Summary
Tropical ecosystems have the highest levels of biodiversity, cycle more water and absorb more carbon than any other terrestrial ecosystem on Earth. Consequently, these ecosystems are ...extremely important components of Earth’s climatic system and biogeochemical cycles. Plant hydraulics is an essential discipline to understand and predict the dynamics of tropical vegetation in scenarios of changing water availability. Using published plant hydraulic data we show that the trade‐off between drought avoidance (expressed as deep‐rooting, deciduousness and capacitance) and hydraulic safety (P50 – the water potential when plants lose 50% of their maximum hydraulic conductivity) is a major axis of physiological variation across tropical ecosystems. We also propose a novel and independent axis of hydraulic trait variation linking vulnerability to hydraulic failure (expressed as the hydraulic safety margin (HSM)) and growth, where inherent fast‐growing plants have lower HSM compared to slow‐growing plants. We surmise that soil nutrients are fundamental drivers of tropical community assembly determining the distribution and abundance of the slow‐safe/fast‐risky strategies. We conclude showing that including either the growth‐HSM or the resistance‐avoidance trade‐off in models can make simulated tropical rainforest communities substantially more vulnerable to drought than similar communities without the trade‐off. These results suggest that vegetation models need to represent hydraulic trade‐off axes to accurately project the functioning and distribution of tropical ecosystems.
Foliar water uptake (FWU) is a common water acquisition mechanism for plants inhabiting temperate fog-affected ecosystems, but the prevalence and consequences of this process for the water and carbon ...balance of tropical cloud forest species are unknown.
We performed a series of experiments under field and glasshouse conditions using a combination of methods (sap flow, fluorescent apoplastic tracers and stable isotopes) to trace fog water movement from foliage to belowground components of Drimys brasiliensis. In addition, we measured leaf water potential, leaf gas exchange, leaf water repellency and growth of plants under contrasting soil water availabilities and fog exposure in glasshouse experiments to evaluate FWU effects on the water and carbon balance of D. brasiliensis saplings.
Fog water diffused directly through leaf cuticles and contributed up to 42% of total foliar water content. FWU caused reversals in sap flow in stems and roots of up to 26% of daily maximum transpiration. Fog water transported through the xylem reached belowground pools and enhanced leaf water potential, photosynthesis, stomatal conductance and growth relative to plants sheltered from fog.
Foliar uptake of fog water is an important water acquisition mechanism that can mitigate the deleterious effects of soil water deficits for D. brasiliensis.
Understanding the mechanisms controlling the distribution of biomes remains a challenge. Although tropical biome distribution has traditionally been explained by climate and soil, contrasting ...vegetation types often occur as mosaics with sharp boundaries under very similar environmental conditions. While evidence suggests that these biomes are alternative states, empirical broad‐scale support to this hypothesis is still lacking. Using community‐level field data and a novel resource‐niche overlap approach, we show that, for a wide range of environmental conditions, fire feedbacks maintain savannas and forests as alternative biome states in both the Neotropics and the Afrotropics. In addition, wooded grasslands and savannas occurred as alternative grassy states in the Afrotropics, depending on the relative importance of fire and herbivory feedbacks. These results are consistent with landscape scale evidence and suggest that disturbance is a general factor driving and maintaining alternative biome states and vegetation mosaics in the tropics.
Many tropical montane cloud forest (TMCF) trees are capable of foliar water uptake (FWU) during leaf-wetting events. In this study, we tested the hypothesis that maintenance of leaf turgor during ...periods of fog exposure and soil drought is related to species’ FWU capacity.
We conducted several experiments using apoplastic tracers, deuterium labeling and leaf immersion in water to evaluate differences in FWU among three common TMCF tree species. We also measured the effect of regular fog exposure on the leaf water potential of plants subjected to soil drought and used these data to model species’ response to long-term drought.
All species were able to absorb water through their leaf cuticles and/or trichomes, although the capacity to do so differed between species. During the drought experiment, the species with higher FWU capacity maintained leaf turgor for a longer period when exposed to fog, whereas the species with lower FWU exerted tighter stomatal regulation to maintain leaf turgor. Model results suggest that without fog, species with high FWU are more likely to lose turgor during seasonal droughts.
We show that leaf-wetting events are essential for trees with high FWU, which tend to be more anisohydric, maintaining leaf turgor during seasonal droughts.
