Tropical savannas and dry forests Pennington, R. Toby; Lehmann, Caroline E.R.; Rowland, Lucy M.
CB/Current biology,
05/2018, Letnik:
28, Številka:
9
Journal Article
Recenzirano
Odprti dostop
In the tropics, research, conservation and public attention focus on rain forests, but this neglects that half of the global tropics have a seasonally dry climate. These regions are home to dry ...forests and savannas (Figures 1 and 2), and are the focus of this Primer. The attention given to rain forests is understandable. Their high species diversity, sheer stature and luxuriance thrill biologists today as much as they did the first explorers in the Age of Discovery. Although dry forest and savanna may make less of a first impression, they support a fascinating diversity of plant strategies to cope with stress and disturbance including fire, drought and herbivory. Savannas played a fundamental role in human evolution, and across Africa and India they support iconic megafauna.
Pennington et al. introduce seasonally dry biomes in the tropics – savannahs and dry forests.
• Biomass and area ratios between leaves, stems and roots regulate many physiological and ecological processes. The Huber value Hv
(sapwood area/leaf area ratio) is central to plant water balance and ...drought responses. However, its coordination with key plant functional traits is poorly understood, and prevents developing trait-based prediction models.
• Based on theoretical arguments, we hypothesise that global patterns in Hv
of terminal woody branches can be predicted from variables related to plant trait spectra, that is plant hydraulics and size and leaf economics.
• Using a global compilation of 1135 species-averaged Hv
, we show that Hv
varies over three orders of magnitude. Higher Hv
are seen in short small-leaved low-specific leaf area (SLA) shrubs with low Ks
in arid relative to tall large-leaved high-SLA trees with high Ks
in moist environments. All traits depend on climate but climatic correlations are stronger for explanatory traits than Hv
. Negative isometry is found between Hv
and Ks
, suggesting a compensation to maintain hydraulic supply to leaves across species.
• This work identifies the major global drivers of branch sapwood/leaf area ratios. Our approach based on widely available traits facilitates the development of accurate models of above-ground biomass allocation and helps predict vegetation responses to drought.
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.
Forest ecosystem models based on heuristic water stress functions poorly predict tropical forest response to drought partly because they do not capture the diversity of hydraulic traits (including ...variation in tree size) observed in tropical forests. We developed a continuous porous media approach to modeling plant hydraulics in which all parameters of the constitutive equations are biologically interpretable and measurable plant hydraulic traits (e.g., turgor loss point πtlp, bulk elastic modulus ε, hydraulic capacitance Cft, xylem hydraulic conductivity ks,max, water potential at 50 % loss of conductivity for both xylem (P50,x) and stomata (P50,gs), and the leaf : sapwood area ratio Al : As). We embedded this plant hydraulics model within a trait forest simulator (TFS) that models light environments of individual trees and their upper boundary conditions (transpiration), as well as providing a means for parameterizing variation in hydraulic traits among individuals. We synthesized literature and existing databases to parameterize all hydraulic traits as a function of stem and leaf traits, including wood density (WD), leaf mass per area (LMA), and photosynthetic capacity (Amax), and evaluated the coupled model (called TFS v.1-Hydro) predictions, against observed diurnal and seasonal variability in stem and leaf water potential as well as stand-scaled sap flux. Our hydraulic trait synthesis revealed coordination among leaf and xylem hydraulic traits and statistically significant relationships of most hydraulic traits with more easily measured plant traits. Using the most informative empirical trait–trait relationships derived from this synthesis, TFS v.1-Hydro successfully captured individual variation in leaf and stem water potential due to increasing tree size and light environment, with model representation of hydraulic architecture and plant traits exerting primary and secondary controls, respectively, on the fidelity of model predictions. The plant hydraulics model made substantial improvements to simulations of total ecosystem transpiration. Remaining uncertainties and limitations of the trait paradigm for plant hydraulics modeling are highlighted.
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
Considerable uncertainty surrounds the fate of Amazon rainforests in response to
climate change.
Here, carbon (C) flux predictions of five terrestrial biosphere models (Community
Land Model version ...3.5 (CLM3.5), Ecosystem Demography model version 2.1 (ED2),
Integrated BIosphere Simulator version 2.6.4 (IBIS), Joint UK Land Environment
Simulator version 2.1 (JULES) and Simple Biosphere model version 3 (SiB3)) and a
hydrodynamic terrestrial ecosystem model (the
Soil–Plant–Atmosphere (SPA) model) were evaluated against
measurements from two large-scale Amazon drought experiments.
