms. Gas inside wood may also play an important role in physiological activities such as water transport, storage, photosynthesis and respiration, but it is still unknown whether these roles are or are not secondary to the mechanical support.

Solanales Stem Biomechanical Properties Are Primarily Determined by Morphology Rather Than Internal Structural Anatomy and Cell Wall Composition

Self-supporting plants and climbers exhibit differences in their structural and biomechanical properties. We hypothesized that such fundamental differences originate at the level of the material properties. In this study, we compared three non-woody members of the Solanales exhibiting different growth habits: (1) a self-supporting plant (potato, Solanum tuberosum), (2) a trailing plant (sweet potato, Ipomoea batatas), and (3) a twining climber (morning glory, Ipomoea tricolor). The mechanical properties investigated by materials analyses were combined with structural, biochemical, and immunohistochemical analyses. Generally, the plants exhibited large morphological differences, but possessed relatively similar anatomy and cell wall composition. The cell walls were primarily composed of hemicelluloses (~60%), with α-cellulose and pectins constituting ~25% and 5%-8%, respectively. Immunohistochemistry of specific cell wall components suggested only minor variation in the occurrence and localization between the species, although some differences in hemicellulose distribution were observed. According to tensile and flexural tests, potato stems were the stiffest by a significant amount and the morning glory stems were the most compliant and showed differences in two- and three-orders of magnitude; the differences between their effective Young's (Elastic) modulus values (geometry-independent parameter), on the other hand, were substantially lower (at the same order of magnitude) and sometimes not even significantly different. Therefore, although variability exists in the internal anatomy and cell wall composition between the different species, the largest differences were seen in the morphology, which appears to be the primary determinant of biomechanical function. Although this does not exclude the possibility of different mechanisms in other plant groups, there is apparently less constraint to modifying stem morphology than anatomy and cell wall composition within the Solanales.

Within-Site Variability of Liana Wood Anatomical Traits: A Case Study in Laussat, French Guiana

Research Highlights: We investigated the variability of vessel diameter distributions within the liana growth form among liana individuals originating from a single site in Laussat, French Guiana. Background and Objectives: Lianas (woody vines) are key components of tropical forests. Lianas are believed to be strong competitors for water, thanks to their presumed efficient vascular systems. However, unlike tropical trees, lianas are overlooked in field data collection. As a result, lianas are often referred to as a homogeneous growth form while little is known about the hydraulic architecture variation among liana individuals. Materials and Methods: We measured several wood hydraulic and structural traits (e.g., basic specific gravity, vessel area, and vessel diameter distribution) of 22 liana individuals in a single sandy site in Laussat, French Guiana. We compared the liana variability of these wood traits and the correlations among them with an existing liana pantropical dataset and two published datasets of trees originating from different, but species-rich, tropical sites. Results: Liana vessel diameter distribution and density were heterogeneous among individuals: there were two orders of magnitude difference between the smallest (4 µm) and the largest (494 µm) vessel diameters, a 50-fold difference existed between extreme vessel densities ranging from 1.8 to 89.3 vessels mm−2, the mean vessel diameter varied between 26 µm and 271 µm, and the individual theoretical stem hydraulic conductivity estimates ranged between 28 and 1041 kg m−1 s−1 MPa−1. Basic specific gravity varied between 0.26 and 0.61. Consequently, liana wood trait variability, even within a small sample, was comparable in magnitude with tree surveys from other tropical sites and the pantropical liana dataset. Conclusions: This study illustrates that even controlling for site and soil type, liana traits are heterogeneous and cannot be considered as a homogeneous growth form. Our results show that the liana hydraulic architecture heterogeneity across and within sites warrants further investigation in order to categorize lianas into functional groups in the same way as trees

Biomass and nitrogen distribution ratios reveal a reduced root investment in temperate lianas vs. self-supporting plants

BACKGROUND AND AIMS

The reliance on external support by lianas has been hypothesized to imply a reduction in the biomass cost of stem construction and root anchorage, and an increased investment in leaves, relative to self-supporting plants. These evolutionary trade-offs have not been adequately tested in an ontogenetic context and on the whole-plant scale. Moreover, the hypothesis may be extended to other potentially limiting resources, such as nitrogen (N.).