Wood properties influence the leaf life span (LL) of tree crowns. As lignin is an important component of wood and the water transport system, we investigated its relationship with embolism resistance ...and the LL of several tree species in a seasonally dry tropical ecosystem.
We determined total lignin and the monomer contents of guaiacyl (G) and syringyl (S) and related them to wood traits and xylem vulnerability to embolism (Ψ50) for the most common species of the Brazilian semiarid, locally known as Caatinga.
Leaf life span was negatively related to Ψ50 and positively related to S : G, which was negatively related to Ψ50. This means that greater S : G increases LL by reducing Ψ50. Lignin content was not correlated with any variable.
We found two apparently unrelated axes of drought resistance. One axis, associated with lignin monomeric composition, increases LL in the dry season as a result of lower xylem embolism vulnerability. The other, associated with wood density and stem water content, helps leafless trees to withstand drought and allows them to resprout at the end of the dry season. The monomeric composition of lignin (S : G) is therefore an important functional wood attribute affecting several key functional aspects of tropical tree species in a semiarid climate.
Plants allocate nutrients to specific leaf cell types; eudicots are thought to predominantly allocate phosphorus (P) to epidermal/bundle sheath cells. However, three Proteaceae species have been ...shown to preferentially allocate P to mesophyll cells instead. These Proteaceae species are highly adapted to P‐impoverished habitats, with exceptionally high photosynthetic P‐use efficiencies (PPUE). We hypothesized that preferential allocation of P to photosynthetic mesophyll cells is an important trait in species adapted to extremely P‐impoverished habitats, contributing to their high PPUE. We used elemental X‐ray mapping to determine leaf cell‐specific nutrient concentrations for 12 Proteaceae species, from habitats of strongly contrasting soil P concentrations, in Australia, Brazil, and Chile. We found that only species from extremely P‐impoverished habitats preferentially allocated P to photosynthetic mesophyll cells, suggesting it has evolved as an adaptation to their extremely P‐impoverished habitat and that it is not a family‐wide trait. Our results highlight the possible role of soil P in driving the evolution of ecologically relevant nutrient allocation patterns and that these patterns cannot be generalized across families. Furthermore, preferential allocation of P to photosynthetic cells may provide new and exciting strategies to improve PPUE in crop species.
Phosphorus (P) is a pivotal plant nutrient. The model for leaf cell‐specific P allocation claims that eudicots allocate P to epidermal/bundle sheath cells. However, our results show that Proteaceae from severely P‐impoverished habitats allocate P to photosynthetic mesophyll cells instead, whereas Proteaceae from P‐richer habitats do not. This preferential allocation of P to photosynthetic cells partially explains the exceptionally high photosynthetic P‐use efficiency of species adapted to P‐impoverished habitats. Knowledge of this trait provides new pathways to improve P‐use efficiency in crop species and provides, for the first time, an unparalleled evaluation into the role of soil P in driving the evolution of ecologically highly relevant nutrient allocation patterns. Furthermore, we demonstrate that cell‐type‐specific nutrient allocation patterns cannot be generalized across eudicot families, in contrast to current models.
The fate of tropical forests under future climate change is dependent on the capacity of their trees to adjust to drier conditions. The capacity of trees to withstand drought is likely to be ...determined by traits associated with their hydraulic systems. However, data on whether tropical trees can adjust hydraulic traits when experiencing drought remain rare. We measured plant hydraulic traits (e.g. hydraulic conductivity and embolism resistance) and plant hydraulic system status (e.g. leaf water potential, native embolism and safety margin) on >150 trees from 12 genera (36 species) and spanning a stem size range from 14 to 68 cm diameter at breast height at the world's only long‐running tropical forest drought experiment. Hydraulic traits showed no adjustment following 15 years of experimentally imposed moisture deficit. This failure to adjust resulted in these drought‐stressed trees experiencing significantly lower leaf water potentials, and higher, but variable, levels of native embolism in the branches. This result suggests that hydraulic damage caused by elevated levels of embolism is likely to be one of the key drivers of drought‐induced mortality following long‐term soil moisture deficit. We demonstrate that some hydraulic traits changed with tree size, however, the direction and magnitude of the change was controlled by taxonomic identity. Our results suggest that Amazonian trees, both small and large, have limited capacity to acclimate their hydraulic systems to future droughts, potentially making them more at risk of drought‐induced mortality.
