Struggle in the flood: tree responses to flooding stress in four tropical floodplain systems

Double stress of waterlogging and drought drives forest-savanna coexistence

Forest-savanna boundaries are ecotones that support complex ecosystem functions and are sensitive to biotic/abiotic perturbations. What drives their distribution today and how it may shift in the future are open questions. Feedbacks among climate, fire, herbivory, and land use are known drivers. Here, we show that alternating seasonal drought and waterlogging stress favors the dominance of savanna-like ecosystems over forests. We track the seasonal water-table depth as an indicator of water stress when too deep and oxygen stress when too shallow and map forest/savanna occurrence within this double-stress space in the neotropics. We find that under a given annual precipitation, savannas are favored in landscape positions experiencing double stress, which is more common as the dry season strengthens (climate driver) but only found in waterlogged lowlands (terrain driver). We further show that hydrological changes at the end of the century may expose some flooded forests to savanna expansion, affecting biodiversity and soil carbon storage. Our results highlight the importance of land hydrology in understanding/predicting forest-savanna transitions in a changing world.

Functional responses to multiple sequential abiotic stress (waterlogging-drought) in three woody taxa with different root systems and stress tolerance

There is generally a trade-off in the resistance to drought and to waterlogging. However, several species are sequentially subjected to both stressors in many environments. We evaluated the ecophysiological strategies to cope with multiple sequential stress of waterlogging and drought (W + D) of three taxa differing in stress resistance and root morphology: the phreatophic Eucalyptus camaldulensis (Ec) and two shallow-rooted willow clones: Salix matsudana x Salix alba (SmxSa) and Salix nigra (Sn4). Individuals of the three taxa were grown in pots and assigned to either of four treatments: Control (well-watered plants), well-watered followed by drought (C + D); waterlogged for 15 days followed by drought (W15d + D) and waterlogged for 30 days followed by drought (W30d + D). Biomass allocation, growth (diameter, height, length of leaves, and roots), specific leaf area, stomatal conductance, water potential, hydraulic conductivity of roots and branches, leaf C13 and root cortical aerenchyma formation were determined at different stages of the experiment. Ec growth was not affected by W + D, developing tolerance strategies at leaf and whole plant levels. Differential effects of W + D were observed in both Salix clones depending on the time of waterlogging. In Sn4 and SmxSa, the root biomass was affected in W15d + D treatment, but a root tolerance response (aerenchyma and adventitious root formation) was observed in W30d + D. In the three taxa, and contrary to expectations, the previous exposure to a waterlogging period did not increase the susceptibility of the plants to a subsequent drought event. On the contrary, we found tolerance, which depended on the time of waterlogging exposure.

Biophysical and biochemical features' feedback associated with a flood episode in a tropical river basin model

Global climate change scenarios such as frequent and extreme floods disturb the river basins by destructing the vegetation resulting in rehabilitation procedures being more costly. Thus, understanding the recovery and regeneration of vegetation followed by extreme flood events is critical for a successful rehabilitation process. Spatial and temporal variation of biochemical and biophysical features derived from remote sensing technology in vegetation can be incorporated to understand the recovery and regeneration of vegetation. The present study explores the flood impact on vegetation caused by major river basins in Sri Lanka (a model tropical river basin) by comparing pre-flood and post-flood cases. The study utilized enhanced vegetation index (EVI), normalized difference vegetation index (NDVI), the fraction of photosynthetically active radiation (FPAR), and gross primary productivity (GPP) of the Moderate Resolution Imaging Spectroradiometer (MODIS) platform. A remarkable decline in EVI, LAI, FPAR, GPP, and vegetation condition index was observed in the post-flood case. Notably, coupled GPP-EVI and GPP-LAI portrayed dependency of features and showed a significant impact triggered by the flood episode by narrowing the feature in post-flood events. EVI depicted the highest regeneration (0.333) while GPP presented the lowest regeneration (0.093) after the flood event. Further, it was revealed that 1.18 years have been on the regeneration. The regeneration of GPP and LAI remained low comparatively justifying the magnitude and impact of the flood event. The study revealed successful implications of vegetation indices on flood basin management of small to large tropical river basins.

