Morphological and physiological adjustment to N and P fertilization in nutrient-limited Metrosideros polymorpha canopy trees in Hawaii

Effects of groundwater depth on ecological stoichiometric characteristics of assimilated branches and soil of two desert plants

Groundwater plays a crucial role in regulating plant growth in arid regions and has significant effects on plant physiological mechanisms. However, research on the influence of groundwater change on plant ecological stoichiometry is still limited. Therefore, this study was carried out to obtain the variations in assimilated branches and soil ecological stoichiometry of two dominant species in the Gurbantunggut Desert (Haloxylon ammodendron and Haloxylon persicum) at different groundwater depths to reveal the responses of desert plants to groundwater depth changes. The results showed that (1) H. persicum branches' stress tolerance indicators (C:N, C:P) are higher, while nutritional indicators (N:P) are lower. The soil nutrient of H. ammodendron is richer. (2) The ecological stoichiometry varied significantly along the groundwater gradient. With the deepening of groundwater, the branches C, N and P increased, and the variation in element ratio was inconsistent. Most of the soil properties was inversely proportional to the depth of groundwater. (3) Groundwater depth was a vital environmental factor affecting the assimilated branches ecological stoichiometry. Soil properties also had a significant influence on element accumulation in assimilated branches. (4) Regulating the allocation of branches ecological stoichiometry is an adaptation of two Haloxylon species to cope with local hydrological conditions changes. These findings provide novel insights into desert plant responses to different groundwater conditions within fragile desert ecosystems and may have implications for the implementation of effective measures related to the stability and sustainability of desert ecosystems.

Intraspecific and sex-dependent variation of leaf traits along altitude gradient in the endangered dioecious tree Taxus fuana Nan Li & R.R. Mill

Plant intraspecific trait variation (ITV) including sex-dependent differences are matters of many ecological consequences, from individual to ecosystem, especially in endangered and rare species. Taxus fuana is an endangered dioecious species with small and isolated populations endemic to the Himalayas region. Little is known about its trait variation between sexes, and among populations. In this study, 18 leaf traits from 179 reproductive trees (males and females) along the altitude (2600-3200m a.s.l.) of the T. fuana populations distributed in Gyirong County, Tibet, China, were measured. ITV and sources of variation in leaf traits were assessed. The relationship between leaf traits of males and females and altitude was analyzed separately. Variations in leaf traits of T. fuana ranged from 3.1% to 24.2%, with the smallest in leaf carbon content and the largest in leaf thickness to area ratio. On average 78.13% of the variation in leaf traits was from within populations and 21.87% among populations. The trends in leaf width, leaf nitrogen to phosphorus ratio, leaf carbon to nitrogen ratio, leaf carbon isotope ratio, and leaf nitrogen isotope ratio in relation to altitude were the same for males and females. Leaf length to width ratio varied significantly with altitude only in males, while leaf phosphorus content, leaf nitrogen content, and leaf carbon to phosphorus ratio varied significantly with altitude only in females. The correlation coefficients of most leaf traits of females with altitude were larger than that of males. In the relationship between leaf traits, there was a high similarity among males and females, but the altitude accounted for more explanation in females than in males. Our results suggested that the variation in leaf traits of T. fuana was small and did not dominate the interspecific competition in the local communities. Adaptation to the altitude gradient of T. fuana might be through altering nutrient storage processes and water use efficiency. Adaptation of male and female T. fuana to environmental changes showed differences, where the males were more tolerant and the females responded greatly to altitude. The differences in adaptation strategies between male and female T. fuana may be detrimental to the maintenance of their populations.

