On the role of ecological adaptation and geographic distribution in the response of trees to climate change

Abstract

Predicting how increases in surface temperature will modulate the response of plants to rising atmospheric CO2 concentrations is an increasingly urgent aspect of climate change research. Plant responses to elevated CO2 have been well documented over the last 40 years, and the mechanisms underlying these responses are well understood. Elevated CO2 affects plants mainly by increasing photosynthesis and decreasing stomatal conductance (Ainsworth and Rogers 2007). However, the scaling up of these primary, leaf-level CO2 responses to the whole plant and canopy levels is moderated by the plant’s growth characteristics (e.g., sink strength, biomass partitioning), and the availability of soil water and nutrients (Long et al. 2004). Unlike elevated CO2, temperature has effects that extend beyond direct leaf-level responses. Temperature affects plant growth through a number of processes at varying scales, including photosynthesis, respiration, meristem initiation, cell division, water transport and phenology (Berry and Bjorkman 1980, Atkin and Tjoelker 2003, Thomas et al. 2007, Way 2011). Importantly, the response of biological activity to temperature is not linear, and it has long been known that photosynthetic thermal responses depend on the plant’s ecological adaptation (Pearcy and Harrison 1974). Consequently, the response of plant growth to increasing temperature alone (not to mention its interaction with elevated CO2) is complicated by ecological adaptation of the species or genotype examined. This, in turn, will determine the thermal optima of temperaturesensitive processes and their potential for acclimation. Nevertheless, a survey of the literature indicates that growth in most tree species responds positively to warmer growth temperature, with deciduous species showing a greater positive response than evergreen trees (Way and Oren 2010). However, this generalization does not always hold; for example, Ghannoum et al. (2010) found that evergreen eucalypts (Eucalyptus saligna and Eucalyptus sideroxylon) had a strong positive growth response to higher temperatures. In this issue, Wertin et al. (2011) demonstrate that the deciduous, temperate species Quercus rubra (northern red oak) showed a negative growth response to elevated temperature, which was strong enough to negate growth enhancements of high CO2. While elevated CO2 alone increased total biomass by 38% relative to the ambient CO2 and ambient temperature (control) treatment, plants grown at elevated CO2 and a 3°C warming had similar biomass to their control counterparts, and plants exposed to elevated CO2 and a 6°C warming had 12% less biomass than control plants (Wertin et al. 2011). Declines in growth were associated with reduced net photosynthetic rates and photosynthetic capacity at higher temperatures, as well as higher dark respiration rates (Wertin et al. 2011). Since respiration usually acclimates to temperature more strongly and quickly than photosynthesis (Gunderson et al. 2000, Campbell et al. 2007, Ow et al. 2008, Way and Sage 2008a, 2008b, Way and Oren 2010), this result is surprising. These recent warming studies highlight the gaps in our understanding and the need to synthesize alternative hypotheses that can form the basis of future experiments in this field. One potential explanation for the diversity in results from warming experiments is presented by Wertin et al. (2011): populations from the equatorial distribution limit of a species may be more prone to warming-related growth declines than populations from the poleward distribution limit. While poleward range limits have received more research attention, equatorial range limits are where negative impacts of climate Commentary