Understanding how drought affects the risk of increased mortality in tropical rain forests

Lead Research Organisation: University of Edinburgh


Predicting the effects of climate change, and especially drought, on rain forest tree mortality and the associated emissions of carbon dioxide (CO2) is an urgent and high-priority task which this project seeks to address. Increases in tree mortality have the potential to substantially increase total CO2 emissions to the atmosphere, but to date our models are not capable of representing the mortality process reliably during drought and we propose to combine new data and modelling to address this deficiency.
The incidence of extreme drought events has increased in recent years, and climate predictions suggest that some tropical regions may be at risk this century. Severe drought has been associated with El Nino events in tropical South America and in SE Asia in the last 30 years. More recently, two 1-in-100 yr drought events have occurred in Amazonia in the past 10 years, adding weight to concerns about future shifts in climate and their impacts.
At the same time, the incidence of widespread increased tree mortality associated with drought has been recognised as globally important. Severe drought in tropical rain forests can have a large impact. For example, in Amazonia, the regional drought of 2005 is thought to have halted the ongoing large net carbon sink by reducing tree growth and increasing tree mortality. At a larger, pan-tropical scale, observations of the impact of severe drought on tropical rain forests have yielded a startling result: not only do mortality rates increase by up to 12 fold during drought, but the impacts differ substantially between SE Asia and Amazonia. Apparently the rain forest trees of SE Asia are more vulnerable to drought than those of Amazonia. In addition, some taxa and tree sizes (e.g. species and genera, and especially large trees) differ in their vulnerability. If we are to understand the effects of drought on the world's rain forests, and to predict their future composition and functioning (e.g. in how they affect atmospheric CO2 concentration), then we need to know why regions and species differ in their vulnerability to drought.
To make these predictions we need to incorporate ecological understanding into vegetation models that can be coupled to global climate models, to form Earth System Models (ESMs). The only way to enable these vegetation models to represent ecology properly is to make measurements in natural rain forests. To understand the impact of drought we must go a step further and experimentally manipulate the moisture available to the forest, in order to understand the responses of each key process (e.g. respiration, photosynthesis etc). Large-scale drought experiments are scientifically powerful, but very rare in any biome. We have created a unique opportunity in this project to combine the results from two tropical rain forest drought experiments, in Amazonia and Borneo. The combination of experimental and modelling expertise in our team is particularly strong and we wish to use it to make a substantial advance in the prediction of the impacts of drought on 21st century rain forest functioning.
We will first use our models to test for physical differences (soils or climate) in Borneo and Amazonia. Secondly we will focus on differences in mortality risk among tree taxa (species or genera) within and between regions, as some are more vulnerable than others to drought. We will focus on measuring whether mortality is associated with the loss of supply of water or carbon, or a mixture of both, and incorporate our results into our models.

In summary, we will use a powerful combination of tropical rain forest field experiments and global vegetation modelling to explain large observed differences in rain forest tree vulnerability to drought across Borneo and Amazonia. The outcome will have pan-tropical application and we will use it to improve predictions of how climate change will affect the global role of tropical rain forests in the 21st century carbon cycle.

Planned Impact

Drought has become a touchstone issue in environmental science and governance in recent years. One focus for this has been tropical rain forest because of the strong feedbacks in this biome among climate, climate change, ecosystem functioning, biodiversity loss, land use change and regional economies. This means that the users of the outputs from our project potentially include scientists, policymakers and the wider public.

Scientists focusing on the forest carbon cycle, climate sensitivity and wider forest ecology.
We expect that many of the needs of this community will be met through normal routes (paper publication, conference leadership and presentation, guest seminars, and our partnership with the Met Office, see case for support and impact plan) and other stakeholders such as the Brazilian climate science teams at INPE. Our team has a strong track record in publishing in leading journals (recent examples include: Global Change Biology, New Phytologist, Science, Nature, Philosophical Transactions of the Royal Society), and our track record is strong in collaboration and data sharing.

Policy makers and policy influencers interested in this field.
There is widespread interest in the importance of rain forests for climate, ecosystem service provision and local-to-national economies, based on land use practice. PI-Meir has recently led a related NERC-funded 'ESPA' project (NE/G0085311/1) to examine forest ecosystem services and their relationship with poverty alleviation, building a network of major players across S. America, as well as key NGOs. He is also Co-I on a new EU project ('Amazalert') designed to understand these same links with outputs to the Brazilian, UK and EU governing bodies interested in climate change and forest conservation, as well as economic development. Project Co-I Sitch is a contributor to the IPCC 5th Assessment Report, which itself will feed into the above processes and into normal IPCC policy-impact channels.