Carbon, Nitrogen, and Phosphorus Stoichiometry and Plant Growth Strategy as Related to Land-Use in Hangzhou Bay Coastal Wetland, China

Ecological stoichiometry can not only instruct soil nutrient stocks and availability, but also indicated plant growth strategy and adaptability to environmental changes or stress. This study was carried out to examine the plant-soil Carbon (C), Nitrogen (N), and Phosphorus (P) stoichiometry distributions and patterns in three tidal wetlands [mudflat (MF), native Phragmites australis-dominated community wetland (NW), invasive Spartina alterniflora-dominated community wetland (IW)], and one reclaimed P. australis-dominated community wetland (RW) in Hangzhou Bay coastal wetland. The results showed that land-uses have more effect on C and N contents, and C:N and N:P ratios in plant than in soil, P content and C:P ratios more affected by plant organ and soil depth. Compared to land-use, both plant organ and soil depth have stronger effects on C, N, and P stoichiometry. Among tidal wetlands, plant N content and C:P, N:P ratios were significantly higher in NW than in IW. In contrast, plant C, N, and P contents and C:P and N:P ratios were significantly lower in RW, and plant C:N was higher. Soil C, N, and P stocks were similar between tidal wetlands, and were significant higher than those of RW, indicating that reclamation were not beneficial to soil nutrient storage. In the NW, soil N availability was relatively high, and P availability was relatively low; and leaf N:P was 15.33, which means vegetation was co-limited by N and P nutrients. In addition, plants in the NW mainly adopted a conservative growth strategy, with a significantly low aboveground biomass of 1469.35 g·m². In the RW, soil N availability was relatively low, P availability was relatively high, and leaf N:P was 3, which means vegetation was limited by N nutrient. In addition, plants in the RW mainly adopted a rapid growth strategy, with a significantly high aboveground biomass of 3261.70 g·m². In the IW, soil N availability was relatively low, soil P availability was relatively high, and leaf N:P was 5.13, which means vegetation was limited by N nutrient. The growth strategy and aboveground biomass (2293.67 g·m²) of the IW were between those of the NW and RW. Our results provide a reference for nutrient management and evaluating the impacts of land-use types on coastal wetland ecosystems.

Early detection of a tree pathogen using airborne remote sensing

Native forests of Hawai'i Island are experiencing an ecological crisis in the form of Rapid 'Ōhi'a Death (ROD), a recently characterized disease caused by two fungal pathogens in the genus Ceratocystis. Since approximately 2010, this disease has caused extensive mortality of Hawai'i's keystone endemic tree, known as 'ōhi'a (Metrosideros polymorpha). Visible symptoms of ROD include rapid browning of canopy leaves, followed by death of the tree within weeks. This quick progression leading to tree mortality makes early detection critical to understanding where the disease will move at a timescale feasible for controlling the disease. We used repeat laser-guided imaging spectroscopy (LGIS) of forests on Hawai'i Island collected by the Global Airborne Observatory (GAO) in 2018 and 2019 to derive maps of foliar trait indices previously found to be important in distinguishing between ROD-infected and healthy 'ōhi'a canopies. Data from these maps were used to develop a prognostic indicator of tree stress prior to the visible onset of browning. We identified canopies that were green in 2018, but became brown in 2019 (defined as "to become brown"; TBB), and a corresponding set of canopies that remained green. The data mapped in 2018 showed separability of foliar trait indices between TBB and green 'ōhi'a, indicating early detection of canopy stress prior to the onset of ROD. Overall, a combination of linear and non-linear analyses revealed canopy water content (CWC), foliar tannins, leaf mass per area (LMA), phenols, cellulose, and non-structural carbohydrates (NSC) are primary drivers of the prognostic spectral capability which collectively result in strong consistent changes in leaf spectral reflectance in the near-infrared (700-1300 nm) and shortwave-infrared regions (1300-2500 nm). Results provide insight into the underlying foliar traits that are indicative of physiological responses of M. polymorpha trees infected with Ceratocycstis and suggest that imaging spectroscopy is an effective tool for identifying trees likely to succumb to ROD prior to the onset of visible symptoms.