METHODS

Plants belonging to five con-familiar pairs of temperate liana/shrub species were cultivated in 120 L barrels and sequentially harvested over up to three growing seasons. To account for the ontogenetic drift, organ biomass and nitrogen fractions were adjusted for plant biomass and N pool, respectively.

KEY RESULTS

Lianas invested, on average, relatively less biomass in the root fraction in comparison with shrubs. This was offset by only insignificant increases in leaf or stem investment. Even though liana stems and roots showed higher N concentration in comparison with shrubs, plant N distribution was mostly driven by, and largely matched, the pattern of biomass distribution. Lianas also showed a greater relative growth rate than shrubs. The differences between the growth forms became apparent only when ontogenetic drift was controlled for. These results were confirmed regardless of whether reproductive biomass was included in the analysis.

CONCLUSIONS

Our results suggest that temperate lianas, in spite of their diverse, species-specific resource distribution patterns, preferentially allocate resources to above-ground organs at the expense of roots. By identifying this trade-off and demonstrating the lack of a general trend for reduction in stem investment in lianas, we significantly modify the prevailing view of liana allocation strategies and evolutionary advantages. Such a resource distribution pattern, along with the cheap unit leaf area and stem unit length construction, situates lianas as a group close to the fast acquisition/rapid growth end of the life strategy spectrum.

Modeling the impact of liana infestation on the demography and carbon cycle of tropical forests

There is mounting empirical evidence that lianas affect the carbon cycle of tropical forests. However, no single vegetation model takes into account this growth form, although such efforts could greatly improve the predictions of carbon dynamics in tropical forests. In this study, we incorporated a novel mechanistic representation of lianas in a dynamic global vegetation model (the Ecosystem Demography Model). We developed a liana-specific plant functional type and mechanisms representing liana-tree interactions (such as light competition, liana-specific allometries, and attachment to host trees) and parameterized them according to a comprehensive literature meta-analysis. We tested the model for an old-growth forest (Paracou, French Guiana) and a secondary forest (Gigante Peninsula, Panama). The resulting model simulations captured many features of the two forests characterized by different levels of liana infestation as revealed by a systematic comparison of the model outputs with empirical data, including local census data from forest inventories, eddy flux tower data, and terrestrial laser scanner-derived forest vertical structure. The inclusion of lianas in the simulations reduced the secondary forest net productivity by up to 0.46 t_C  ha^-1  year^-1 , which corresponds to a limited relative reduction of 2.6% in comparison with a reference simulation without lianas. However, this resulted in significantly reduced accumulated above-ground biomass after 70 years of regrowth by up to 20 t_C /ha (19% of the reference simulation). Ultimately, the simulated negative impact of lianas on the total biomass was almost completely cancelled out when the forest reached an old-growth successional stage. Our findings suggest that lianas negatively influence the forest potential carbon sink strength, especially for young, disturbed, liana-rich sites. In light of the critical role that lianas play in the profound changes currently experienced by tropical forests, this new model provides a robust numerical tool to forecast the impact of lianas on tropical forest carbon sinks.

Are hydraulic patterns of lianas different from trees? New insights from Hedera helix

Lianas form long and flexible but disproportionately narrow stems, and thus require particular strategies to maintain the integrity of xylem water transport and ensure supply to large crown areas. The hydraulic architecture of lianas and the respective within-plant coordination of transport efficiency and safety, and the underlying anatomical variations in xylem, are largely unexplored. We analysed Hedera helix, a liana widespread in European temperate forests, with respect to hydraulic and xylem anatomical variations between the main stem and branches, between juvenile and adult life phases, and along the vertical axis. Main stems were significantly less embolism resistant but exhibited a higher hydraulic conductivity than branches. In branches, the cell turgor loss point of leaves decreased, while the embolism resistance and conductivity of xylem, as well as conduit diameters, increased with height. High water-transport capacities allow ivy to compensate for the small cross-section of stems, while the limited resistance to drought-induced xylem dysfunction of the main stem is probably linked to conservative stomatal regulation. Pronounced differences in xylem anatomy, hydraulic efficiency, and safety between the main stem and branches and along the vertical axis are surprisingly similar to those of self-supporting plants, and indicate that the coordination of carbon and water economies requires similar internal adjustments in tall plants.

