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

Lead Research Organisation: University of Exeter
Department Name: Geography


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 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 agricultural 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 minimal support to extend this product after current funding under NE/GO18287/1 runs out, and importantly to also link our on-going data archiving and quality checking with a new NERC-funded data portal provided by one of our project partners.


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Bastos A (2018) Impact of the 2015/2016 El Niño on the terrestrial carbon cycle constrained by bottom-up and top-down approaches. in Philosophical transactions of the Royal Society of London. Series B, Biological sciences

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Eller CB (2018) Modelling tropical forest responses to drought and El Niño with a stomatal optimization model based on xylem hydraulics. in Philosophical transactions of the Royal Society of London. Series B, Biological sciences

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Fisher J (2014) Modeling the Terrestrial Biosphere in Annual Review of Environment and Resources

Description When exposed to long-term drought, tropical trees die due to plant hydraulic stress rather than carbon starvation. Also it's the tall trees that are most succeptable.
Exploitation Route Results from the empirical component of this grant are highly relevant to understand the mechanisms of drought mortality.
Sectors Environment