The fate of tropical forests under future climate change is dependent on the capacity of their trees to adjust to drier conditions. Following 15 years of experimentally imposed moisture deficit, Amazon trees showed no adjustment in their hydraulic traits to moisture deficit. This failure to adjust resulted in these drought‐stressed trees experiencing significantly reduced water potential and increased hydraulic failure. Both, small and large trees equally, could not adapt to moisture deficit. Our results suggest Amazon trees have a limited capacity to adjust to future droughts.
Summary
The mangrove Avicennia marina adjusts internal salt concentrations by foliar salt secretion. Deliquescence of accumulated salt causes leaf wetting that may provide a water source for ...salt‐secreting plants in arid coastal wetlands where high nocturnal humidity can usually support deliquescence whereas rainfall events are rare. We tested the hypotheses that salt deliquescence on leaf surfaces can drive top‐down rehydration, and that such absorption of moisture from unsaturated atmospheres makes a functional contribution to dry season shoot water balances.
Sap flow and water relations were monitored to assess the uptake of atmospheric water by branches during shoot wetting events under natural and manipulated microclimatic conditions.
Reverse sap flow rates increased with increasing relative humidity from 70% to 89%, consistent with function of salt deliquescence in harvesting moisture from unsaturated atmospheres. Top‐down rehydration elevated branch water potentials above those possible from root water uptake, subsidising transpiration rates and reducing branch vulnerability to hydraulic failure in the subsequent photoperiod.
Absorption of atmospheric moisture harvested through deliquescence of salt on leaf surfaces enhances water balances of Avicennia marina growing in hypersaline wetlands under arid climatic conditions. Top‐down rehydration from these frequent, low intensity wetting events contributes to prevention of carbon starvation and hydraulic failure during drought.
The functional trait approach has, as a central tenet, that plant traits are functional and shape individual performance, but this has rarely been tested in the field. Here, we tested the ...individual-based trait approach in a hyperdiverse Amazonian tropical rainforest and evaluated intraspecific variation in trait values, plant strategies at the individual level, and whether traits are functional and predict individual performance.
We evaluated > 1300 tree saplings belonging to > 383 species, measured 25 traits related to growth and defense, and evaluated the effects of environmental conditions, plant size, and traits on stem growth.
A total of 44% of the trait variation was observed within species, indicating a strong potential for acclimation. Individuals showed two strategy spectra, related to tissue toughness and organ size vs leaf display. In this nutrient- and light-limited forest, traits measured at the individual level were surprisingly poor predictors of individual growth performance because of convergence of traits and growth rates.
Functional trait approaches based on individuals or species are conceptually fundamentally different: the species-based approach focuses on the potential and the individual-based approach on the realized traits and growth rates. Counterintuitively, the individual approach leads to a poor prediction of individual performance, although it provides a more realistic view on community dynamics.
Faster growth in tropical trees is usually associated with higher mortality rates, but the mechanisms underlying this relationship are poorly understood. In this study, we investigate how tree growth ...patterns are linked with environmental conditions and hydraulic traits, by monitoring the cambial growth of 9 tropical cloud forest tree species coupled with numerical simulations using an optimization model. We find that fast‐growing trees have lower xylem safety margins than slow‐growing trees and this pattern is not necessarily linked to differences in stomatal behaviour or environmental conditions when growth occurs. Instead, fast‐growing trees have xylem vessels that are more vulnerable to cavitation and lower density wood. We propose the growth ‐ xylem vulnerability trade‐off represents a wood hydraulic economics spectrum similar to the classic leaf economic spectrum, and show through numerical simulations that this trade‐off can emerge from the coordination between growth rates, wood density, and xylem vulnerability to cavitation. Our results suggest that vulnerability to hydraulic failure might be related with the growth‐mortality trade‐off in tropical trees, determining important life history differences. These findings are important in furthering our understanding of xylem hydraulic functioning and its implications on plant carbon economy.
We investigate the relationships between tropical cloud forest trees hydraulic traits and their growth patterns. Our results indicate the existence of a growth‐hydraulic safety trade‐off that has several important ecological and physiological implications. This trade‐off can be explained using the fast–slow plant economic theory and represented with a profit optimization model. Our findings are an important contribution to the development of a wood economic spectrum theory for tropical trees
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