Variance in biomass-allocation fractions is explained by distribution in European trees

Intraspecific variability in ecological traits confers the ability of a species to adapt to an ever-changing environment. Fractions of biomass allocation in plants (BAFs) represent both ecological traits and direct expressions of investment strategies and so have important implications on plant fitness, particularly under current global change. We combined data on BAFs of trees in > 10 000 forest plots with their distributions in Europe. We aimed to test whether plant species with wider distributions have more or less variable intraspecific variance of the BAFs foliage-woody biomass and shoot-root ratios than species with limited distribution. Irrespective of corrections for tree age and phylogenetic relatedness, the standard deviation in BAFs was up to three times higher in species with the most extensive distributions than in those with the least extensive distribution due to a higher genetic diversity. Variance in BAFs also increased with latitude. We show that a combination of 36% tree genetic diversity and 64% environmental variability explains variance in BAFs and implies that changes in genetic diversity occur quickly. Genetic diversity should thus play a key role in regulating species responses to future climate change. Loss of habitat, even if transient, could induce a loss of genetic diversity and hinder species survival.

Testing ecological theory with lianas

Contents Summary 366 I. Introduction 366 II. Testing ecological theory: effects of the environment on lianas 369 III. A unified explanation for liana distribution and the maintenance of liana diversity 370 IV. Testing ecological theory: effects of lianas on the environment 373 V. Theoretical effects of lianas on forest diversity 375 VI. Lianas and trophic interactions in forests 375 VII. Unresolved challenges in liana ecology 376 VIII. Conclusions 377 Acknowledgements 377 References 377 SUMMARY: Lianas constitute a diverse polyphyletic plant group that is advancing our understanding of ecological theory. Specifically, lianas are providing new insights into the mechanisms that control plant distribution and diversity maintenance. For example, there is now evidence that a single, scalable mechanism may explain local, regional, and pan-tropical distribution of lianas, as well as the maintenance of liana species diversity. The ability to outcompete trees under dry, stressful conditions in seasonal forests provides lianas a growth advantage that, over time, results in relatively high abundance in seasonal forests and low abundance in aseasonal forests. Lianas may also gain a similar growth advantage following disturbance, thus explaining why liana density and diversity peak following disturbance at the local, forest scale. The study of ecology, however, is more than the effect of the environment on organisms; it also includes the effects of organisms on the environment. Considerable empirical evidence now indicates that lianas substantially alter their environment by consuming resources, suppressing tree performance, and influencing emergent properties of forests, such as ecosystem functioning, plant and animal diversity, and community composition. These recent studies using lianas are transcending classical tropical ecology research and are now providing novel insights into fundamental ecological theory.

Tropical dry forest trees and lianas differ in leaf economic spectrum traits but have overlapping water-use strategies

Tree species in tropical dry forests employ a wide range of strategies to cope with seasonal drought, including regulation of hydraulic function. However, it is uncertain if co-occurring lianas also possess a diversity of strategies. For a taxonomically diverse group of 14 tree and 7 liana species, we measured morphological and hydraulic functional traits during an unusual drought and under non-drought conditions to determine (i) if trees have different water-use strategies than lianas and (ii) if relationships among these traits can be used to better understand how tree and liana species regulate diurnal leaf water potential (Ψdiurnal). In this Costa Rican tropical dry forest, lianas and trees had overlapping water-use strategies, but differed in many leaf economic spectrum traits. Specifically, we found that both lianas and trees employed a diversity of Ψdiurnal regulation strategies, which did not differ statistically. However, lianas and trees did significantly differ in terms of certain traits including leaf area, specific leaf area, petiole length, wood vessel diameter and xylem vessel density. All liana and tree species we measured fell along a continuum of isohydric (partial) to anisohydric (strict or extreme) Ψdiurnal regulation strategies, and leaf area, petiole length, stomatal conductance and wood vessel diameter correlated with these strategies. These findings contribute to a trait-based understanding of how plants regulate Ψdiurnal under both drought stress and sufficient water availability, and underscore that lianas and trees employ a similarly wide range of Ψdiurnal regulation strategies, despite having vastly different growth forms.

