Using plant hydraulic scaling to predict the drought vulnerability of the world's tallest tropical trees

Tropical rainforests are one of the planets most important stores of carbon, as well as being essential to water cycling at large scales. Within tropical forests the largest trees, with diameters exceeding 70 cm, store between 25-45% of the carbon, yet represent <4% of the total number of trees. These large trees also transport disproportionately more water than smaller individuals do, making them a conservation priority for the future.

Large tropical trees are likely to be very old, with many between 200-500 years and some estimated to be >1400 years old. Therefore, they have survived historical extreme climate events, including drought. Yet, recent evidence suggests water transport limitations are likely to make larger trees more vulnerable to the more extreme, more frequent drought events, which are predicted for the future. However, we still do not understand how large trees manage to overcome the huge resistances associated with transporting water such large vertical distances, against gravity, which substantially increase the hydraulic stress the tree experiences in a given climate. This information is essential to understanding how vulnerable these iconic tropical trees will be to the predicted future increases in drought frequency and intensity. Large trees can minimise the effects of increasing resistance to water transport with height through changing multiple leaf and stem hydraulic traits vertically through their stem and canopy. However, data on these vertical changes are rare and do not exist for tropical trees. Consequently, there is limited knowledge concerning whether trees can or cannot compensate for the negative effects being taller has on their water transport capacity and therefore their vulnerability to future drought events.

In this project we will combine novel measurements of vertical changes in tree anatomical, structural and hydraulic properties on the world's tallest tropical trees, in two different tropical regions - Amazonia and Borneo - to achieve the following aims:

Aim 1: Determine how vertical changes in tree hydraulic and anatomical traits regulate the capacity of tall trees to maintain water transport to their leaves under different environmental conditions.
Aim 2: Determine if key structural and architectural properties of tropical trees control the vertical gradients of plant hydraulic and anatomical properties.
Aim 3: Determine how accounting for vertical gradients in hydraulic properties in tall tropical trees alters predictions of tropical forest water and carbon cycling.

To achieve these aims we will study the tallest tropical trees in the world. This will include trees in Amazonia discovered in 2019 that reach 88.5 m tall, ~30m taller than any other tree recorded in the neotropics. We will compare these to equivalent sized trees in Borneo from the dipterocarp family, the family containing the tallest angiosperm species in the world. On these trees we will measure vertical gradients in hydraulic and anatomical traits on 60 trees varying in height from 20-90 m. These trees will come from eight dominant species in Brazil and Borneo, allowing us to contrast the hydraulic adaptations of trees species from drier, more seasonal climates (Brazil), to those of species that have evolved in wetter, a-seasonal climates (Borneo). To realise the three aims above, our novel vertical hydraulic trait measurements will be combined with measures of whole-tree water transport and storage, tree architectural data derived from state-of-the-art ground-based laser scanning and vegetation models. Combining these techniques will allow us to make a step-change in our current understanding of the limits to water transport in the world's tallest tropical trees and the impact this may have on carbon and water cycling under future climate scenarios.

Non-academic beneficiaries:

The governments within Brazil and Borneo: Tropical forests are a major global resource, but of particular value to the governments of countries like Brazil and Malaysia. Effectively conserving these areas is however complex and requires prioritising set areas for protection, whilst allowing other areas to be used for food production, housing and industry. This project will generate new scientific information on the value of prioritising areas for conservation that contain the largest tropical trees. This is critical, as often areas containing the largest tree species are priority areas for the logging industry. Therefore, we hope to be able to generate the information needed to inform new practices for landscape design to buffer against drought stress in tropical regions through using our research to help evaluate the best areas to conserve, based off forest size distribution.

Reserve managers: Managing reserves in tropical forests is complex and requires balancing competing land-use pressures. Both the reserves in which we will work during this project are reliant on ecotourism. In particular, the large trees in both the Tumucumaque and Sepilok reserves are of particular importance, as a tourist attraction in themselves and for sustaining charismatic fauna, such as orangutan populations in Borneo. Scientific information about the vulnerability of these large trees to future changes in climate is therefore essential to setting conservation priorities and mitigation policies within these areas to limit the effects of climate change on the loss of natural resources.

The IPCC and climate policy makers: Predicting the impacts of climate change on tropical forests is central to accurately forecasting future global climate change trajectories and setting effective climate mitigation policy. Our work will directly contribute to improving the accuracy of prediction of climate-carbon cycle feedbacks in tropical forests. CoI Sitch is an IPCC author and both he and PI Rowland collaborate with multiple other IPCC authors including Profs Betts, Collins, Cox and Friedlingstein and Dr. Jones. Consequently, we can communicate the results from this project directly to IPCC scientists, generating substantial potential for our modelling results to influence climate change policy at the highest level.

Local communities within Brazil and Borneo: Brazil and Borneo both contain highly vulnerable local communities, who are reliant on the resources that natural ecosystems, including tropical forests, provide for subsistence. The impact of climate change on these ecosystems is therefore directly relevant to the livelihoods of these local communities. To mitigate the impacts of climate change on local communities such as these, therefore requires an in-depth understanding of how the resources tropical forests provide may change in the future. This project will directly contribute to this knowledge and enable more effective plans to be created to secure the resources necessary for these local communities in the future.

The general public: Tropical forests are an essential global resource and therefore their future persistence is of value to the entire global population. To disseminate the results of our research to the public we will work with the Eden project, who receive >1 million visitors per year, alongside a further >50,000 school pupils and 4 million annual visitors to their website. PI Rowland has been working as a science advisor to the Eden project since 2016. We will build on this existing relationship during this project to provide new school resources, tailored to the UK curriculum and new public displays within the Eden tropical biome and on their website to highlight the value of prioritising the conservation of tropical forests, particularly areas containing the tallest tropical tree species.