Machine learning for the prediction of L. chinensis carbon, nitrogen and phosphorus contents and understanding of mechanisms underlying grassland degradation

The grasslands of Western Jilin Province in China have experienced severe degradation during the last 50 years. Radial basis function neural networks (RBFNN) and support vector machines (SVM) were used to predict the carbon, nitrogen, and phosphorus contents of Leymus chinensis (L. chinensis) and explore the degree of grassland degradation using the matter-element extension model. Both RBFNN and SVM demonstrated good prediction accuracy. The results indicated that there was severe degradation, as samples were mainly concentrated in the 3rd and 4th levels. The growth of L. chinensis was shown to be limited by either nitrogen, phosphorus, or both during different stages of degradation. The soil chemistry changed noticeably as degradation aggravated, which represents a destabilization of L. chinensis community homeostasis. Soil salinization aggravates soil nutrient loss and decreases the bioavailability of soil nutrients. This, along with the destabilization of C/N, C/P and N/P ratios, weakens the photosynthetic ability and productivity of L. chinensis. This conclusion was supported by observations that L. chinensis is gradually being replaced by a Chloris virgata, Puccinellia tenuiflora and Suaeda acuminate mixed community.

Leaf age dependent changes in within-canopy variation in leaf functional traits: a meta-analysis

Within-canopy variation in leaf structural and photosynthetic characteristics is a major means by which whole canopy photosynthesis is maximized at given total canopy nitrogen. As key acclimatory modifications, leaf nitrogen content (N A) and photosynthetic capacity (A A) per unit area increase with increasing light availability in the canopy and these increases are associated with increases in leaf dry mass per unit area (M A) and/or nitrogen content per dry mass and/or allocation. However, leaf functional characteristics change with increasing leaf age during leaf development and aging, but the importance of these alterations for within-canopy trait gradients is unknown. I conducted a meta-analysis based on 71 canopies that were sampled at different time periods or, in evergreens, included measurements for different-aged leaves to understand how within-canopy variations in leaf traits (trait plasticity) depend on leaf age. The analysis demonstrated that in evergreen woody species, M A and N A plasticity decreased with increasing leaf age, but the change in A A plasticity was less suggesting a certain re-acclimation of A A to altered light. In deciduous woody species, M A and N A gradients in flush-type species increased during leaf development and were almost invariable through the rest of the season, while in continuously leaf-forming species, the trait gradients increased constantly with increasing leaf age. In forbs, N A plasticity increased, while in grasses, N A plasticity decreased with increasing leaf age, reflecting life form differences in age-dependent changes in light availability and in nitrogen resorption for growth of generative organs. Although more work is needed to improve the coverage of age-dependent plasticity changes in some plant life forms, I argue that the age-dependent variation in trait plasticity uncovered in this study is large enough to warrant incorporation in simulations of canopy photosynthesis through the growing period.

Response of photosynthetic carbon gain to ecosystem retrogression of vascular plants and mosses in the boreal forest

In the long-term absence of rejuvenating disturbances, forest succession frequently proceeds from a maximal biomass phase to a retrogressive phase characterized by reduced nutrient availability [notably nitrogen (N) and phosphorus (P)] and net primary productivity. Few studies have considered how retrogression induces changes in ecophysiological responses associated with photosynthetic carbon (C) gain, and only for trees. We tested the hypothesis that retrogression would negatively impact photosynthetic C gain of four contrasting species, and that this impact would be greater for vascular plants (i.e., trees and shrubs) than for non-vascular plants (i.e., mosses). We used a 5,000-year-old chronosequence of forested islands in Sweden, where retrogression occurs in the long-term absence of lightning-ignited wildfires. Despite fundamental differences in plant form and ecological niche among species, vascular plants and mosses showed similar ecophysiological responses to retrogression. The most common effects of retrogression were reductions in photosynthesis and respiration per unit foliar N, increases in foliar N, δ(13)C and δ(15)N, and decreases in specific leaf areas. In contrast, photosynthesis per unit mass or area generally did not change along the chronosequence, but did vary many-fold between vascular plants and mosses. The consistent increases in foliar N without corresponding increases in mass- or area-based photosynthesis suggest that other factor(s), such as P co-limitation, light conditions or water availability, may co-regulate C gain in retrogressive boreal forests. Against our predictions, traits of mosses associated with C and N were generally highly responsive to retrogression, which has implications for how mosses influence ecosystem processes in boreal forests.

