Session: Last Millennium & Past2K
Author: Guangqi Li / g.li2@reading.ac.uk / Centre for Past Climate Change and School of Archaeology, Geography and Environmental Sciences (SAGES), Reading University, Reading, UK
Co-author: Sandy P. Harrison, Centre for Past Climate Change and School of Archaeology, Geography and Environmental Sciences (SAGES), Reading University, Reading, UK;
                    I. Colin Prentice, AXA Chair of Biosphere and Climate Impacts, Department of Life Sciences, Imperial College London, Silwood Park Campus, Ascot, UK;

Abstract:
The derivation of Last Millennium climate reconstructions from tree-ring series is based on several assumptions including that (a) ontogenetic effects on radial growth can be removed statistically, (b) tree growth at a given site is controlled by a single climate variable or at most a simple combination of two variable, (c) the statistical relationship between radial growth and climate variables is invariant through time, (d) changing CO2 concentrations [CO2] have negligible impacts on growth, and (e) carbon allocation to stem growth is a constant proportion of total productivity. Here we show that inherent temporal sampling biases mean that standard techniques to account for ontogeny still preserve the impacts of changes in long-term mean climate on growth. We also show that tree growth is always controlled by multiple climate variables, including light, atmospheric drought, soil moisture and growing season temperature. The relationship between growing season temperature and tree growth is non-monotonic, such that increasing temperature has positive effects on radial growth in cool climates but negative effects on growth in more temperate regions. The strength of the relationship between any one climate factor and growth varies spatially, but more importantly is not invariant through time. Finally, we show that while changes in [CO2] increase photosynthesis and gross primary production, this is not always reflected in increased stem growth. Tree responses to changing [CO2] involve changes in carbon allocation to leaves and rooting systems. Given that changes in [CO2] affect water-use efficiency, changes in allocation are also expected as a response to persistent drought and will therefore modulate the apparent relationship between stem growth and climate. While these analyses raise serious issues about the reliability of climate reconstructions based on tree-ring series, we propose a way forward through process-based modelling. By building on ecophysiological theory, process-based modelling avoids unrealistic assumptions about tree growth, and should allow more soundly based interpretations of tree-ring data and comparisons with palaeoclimate simulations.