Yield and resource use efficiency of Plukenetia volubilis plants at two distinct growth stages as affected by irrigation and fertilization

This study is to test how seedlings (vegetative) and large plants (reproductive) of an oilseed crop (Plukenetia volubilis) responded to regulated deficit irrigation techniques (conventional deficit irrigation, DI; alternative partial root-zone irrigation, APRI) in a tropical humid monsoon area. Seedlings were more sensitive to water deficit than large plants. Although APRI did better than DI in saving water for both seedlings and large plants at the same amount of irrigation, full irrigation (FI) is optimal for faster seedling growth at the expense of water-use efficiency (WUE). The seed number per unit area was responsible for the total seed oil yield, largely depending on the active process of carbon and nitrogen storages at the whole-plant level. The magnitude of the increase in total seed and seed oil yield by fertilization was similar under different irrigation regimes. Compared with FI, DI can save water, but reduced the total seed yield and had lower agronomic nutrient-use efficiency (NUEagr); whereas APRI had similar total seed yield and NUEagr, but reduced water use greatly. Although the dual goal of increasing the yield and saving water was not compatible, maintaining a high yield and NUEagr at the cost of WUE is recommended for P. volubilis plantation in t he water-rich areas.

Exploring the bark thickness-stem diameter relationship: clues from lianas, successive cambia, monocots and gymnosperms

Bark thickness is ecologically crucial, affecting functions from fire protection to photosynthesis. Bark thickness scales predictably with stem diameter, but there is little consensus on whether this scaling is a passive consequence of growth or an important adaptive phenomenon requiring explanation. With a comparative study across 913 species, we test the expectation that, if bark thickness-stem diameter scaling is adaptive, it should be possible to find ecological situations in which scaling is predictably altered, in this case between species with different types and deployments of phloem. 'Dicots' with successive cambia and monocots, which have phloem-free bark, had predictably thinner inner (mostly living) bark than plants with single cambia. Lianas, which supply large leaf areas with limited stem area, had much thicker inner bark than self-supporting plants. Gymnosperms had thicker outer bark than angiosperms. Inner bark probably scales with plant metabolic demands, for example with leaf area. Outer bark scales with stem diameter less predictably, probably reflecting diverse adaptive factors; for example, it tends to be thicker in fire-prone species and very thin when bark photosynthesis is favored. Predictable bark thickness-stem diameter scaling across plants with different photosynthate translocation demands and modes strongly supports the idea that this relationship is functionally important and adaptively significant.

Liana effects on biomass dynamics strengthen during secondary forest succession

Secondary forests are important carbon sinks, but their biomass dynamics vary markedly within and across landscapes. The biotic and abiotic drivers of this variation are still not well understood. We tested the effects of soil resource availability and competition by lianas on the biomass dynamics of young secondary tropical forests in Panama and assessed the extent to which liana effects were mediated by soil resource availability. Over a five-year period, growth, mortality, and recruitment of woody plants of ≥1 cm diameter were monitored in 84 plots in 3-30-year-old secondary forests across the Agua Salud site in central Panama. Biomass dynamics and the effects of lianas and soil resources were examined using (generalized) linear mixed-effect models and a model averaging approach. There was strong spatial and temporal variation in liana biomass within and across the plots. The relative biomass of lianas had a strong negative effect on overall tree growth, growth of understory trees decreased with soil fertility and dry season soil water content, and the effect of lianas on tree mortality varied with soil fertility. Tree recruitment was not associated with any of the predictor variables. Our model indicates that tree biomass growth across our landscape was reduced with 22% due to competition with lianas, and that the effect of lianas increased during succession, from 19% after five years to 32% after 30 years. The projected liana-induced growth reduction after 60 years was 47%, which was consistent with data from a nearby site. Our study shows that the observed liana proliferation across tropical forests may reduce the sequestration and storage of carbon in young secondary forests, with important implications for the carbon balance of tropical forest landscapes and consequently for global climate change. Our study highlights the need to incorporate lianas and soil variables in research on the biomass dynamics of secondary forest across tropical landscapes, and the need for well-replicated longitudinal studies to cover landscape-level variability in the relevant abiotic and biotic components.

