Special Feature Articles: Amazonian rain forests and drought: response and vulnerability
Assessing uncertainties in a second-generation dynamic vegetation model caused by ecological scale limitations
Patrick Meir, F. Ian Woodward
Rosie Fisher, Nate McDowell, Drew Purves, Paul Moorcroft, Stephen Sitch, Peter Cox, Chris Huntingford, Patrick Meir, F. Ian Woodward
Drought impacts on the Amazon forest: the remote sensing perspective Gregory P. Asner, Ane Alencar
Development of probability density functions for future South American rainfall
Effect of 7 yr of experimental drought on vegetation dynamics and biomass storage of an eastern Amazonian rainforest
Tim E. Jupp, Peter M. Cox, Anja Rammig, Kirsten Thonicke, Wolfgang Lucht, Wolfgang Cramer
Antonio Carlos Lola da Costa et al.
Soil moisture depletion under simulated drought in the Amazon: impacts on deep root uptake Daniel Markewitz, Scott Devine, Eric A. Davidson, Paulo Brando, Daniel C. Nepstad
Shifts in plant respiration and carbon use efficiency at a large-scale drought experiment in the eastern Amazon D. B. Metcalfe et al.
Effects of water and nutrient availability on fine root growth in eastern Amazonian forest regrowth, Brazil Tâmara Thaiz Santana Lima, Izildinha Souza Miranda, Steel Silva Vasconcelos
Drought–mortality relationships for tropical forests Oliver L. Phillips et al.
Multiple mechanisms of Amazonian forest biomass losses in three dynamic global vegetation models under climate change David Galbraith, Peter E. Levy, Stephen Sitch, Chris Huntingford, Peter Cox, Mathew Williams, Patrick Meir
Estimating the risk of Amazonian forest dieback Anja Rammig et al.
The climatic sensitivity of the forest, savanna and forest– savanna transition in tropical South Marina Hirota, Carlos Nobre, Marcos Daisuke Oyama, Mercedes MC Bustamante
Predicting moisture dynamics of fine understory fuels in a moist tropical rainforest system: results of a pilot study undertaken to identify proxy variables useful for rating fire danger David Ray, Dan Nepstad, Paulo Brando
Remote sensing detection of droughts in Amazonian forest canopies Liana O. Anderson, Yadvinder Malhi, Luiz E. O. C. Aragão, Richard Ladle, Egidio Arai, Nicolas Barbier, Oliver Phillips
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Introduction The Amazonian rain forest carries out about 15% of terrestrial photosynthesis and contains about 25% of the world's terrestrial diversity — characteristics that are likely to be highly susceptible to drought — with recent indications of drying and simulations of future long term drought. The Special Feature in New Phytologist (187:3) brings together information from field observations and experiments, simulation models and satellite remote sensing to investigate the sensitivity of the Amazonian rain forest to drought. The overall picture is of a forest that is negatively impacted by drought, especially long term drought, with larger trees disproportionally at risk and with risks extending for some years following the termination of drought.
Research review Drought impacts on the Amazon forest: the remote sensing perspective Author for correspondence: Gregory P. Asner Tel: +1 650 4621047 Email: gpa@stanford.edu
Gregory P. Asner and Ane Alencar
Summary New Phytologist (2010) 187: 569–578 doi: 10.1111/j.1469-8137.2010.03310.x
Keywords: advanced very high resolution radiometer, Amazon basin, Brazil, green-up, Landsat, moderate resolution imaging spectroradiometer, phenology, tropical forest.
Drought varies spatially and temporally throughout the Amazon basin, challenging efforts to assess ecological impacts via field measurements alone. Remote sensing offers a range of regional insights into drought-mediated changes in cloud cover and rainfall, canopy physiology, and fire. Here, we summarize remote sensing studies of AmazĂ´nia which indicate that: fires and burn scars are more common during drought years; hydrological function including floodplain area is significantly affected by drought; and land use affects the sensitivity of the forest to dry conditions and increases fire susceptibility during drought. We highlight two controversial areas of research centering on canopy physiological responses to drought and changes in subcanopy fires during drought. By comparing findings from field and satellite studies, we contend that current remote sensing observations and techniques cannot resolve these controversies using current satellite observations. We conclude that studies integrating multiple lines of evidence from physiological, disturbance-fire, and hydrological remote sensing, as well as field measurements, are critically needed to narrow our uncertainty of basinlevel responses to drought and climate change.
