Background and aims The survival and coexistence of plants in water-limited environments are related to their ability to coordinate water acquisition and regulation of water loss. To assess the ...coordination among below and aboveground hydraulic traits and the diversity of water-use strategies, we evaluated rooting depth and several leaf hydraulic traits of 15 species in campos rupestres, a seasonally-dry biodiversity hotspot in central Brazil. Methods We assessed the depth of plant water acquisition by excavating roots and analyzing the stable isotope composition of hydrogen (δD) and oxygen (δ18O) in the xylem and soil water. We also measured mid-morning stomatal conductance, leaf-water potential at turgor loss point (ΨTLP) and pre-dawn leaf water potentials (ΨPD) during wet and dry seasons. Results We demonstrated that rooting depth is a good predictor of seasonal variations in stomatal conductance and ΨPD. Shallow-rooted plants had greater variation in stomatal conductance and ΨPD than deep-rooted plants. Woody plants with shallower roots also had lower ΨTLP than deep-rooted plants, revealing higher drought resistance. Conclusion We demonstrate that shallow-rooted species, more exposed to variation in water availability, have mechanisms to confer drought resistance through turgor maintenance. Our results support the theory of hydrological niche segregation and its underlying trade-offs related to drought resistance.
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.
The relationship between rooting depth and above‐ground hydraulic traits can potentially define drought resistance strategies that are important in determining species distribution and coexistence in ...seasonal tropical forests, and understanding this is important for predicting the effects of future climate change in these ecosystems.
We assessed the rooting depth of 12 dominant tree species (representing c. 42% of the forest basal area) in a seasonal Amazon forest using the stable isotope ratios (δ18O and δ2H) of water collected from tree xylem and soils from a range of depths. We took advantage of a major ENSO‐related drought in 2015/2016 that caused substantial evaporative isotope enrichment in the soil and revealed water use strategies of each species under extreme conditions. We measured the minimum dry season leaf water potential both in a normal year (2014; Ψnon‐ENSO) and in an extreme drought year (2015; ΨENSO). Furthermore, we measured xylem hydraulic traits that indicate water potential thresholds trees tolerate without risking hydraulic failure (P50 and P88).
We demonstrate that coexisting trees are largely segregated along a single hydrological niche axis defined by root depth differences, access to light and tolerance of low water potential. These differences in rooting depth were strongly related to tree size; diameter at breast height (DBH) explained 72% of the variation in the δ18Oxylem. Additionally, δ18Oxylem explained 49% of the variation in P50 and 70% of P88, with shallow‐rooted species more tolerant of low water potentials, while δ18O of xylem water explained 47% and 77% of the variation of minimum Ψnon‐ENSO and ΨENSO.
We propose a new formulation to estimate an effective functional rooting depth, i.e. the likely soil depth from which roots can sustain water uptake for physiological functions, using DBH as predictor of root depth at this site. Based on these estimates, we conclude that rooting depth varies systematically across the most abundant families, genera and species at the Tapajós forest, and that understorey species in particular are limited to shallow rooting depths.
Our results support the theory of hydrological niche segregation and its underlying trade‐off related to drought resistance, which also affect the dominance structure of trees in this seasonal eastern Amazon forest.
Synthesis. Our results support the theory of hydrological niche segregation and demonstrate its underlying trade‐off related to drought resistance (access to deep water vs. tolerance of very low water potentials). We found that the single hydrological axis defining water use traits was strongly related to tree size, and infer that periodic extreme droughts influence community composition and the dominance structure of trees in this seasonal eastern Amazon forest.
Our results support the theory of hydrological niche segregation and demonstrate its underlying trade‐off related to drought resistance (access to deep water vs. tolerance of very low water potentials). We found that the single hydrological axis defining water use traits was strongly related to tree size, and infer that periodic extreme droughts influence community composition and the dominance structure of trees in this seasonal eastern Amazon forest.
Reducing uncertainties in the response of tropical forests to global change requires understanding how intra- and interannual climatic variability selects for different species, community functional ...composition and ecosystem functioning, so that the response to climatic events of differing frequency and severity can be predicted.