Unravelling roots of lianas: a case study in Sapindaceae

Background and Aims Roots are key in the evolution of plants, being in charge of critical functions, such as water and nutrient uptake and anchorage of the plant body. Stems of lianescent Sapindaceae conform to the anatomical patterns typical of climbing plants, having cambial variants in their stems and vessel dimorphism in their wood. The roots of these lianas, however, are largely unexplored, so we do not know whether the plant habit has as strong an impact on their anatomy as on the anatomy of their stems. Our aim was, therefore, to thoroughly explore the anatomy of liana roots, underground organs under selective pressure completely different from that experienced by the stems. Methods We studied mature roots of 14 species belonging to five of the six genera currently recognized in the lianoid tribe Paullinieae (Sapindaceae) using traditional methods for macro- and microscopic analyses, as well as micro-computed tomography (micro-CT) techniques. Key Results Roots were shown to be strongly shaped by the lianescent habit in Paullinieae, exhibiting traits of the lianescent vascular syndrome in terms of both wood and overall anatomy. The only way to distinguish root from stem in secondary growth is by the exarch protoxylem position in the roots, as opposed to the endarch position typical of the stems. The most conspicuous trait of the lianescent vascular syndrome, which is the presence of vessel dimorphism, is evident in all roots, and we hypothesize that it helps to create an efficient, safe pathway for water conduction from this organ towards the stems. Other anatomical features present were parenchyma bands, present in the wood of almost all of the analysed species, except for Thinouia and Urvillea, where parenchyma-like fibre bands alternating with ordinary fibres are present. The majority of the roots showed no cambial variants. However, lobed roots were found in Urvillea rufescens and phloem wedges were observed in Serjania lethalis and Serjania caracasana. Neo-formed peripheral vascular strands and cylinders were common in mature roots of Serjania caracasana, and vascular connections were found uniting the peripheral and central vascular cylinders through phloem wedges, as revealed by anatomical and micro-CT analyses. The vascular connections likely represent another key mechanism to create a network that increases the area of vascular tissue and contributes as an additional conduction pathway within these thick roots. Conclusions Some traits from the lianescent vascular syndrome, such as vessel dimorphism, are present in the roots of lianescent Sapindaceae, while others, such as cambial variants common in the stems, are largely absent.

Short and Long-Term Soil Moisture Effects of Liana Removal in a Seasonally Moist Tropical Forest

Lianas (woody vines) are particularly abundant in tropical forests, and their abundance is increasing in the neotropics. Lianas can compete intensely with trees for above- and belowground resources, including water. As tropical forests experience longer and more intense dry seasons, competition for water is likely to intensify. However, we lack an understanding of how liana abundance affects soil moisture and hence competition with trees for water in tropical forests. To address this critical knowledge gap, we conducted a large-scale liana removal experiment in a seasonal tropical moist forest in central Panama. We monitored shallow and deep soil moisture over the course of three years to assess the effects of lianas in eight 0.64 ha removal plots and eight control plots. Liana removal caused short-term effects in surface soils. Surface soils (10 cm depth) in removal plots dried more slowly during dry periods and accumulated water more slowly after rainfall events. These effects disappeared within four months of the removal treatment. In deeper soils (40 cm depth), liana removal resulted in a multi-year trend towards 5–25% higher soil moisture during the dry seasons with the largest significant effects occurring in the dry season of the third year following treatment. Liana removal did not affect surface soil temperature. Multiple and mutually occurring mechanisms may be responsible for the effects of liana removal on soil moisture, including competition with trees, and altered microclimate, and soil structure. These results indicate that lianas influence hydrologic processes, which may affect tree community dynamics and forest carbon cycling.