Effect of 7 yr of experimental drought on vegetation dynamics and biomass storage of an eastern Amazonian rainforest Antonio Carlos Lola da Costa , David Galbraith, Samuel Almeida , Bruno Takeshi Tanaka Portela, Mauricio da Costa, João de Athaydes Silva Junior, Alan P. Braga, Paulo H. L. de Gonçalves , Alex AR de Oliveira, Rosie Fisher, Oliver L. Phillips, Daniel B. Metcalfe, Peter Levy and Patrick Meir
Summary Author for correspondence: David Galbraith Tel: +44 131 445 8594 Email: darga@ceh.ac.uk
At least one climate model predicts severe reductions of rainfall over Amazonia during this century. Long-term throughfall exclusion (TFE) experiments represent the best available means to investigate the resilience of the Amazon rainforest to such droughts.
New Phytologist (2010) 187: 579–591 doi: 10.1111/j.1469-8137.2010.03309.x
Results are presented from a 7 yr TFE study at Caxiuanã National Forest, eastern Amazonia. We focus on the impacts of the drought on tree mortality, wood production and above-ground biomass.
KEYWORDS Amazon rainforest • biomass • drought • tree mortality • wood production
Tree mortality in the TFE plot over the experimental period was 2.5% yr−1, compared with 1.25% yr−1 in a nearby control plot experiencing normal rainfall. Differences in stem mortality between plots were greatest in the largest (> 40 cm diameter at breast height (dbh)) size class (4.1% yr−1 in the TFE and 1.4% yr−1 in the control). Wood production in the TFE plot was c. 30% lower than in the control plot. Together, these changes resulted in a loss of 37.8 ± 2.0 Mg carbon (C) ha−1 in the TFE plot (2002–2008), compared with no change in the control. These results are remarkably consistent with those from another TFE (at Tapajós National Forest), suggesting that eastern Amazonian forests may respond to prolonged drought in a predictable manner.
Soil moisture depletion under simulated drought in the Amazon: impacts on deep root uptake Daniel Markewitz, Scott Devine, Eric A. Davidson, Paulo Brando and Daniel C. Nepstad
Summary Author for correspondence: Daniel Markewitz Tel: +1 706 542 0133 Email: dmarke@warnell.uga.edu
New Phytologist (2010) 187: 592–607 doi: 10.1111/j.1469-8137.2010.03391.x
KEYWORDS Amazon • deep root uptake • drought • HYDRUS • soil moisture • throughfall exclusion
Deep root water uptake in tropical Amazonian forests has been a major discovery during the last 15 yr. However, the effects of extended droughts, which may increase with climate change, on deep soil moisture utilization remain uncertain. The current study utilized a 1999–2005 record of volumetric water content (VWC) under a throughfall exclusion experiment to calibrate a one-dimensional model of the hydrologic system to estimate VWC, and to quantify the rate of root uptake through 11.5 m of soil. Simulations with root uptake compensation had a relative root mean square error (RRMSE) of 11% at 0–40 cm and < 5% at 350–1150 cm. The simulated contribution of deep root uptake under the control was c. 20% of water demand from 250 to 550 cm and c. 10% from 550 to 1150 cm. Furthermore, in years 2 (2001) and 3 (2002) of throughfall exclusion, deep root uptake increased as soil moisture was available but then declined to near zero in deep layers in 2003 and 2004. Deep root uptake was limited despite high VWC (i.e. > 0.30 cm3 cm−3). This limitation may partly be attributable to high residual water contents (θr) in these highclay (70–90%) soils or due to high soil-to-root resistance. The ability of deep roots and soils to contribute increasing amounts of water with extended drought will be limited.
Shifts in plant respiration and carbon use efficiency at a large-scale drought experiment in the eastern Amazon D. B. Metcalfe, P. Meir, L. E. O. C. Aragão, R. Lobo-do-Vale, D. Galbraith, R. A. Fisher, M. M. Chaves, J. P. Maroco, A. C. L. da Costa, S. S. de Almeida, A. P. Braga, P. H. L. Gonçalves, J. de Athaydes, M. da Costa, T. T. B. Portela, A. A. R. de Oliveira, Y. Malhi and M. Williams Summary Author for correspondence: Daniel B. Metcalfe Tel.: + 44 1865 285182 Email: daniel.metcalfe@ouce.ox.ac.uk
The effects of drought on the Amazon rainforest are potentially large but remain poorly understood. Here, carbon (C) cycling after 5 yr of a large-scale through-fall exclusion (TFE) experiment excluding about 50% of incident rainfall from an eastern Amazon rainforest was compared with a nearby control plot.