Model predictions agreed with the observed C fluxes in the control plots of both
experiments, but poorly replicated the responses to the drought treatments. Most
notably, with the exception of ED2, the models predicted negligible reductions
in aboveground biomass in response to the drought treatments, which was in
contrast to an observed c. 20% reduction at both sites.
For ED2, the timing of the decline in aboveground biomass was accurate, but the
magnitude was too high for one site and too low for the other.
Three key findings indicate critical areas for future research and model
development. First, the models predicted declines in autotrophic respiration
under prolonged drought in contrast to measured increases at one of the sites.
Secondly, models lacking a phenological response to drought introduced bias in
the sensitivity of canopy productivity and respiration to drought. Thirdly, the
phenomenological water-stress functions used by the terrestrial biosphere models
to represent the effects of soil moisture on stomatal conductance yielded
unrealistic diurnal and seasonal responses to drought.
Many tropical rain forest regions are at risk of increased future drought. The net effects of drought on forest ecosystem functioning will be substantial if important ecological thresholds are ...passed. However, understanding and predicting these effects is challenging using observational studies alone. Field-based rainfall exclusion (canopy throughfall exclusion; TFE) experiments can offer mechanistic insight into the response to extended or severe drought and can be used to help improve model-based simulations, which are currently inadequate. Only eight TFE experiments have been reported for tropical rain forests. We examine them, synthesizing key results and focusing on two processes that have shown threshold behavior in response to drought: (1) tree mortality and (2) the efflux of carbon dioxdie from soil, soil respiration. We show that: (a) where tested using large-scale field experiments, tropical rain forest tree mortality is resistant to long-term soil moisture deficit up to a threshold of 50% of the water that is extractable by vegetation from the soil, but high mortality occurs beyond this value, with evidence from one site of increased autotrophic respiration, and (b) soil respiration reaches its peak value in response to soil moisture at significantly higher soil moisture content for clay-rich soils than for clay-poor soils. This first synthesis of tropical TFE experiments offers the hypothesis that low soil moisture–related thresholds for key stress responses in soil and vegetation may prove to be widely applicable across tropical rain forests despite the diversity of these forests.
Current policy is driving renewed impetus to restore forests to return ecological function, protect species, sequester carbon and secure livelihoods. Here we assess the contribution of tree planting ...to ecosystem restoration in tropical and sub-tropical Asia; we synthesize evidence on mortality and growth of planted trees at 176 sites and assess structural and biodiversity recovery of co-located actively restored and naturally regenerating forest plots. Mean mortality of planted trees was 18% 1 year after planting, increasing to 44% after 5 years. Mortality varied strongly by site and was typically
20% higher in open areas than degraded forest, with height at planting positively affecting survival. Size-standardized growth rates were negatively related to species-level wood density in degraded forest and plantations enrichment settings. Based on community-level data from 11 landscapes, active restoration resulted in faster accumulation of tree basal area and structural properties were closer to old-growth reference sites, relative to natural regeneration, but tree species richness did not differ. High variability in outcomes across sites indicates that planting for restoration is potentially rewarding but risky and context-dependent. Restoration projects must prepare for and manage commonly occurring challenges and align with efforts to protect and reconnect remaining forest areas. The abstract of this article is available in Bahasa Indonesia in the electronic supplementary material. This article is part of the theme issue 'Understanding forest landscape restoration: reinforcing scientific foundations for the UN Decade on Ecosystem Restoration'.
The tropics are predicted to become warmer and drier, and understanding the sensitivity of tree species to drought is important for characterizing the risk to forests of climate change. This study ...makes use of a long-term drought experiment in the Amazon rainforest to evaluate the role of leaf-level water relations, leaf anatomy and their plasticity in response to drought in six tree genera.
The variables (osmotic potential at full turgor, turgor loss point, capacitance, elastic modulus, relative water content and saturated water content) were compared between seasons and between plots (control and through-fall exclusion) enabling a comparison between short- and long-term plasticity in traits. Leaf anatomical traits were correlated with water relation parameters to determine whether water relations differed among tissues.
