Do dryland ecosystems control variability and recent trends in the land CO2 sink?

Lead Research Organisation: University of Exeter
Department Name: Geography


Drylands store modest amounts of carbon in their soils and vegetation per unit area; however, as they cover 40% of the global land mass they store globally significant amounts of carbon overall. Rainfall is very variable between years, and these ecosystems respond rapidly to water availability. Therefore, carbon storage also varies greatly through time. Recent work has shown that dryland ecosystems may control variations in the global carbon cycle; however, there are significant uncertainties associated with understanding of carbon dynamics in dryland soils and vegetation, especially as climates change. Some researchers argue that drylands do not have the capacity to store or lose enough carbon to make any difference to the global amounts of carbon that are taken up by plants or soils, or lost to the atmosphere. Others argue that drylands may provide the 'missing-sink' in the global carbon cycle - helping to explain the stores of carbon needed to balance the global carbon budget, and that especially during wetter years, they have the capacity to store much more carbon than has previously been estimated.

The current lack of consensus on the role of drylands in the global carbon cycle is hindering scientific ability to constrain the global carbon budget and understand future trends in the ability of the terrestrial carbon sink to mitigate climate change. This project will address this disagreement, providing robust analysis of existing data across a gradient of aridity and a range of plant types in a dryland region that contains the highest density of existing monitoring sites in the world. We will also undertake highly novel fieldwork, collecting new data which allow us to understand uncertainty in existing datasets that describe carbon storage and loss. We will then use new observations to evaluate which remotely sensed products, from existing satellite networks, are the most accurate at representing differences in carbon stocks in drylands. This element of the project is fundamental to understanding our planet, as it will enable more accurate global predictions of the carbon cycle and how this affects the global climate.

Global modelling work argues that marginal, dryland ecosystems may control the global variability of carbon storage and loss and may also exert a profound control on the long-term trends of carbon storage and loss between the Earth and the atmosphere. This proposal will improve the empirical foundations of such model predictions. We contend that the predictions are likely to be true, but as yet have not been validated, nor understood well, in terms of the mechanisms that might underpin these controls. We will model the dynamics of vegetation in drylands, to test which vegetation models make the best predictions of growth and dieback, through wet and dry periods observed in the data that we collect in the first part of the project. Once we have established which vegetation models perform best, we will populate these models with appropriate parameters to predict how vegetation might respond to future climates, thus ensuring that the next step - to improve global model predictions of carbon loss and storage is made via a dialogue between empirical data collection and modelling.

The project will deliver a fundamental improvement in our understanding of the carbon cycle in drylands, demonstrating empirically whether or not these landscapes have the capacity to control inter-annual variability and long-term trends in the land carbon sink. It will allow us to develop field techniques that can be exported to other landscapes, to constrain the uncertainty associated with measurements of ecosystem change. It will further allow us to understand and then recommend which globally available remote sensing products are best at characterising change in above-ground carbon stores in drylands. Finally it will permit us to make significant, data-based improvements to predictions of the global carbon cycle.

Planned Impact

Who could potentially benefit from the proposed research over different timescales and how might they benefit?

We have worked with a number of project partners and potential stakeholders for > 15 years on NERC, US National Science Foundation and other research funder projects. Through this work we have identified that the key beneficiaries from the proposed research would include Project Partners, the Global Fluxnet community, the Met Office and Hadley Centre scientists, US statutory agencies and major land owners/users in drylands throughout the world. From project inception we will work with these groups to identify further areas of impact that the project might deliver and further questions that might be targeted to reach wider groups during the project.

In the short-term, during the project lifespan, Project Partners will be the key beneficiaries, as we integrate our science findings and methodological advances within the research programs that span the 28 field sites that we propose to work at. Our assessments of aboveground biomass and replicated measures of net ecosystem exchange will allow these partners to re-evaluate how they measure this important element of the carbon cycle. Through the Global network of Fluxnet scientists, we will also impact a much larger group of researchers, numbering some thousand, who operate the ca. 680 flux towers that run worldwide. Constraining the understanding of uncertainty associated with this Global dataset, would be a major impact of this work. Towards the end of the project, we will present findings to scientists at the Met Office and equivalent institutes Globally. As an influential community who advise Global policy on climate change, we will speak out to a number of IPCC authors who are based in Exeter and further afield, to ensure that the work that we undertake is delivered to this important group of Earth System modellers. We fully anticipate that this group will integrate our findings into improved Global predictions of climate change.

In the medium term, during the last two years of the project and post-project we will engage stakeholders, from the Drone manufacturing industry, to share our findings through workshops and presentations to trade fairs that demonstrate the value of our methodological advances. This fast growing industry is dynamic and therefore any state-of-the-art developments that we can provide will soon be integrated into the kind of products that are available 'off-the-shelf'. We will also engage the community involved in manufacturing equipment to measure net ecosystem exchange. Our technical advances delivered on this project will appeal to those businesses who are trying to make such hardware more affordable, portable and therefore deployable in remote locations, especially in developing countries.

We also understand the value of public engagement with science and will deliver this through a number of approaches including (1) Online capture of all science and popular science talks via YouTube and Vimeo, linked to project blogs written by staff, (2) Online versions of project workshops (again via YouTube and Vimeo) to ensure that free knowledge exchange of our methodological advances are available to Global audiences, (3) attendance at science fairs, hosted by the University of Exeter to engage wider publics with our research.

In the long-term our research will benefit statutory bodies whose role it is to advise US Govt. on land management policy. We already work with the Bureau of Land Management and the US Fish and Wildlife Service, whose practice of rangeland management will be directly impacted by our improved understanding of vegetation carbon storage and how this varies with plant functional type. These bodies will in turn, be able to advise individual farmers/ranchers as to the benefits of different vegetation management, using our evidence of the implications for carbon budgets to support their advice.
Title Allometric modelling of plant biomass from drone-acquired photographs: drone images, ground control marker coordinates and biomass data from 36 sites, 2016-2020 
Description This dataset contains RGB photographs acquired from drone surveys. There are 741 harvest plots from 38 surveys at 36 sites around the world. Each site was approximately 1 ha in area. Included with the photographic images are the coordinates of ground control markers, biomass, taxonomic and location data for harvest plots and ancillary metadata. The observations can be used to obtain allometric size-biomass models. This work was supported by the Natural Environment Research Council award number NE/R00062X/1 as part of the project 'Do dryland ecosystems control variability and recent trends in the land CO2 sink?' 
Type Of Material Database/Collection of data 
Year Produced 2020 
Provided To Others? Yes  
Title Field measurements of aboveground biomass, canopy area, stem diameter and sapwood area of Juniperus monosperma trees in New Mexico, collected in 2018 and 2019 
Description The dataset consists of observations of aboveground biomass, canopy area, maximum height, stem diameter and sapwood area of Juniperus monosperma (Oneseed Juniper) trees, measured at a site in central New Mexico in 2018 and 2019. In total, 200 stems for sapwood area were measured, and 18 trees for full biomass determinations. 
Type Of Material Database/Collection of data 
Year Produced 2019 
Provided To Others? Yes