Live fast, die young: are tree life cycles accelerating due to global change?
Dr Roel Brienen (SoG), Prof. Emanuel Gloor (SoG), Dr. David Galbraith (SoG)Contact email: firstname.lastname@example.org
Tree mortality has accelerated widely over recent decades (Allen etal., 2010), in some areas almost in line with productivity increases (Brienen etal., 2015), and with increases in atmospheric CO2 and temperature (Van Mantgem etal., 2009). Increases in tree mortality affect the functioning of forests and may constitute an important positive feedback on the global carbon cycle; it could significantly weaken or even reverse the carbon sink of forests in the future (Bugmann & Bigler, 2011; Friend etal., 2013). Thus, an accurate understanding of the causes behind these mortality increases is crucial.
Many studies have focused on climate driven mortality mechanisms (McDowell, 2011), but significantly less attention is given to possible intrinsic demographic processes that may partially drive these mortality increases. Tree mortality has been shown to be related to patterns of forest productivity such that forests with greater productivity also exhibit greater mortality rates, and visa versa (Stephenson & van Mantgem, 2005). Thus, greater productivity directly or indirectly affects tree longevity, or tree life spans. As forest productivity is increasing in many forests worldwide (Pan etal., 2011), this could possibly be one of the contributing mechanisms to observed increases in mortality. A recent study by Brienen et al (2015) shows that productivity and mortality increases in the Amazon rainforest have gone up at about the same rate over the past 30 years, could be due to faster completion of trees’ life-cycles; i.e., the “live fast, die young” mechanism. However, this phenomenon remains so far unproven.
Fig. 1 Relationship between tree growth and maximum observed ages in tree rings. Panel a) shows that those trees that grew fast historically (green lines) died at a younger age compared to slow growing trees (red lines). Figure from Bigler and Velben 2008).Panel b) shows that variation in maximum tree ages across various beech populations is negatively related to temperature (Graph from Di Fillipo et al. 2015).
Despite evidence for the existence of a negative relationship between productivity and turnover at various organisational levels, it is much less clear to what degree these relationships exist at the individual tree level, as it is hard to follow trees over their full life span which can last 100s to 1000s of years. One easy technique however to study these relationships is the use of tree rings. Application of this method has shown that even at the individual tree level (i.e., within a single species) faster growth of trees seems to lead to shorter tree longevity. The first study to convincingly demonstrate this was by Bigler and Veblen (2009) who sampled dead conifer trees in the Alps and showed that older trees grew much slower over their life time compared to younger trees (see Fig. 1a). More recently Di Filippo et al. (2015) show that the same relationship holds for temperate broadleaf trees. Interestingly variation in tree turnover is closely related to variation in temperature. As show in Figure 1b, for every degree of warming tree longevity in beech trees decreased by 30 years. If this relationship holds up in a warming world, tree turnover rates may be expected to continue to accelerate, thereby intrinsically changing forest dynamics and the capacity of forests to act as a carbon sink.
Fig. 2 Approaches used in this study consist of sampling and analysing tree ring data from new sites (a and b) and use the extensive dataset of the International Tree Ring DataBase (ITRDB) available online (see http://www.ncdc.noaa.gov/data-access/paleoclimatologydata/datasets/tree-ring). This dataset contains tree ring data from over 4000 sites, and hundreds of species.
It remains still unclear to what degree the same relationships between tree productivity and lifespan, or temperature and tree life spans holds up in other species, and in other biomes. For example, do the same principles apply in a tropical rainforest? Other open questions involve the unclear role of temperature in this relationship. Does temperature influence tree longevities directly, or is the temperature effect simply a result of increasing productivity with increases in temperature? And how do trees growing at the warm extreme of their nature range respond to change in temp (e.g., at the southern margins in Northern Hemisphere)? These are questions that have not been addressed before, but are important to understand the future response of forests to climate change.
The aim of this Phd is to use existing and new tree ring data to shed light on the following questions;
- How common is the relationship between tree productivity (growth) and tree lifespan across a range of species? Does it vary between needle trees and broadleaf trees?