New Phytologist (2010) 187: 608–621 doi: 10.1111/j.1469-8137.2010.03319.x
Principal C stocks and fluxes were intensively measured in 2005. Additional minor components were either quantified in later site measurements or derived from the available literature.
KEYWORDS Amazon rain forest • carbon cycling • carbon dioxide • carbon use efficiency • drought • gross primary productivity • net primary productivity • partitioning
Total ecosystem respiration (Reco) and total plant C expenditure (PCE, the sum of net primary productivity (NPP) and autotrophic respiration (Rauto)), were elevated on the TFE plot relative to the control. The increase in PCE and Reco was mainly caused by a rise in Rauto from foliage and roots. Heterotrophic respiration did not differ substantially between plots. NPP was 2.4 ± 1.4 t C ha−1 yr−1 lower on the TFE than the control. Ecosystem carbon use efficiency, the proportion of PCE invested in NPP, was lower in the TFE plot (0.24 ± 0.04) than in the control (0.32 ± 0.04). Drought caused by the TFE treatment appeared to drive fundamental shifts in ecosystem C cycling with potentially important consequences for long-term forest C storage.
Effects of water and nutrient availability on fine root growth in eastern Amazonian forest regrowth, Brazil Tâmara Thaiz Santana Lima, Izildinha Souza Miranda and Steel Silva Vasconcelos
Summary Author for correspondence: Tâmara Thaiz Santana Lima Tel: +55 91 32105236 Email: thaiz.lima@gmail.com
Fine root dynamics is widely recognized as an important biogeochemical process, but there are few data on fine root growth and its response to soil resource availability, especially for tropical forests.
New Phytologist (2010) 187: 622–630 doi: 10.1111/j.1469-8137.2010.03299.x
We evaluated the response of fine root dynamics to altered availability of soil water and nutrients in a 20-yr-old forest regrowth in eastern Amazonia. In one experiment the dry season reduction in soil moisture was alleviated by irrigation. In the other experiment, nutrient supply was reduced by litter removal. We used the ingrowth core technique to measure fine root mass growth, length growth, mortality and specific root length.
KEYWORDS belowground biomass • drought • irrigation • litter removal • rainfall seasonality
Dry-season irrigation had no significant effect on mass and length of live and dead roots, whereas litter removal reduced mass and length of live roots. For both irrigation and litter removal experiments, root growth was significantly greater in the dry season than in the wet season. Increased root growth was associated with decreased soil water availability. However, root growth did not increase in response to nutrient reduction in litter removal plots. Overall, our results suggest that belowground allocation may differ according to the type of soil resource limitation.
Drought–mortality relationships for tropical forests Oliver L. Phillips, Geertje van der Heijden, Simon L. Lewis, Gabriela López-González, Luiz E. O. C. Aragão, Jon Lloyd, Yadvinder Malhi, Abel Monteagudo, Samuel Almeida, Esteban Alvarez Dávila, Iêda Amaral, Sandy Andelman, Ana Andrade, Luzmila Arroyo, Gerardo Aymard, Tim R. Baker, Lilian Blanc, Damien Bonal, Átila Cristina Alves de Oliveira, Kuo-Jung Chao, Nallaret Dávila Cardozo, Lola da Costa, Ted R. Feldpausch, Joshua B. Fisher, Nikolaos M. Fyllas, Maria Aparecida Freitas, David Galbraith, Emanuel Gloor, Niro Higuchi, Eurídice Honorio, Eliana Jiménez, Helen Keeling, Tim J. Killeen, Jon C. Lovett , Patrick Meir, Casimiro Mendoza, Alexandra Morel, Percy Núñez Vargas, Sandra Patiño, Kelvin S-H. Peh, Antonio Peña Cruz, Adriana Prieto, Carlos A. Quesada, Fredy Ramírez, Hirma Ramírez, Agustín Rudas, Rafael Salamão, Michael Schwarz, Javier Silva, Marcos Silveira, J. W. Ferry Slik, Bonaventure Sonké, Anne Sota Thomas, Juliana Stropp, James R. D. Taplin, Rodolfo Vásquez and Emilio Vilanova
Summary Author for correspondence: Oliver L. Phillips Tel: +44 (0)113 343 6832 Email: o.phillips@leeds.ac.uk
The rich ecology of tropical forests is intimately tied to their moisture status. Multisite syntheses can provide a macro-scale view of these linkages and their susceptibility to changing climates. Here, we report pan-tropical and regional-scale analyses of tree vulnerability to drought.