Changes in floodplain hydrology following serial damming of the Tocantins River in the eastern Amazon

Riparian forests are ecotones that link aquatic and terrestrial habitats, providing ecosystem services including sediment control and nutrient regulation. Riparian forest function is intimately linked to river hydrology and floodplain dynamics, which can be severely altered by dams. The Tocantins River in the eastern Amazon has six mega-dams along its course. To understand the large-scale and cumulative impacts of multiple dams on the Tocantins floodplain, we quantified landscape-scale changes in floodplain extent, hydroperiod, and flood timing on a 145-km stretch of the river downstream of five dams. We used water level data from 1985 to 2019 to compare daily floodplain inundation dynamics before and after damming. We also developed models to examine the impacts of climate and land use change on hydrology of the Tocantins River. After installation of the first dam in 1998, an average of 82.3 km² (63%) of the floodplain no longer flooded, overall average hydroperiod decreased by 15 days (11%), and flooding started an average of five days earlier. After all five dams were installed, 72% of the average pre-dam flooded area no longer flooded, average hydroperiod had decreased by 35%, and average inundation onset occurred 12 days later. These changes in floodplain hydrology appeared to be driven primarily by dam operations as we found no significant changes in precipitation over the study period. Increasing loss of natural vegetation in the watershed may play a role in changed hydrology but cannot explain the abrupt loss of floodplain extent after the first dam was installed. This is one of few studies to quantify dam-induced floodplain alteration at a landscape scale and to investigate impacts of multiple dams on a landscape. Our results indicate that the Tocantins River floodplain is undergoing drastic hydrologic alteration. The impacts of multiple dams are cumulative and non-linear, especially for hydroperiod and flood timing.

Non-Structural Flood Management in European Rural Mountain Areas—Are Scientists Supporting Implementation?

Mountain areas are highly exposed to flood risks. The latter are increasing in the context of climate change, urbanization, and land use changes. Non-structural approaches such as nature-based solutions can provide opportunities to reduce the risks of such natural hazards and provide further ecological, social, and economic benefits. However, few non-structural flood mitigation measures are implemented in rural mountain areas so far. The objective of this paper is to investigate if the scientific boundaries limit the implementation of non-structural flood management in rural mountain areas. In the study, we statistically analyzed the knowledge about flood management through a systematic literature review and expert surveys, with a focus on European rural mountain areas. Both methods showed that scientific knowledge is available for decision makers and that nature-based solutions are efficient, cost-effective, multifunctional, and have potential for large-scale implementation.

Exploring the relationship between soil and plant evolutionary diversity in the Roraima table mountain OCBIL, Guayana Highlands

OCBIL theory aims to develop hypotheses that explain the evolutionary ecology of biota on old, climatically buffered, infertile landscapes (OCBILs). The table mountain OCBILs of the Guayana Highlands are a major centre of Neotropical plant diversity; however, knowledge about the underlying mechanisms of plant community assembly at the mountaintops is still limited. We evaluated the effect of a fine-scale geoenvironmental gradient on the phylogenetic metrics of plant communities on the iconic, highly isolated Roraima table mountain of the Guayana Highlands. We selected three specific geoenvironments: Bonnetia-shrubby rupestrian grassland, peaty rupestrian grassland and sandy rupestrian grassland. We evaluated evolutionary history using species richness and phylogenetic metrics and analysed effects of the soil on phylogenetic metrics using linear models. Of the 55 species surveyed, 40% are endemic to the Guayana Highlands. Bonnetia-shrubby rupestrian grassland showed higher species richness (47) than peaty rupestrian grassland (30) and sandy rupestrian grassland (24). We observed significant differences in phylogenetic diversity and structure among geoenvironments. The highest phylogenetic diversity was found for Bonnetia-shrubby rupestrian grassland and sandy rupestrian grassland. Peaty rupestrian grassland had significantly lower mean pairwise phylogenetic distance and all standardized phylogenetic metrics. The values of standardized phylogenetic metrics were close to zero, suggesting random phylogenetic structure. The linear models revealed that soil texture explained most of the variation in phylogenetic metrics. Our results might be related to the long geological history of tepuis and the accumulation of lineages over tens of millions of years, which supports the OCBIL theory.

Research Trends and Methodological Approaches of the Impacts of Windstorms on Forests in Tropical, Subtropical, and Temperate Zones: Where Are We Now and How Should Research Move Forward?

