The fate of forests on a warming planet: assessing climate sensitivity of tree species using tree rings

Forests play an important role in the regulation of the earth’s climate and in our efforts to limit atmospheric CO2 rises. For example, over the past decades forests have been responsible for the uptake of about a third of fossil fuel emissions. To what degree forests will continue to provide such services depends on the response of trees to a heating planet. Globally temperatures are rising at unprecedented rates and droughts are predicted to become more frequent and will increase in intensity. These changes will affect tree growth and survival, but which species survive and which die remains still poorly understood. In this project, you will study what characteristics determines species’ vulnerability to climate.
Different tree species show strong differences in their responses to climate. While some trees succumb even under a mild drought others are able to survive and even continue growing under severe droughts. These differences are due to differences in trees’ specific properties or traits. One important plant trait explaining differences in trees climate response is that linked to their water transport, and the trade-off between water transport efficiency and safety (Sperry et al. 2008). Trees that are more efficient at water transport due to greater hydraulic conductance are believed to be more sensitive to inter-annual variation in climate. In contrast, trees with more conservative water use strategies and a “safer” hydraulic transport system may maintain a positive water balance throughout drought periods. Recent studies show that forests with more diverse hydraulic traits are more buffered to changing drought conditions. Thus, hydraulic traits play an important role in forest functioning.
At the same time, there is also evidence for an important role of carbohydrate reserves (eg. starch) for tree functioning. Carbohydrate reserves can be used in bad years thus providing resilience to inter-annual variation in climate. Thus, besides hydraulics other traits may also play an important role in protecting trees from the impacts of strong year-to-year climate variation.

The aim of this PhD project is to
1. Assess the global linkage between climate sensitivity of tree species and their functional traits;
2. Evaluate how functional traits and climate sensitivity change with tree size and age;
3. Provide recommendations to improve model predictions of trees’ responses to future climate using functional traits.

Despite, recent progress in understanding trees’ physiological responses to variation in climate, the governing principles to predict trees’ resilience to climate remains poorly quantified. In this study, you will provide the first global assessment of trees’ climate sensitivity and link these to species’ functional traits. To this end, you will use the International Tree Ring Data Bank (ITRDB) to assess the inter-annual response of trees to climate, and link those responses to the trees functional characteristics, especially those traits related to hydraulics. In addition to using existing datasets, you will in this PhD also collect new tree ring and trait data on a few tropical and temperate tree species. This will involve fieldwork in the tropics (most likely in the Amazon) and in Europe.

You will work under the supervision of a strong team of earth system dynamics experts within the Ecology and Global Change research group of the School of Geography. Direct daily supervision will be done by Dr. Roel Brienen, Prof. Emanuel Gloor and Dr. David Galbraith. You will also benefit from working within a highly active and multidisciplinary group of scientists in the Leeds Ecosystem, Atmosphere & Forest (LEAF). The school of geography has excellent and state-of-the-art laboratory facilities including a full equipped tree ring lab.