Testing the limits: Effects of climate and competition on conifer distributions

Background/Question/Methods

Global climate change is expected to alter species’ distributions, but the extent to which climate determines species’ range limits remains poorly understood. Biotic interactions such as competition and facilitation also affect species distributions, and climate and biotic factors will likely interact to determine range shift dynamics under climate change. We investigate the importance of climate and competitive interactions in determining range limits of Pacific Northwestern conifers (Abies amabilis, Tsuga heterophylla, Tsuga mertensiana) by transplanting seeds and seedlings into different competitive environments (canopy gaps and nongaps, with understory competition removed or not) across an elevational gradient at Mount Rainier, Washington, USA. We also investigate the importance of seed source elevation by transplanting seeds from upper, middle, and lower portions of the range across the elevational gradient. We quantified germination, survival, and growth of tree seedlings in the middle of their ranges, at their upper and lower limits, and beyond their current ranges. In addition, we measured key climatic explanatory variables, such as snowmelt date and temperature, as well as soil moisture and light availability, which are resources for which trees may compete. 

Results/Conclusions

For all three focal species, germination and survival varied by planted elevation and competitive environment. A common assumption is that species perform best in the middle of their ranges; however we did not find evidence of this. For example, T. heterophylla seedling germination and survival were highest at its upper range limit. Further, the effect of competition varied with elevation, particularly for A. amabilis and T. mertensiana, two high-elevation species whose ranges extend to treeline. When planted below their natural lower limits, these species experienced highest survival in low-competition treatments (canopy gaps, with understory vegetation removed). Above their upper limits at treeline, however, survival was highest in “high-competition” treatments (nongaps), suggesting that facilitation is more important than competition at treeline. Snowmelt date varied with competitive environment, particularly at high elevations, with earlier snowmelt under closed canopies than in canopy gaps, so differences in survival may be due to microclimate variation caused by vegetation. Abies amabilis, the species with the broadest elevational range, was the only focal species for which seed source elevation also affected germination and survival. Across all planted elevations, seedling germination and survival was highest for seeds collected from the upper limit of this species. Taken together, our results suggest that biotic interactions will play an important role in range shift dynamics under future climate change.