Here we present an extensive dataset of hydraulic traits of dominant species in two tropical Amazon forests with contrasting precipitation regimes – low seasonality forest (LSF) and high seasonality forest (HSF) – and relate them to community and ecosystem response to the El Niño–Southern Oscillation (ENSO) of 2015.
Hydraulic traits indicated higher drought tolerance in the HSF than in the LSF. Despite more intense drought and lower plant water potentials in HSF during the 2015-ENSO, greater xylem embolism resistance maintained similar hydraulic safety margin as in LSF. This likely explains how ecosystem-scale whole-forest canopy conductance at HSF maintained a similar response to atmospheric drought as at LSF, despite their water transport systems operating at different water potentials.
Our results indicate that contrasting precipitation regimes (at seasonal and interannual time scales) select for assemblies of hydraulic traits and taxa at the community level, which may have a significant role in modulating forest drought response at ecosystem scales.
Tropical tree species span a range of life‐history strategies within a fast–slow continuum. The position of a species within this continuum is thought to reflect a negative relationship between ...growth and storage, with fast‐growing species allocating more carbon to growth and slow‐growing species investing more in storage. For tropical species, the relationship between storage and life‐history strategies has been largely studied on seedlings and less so in adult trees.
We evaluated how stored non‐structural carbohydrates (NSC) vary across adult trees spanning the fast–slow continuum in the Peruvian Amazon by: (a) analysing whole‐tree NSC in two species of contrasting growth and (b) investigating the relationships with key life‐history traits across a broader set of species.
Our results are consistent with a growth–storage trade‐off. The analysis of whole‐tree NSC revealed that the slow‐growing Eschweilera coriacea stored about 2.7 times as much NSC as the fast‐growing Bixa arborea due to markedly higher storage in woody stems and roots. B. arborea also had higher seasonality in NSC, reflecting its strong seasonality in stem growth. Across a range of species, stem starch was negatively related to species growth rate and positively related to wood density.
Given the role of NSC in mediating plants' response to stress, our results suggest that slow‐growing species with greater storage reserves may be more resilient to drought than fast‐growing species.
Resumo
Espécies de árvores tropicais possuem diferentes histórias de vida, que ocorrem ao longo de um contínuo ‘rápido‐lento’ de estratégias de crescimento. Acredita‐se que a posição de uma espécie dentro desse contínuo, reflita uma relação negativa entre crescimento e armazenamento, com espécies de crescimento rápido alocando mais carbono para o crescimento e espécies de crescimento lento investindo mais em armazenamento. Para espécies tropicais, a relação entre armazenamento e as estratégias ao longo do contínuo de histórias de vida tem sido amplamente estudada em plântulas, e menos em árvores adultas.
Neste trabalho avaliamos como os estoques de carbono (carboidratos não estruturais, CNE) variam entre árvores adultas ao longo do contínuo rápido‐lento, coletadas na Amazônia Peruana (i) analisando as concentrações de CNE na árvore inteira em duas espécies com estratégias diferentes de crescimento e (ii) investigando as relações com as principais características da história de vida em um conjunto mais amplo de espécies.
Nossos resultados indicam que a alocação de carbono para crescimento ou estoque faz parte da estratégia das espécies. A análise de CNE em toda a árvore revelou que Eschweilera coriacea (crescimento lento), armazena cerca de 2,7 vezes mais CNE do que Bixa arborea (crescimento rápido). Essa diferença no tamanho dos estoques entre as espécies ocorre principalmente devido ao maior armazenamento de CNE em órgãos de reserva (tronco e raízes) da espécie lenta. Entre as duas espécies, B. arborea é a que apresenta maior variação nas concentrações de CNE entre as estações, refletindo sua forte sazonalidade no crescimento. Avaliando um conjunto maior de espécies, encontramos que o amido do tronco é negativamente relacionado à taxa de crescimento das espécies e positivamente relacionado à densidade da madeira.