Windstorm is one of the destructive natural disturbances, but the scale-link extent to which recurrent windstorms influenced forests ecosystems is poorly understood in a changing climate across regions. We reviewed the synergistic impacts of windstorms on forests and assessed research trends and methodological approaches from peer-reviewed articles published from 2000 to 2020 in tropical (TRF), subtropical (SUF), and temperate (TEF) forests/zones, based on the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines. Overall, the majority of the reviewed studies were conducted in TRF (i.e., 40%), intermediate in SUF (i.e., 34%), and the lowest in TEF (i.e., 26%). Among the four levels of biological organization, the species-population and community-ecosystem levels had the highest number of study cases, while the molecular-cellular-individual and landscape levels had the lowest study cases in all forest types. Most of the articles reviewed dealt largely on tree mortality/survival and regeneration/succession for TRF, tree mortality/survival and species composition/richness/diversity for SUF, and stem density, gap dynamics, and regeneration/succession for TEF. However, research on the effects of windstorms on mycorrhizal symbioses, population genetics, and physiological adaptation, element fluxes via litterfall, litter decomposition, belowground processes, biological invasion, and tree health are less common in all forest types. Further, most of the studies were conducted in permanent plots but these studies mostly used observational design, while controlled studies are obviously limited. Consequently, more observational and controlled studies are needed on the topic reviewed, particularly studies at the molecular-cellular-individual and landscape levels, to help inform forest management decision-making about developing sustainable and resilient forests amid climate change.

Functional Divergence between Várzea and Igapó Forests: A Study of Functional Trait Diversity in the Colombian Orinoco Basin

Research Highlights: Functional diversity studies help to better understand how organisms respond to different environmental conditions. Conditions in tropical flooded forests are highly variable, including levels of nutrient availability, pH, and flood depth, but few studies have explored the impact of variation in these factors on plant functional diversity. Background and Objectives: In the Orinoco basin, as in the Amazon, floodplain forests have been classified into várzea (white-water rivers, with nutrient-rich soils) and igapó (black-water rivers, associated with nutrient-poor soils). We evaluated the functional diversity of plant species in várzea and igapó, as well as the influence of external and internal filters on the plant community assembly of each forest type, and compared our results with studies in the Amazon basin. Materials and Methods: Six functional traits were recorded in the várzea and igapó forests of the Colombian Orinoco basin (one-hectare plot for each forest type, with no replicates). We evaluated plant species diversity (richness, Fisher’s α, Shannon and Simpson indices), as well as functional diversity (functional richness, functional evenness, functional divergence, and functional dispersion) and the influence of external and internal filters, based on a comparison of variance at different organizational levels. Results: A high functional differentiation between várzea and igapó was found, as well as a high functional divergence within each forest type. We also observed a greater influence of internal filters on the community assembly of both forest types, compared to external filters. Functional traits such as wood density and leaf dry matter content, showed the same patterns as the várzea and igapó forests in the Amazon. Conclusions: Despite the low taxonomic and functional richness, there is high functional divergence within flooded forests. We also show that in forests under stress (e.g., from flooding), internal filters can be key in assembling communities and promote high functional divergence. Given that the functional diversity of the várzea and igapó in the Orinoco is largely unexplored, we highlight the need for more research for the effective conservation of these flooded forests.

Convergent evolution of tree hydraulic traits in Amazonian habitats: implications for community assemblage and vulnerability to drought

Amazonian droughts are increasing in frequency and severity. However, little is known about how this may influence species-specific vulnerability to drought across different ecosystem types. We measured 16 functional traits for 16 congeneric species from six families and eight genera restricted to floodplain, swamp, white-sand or plateau forests of Central Amazonia. We investigated whether habitat distributions can be explained by species hydraulic strategies, and if habitat specialists differ in their vulnerability to embolism that would make water transport difficult during drought periods. We found strong functional differences among species. Nonflooded species had higher wood specific gravity and lower stomatal density, whereas flooded species had wider vessels, and higher leaf and xylem hydraulic conductivity. The P₅₀ values (water potential at 50% loss of hydraulic conductivity) of nonflooded species were significantly more negative than flooded species. However, we found no differences in hydraulic safety margin among species, suggesting that all trees may be equally likely to experience hydraulic failure during severe droughts. Water availability imposes a strong selection leading to differentiation of plant hydraulic strategies among species and may underlie patterns of adaptive radiation in many tropical tree genera. Our results have important implications for modeling species distribution and resilience under future climate scenarios.