Cellulose (delta)18O is an index of leaf-to-air vapor pressure difference (VPD) in tropical plants

Cellulose in plants contains oxygen that derives in most cases from precipitation. Because the stable oxygen isotope composition, δ(18)O, of precipitation is associated with environmental conditions, cellulose δ(18)O should be as well. However, plant physiological models using δ(18)O suggest that cellulose δ(18)O is influenced by a complex mix of both climatic and physiological drivers. This influence complicates the interpretation of cellulose δ(18)O values in a paleo-context. Here, we combined empirical data analyses with mechanistic model simulations to i) quantify the impacts that the primary climatic drivers humidity (e(a)) and air temperature (T(air)) have on cellulose δ(18)O values in different tropical ecosystems and ii) determine which environmental signal is dominating cellulose δ(18)O values. Our results revealed that e(a) and T(air) equally influence cellulose δ(18)O values and that distinguishing which of these factors dominates the δ(18)O values of cellulose cannot be accomplished in the absence of additional environmental information. However, the individual impacts of e(a) and T(air) on the δ(18)O values of cellulose can be integrated into a single index of plant-experienced atmospheric vapor demand: the leaf-to-air vapor pressure difference (VPD). We found a robust relationship between VPD and cellulose δ(18)O values in both empirical and modeled data in all ecosystems that we investigated. Our analysis revealed therefore that δ(18)O values in plant cellulose can be used as a proxy for VPD in tropical ecosystems. As VPD is an essential variable that determines the biogeochemical dynamics of ecosystems, our study has applications in ecological-, climate-, or forensic-sciences.

Leaf nitrogen:phosphorus stoichiometry across Chinese grassland biomes

Leaf N and P stoichiometry covaries with many aspects of plant biology, yet the drivers of this trait at biogeographic scales remain uncertain. Recently we reported the patterns of leaf C and N based on systematic census of 213 species over 199 research sites in the grassland biomes of China. With the expanded analysis of leaf P, here we report patterns of leaf P and N:P ratios, and analyze the relative contribution of climatic variables and phylogeny in structuring patterns of leaf N:P stoichiometry. Average values of leaf P and N:P ratio were 1.9 mg g(-1) and 15.3 (mass ratio), respectively, consistent with the previous observation of a higher N:P ratio in China's flora than the global averages (ca. 13.8), resulting from a lower leaf P. Climatic variables had very little direct correlation with leaf P and N:P ratios, with growing season precipitation and temperature together explaining less than 2% of the variation, while inter-site differences and within-site phylogenetic variation explained 55 and 26% of the total variation in leaf P and N:P ratios. Across all sites and species, leaf N and P were highly positively correlated at all levels. However, the within-site, within-species covariations of leaf N and P were weaker than those across sites and across species. Leaf N and P relationships are driven by both variation between sites at the landscape scale (explaining 58% of the variance) and within sites at the local scale (explaining 24%), while the climatic factors exerted limited influence (explaining less than 3%). In addition, leaf N:P ratios in two dominant genera Kobresia and Stipa had different responses to precipitation. This study suggests that geographic variation and between-species variation, rather than climatic variation, are the major determinants of grassland foliar stoichiometry at the biome level.

Richness and species composition of arboreal arthropods affected by nutrients and predators: a press experiment

A longstanding goal for ecologists is to understand the processes that maintain biological diversity in communities, yet few studies have investigated the combined effects of predators and resources on biodiversity in natural ecosystems. We fertilized nutrient limited plots and excluded insectivorous birds in a randomized block design, and examined the impacts on arthropods associated with the dominant tree in the Hawaiian Islands, Metrosideros polymorpha (Myrtaceae). After 33 months, the species load (per foliage mass) of herbivores and carnivores increased with fertilization, but rarified richness (standardized to abundance) did not change. Fertilization depressed species richness of arboreal detritivores, and carnivore richness dropped in caged, unfertilized plots, both because of the increased dominance of common, introduced species with treatments. Herbivore species abundance distributions were more equitable than other trophic levels following treatments, and fertilization added specialized native species without changing relativized species richness. Overall, bird removal and nutrient addition treatments on arthropod richness acted largely independently, but with countervailing influences that obscured distinct top-down and bottom-up effects on different trophic levels. This study demonstrates that species composition, biological invasions, and the individuality of species traits may complicate efforts to predict the interactive effects of resources and predation on species diversity in food webs.