New Phytologist (2010) 187: 631–646 doi: 10.1111/j.1469-8137.2010.03359.x
We assembled available data on tropical forest tree stem mortality before, during, and after recent drought events, from 119 monitoring plots in 10 countries concentrated in Amazonia and Borneo.
KEYWORDS Amazon • Borneo • drought • lags • mortality • RAINFOR • trees • tropics
In most sites, larger trees are disproportionately at risk. At least within Amazonia, low wood density trees are also at greater risk of drought-associated mortality, independent of size. For comparable drought intensities, trees in Borneo are more vulnerable than trees in the Amazon. There is some evidence for lagged impacts of drought, with mortality rates remaining elevated 2 yr after the meteorological event is over. These findings indicate that repeated droughts would shift the functional composition of tropical forests toward smaller, denser-wooded trees. At very high drought intensities, the linear relationship between tree mortality and moisture stress apparently breaks down, suggesting the existence of moisture stress thresholds beyond which some tropical forests would suffer catastrophic tree mortality.
Multiple mechanisms of Amazonian forest biomass losses in three dynamic global vegetation models under climate change David Galbraith, Peter E. Levy, Stephen Sitch, Chris Huntingford, Peter Cox, Mathew Williams 1 and Patrick Meir Summary Author for correspondence: David Galbraith Tel: +44 (0) 131 445 8594 Email: darga@ceh.ac.uk
New Phytologist (2010) 187: 647–665 doi: 10.1111/j.1469-8137.2010.03350.x
KEYWORDS Amazon 'die-back' • Amazon drought • CO2 fertilization • dynamic global vegetation models (DGVMs) • elevated temperatures • photosynthesis • plant respiration
The large-scale loss of Amazonian rainforest under some future climate scenarios has generally been considered to be driven by increased drying over Amazonia predicted by some general circulation models (GCMs). However, the importance of rainfall relative to other drivers has never been formally examined. Here, we conducted factorial simulations to ascertain the contributions of four environmental drivers (precipitation, temperature, humidity and CO2) to simulated changes in Amazonian vegetation carbon (Cveg), in three dynamic global vegetation models (DGVMs) forced with climate data based on HadCM3 for four SRES scenarios. Increased temperature was found to be more important than precipitation reduction in causing losses of Amazonian Cveg in two DGVMs (Hyland and TRIFFID), and as important as precipitation reduction in a third DGVM (LPJ). Increases in plant respiration, direct declines in photosynthesis and increases in vapour pressure deficit (VPD) all contributed to reduce Cveg under high temperature, but the contribution of each mechanism varied greatly across models. Rising CO2 mitigated much of the climate-driven biomass losses in the models. Additional work is required to constrain model behaviour with experimental data under conditions of high temperature and drought. Current models may be overly sensitive to long-term elevated temperatures as they do not account for physiologi-
Assessing uncertainties in a second-generation dynamic vegetation model caused by ecological scale limitations Rosie Fisher, Nate McDowell, Drew Purves, Paul Moorcroft, Stephen Sitch, Peter Cox, Chris Huntingford, Patrick Meir and F. Ian Woodward Summary Author for correspondence: Rosie Fisher Tel: +1 505 6656006 Email: rosieafisher@gmail.com
New Phytologist (2010) 187: 666–681 doi: 10.1111/j.1469-8137.2010.03340.x
KEYWORDS Amazon • competition • competitive exclusion • dynamic global vegetation model (DGVM) • ecosystem demography • migration • perfect plasticity • scaling
Second-generation Dynamic Global Vegetation Models (DGVMs) have recently been developed that explicitly represent the ecological dynamics of disturbance, vertical competition for light, and succession. Here, we introduce a modified secondgeneration DGVM and examine how the representation of demographic processes operating at two-dimensional spatial scales not represented by these models can influence predicted community structure, and responses of ecosystems to climate change. The key demographic processes we investigated were seed advection, seed mixing, sapling survival, competitive exclusion and plant mortality. We varied these parameters in the context of a simulated Amazon rainforest ecosystem containing seven plant functional types (PFTs) that varied along a trade-off surface between growth and the risk of starvation induced mortality. Varying the five unconstrained parameters generated community structures ranging from monocultures to equal co-dominance of the seven PFTs. When exposed to a climate change scenario, the competing impacts of CO2 fertilization and increasing plant mortality caused ecosystem biomass to diverge substantially between simulations, with mid-21st century biomass predictions ranging from 1.5 to 27.0 kg C m−2. Filtering the results using contemporary observation ranges of biomass, leaf area index (LAI), gross primary productivity (GPP) and net primary productivity (NPP) did not substantially constrain the potential outcomes. We conclude that demographic processes represent a large source of uncertainty in DGVM predictions.