Stomatal behavior following mid- or long-term exposure to high relative air humidity: A review

High relative air humidity (RH ≥ 85%) is frequent in controlled environments, and not uncommon in nature. In this review, we examine the high RH effects on plants with a special focus on stomatal characters. All aspects of stomatal physiology are attenuated by elevated RH during leaf expansion (long-term) in C₃ species. These include impaired opening and closing response, as well as weak diel oscillations. Consequently, the high RH-grown plants are not only vulnerable to biotic and abiotic stress, but also undergo a deregulation between CO₂ uptake and water loss. Stomatal behavior of a single leaf is determined by the local microclimate during expansion, and may be different than the remaining leaves of the same plant. No effect of high RH is apparent in C₄ and CAM species, while the same is expected for species with hydropassive stomatal closure. Formation of bigger stomata with larger pores is a universal response to high RH during leaf expansion, whereas the effect on stomatal density appears to be species- and leaf side-specific. Compelling evidence suggests that ABA mediates the high RH-induced stomatal malfunction, as well as the stomatal size increase. Although high RH stimulates leaf ethylene evolution, it remains elusive whether or not this contributes to stomatal malfunction. Most species lose stomatal function following mid-term (4-7 d) exposure to high RH following leaf expansion. Consequently, the regulatory role of ambient humidity on stomatal functionality is not limited to the period of leaf expansion, but holds throughout the leaf life span.

Determinants of tree cover in tropical floodplains

Tree cover differentiates forests from savannas and grasslands. In tropical floodplains, factors differentiating these systems are poorly known, even though floodplains cover 10% of the tropical landmass. Seasonal inundation potentially presents trees with both challenges (soil anoxia) and benefits (moisture and nutrient deposition), the relative importance of which may depend on ecological context, e.g. if floods alleviate water stress more in more arid ecosystems. Here, we use remotely sensed data across 13 large tropical and sub-tropical floodplain ecosystems on five continents to show that climatic water balance (i.e. precipitation-potential evapotranspiration) strongly increases floodplain tree cover in interaction with flooding, fire and topography. As predicted, flooding increases tree cover in more arid floodplains, but decreases tree cover in climatically wetter ones. As in uplands, frequent fire reduced tree cover, particularly in wet regions, but-in contrast with uplands-lower elevation and sandier soils decreased tree cover. Our results suggest that predicting the impacts of changing climate, land use and hydrology on floodplain ecosystems depends on considering climate-disturbance interactions. While outright wetland conversion proceeds globally, additional anthropogenic activities, including alteration of fire frequencies and dam construction, will also shift floodplain tree cover, especially in wet climates.

Trait divergence and habitat specialization in tropical floodplain forests trees

Habitat heterogeneity of tropical forests is thought to lead to specialization in plants and contribute to the high diversity of tree species in Amazonia. One prediction of habitat specialization is that species specialized for resource-rich habitats will have traits associated with high resource acquisition and fast growth while species specialized for resource-poor habitats will have traits associated with high resource conservation and persistence but slow growth. We tested this idea for seven genera and for twelve families from nutrient-rich white-water floodplain forest (várzea) and nutrient-poor black-water (igapó) floodplain forest. We measured 11 traits that are important for the carbon and nutrient balance of the trees, and compared trait variation between habitat types (white- and black-water forests), and the effect of habitat and genus/family on trait divergence. Functional traits of congeneric species differed between habitat types, where white-water forest species invested in resource acquisition and productive tissues, whereas black-water forest species invested in resource conservation and persistent tissues. Habitat specialization is leading to the differentiation of floodplain tree species of white-water and black-water forests, thus contributing to a high diversity of plant species in floodplain forests.

Environmental filtering of eudicot lineages underlies phylogenetic clustering in tropical South American flooded forests

The phylogenetic community assembly approach has been used to elucidate the role of ecological and historical processes in shaping tropical tree communities. Recent studies have shown that stressful environments, such as seasonally dry, white-sand and flooded forests tend to be phylogenetically clustered, arguing for niche conservatism as the main driver for this pattern. Very few studies have attempted to identify the lineages that contribute to such assembly patterns. We aimed to improve our understanding of the assembly of flooded forest tree communities in Northern South America by asking the following questions: are seasonally flooded forests phylogenetically clustered? If so, which angiosperm lineages are over-represented in seasonally flooded forests? To assess our hypotheses, we investigated seasonally flooded and terra firme forests from the Magdalena, Orinoco and Amazon Basins, in Colombia. Our results show that, regardless of the river basin in which they are located, seasonally flooded forests of Northern South America tend to be phylogenetically clustered, which means that the more abundant taxa in these forests are more closely related to each other than expected by chance. Based on our alpha and beta phylodiversity analyses we interpret that eudicots are more likely to adapt to extreme environments such as seasonally flooded forests, which indicates the importance of environmental filtering in the assembly of the Neotropical flora.