- What is the ultimate cause behind this relationship? Do trees simply have maximum size they can reach, or do they senescence faster? What is the effect of climate (temperature and rainfall) versus resource availability on this relationship?
- How do predicted changes in climate and productivity affect forest turnover rates, and carbon storage?
Your research methods may involve the following approaches:
- Analysing data from the International Tree Ring DataBase (ITRDB,http://www.ncdc.noaa.gov/data-access/paleoclimatology-data/datasets/tree-ring). This dataset contains over 100.000 tree ring records from more than 4000 sites, and 200 species from across the globe (see Fig 2c). You will use this dataset to map global patterns of (maximum) tree ages and lifespan, and study how this varies between taxa and across biomes, and unravel the drivers in variation in tree ages and lifespan by linking tree ring data to environmental conditions (eg. soil and climate), and to estimates of tree growth and biome specific productivity and mortality data.
- Collecting new tree ring data across resource or productivity-gradients and across climatic gradients to assess the effects of resources availability and climate on tree lifespans and theirrelationship with tree productivity (growth). This could be done for example, by sampling one or more species along a gradient from the optimum of their natural range to the limits of their range. Sites could include altitudinal or latitudinal gradients, either in the tropics, temperate or arctic regions.
Potential for high impact outcome
Increases in tree turnover have an immediate effect on the carbon sink capacity of forests and the research topic of this PhD study is thus of direct relevance for predictions of future levels of atmospheric CO2 and the rate of climate change. Our approach is new as very few studies have looked at these types of mechanisms as a possible explanation for trends in mortality increases, and it is anticipated that this study will generate several important papers of which some publishable in high impact journals.
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. 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/).
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.
Allen, C.D., Macalady, A.K., Chenchouni, H., Bachelet, D., McDowell, N., Vennetier, M., Kitzberger, T., Rigling, A., Breshears, D.D. & Hogg, E. (2010) A global overview of drought and heat-induced tree mortality reveals emerging climate change risks for forests. Forest Ecology and Management, 259, 660-684.
Bigler, C. & Veblen, T.T. (2009) Increased early growth rates decrease longevities of conifers in subalpine forests. Oikos, 118, 1130-1138. Brienen, R., Phillips, O., Feldpausch, T., Gloor, E., Baker, T., Lloyd, J., Lopez-Gonzalez, G., Monteagudo-Mendoza, A., Malhi, Y. & Lewis, S. (2015) Long-term decline of the Amazon carbon sink. Nature, 519, 344-348.
Bugmann, H. & Bigler, C. (2011) Will the CO2 fertilization effect in forests be offset by reduced tree longevity? Oecologia, 165, 533-544.
Di Filippo, A., Pederson, N., Baliva, M., Brunetti, M., Dinella, A., Kitamura, K., Knapp, H.D., Schirone, B. & Piovesan, G. (2015) The longevity ofbroadleaf deciduous trees in Northern Hemisphere temperate forests: insights from tree-ring series. Frontiers in Ecology and Evolution,3, 46.
Friend, A.D., Lucht, W., Rademacher, T.T., Keribin, R., Betts, R., Cadule, P., Ciais, P., Clark, D.B., Dankers, R. & Falloon, P.D. (2013) Carbon residence time dominates uncertainty in terrestrial vegetation responses to future climate and atmospheric CO2. Proceedings of the National Academy of Sciences, 201222477.
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.
Stephenson, N.L. & van Mantgem, P.J. (2005) Forest turnover rates follow global and regional patterns of productivity. Ecology Letters, 8, 524-531. Van Mantgem, P.J., Stephenson, N.L., Byrne, J.C., Daniels, L.D., Franklin, J.F., Fulé, P.Z., Harmon, M.E., Larson, A.J., Smith, J.M. & Taylor, A.H. (2009) Widespread increase of tree mortality rates in the western United States. Science, 323, 521-524.
Related undergraduate subjects:
- Environmental science