Development of probability density functions for future South American rainfall Tim E. Jupp, Peter M. Cox, Anja Rammig, Kirsten Thonicke, Wolfgang Lucht and Wolfgang Cramer
Summary Author for correspondence: Tim E. Jupp Tel: +44 01392 263642 Email: t.e.jupp@exeter.ac.uk
We estimate probability density functions (PDFs) for future rainfall in five regions of South America, by weighting the predictions of the 24 Coupled Model Intercomparison Archive Project 3 (CMIP3) General Circulation Models (GCMs). The models are rated according to their relative abilities to reproduce the inter-annual variability in seasonal rainfall.
New Phytologist (2010) 187: 682–693 doi: 10.1111/j.1469-8137.2010.03368.x
The relative weighting of the climate models is updated sequentially according to Bayes' theorem, based on the biases in the mean of the predicted time-series and the distributional fit of the bias-corrected time-series.
KEYWORDS Amazonia • Bayesian statistics • climate change • forest dieback • probability • vegetation modelling
Depending on the season and the region, we find very different rankings of the GCMs, with no single model doing well in all cases. However, in some regions and seasons, differential weighting of the models leads to significant shifts in the derived rainfall PDFs. Using a combination of the relative model weightings for each season we have also derived a set of overall model weightings for each region that can be used to produce PDFs of forest biomass from the simulations of the Lund–Potsdam–Jena Dynamic Global Vegetation Model for managed land (LPJmL).
Estimating the risk of Amazonian forest dieback Anja Rammig, Tim Jupp, Kirsten Thonicke, Britta Tietjen, Jens Heinke, Sebastian Ostberg, Wolfgang Lucht, Wolfgang Cramer and Peter Cox Summary Author for correspondence: Anja Rammig Tel: +49 (0) 331 288 2454 Email: anja.rammig@pik-potsdam.de
Climate change will very likely affect most forests in Amazonia during the course of the 21st century, but the direction and intensity of the change are uncertain, in part because of differences in rainfall projections. In order to constrain this uncertainty, we estimate the probability for biomass change in Amazonia on the basis of rainfall projections that are weighted by climate model performance for current conditions.
New Phytologist (2010) 187: 694–706 doi: 10.1111/j.1469-8137.2010.03318.x
We estimate the risk of forest dieback by using weighted rainfall projections from 24 general circulation models (GCMs) to create probability density functions (PDFs) for future forest biomass changes simulated by a dynamic vegetation model (LPJmL).
KEYWORDS Amazonia • Bayesian statistics • climate change • forest dieback • probability • vegetation modelling
Our probabilistic assessment of biomass change suggests a likely shift towards increasing biomass compared with nonweighted results. Biomass estimates range between a gain of 6.2 and a loss of 2.7 kg carbon m−2 for the Amazon region, depending on the strength of CO2 fertilization. The uncertainty associated with the long-term effect of CO2 is much larger than that associated with precipitation change. This underlines the importance of reducing uncertainties in the direct effects of CO2 on tropical ecosystems.