Dado o papel do CNE na mediação da resposta ao estresse das plantas, nossos resultados sugerem que as espécies de crescimento lento, com maior densidade da madeira e maiores estoques de CNE podem ser mais resistentes à seca do que as espécies de crescimento rápido e baixa densidade da madeira.
Resumen
Las especies de árboles tropicales tienen diferentes ciclos vitales, que ocurren a lo largo de un continuo de estrategias de crecimiento de rápidas‐lentas. Se cree que la posición de una especie dentro de este continuo refleja una relación negativa entre crecimiento y almacenamiento, las especies de crecimiento rápido invierten más carbono en crecimiento y las de crecimiento lento en almacenamiento. Para las especies tropicales, la relación entre el almacenamiento y las estrategias a lo largo de sus ciclos vitales se ha estudiado ampliamente en las plántulas y menos en los árboles adultos.
En este trabajo evaluamos cómo el almacenamiento de carbono (carbohidratos no estructurales, CNE) varían entre árboles adultos a lo largo del continuo rápido‐lento, recogidos en la Amazonía Peruana (i) analizando las concentraciones de CNE en todo el árbol de dos especies de crecimiento contrastantes e (ii) investigando las relaciones con las características principales de los ciclos vitales en un conjunto más amplio de especies.
Nuestros resultados indican que la asignación de carbono al crecimiento o almacenamiento es parte de la estrategia de la especie. El análisis de CNE de todo el árbol reveló que Eschweilera coriacea (crecimiento lento) almacena aproximadamente 2,7 veces más CNE que Bixa arborea (crecimiento rápido). Esta diferencia en el tamaño de los reservorios entre especies se debe principalmente al mayor almacenamiento de NSC en los órganos de reserva (tronco y raíces) de las especies lentas. Entre las dos especies, B. arborea es la que presenta mayor variación en las concentraciones de CNE entre estaciones, lo que refleja su fuerte estacionalidad en el crecimiento. Al evaluar un conjunto más grande de especies, encontramos que el almidón del tronco está relacionado negativamente con la tasa de crecimiento de las especies y positivamente con la densidad de la madera.
Dado el papel de los CNE en la mediación de la respuesta al estrés de las plantas, nuestros resultados sugieren que las especies de crecimiento lento con mayor densidad de madera y mayores reservas de CNE pueden ser más resistentes a la sequía que las especies de crecimiento rápido y baja densidad de madera.
A free Plain Language Summary can be found within the Supporting Information of this article.
A free Plain Language Summary can be found within the Supporting Information of this article.
Climate change is predicted to affect the water balance of several ecosystems mostly through changes in the energy budget and hydrological input (rainfall frequency, intensity, and timing). Changes ...in rainfall patterns and cloudiness directly affect incoming radiation, atmospheric water vapor saturation deficit and soil water availability, the main variables controlling the rates of water uptake and transport along the soil–plant–atmosphere continuum (SPAC). Developing a predictive framework about vegetation responses to a changing climate is challenging because it involves complex non-linear interactions between these environmental variables and species-specific responses. By examining the hydraulic traits of functional groups within plant communities we can better predict the impacts of changes in rainfall regimes within functional groups and therefore, generate more realistic predictions of ecosystem carbon and water balance changes due to local and regional changes in precipitation regimes. In this review, we discuss several aspects of plant hydraulic functioning and then explore how predicted changes in precipitation regimes may affect tree water and carbon balance. We examine the impacts of changes in rainfall patterns on the SPAC and also explore the multiple ways that plants can absorb and transport water. Finally, we explore the diversity of hydraulic traits and potential mechanisms causing large-scale drought-induced mortality recently observed in several ecosystems.