The key findings were: osmotic adjustment occurred in response to the long-term drought treatment; species resistant to drought stress showed less osmotic adjustment than drought-sensitive species; and water relation traits were correlated with tissue properties, especially the thickness of the abaxial epidermis and the spongy mesophyll.
These findings demonstrate that cell-level water relation traits can acclimate to long-term water stress, and highlight the limitations of extrapolating the results of short-term studies to temporal scales associated with climate change.
Revealing the mechanisms of environmental niche partitioning within lowland tropical forests is important for understanding the drivers of current species distributions and potential vulnerability to ...environmental change. Tropical forest structure and species composition change across edaphic gradients in Borneo over short distances. However, our understanding of how edaphic conditions affect tree physiology and whether these relationships drive niche partitioning within Bornean forests remains incomplete.
This study evaluated how leaf physiological function changes with nutrient availability across a fine‐scale edaphic gradient and whether these relationships vary according to tree height. Furthermore, we tested whether intraspecific leaf trait variation allows generalist species to populate a wider range of environments.
We measured leaf traits of 218 trees ranging in height from 4 to 66 m from 13 dipterocarp species within four tropical forest types (alluvial, mudstone, sandstone and kerangas) occurring along an <5 km edaphic gradient in North Borneo. The traits measured included saturating photosynthesis (Asat), maximum photosynthetic capacity (Vcmax), leaf dark respiration (Rleaf), leaf mass per area (LMA), leaf thickness, minimum stomatal conductance (gdark) and leaf nutrient concentrations (N, P, Ca, K and Mg).
Across all species, leaf traits varied consistently in response to soil nutrient availability across forest types except Rleaf_mass, Mgleaf and Caleaf. Changes in photosynthesis and respiration rates were related to different leaf nutrients across forest types, with greater nutrient‐use efficiency in more nutrient‐poor environments. Generalist species partially or fully compensated reductions in mass‐based photosynthesis through increasing LMA in more nutrient‐poor environments.
Leaf traits also varied with tree height, except Vcmax_mass, but only in response to height‐related modifications of leaf morphology (LMA and leaf thickness). These height–trait relationships did not vary across the edaphic gradient, except for Asat, Nleaf, Pleaf and Kleaf.
Our results highlight that modification of leaf physiological function and morphology act as important adaptations for Bornean dipterocarps in response to edaphic and vertical environmental gradients. Meanwhile, multiple nutrients appear to contribute to niche partitioning and could drive species distributions and high biodiversity within Bornean forest landscapes.
Read the free Plain Language Summary for this article on the Journal blog.
Abstrak
Pendedahan mekanisma pembahagian nic alam sekitar di hutan tropika tanah rendah adalah penting untuk memahami pemacu taburan dan kerentanan spesies semasa terhadap perubahan alam sekitar. Struktur hutan tropika dan komposisi spesies berubah mengikut faktor edafik dalam jarak dekat. Walau bagaimanapun, pemahaman kami tentang bagaimana keadaan edafik mempengaruhi fisiologi pokok dan sama ada ia memacu pembahagian nic dalam hutan Borneo masih tidak lengkap.
Kajian ini menilai bagaimana perubahan fungsi fisiologi daun dengan ketersediaan nutrien berdasarkan faktor edafik dan sama ada ia berbeza mengikut ketinggian pokok. Malah, kami menguji sama ada ciri‐ciri variasi intraspesifik daun membolehkan spesies umum mendiami persekitaraan yang lebih luas.
Kami mengukur 14 ciri‐ciri daun bagi 218 pokok yang berketinggian dari 4 hingga 66 meter, melibatkan 13 spesies dipterokap dalam empat jenis hutan tropika (aluvium, batu lumpur, batu pasir, kerangas) yang wujud di sepanjang kecerunan edafik (<5 km) di Borneo Utara. Ciri‐ciri yang diukur termasuk fotosintesis menepu (Asat), kapasiti maksimum fotosintesis (Vcmax), respirasi gelap daun (Rleaf), jisim daun per permukaan (LMA), ketebalan daun, konduksian minimum stomata (gdark) dan kepekatan nutrien daun (N, P, Ca, K, Mg).
