Global warming is expected to have a wide range of ecological and evolutionary impacts on natural plant populations.  Several recent studies have transplanted genotypes collected from northern populations into more southern, warmer locations to simulate the conditions populations will experience under global warming.  If genotypes experience fitness declines when planted into warmer environments, then global warming may cause population declines, range shifts, and possibly even extinctions. Results from these types of transplant experiments have been informative for studying both the ecological and evolutionary consequences of global warming; however, many other factors also vary across latitudinal gradients (e.g., species interactions, soil characteristics, precipitation, and photoperiod) and may contribute to observed differences between northern and southern transplant sites.  Here we use dendrochronological techniques to more specifically test for associations between temperature and plant growth, while minimizing the effects of these other potentially confounding variables.  We used two existing common gardens (established in 1967) consisting of 41 Larix laricina genotypes that had been collected from across a broad latitudinal gradient and historical climate data from each source location to investigate how genotypes from different historical climates perform under relatively colder versus warmer environmental conditions. We assessed survival, measured size (diameter breast height = DBH), estimated ice damage and herbivory, and cored all trees. 

Results/Conclusions

We found that genotypes collected from warmer environments had greater DBH and survival than genotypes collected from cooler habitats, especially at the southern transplant site (source population temperature x recipient site interaction F_1,53=5.24, P<0.03). All genotypes showed greater growth (F_1,53 =11.05, P<0.002) and tended to have increased survival at the more northern transplant site, however, suggesting that warming may be detrimental to Larix growth and fitness.  Interestingly, genotypes collected from northern environments were more susceptible to ice damage induced mortality at the southern transplant site than genotypes from southern environments (F_1,60 = 4.63, P<0.04).  The association between historical temperatures of the source population and survival/growth at the recipient site and the strong effects of ice damage suggest that temperature variation between sites is likely responsible for growth/survival differences; however, we also are currently measuring tree rings to more closely correlate growth responses to temperature. This dendrochronological technique will allow us to specifically attribute differences in growth to temperature or other climatic factors that vary across both space (transplant sites) and time (annual variation in temperature, precipitation, etc.).