Biomass allocation and long-term growth patterns of temperate lianas in comparison with trees

The host-dependent support habit of lianas is generally interpreted as a strategy designed to reduce resource investment in mechanical tissues; this allows preferential allocation to leaf and stem extension, thereby enhancing productivity and competitive abilities. However, this hypothesis has not been rigorously tested. We examined the aboveground allometries regarding biomass allocation (leaf mass and current-year stem mass (approximated as biomass allocated to extension growth) vs total aboveground mass) and long-term apparent growth patterns (height and aboveground mass vs age, i.e. numbers of growth rings) for nine deciduous liana species in Japan. Lianas had, on average, three- and five-fold greater leaf and current-year stem mass, respectively, than trees for a given aboveground mass, whereas the time course to reach the forest canopy was comparable and biomass accumulation during that period was only one-tenth that of co-occurring canopy trees. The balance between the lengths of yearly stem extension and existing older stems indicated that lianas lost c. 75% of stem length during growth to the canopy, which is probably a consequence of the host-dependent growth. Our observations suggest that, although lianas rely on hosts mechanically, allowing for short-term vigorous growth, this habit requires a large cost and could limit plant growth over protracted periods.

Species divergence and phylogenetic variation of ecophysiological traits in lianas and trees

The climbing habit is an evolutionary key innovation in plants because it is associated with enhanced clade diversification. We tested whether patterns of species divergence and variation of three ecophysiological traits that are fundamental for plant adaptation to light environments (maximum photosynthetic rate [A_max], dark respiration rate [R_d], and specific leaf area [SLA]) are consistent with this key innovation. Using data reported from four tropical forests and three temperate forests, we compared phylogenetic distance among species as well as the evolutionary rate, phylogenetic distance and phylogenetic signal of those traits in lianas and trees. Estimates of evolutionary rates showed that R_d evolved faster in lianas, while SLA evolved faster in trees. The mean phylogenetic distance was 1.2 times greater among liana species than among tree species. Likewise, estimates of phylogenetic distance indicated that lianas were less related than by chance alone (phylogenetic evenness across 63 species), and trees were more related than expected by chance (phylogenetic clustering across 71 species). Lianas showed evenness for R_d, while trees showed phylogenetic clustering for this trait. In contrast, for SLA, lianas exhibited phylogenetic clustering and trees showed phylogenetic evenness. Lianas and trees showed patterns of ecophysiological trait variation among species that were independent of phylogenetic relatedness. We found support for the expected pattern of greater species divergence in lianas, but did not find consistent patterns regarding ecophysiological trait evolution and divergence. R_d followed the species-level pattern, i.e., greater divergence/evolution in lianas compared to trees, while the opposite occurred for SLA and no pattern was detected for A_max. R_d may have driven lianas' divergence across forest environments, and might contribute to diversification in climber clades.

Gelatinous fibers and variant secondary growth related to stem undulation and contraction in a monkey ladder vine, Bauhinia glabra (Fabaceae)

PREMISE OF THE STUDY

Some of the most striking stem shapes occur in species of Bauhinia (Fabaceae) known as monkey ladder vines. Their mature stems are flattened and develop regular undulations. Although stems have variant (anomalous) secondary growth, the mechanism causing the undulations is unknown.

METHODS

We measured stem segments over time (20 mo), described stem development using light microscopy, and correlated the changes in stem shape with anatomy.

KEY RESULTS

Growing stems are initially straight and bear tendrils on short axillary branches. The inner secondary xylem has narrow vessels and lignified fibers. As stems age, they become flattened and increasingly undulated with the production of two lobes of outer secondary xylem (OX) with wide vessels and only gelatinous fibers (G-fibers). Similar G-fibers are present in the secondary phloem and the cortical sclerified layer. In transverse sections, the concave side of each undulation has a greater area and quantity of G-fibers than the opposite convex side. Some older stems are not undulated and have less lobing of OX. Undulation causes a shortening of the stem segments: up to 28% of the original length.

CONCLUSIONS

Uneven distribution of G-fibers produces tensions that are involved in the protracted development of undulations. While young extending shoots attach by lateral branch tendrils, older stems may maintain their position in the canopy using undulations and persistent branch bases as gripping devices. Flattened and undulated stems with G-fibers produce flexible woody stems.