The climatic sensitivity of the forest, savanna and forest– savanna transition in tropical South America Marina Hirota, Carlos Nobre, Marcos Daisuke Oyama and Mercedes MC Bustamante
Summary Author for correspondence: Marina Hirota Tel: +55 12 3186 9521 Email: marina.hirota@cptec.inpe.br
New Phytologist (2010) 187: 707–719 doi: 10.1111/j.1469-8137.2010.03352.x
KEYWORDS climate change • conceptual modeling • forest–savanna boundary • natural fires • South America
We used a climate–vegetation–natural fire (CVNF) conceptual model to evaluate the sensitivity and vulnerability of forest, savanna, and the forest–savanna transition to environmental changes in tropical South America. Initially, under current environmental conditions, CVNF model results suggested that, in the absence of fires, tropical forests would extend c. 200 km into the presently observed savanna domain. Environmental changes were then imposed upon the model in temperature, precipitation and lightning strikes. These changes ranged from 2 to 6°C warming, +10 to −20% precipitation change and 0 to 15% increase in lightning frequency, which, in aggregate form, represent expected future climatic changes in response to global warming and deforestation. The most critical vegetation changes are projected to take place over the easternmost portions of the basin, with a widening of the forest–savanna transition. The transition width would increase from 150 to c. 300 km, with tree cover losses ranging from 20 to 85%. This means that c. 6% of the areas currently covered by forests could potentially turn into grass-dominated savanna landscapes. The mechanism driving tree cover reduction consists of the combination of less favorable climate conditions for trees and more fire activity. In addition, this sensitivity analysis predicts that the current dry shrubland vegetation of northeast Brazil could potentially turn into a bare soil landscape.
Predicting moisture dynamics of fine understory fuels in a moist tropical rainforest system: results of a pilot study undertaken to identify proxy variables useful for rating fire danger David Ray, Dan Nepstad and Paulo Brando Summary Author for correspondence: David Ray Tel: +1 850 893 9605 Email: dray@ttrs.org
The use of fire as a land management tool in the moist tropics often has the unintended consequence of degrading adjacent forest, particularly during severe droughts. Reliable models of fire danger are needed to help mitigate these impacts.
New Phytologist (2010) 187: 720–732 doi: 10.1111/j.1469-8137.2010.03358.x
Here, we studied the moisture dynamics of fine understory fuels in the east-central Brazilian Amazon during the 2003 dry season. Drying stations established under varying amounts of canopy cover (leaf area index (LAI) = 0 – 5.3) were subjected to a range of water inputs (5–15 mm) and models were developed to forecast litter moisture content (LMC). Predictions were then compared with independent field data.
KEYWORDS Amazon • drought • fine fuels • fire danger • fire model • forest structure • leaf area index (LAI) • rainforest
A multiple linear regression relating litter moisture content to forest structure (LAI), ambient vapor pressure deficit (VPDM) and an index of elapsed time since a precipitation event (d−1) was identified as the best-fit model (adjusted R2 = 0.89). Relative to the independent observations, model predictions were relatively unbiased when the LMC was ≤ 50%, but consistently underestimated the LMC when the observed values were higher. The approach to predicting fire danger based on forest structure and meteorological variables is promising; however, additional information to the LAI, for example forest biomass, may be required to accurately capture the influence of forest structure on understory microclimate.
Remote sensing detection of droughts in Amazonian forest canopies Liana O. Anderson, Yadvinder Malhi, Luiz E. O. C. Aragão, Richard Ladle, Egidio Arai , Nicolas Barbier and Oliver Phillips Summary Author for correspondence: Liana O. Anderson Tel: +44 1392 848556 Email: liana.anderson@ouce.ox.ac.uk
Remote sensing data are a key tool to assess large forested areas, where limitations such as accessibility and lack of field measurements are prevalent. Here, we have analysed datasets from moderate resolution imaging spectroradiometer (MODIS) satellite measurements and field data to assess the impacts of the 2005 drought in Amazonia.
New Phytologist (2010) 187: 733–750 doi: 10.1111/j.1469-8137.2010.03355.x
We combined vegetation indices (VI) and climatological variables to evaluate the spatiotemporal patterns associated with the 2005 drought, and explore the relationships between remotely-sensed indices and forest inventory data on tree mortality.
KEYWORDS Amazon • climate change • drought • MODIS (moderate resolution imaging spectroradiometer) • phenology • tree mortality • tropical forest • vegetation indices
There were differences in results based on c4 and c5 MODIS products. C5 VI showed no spatial relationship with rainfall or aerosol optical depth; however, distinct regions responded significantly to the increased radiation in 2005. The increase in the Enhanced VI (EVI) during 2005 showed a significant positive relationship (P < 0.07) with the increase of tree mortality. By contrast, the normalized difference water index (NDWI) exhibited a significant negative relationship (P < 0.09) with tree mortality. Previous studies have suggested that the increase in EVI during the 2005 drought was associated with a positive response of forest photosynthesis to changes in the radiation income. We discuss the evidence that this increase could be related to structural changes in the canopy.
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