PI-Meir, CoI-Mencuccini and CoI-Sitch, and all project partners will continue to contribute to these networks, but also their national ones, in government research organisations, national museums and federal universities in UK, Europe, USA, Brazil, Malaysia and Japan. In this context, whilst scientific knowledge exchange will be a continuous process during the project we request a small amount of funding to extend our final science meeting by a day to invite a focused group of stakeholders (science, NGO, govt) for dissemination purposes.

Public interest, data management and data provision.
There is increasing public interest in how tropical rain forests affect our economy and our climate; the roles of deforestation, climate change and their impact on biodiversity also grab headlines regularly. More recently the carbon-climate-forest relationship has discernibly increased in profile for younger members of the public. This is our main focus, and is the main area for which we request Impact Plan funding. We request funds to link ongoing work with new school education activities led by our highly successful outreach team at the School of Geosciences. These funds will enable the integration of our science within the Scottish schools 'Curriculum for Excellence' that was rolled out by the Scottish Government in 2010. We request funds to support the time of two designated outreach staff to deliver this work, a tried-and-tested method of cascading knowledge among school pupils of different ages, and to deliver new materials needed to enhance teaching and communicate our results more widely at science communication events. This outreach process will make use of our prize-winning science education website 'Climate Kaleidoscope'. We request support to extend this product and importantly to also link it to our on-going data archiving and quality checking with a new NERC-funded data portal provided by one of our project partners.


10 25 50
Description The key main current findings are:
1. A breakthrough study showing that the risk factor for tree death during drought is a plant hydraulic process. The transport of water in wood can be disrupted and this appears to trigger the cascade of processesleading to death. This finding is important for modelling tree mortality and will lead to future more detailed research.
2. We have shown that photosynthetic capacity of leaves is resistant to drought stress but does acclimate (decline) slightly, only under co-incident high solar exposure.
3. We have confirmed that some species are more drought intolerant than others and that their water status and metabolism differ substantially from tolerant species
4. We have also demonstrated that leaf hydraulic properties are less strongly linked with drought tolerance than xylem properties.
5. We have shown that loss of basal area (biomass) is a strong determinant of loss of transpiration services and that under additional drought the current forest could become critically water-stressed.
6. We have shown that plant respiration responds to the stress of drought eg to support growth in small trees released from shading supression and (ii) to support maintenance.
7. We have shown that long term drought disrupts standard reproductive litter cycles initially, changing the 'state' of carbon fluxes to/from growth and reproduction, but over the long term the forest reproductive cycle recovers, indicating a re-prioritisation of carbon supply to reproduction.
8. We have made the first solid modeling steps to including the main findings on plant hydraulics and water use into a new model structure of JULES: here we have included an optimised stomatal conductance term that also responds to plant hydraulic potential. This is the fore-runner of a fully functional JULES model that is able to predict tree mortality on a semi-mechanistic basis, based on plant hydraulic stress and plant hydraulic vulnerability.
9. We have shown for the first time that Amazon forest leaves are capable of absorbing water from dew or rain on the leaf surface, thus adding up to 10% of water provision during the year (10% of transpiration use).
10. We have shown that key traits governing growth change from metabolic and photosynthetic to drought-related in trees transitioning from normal rainfall to drought conditions.
11. We have demonstrated substantial species based variation in drought stress metrics, underlining the need to understand this range of responses to drought across species if forest-wide responses to drought are to be understood and quantified/predicted.
Exploitation Route Earth system science and vegetation modelling
Land use policy
Sectors Education,Environment,Government, Democracy and Justice,Culture, Heritage, Museums and Collections

Description The work is being linked with the Earth system science community and with EU and Brazilian partners (including policy advice). We are building an impact process with forest risk analysis colleagues in the UK and with government policy development through our international science partners. The outcomes of this work directly enabled a junior team member (post doc) to gain very high profile publications and subsequently win a NERC fellowship award to work further on the experiment funded by this work: thus a strong impact on career trajectory has been enabled by this grant. This success has been followed up by activity on the fellowship directly related to this grant, and has led to further high impacts (high IF journals) publications, including three in 2020. The work of the original team this grant enabled has been extended and its impact underlined by the first two full publications of the representation of our empirical findings (were published in Nature) in the JULES land surface model. This represents full pull through from the detailed fieldwork of the original proposal to the aspired-for significant changes in the structure and functioning of the UK land surface model (JULES) that forms part of the UK Earth system model (UKESM). The work has also attracted media attention. We now have leading papers out that demonstrate how a large scale empirical study has driven the development of the UKESM (national capability); these have emerged in the past year or two, followed by detailed ecophysiological work on water uptake by leaves, and the movement of water from the water table to the leaf, accounting for different ecosystem-component resistances. Further funding for this experiment has now been won from the Met Office, the Royal Society of London, and the NERC.
First Year Of Impact 2015
Sector Education,Environment
Impact Types Cultural,Economic,Policy & public services

