Climate change at experimental range limits: Too hot, too cold, or competition?

Background/Question/Methods

Recruitment limitation by climate, competition, or both is hypothesized to control range limits. Specifically, range limits are expected to be limited in north by climate, especially physiological stress in winter, and in the south by competition.  We compared germination and survival of common eastern temperate tree species in experimentally heated plots located near southern and northern range limits of eastern temperate forest species, in Harvard and Duke Forests. Treatments were implemented in a factorial design, with two light treatments (gap or understory) and three temperatures (ambient, +3C, +5C), to determine the relative importance of climate and competition for recruitment at both sites.

Results/Conclusions

Results were opposed to the predictions from the climate-competition hypothesis for range limits. The largest climatic effect on survival was summer temperatures, which decreased survival for most species.  Despite anomalously cold years during the study, there were no significant effects of winter temperatures for any species. Moreover species from warmer climates survived as well as species found where winter temperatures are severe. Competition with the canopy for light and moisture had a large impact on growth, germination, and survival of all species. There was also a positive interaction between light level and summer temperature for 25% of species, but there was no effect of gap environment on winter survival. Germination also declined with increasing temperatures, but less pronounced than observed for seedling survival. Overall, summer temperatures exerted a stronger regulation of recruitment at range limits than did winter temperatures.  If winter temperatures do not limit northern range limits, then competition may be important at northern and southern boundaries.