What is the fate of forests on a warming planet? Assessing climate sensitivity of tree species using tree rings.

What is the fate of forests on a warming planet? Assessing climate sensitivity of tree species using tree rings.

Supervisors: Profs. Roel Brienen, (SoG), Manuel Gloor (SoG), David Galbraith (SoG)

Contact e-mail: r.brienen@leeds.ac.uk

Research Keywords :     Forests, Ecology, climate change, Resilience, sensitivity, dendrochronology

Trees are amazing organisms. Through ingenious adaptations, trees can grow and survive in an astonishing range of environments from humid tropical rainforests to cold deserts. They are also important for life on earth in various ways. They play a vital role in the regulation of the earth’s climate by affecting the hydrological cycle (Spracklen et al. 2012), cooling the earth (Bonan 2008), and storing large amounts of carbon (Pan et al. 2011). However, the climate in which trees have evolved and grown for centuries is not the same as the climate trees will be facing in the future. Recent studies show that tree mortality is increasing globally (Brienen et al. 2015, Allen et al. 2010, Zu et al.2019), indicating that some trees are unable to cope with climate change. How different species and forests respond to our rapidly changing climate will have important consequences for the vital functions that trees fulfil.

            The response of trees to climate extremes varies widely; while some species succumb even under mild drought, others can survive and even maintain growth under severe droughts (McDowell 2011). These differences are related to the trees’ specific properties or traits. Some important traits believed to control trees’ climate sensitivity are those related to water transport (hydraulic traits), and growth strategy (carbohydrate storage and use). For example, species exhibiting “safer” hydraulic transport systems may maintain a positive water balance throughout drought periods and respond less strong to variation in climate (Anderegg et al. 2018). Similarly, species investing more in building and maintaining carbohydrate reserves (eg. starch) may be more resilient to inter-annual variation in climate than species that prioritise growth (Chapin et al. 1990).

The aim of this project is to reveal why some species are more sensitive to climate than others, and assess what set of characteristics provide the best adaptations to our future climate.

In this project you will be able to provide the first global assessment of the sensitivity of tree growth to climate and link it to species’ functional traits, as well as their evolutionary relatedness. This work is possible because of large databases of tree growth derived from tree ring records and easy access to trait data. Large tree ring databases, including the International Tree Ring Data Base (ITRDB) and data from National Forest Inventories (NFI) allow assessment of the inter-annual growth response of trees to climate (Fig. 1). The inclusion of more and more sites and species in these databases makes it now possible to reveal insight in species climate sensitivity of more species from across the globe, including tropical trees.  At the same time, integration, and expansion of species’ trait databases (eg. TRY, DRYAD), allows linking detailed assessment of species climate sensitivity to an ever-increasing set of functional traits and species phylogeny (i.e., their evolutionary relatedness).  In addition to using these existing datasets, you will also have the opportunity in this PhD to collect new tree ring and trait data to gain field competency skills and complement data for poorly represented species and sites.

Using these large global datasets, you will address several significant questions, including:

• Can we predict species’ growth sensitivity to climate from functional traits, and is there a relationship with species’ phylogenetic relatedness?
• How does the climate sensitivity of tree growth vary across spatial and environmental scales?
• Is the climate sensitivity a good predictor for species mortality risk, and how do these relate to species’ lifespans?

The insights generated will lead to recommendations to improve model predictions of trees’ responses to future climate using functional traits.

Material & methods

You will collate large datasets of tree rings, functional traits and climate from across the world to assess the climate sensitivity of tree species to drought and temperature. The International Tree Ring Database (ITRDB) contains over 100.000 tree ring records from more than 4.000 sites, and 200 species from across the globe (see Fig 1). These data will be complemented with suitable tree ring data from collaborators and from scientific publications. By linking these data to climate records, you will first assess the sensitivity of growth responses (ring widths) to inter-annual variation in climate (precipitation and temperature). Then you will link species’ climate sensitivity to functional trait data (eg. on plant hydraulics, wood characteristics and vessel anatomy, from TRY, DRYAD) to test whether species with more conservative hydraulic systems are more resistant to variation in climate. These existing datasets will be complemented with your own data for a select number of species and sites.

Potential for high impact outcome

Despite the large number of studies looking at plant functional traits, only very few studies have linked such traits to plant performance measures. Tree rings are an easy way of obtaining good estimates of plant performance in the form of ring width responses to interannual variation in climate. In addition, for a select number of species tree ring samples from trees that died -in response to climate or other causes- allow assessing the trees survival strategies and assessing differences between species with different functional traits. This study will provide a very new outlook on the significance of species’ functional traits by linking trait data to tree ring data. This has not been attempted before and will likely result in important, high impact outcome.

Training

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 by Profs. Roel Brienen, Emanuel Gloor and 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. You will have access to a broad spectrum of training workshops put on by the Faculty that include an extensive range of training workshops in numerical modelling, through to managing your degree, to preparing for your viva (http://www.emeskillstraining.leeds.ac.uk/).

Student profile

You are expected to have strong interests in environmental and earth system science and global change. You also should have some background in disciplines such as mathematics, physics, geography, biology, or environmental science. Strong analytical skills are required.

 

References

Anderegg, W. R., Konings, A. G., Trugman, A. T., Yu, K., Bowling, D. R., Gabbitas, R., … & Zenes, N. (2018). Hydraulic diversity of forests regulates ecosystem resilience during drought. Nature, 561, 538–541.

Bonan, Gordon B. “Forests and climate change: forcings, feedbacks, and the climate benefits of forests.” science 320, no. 5882 (2008): 1444-1449.

Chapin III, F. S., Schulze, E. D., & Mooney, H. A. (1990). The ecology and economics of storage in plants. Annual review of ecology and systematics, 21(1), 423-447.

McDowell, N.G. (2011) Mechanisms Linking Drought, Hydraulics, Carbon Metabolism, and Vegetation Mortality. Plant Physiology, 155, 1051-1059.

Pan, Y., Birdsey, R.A., Fang, J., Houghton, R., Kauppi, P.E., Kurz, W.A., Phillips, O.L., Shvidenko, A., Lewis, S.L., Canadell, J.G., Ciais, P., Jackson, R.B., Pacala, S.W., McGuire, A.D., Piao, S., Rautiainen, A., Sitch, S. & Hayes, D. (2011) A Large and Persistent Carbon Sink in the World’s Forests. Science, 333, 988-993.

Ryan, M. G., & Yoder, B. J. (1997). Hydraulic limits to tree height and tree growth. Bioscience, 47(4), 235-242.

Sperry, John S., Frederick C. Meinzer, and KA. McCulloh. “Safety and efficiency conflicts in hydraulic architecture: scaling from tissues to trees.” Plant, Cell & Environment 31.5 (2008): 632-645.

Spracklen, D.V., Arnold, S.R. and Taylor, C.M. (2012) Observations of increased tropical rainfall preceded by air passage over forests. Nature, 489(7415), 282.