Description Human Modified Tropical Forests Programme
Amount £4,000,000 (GBP)
Funding ID NE/K01627X/1 
Organisation Natural Environment Research Council 
Sector Public
Country United Kingdom
Start 09/2013 
End 09/2018
Title Data from: Small tropical forest trees have a greater capacity to adjust carbon metabolism to long-term drought than large canopy trees 
Description The response of small understory trees to long-term drought is vital in determining the future composition, carbon stocks and dynamics of tropical forests. Long-term drought is, however, also likely to expose understory trees to increased light availability driven by drought-induced mortality. Relatively little is known about the potential for understory trees to adjust their physiology to both decreasing water and increasing light availability. We analysed data on maximum photosynthetic capacity (J max, V cmax), leaf respiration (R leaf), leaf mass per area (LMA), leaf thickness and leaf nitrogen and phosphorus concentrations from 66 small trees across 12 common genera at the world's longest running tropical rainfall exclusion experiment and compared responses to those from 61 surviving canopy trees. Small trees increased J max, V cmax, R leaf and LMA (71%, 29%, 32%, 15% respectively) in response to the drought treatment, but leaf thickness and leaf nutrient concentrations did not change. Small trees were significantly more responsive than large canopy trees to the drought treatment, suggesting greater phenotypic plasticity and resilience to prolonged drought, although differences among taxa were observed. Our results highlight that small tropical trees have greater capacity to respond to ecosystem level changes and have the potential to regenerate resilient forests following future droughts. 
Type Of Material Database/Collection of data 
Year Produced 2020 
Provided To Others? Yes  
URL http://datadryad.org/stash/dataset/doi:10.5061/dryad.547d7wm67
Description Collaboration with EU project 'Amazalert' 
Organisation European Commission
Department Seventh Framework Programme (FP7)
Country European Union (EU) 
Sector Public 
PI Contribution We have endeavoured to set up close exchange with an EU project ?Amazalert?. The aim of 'Amazalert' is to understand the climate risk to Amazonian forests, mainly using existing data and by promoting Dynamic Global Vegetation Model (DGVM) development to improve ecosystem process simulation. A second main aim is to achieve policy-level feed-through of the results of this work in relation to land use, ecosystem services and climate in the Amazon. We have been able to bring our drought experiment (funded in this NERC project) to uniquely inform this modelling and policy feed-through process. Activities: several meetings through the year, and close exchange between this NERC project and the EU project, ?Amazalert?.                 Outcomes include wider (international) model take-up of our empirical results and more detailed modelling progress, including the development of a (new) trait based modelling framework and strong science-policy links in Brazil. Subsequent expected outcomes include increased publications emerging from this NERC/EU link, and direct influence on Brazilian government environmental and scientific policy (especially the agricultural and land use research department, Embrapa, who are partners in Amazalert).
Start Year 2013
Description collaboration with Brazilian FACE experiment 
Organisation Brazilian Free-air CO2 Enrichment (FACE) Experiment
Country Brazil 
Sector Academic/University 
PI Contribution Meir?s track record using these and previous NERC funds to install and extend the world?s only long-term ecosystem-scale drought experiment in tropical rain forest led to an invitation to be a founding member of the SSC (scientific steering committee) of a new large scale experiment in the Amazon, designed to test understanding of impacts of elevated CO2 on tropical rain forest functioning (Amazon-FACE). We participated in a Science-Government-Private Sector meeting: ?Face to FACE in the Amazon?, Washington DC, USA, April 2013. This science meeting was funded by the Inter American Development Bank (IDB) and attended by senior IDB staff and the National Science Advisor to the Brazilian Government, as well as scientists. Activities by Meir included: Lead on above ground processes work-package presentation, and discussion lead on large scale experiments in rain forest. Outcomes included a news piece in Nature (Tollefson et al., 2013) and subsequently funding from IDB ($1.5m), Brazilian Government and Brazilian states ($3 m). This funding goes to Brazilian institutions mainly, but will create research opportunities in collaborating countries ? UK, USA, Netherlands, Germany, Australia.
Start Year 2013
Description Schools interational engagement 
Form Of Engagement Activity Participation in an activity, workshop or similar
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Schools
Results and Impact Building a relationship between Peruvian/Andean schools and Edinburgh schools. Providing some basic inexpensive weather station equipment to log weather data at the schools (they are remote and resource-poor), and then connect with this information with schools in Edinburgh. Increase interest in our research, helped build a 'public licence' to do research, enabled teh students to understand the environment about them and international science
Year(s) Of Engagement Activity 2011,2012,2013,2014