Non-structural carbohydrates (NSC) are major substrates for plant metabolism and have been implicated in mediating drought-induced tree mortality. Despite their significance, NSC dynamics in tropical ...forests remain little studied. We present leaf and branch NSC data for 82 Amazon canopy tree species in six sites spanning a broad precipitation gradient. During the wet season, total NSC (NSC
) concentrations in both organs were remarkably similar across communities. However, NSC
and its soluble sugar (SS) and starch components varied much more across sites during the dry season. Notably, the proportion of leaf NSC
in the form of SS (SS:NSC
) increased greatly in the dry season in almost all species in the driest sites, implying an important role of SS in mediating water stress in these sites. This adjustment of leaf NSC balance was not observed in tree species less-adapted to water deficit, even under exceptionally dry conditions. Thus, leaf carbon metabolism may help to explain floristic sorting across water availability gradients in Amazonia and enable better prediction of forest responses to future climate change.
Abstract
Tropical montane cloud forests (TMCFs) are expected to experience more frequent and prolonged droughts over the coming century, yet understanding of TCMF tree responses to moisture stress ...remains weak compared with the lowland tropics. We simulated a severe drought in a throughfall reduction experiment (TFR) for 2 years in a Peruvian TCMF and evaluated the physiological responses of several dominant species (Clusia flaviflora Engl., Weinmannia bangii (Rusby) Engl., Weinmannia crassifolia Ruiz & Pav. and Prunus integrifolia (C. Presl) Walp). Measurements were taken of (i) sap flow; (ii) diurnal cycles of stem shrinkage, stem moisture variation and water-use; and (iii) intrinsic water-use efficiency (iWUE) estimated from foliar δ13C. In W. bangii, we used dendrometers and volumetric water content (VWC) sensors to quantify daily cycles of stem water storage. In 2 years of sap flow (Js) data, we found a threshold response of water use to vapor pressure deficit vapor pressure deficit (VPD) > 1.07 kPa independent of treatment, though control trees used more soil water than the treatment trees. The daily decline in water use in the TFR trees was associated with a strong reduction in both morning and afternoon Js rates at a given VPD. Soil moisture also affected the hysteresis strength between Js and VPD. Reduced hysteresis under moisture stress implies that TMCFs are strongly dependent on shallow soil water. Additionally, we suggest that hysteresis can serve as a sensitive indicator of environmental constraints on plant function. Finally, 6 months into the experiment, the TFR treatment significantly increased iWUE in all study species. Our results highlight the conservative behavior of TMCF tree water use under severe soil drought and elucidate physiological thresholds related to VPD and its interaction with soil moisture. The observed strongly isohydric response likely incurs a cost to the carbon balance of the tree and reduces overall ecosystem carbon uptake.
ENSO effects on the transpiration of eastern Amazon trees Brum, Mauro; Gutiérrez López, Jose; Asbjornsen, Heidi ...
Philosophical transactions - Royal Society. Biological sciences,
10/2018, Letnik:
373, Številka:
1760
Journal Article
Recenzirano
Odprti dostop
Tree transpiration is important in the recycling of precipitation in the Amazon and might be negatively affected by El Niño–Southern Oscillation (ENSO)–induced droughts. To investigate the relative ...importance of soil moisture deficits versus increasing atmospheric demand (VPD) and determine if these drivers exert different controls over tree transpiration during the wet season versus the dry season (DS), we conducted sap flow measurements in a primary lowland tropical forest in eastern Amazon during the most extreme ENSO-induced drought (2015/2016) recorded in the Amazon. We also assessed whether trees occupying different canopy strata contribute equally to the overall stand transpiration (Tstand). Canopy trees were the primary source of Tstand. However, subcanopy trees are still important as they transpired an amount similar to other biomes around the globe. Tree water use was higher during the DS, indicating that during extreme drought trees did not reduce transpiration in response to low soil moisture. Photosynthetically active radiation and VPD exerted an overriding effect on water use patterns relative to soil moisture during extreme drought, indicating that light and atmospheric constraints play a critical role in controlling ecosystem fluxes of water. Our study highlights the importance of canopy and subcanopy trees to the regional water balance and highlights the resilience to droughts that these trees show during an extreme ENSO event.
This article is part of a discussion meeting issue ‘The impact of the 2015/2016 El Niño on the terrestrial tropical carbon cycle: patterns, mechanisms and implications’.
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