Molecular and physiological responses of trees to waterlogging stress

One major effect of global climate change will be altered precipitation patterns in many regions of the world. This will cause a higher probability of long-term waterlogging in winter/spring and flash floods in summer because of extreme rainfall events. Particularly, trees not adapted at their natural site to such waterlogging stress can be impaired. Despite the enormous economic, ecological and social importance of forest ecosystems, the effect of waterlogging on trees is far less understood than the effect on many crops or the model plant Arabidopsis. There is only a handful of studies available investigating the transcriptome and metabolome of waterlogged trees. Main physiological responses of trees to waterlogging include the stimulation of fermentative pathways and an accelerated glycolytic flux. Many energy-consuming, anabolic processes are slowed down to overcome the energy crisis mediated by waterlogging. A crucial feature of waterlogging tolerance is the steady supply of glycolysis with carbohydrates, particularly in the roots; stress-sensitive trees fail to maintain sufficient carbohydrate availability resulting in the dieback of the stressed tissues. The present review summarizes physiological and molecular features of waterlogging tolerance of trees; the focus is on carbon metabolism in both, leaves and roots of trees.

The flood pulse as the underlying driver of vegetation in the largest wetland and fishery of the Mekong Basin

The Tonle Sap is the largest wetland in Southeast Asia and one of the world's most productive inland fisheries. The Mekong River inundates the Tonle Sap every year, shaping a mosaic of natural and agricultural habitats. Ongoing hydropower development, however, will dampen the flood pulse that maintains the Tonle Sap. This study established the current underlying relationship among hydrology, vegetation, and human use. We found that vegetation is strongly influenced by flood duration; however, this relationship was heavily distorted by fire, grazing, and rice cultivation. The expected flood pulse alteration will result in higher water levels during the dry season, permanently inundating existing forests. The reduction of the maximum flood extent will facilitate agricultural expansion into natural habitats. This study is the most comprehensive field survey of the Tonle Sap to date, and it provides fundamental knowledge needed to understand the underlying processes that maintain this important wetland.

Responses to flooding of plant water relations and leaf gas exchange in tropical tolerant trees of a black-water wetland

This review summarizes the research on physiological responses to flooding of trees in the seasonal black-water wetland of the Mapire River in Venezuela. Inter-annual variability was found during 8 years of sampling, in spite of which a general picture emerged of increased stomatal conductance (gs) and photosynthetic rate (PN) during the flooded period to values as high as or higher than in plants in drained wet soil. Models explaining the initial inhibitory responses and the acclimation to flooding are proposed. In the inhibitory phase of flooding, hypoxia generated by flooding causes a decrease in root water absorption and stomatal closure. An increase with flooding in xylem water potential (ψ) suggests that flooding does not cause water deficit. The PN decreases due to changes in relative stomatal and non-stomatal limitations to photosynthesis; an increase in the latter is due to reduced chlorophyll and total soluble protein content. Total non-structural carbohydrates (TNC) accumulate in leaves but their content begins to decrease during the acclimatized phase at full flooding, coinciding with the resumption of high gs and PN. The reversal of the diminution in gs is associated, in some but not all species, to the growth of adventitious roots. The occurrence of morpho-anatomical and biochemical adaptations which improve oxygen supply would cause the acclimation, including increased water absorption by the roots, increased rubisco and chlorophyll contents and ultimately increased PN. Therefore, trees would perform as if flooding did not signify a stress to their physiology.

A plant's perspective of extremes: terrestrial plant responses to changing climatic variability

We review observational, experimental, and model results on how plants respond to extreme climatic conditions induced by changing climatic variability. Distinguishing between impacts of changing mean climatic conditions and changing climatic variability on terrestrial ecosystems is generally underrated in current studies. The goals of our review are thus (1) to identify plant processes that are vulnerable to changes in the variability of climatic variables rather than to changes in their mean, and (2) to depict/evaluate available study designs to quantify responses of plants to changing climatic variability. We find that phenology is largely affected by changing mean climate but also that impacts of climatic variability are much less studied, although potentially damaging. We note that plant water relations seem to be very vulnerable to extremes driven by changes in temperature and precipitation and that heat-waves and flooding have stronger impacts on physiological processes than changing mean climate. Moreover, interacting phenological and physiological processes are likely to further complicate plant responses to changing climatic variability. Phenological and physiological processes and their interactions culminate in even more sophisticated responses to changing mean climate and climatic variability at the species and community level. Generally, observational studies are well suited to study plant responses to changing mean climate, but less suitable to gain a mechanistic understanding of plant responses to climatic variability. Experiments seem best suited to simulate extreme events. In models, temporal resolution and model structure are crucial to capture plant responses to changing climatic variability. We highlight that a combination of experimental, observational, and/or modeling studies have the potential to overcome important caveats of the respective individual approaches.