Ciri‐ciri daun berbeza secara konsisten terhadap ketersediaan nutrien tanah dalam semua jenis hutan kecuali Rleaf_mass, Mgleaf dan Caleaf. Perubahan kadar fotosintesis dan respirasi berhubung kait dengan perbezaan nutrient daun di semua jenis hutan dan kecekapan penggunaan nutrien adalah tinggi dalam kawasan nutrien rendah. Spesies umum mengimbangi pengurangan berkaitan jisim dan fotosintesis secara separa atau penuh dengan meningkatkan LMA dalam kawasan kawasan nutrien rendah.
Ciri‐ciri daun juga berbeza berdasarkan ketinggian pokok kecuali Vcmax_mass apabila ia melibatkan tindak balas perubahan morfologi daun (LMA dan ketebalan daun). Hubung kait ini tidak berbeza mengikut faktor edafik kecuali Asat, Nleaf, Pleaf dan Kleaf.
Dapatan kajian kami menekankan bahawa perubahan fungsi fisiologi dan morfologi daun merupakan strategi penyesuaian penting pokok dipterokap Borneo sebagai tindak balas terhadap faktor edafik serta alam sekitar. Sementara itu, pelbagai nutrien menyumbang kepada proses pembahagian nic dan mampu memacu taburan spesies serta kepelbagaian yang tinggi dalam landskap hutan Borneo.
Resumen
El entendimiento de los mecanismos de partición de nicho ambiental de los bosques tropicales es importante para comprender los factores que controlan las distribuciones de especies presentes actualmente en los bosques y su posible vulnerabilidad a cambios ambientales. La estructura del bosque tropical y la composición de especies varían a través de gradientes edáficos en Borneo en distancias cortas. No obstante, se desconoce cómo las condiciones edáficas afectan la fisiología de los árboles y si estas relaciones impulsan la partición de nicho dentro en los bosques de Borneo.
En este estudio se evalúa la variación fisiológica foliar con la disponibilidad de nutrientes en un gradiente edáfico de escala detallada y si estas relaciones varían según la altura de los árboles. También se evalúa si la variación intraespecífica de rasgos foliares permite que las especies generalistas habiten en un rango más amplio de entornos ambientales.
Se midieron rangos foliares en 218 árboles ‐que varían en altura de 4 a 66 m‐ de 13 especies de dipterocarpáceas dentro de cuatro tipos de bosque tropical (aluial, lutita, arenisca y kerangas) que se encuentran a lo largo de un gradiente edáfico de < 5 km en el norte de Borneo. Los rangos medidos incluyen fotosíntesis en condiciones de saturación por luz (Asat), capacidad máxima para hacer fotosintesis (Vcmax), respiración foliar en la oscuridad (Rfoliar), masa foliar expresada en unidad de área (LMA), espesor de hoja, conductancia estomática mínima (gmin) y concentraciones foliar de nutrientes (N,P, Ca, K, Mg).
En todas las especies, la majoría de los rasgos foliares investigados mostraron una variación consistente con la disponibilidad de nutrientes del suelo en todos los tipos de bosques excepto en los siguientes rasgos: Rfoliar_peso, Mgfoliar y Cafoliar. La variación en tatas de fotosíntesis y respiración está relacionda con diferentes nutrientes foliares en todos los tipos de bosque estudiados, mostrando mayor eficiencia en el uso de nutrientes en los ambientes más infértiles. Las especies generalistas compensaron parcial o totalmente las reducciones en la fotosíntesis expresada en unidades de peso‐ mediante el aumento de LMA en los entornos más infértiles.
Los rasgos foliares presentaron variaciónes relacionadas con la altura de los árboles (excepto Vcmax_mass, pero sólo en respuesta a modificaciones relacionadas con la altura en la morfología de la hoja (LMA y espesor foliar). Las relaciones altura‐rasgo no variaron a lo largo del gradiente edáfico, excepto para Asat, Nfoliar, Pfoliiar y Kfoliar.
Nuestros resultados demuestran que la variación de la fisiología y la morfología foliar son adaptaciones importantes en las especies de la familia de las dipterocarpáceas de Borneo en respuesta a gradientes ambientales verticales y edáficos. Al mismo tiempo, nuestras medidas sugieren que múltiples nutrientes contribuyen a la partición de nicho y podrían controlar la distribución de especies y la alta biodiversidad de los bosques de Borneo.
Read the free Plain Language Summary for this article on the Journal blog.
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