Photosynthesis and reflectance indices for rainforest species in ecosystems undergoing progression and retrogression along a soil fertility chronosequence in New Zealand

Measurements of photosynthesis at saturating irradiance and CO2 partial pressure, Amax, "adjusted" normalised difference vegetation index, RaNDVI, and photochemical reflectance index, RPRI, were made on trees sampled along a soil chronosequence to investigate the relationship between carbon uptake and ecosystem development in relation to nutrient availability. Measurements were made on the three most dominant species at six sites along the sequence in South Westland, New Zealand with soil age ranging from < 6 to 120,000 years resulting from the retreat of the Franz Josef glacier. The decrease in soil phosphorus availability with increasing soil age and high soil nitrogen availability at the two youngest sites, due to the presence of a nitrogen-fixing species, provided marked differences in nutrient availability. Mean Amax was high at the two youngest sites, then decreased markedly with increasing site age. Analysis of the data for individual species within sites revealed separation of groups of species in the response of Amax to Nm and Pm, suggesting complex interactions between the two nutrients. There were strong linear relationships for leaf-level RaNDVI and RPRI with Amax, at high irradiance, showing that measurements of reflectance indices can be used to estimate Amax for foliage with a range in morphology and nutrient concentrations. Notwithstanding the change in species composition from angiosperms to conifers with increasing site age, the presence of nitrogen-fixing species, the variability in foliage morphology from flat leaves to imbricate scales and a wide range in foliar nitrogen and phosphorus concentrations, there were strong positive linear relationships between site average Amax and foliage nitrogen, Nm, and phosphorus, Pm, concentrations on a foliage mass basis. The results provide insights to interpret the regulation of photosynthesis across natural ecosystems with marked gradients in nitrogen and phosphorus availability.

Adaptive radiation of photosynthetic physiology in the Hawaiian lobeliads: light regimes, static light responses, and whole-plant compensation points

Six endemic genera/sections of lobeliads (Campanulaceae) occupy nearly the full range of light regimes on moist sites in the Hawaiian Islands, from open alpine bogs and seacliffs to densely shaded rainforest interiors. To determine whether this clade has undergone a corresponding adaptive radiation in photosynthetic adaptations, we studied the natural light habitats and physiological characteristics of 11 species representing each sublineage. Across species in the field, average photon flux density (PFD) varies from 2.3 to 30.0 mol · m(-2) · d(-1), and maximum assimilation rate (A(max)) ranges from 0.17 to 0.35 μmol CO(2) · g(-1) · s(-1). Across species, A(max), dark respiration rate (R), Michaelis-Menten constant (k), light compensation point, specific leaf area (SLA), maximum carboxylation rate (V(cmax)), maximum rate of electron transport (J(max)), photosynthesis at saturating CO(2) (A(satCO(2))), and carboxylation efficiency (α) all increase significantly and in tightly coupled fashion with PFD, in accord with classical economic theory. Area-based rates have a higher degree of physiological integration with each other and tighter coupling to PFD than the corresponding mass-based rates, despite the energetic importance of the latter. Area-based rates frequently show adaptive cross-over: high-light species outperform low-light species at high PFD and vice versa at low PFD. A(max)-mass has little relationship to leaf mass per unit area (LMA), leaf N content, or leaf lifespan individually, but a multiple regression explains 96% of the variance in A(max)-mass across species in terms of SLA, leaf N content, and average PFD. Instantaneous leaf compensation points range from 0.1 to 1.2% full sunlight, far lower than the ecological (whole-plant) compensation points (ECPs) of 1.1 to 29.0% sunlight calculated based on photosynthetic parameters, leaf longevity, and allocation to leaf vs. nonleaf tissue. The ECPs are much closer to the lower limits of PFD actually experienced by lobeliads, suggesting they may play an important role in restricting species distributions. Taken together, these data provide evidence for an adaptive radiation in photosynthetic traits that is strongly correlated with-and indeed may help determine-the light regime that each species inhabits.