Competition modulates climate sensitivity of Pseudotsuga menziesii

Background/Question/Methods

Despite strong experimental and observational evidence suggesting that competition will significantly affect vegetation responses to climate change, current research has largely ignored the role of competition in moderating climate-growth relationships of mature trees. Given consistent observations from tree-ring studies demonstrating a link between climate sensitivity (the degree of response to annual climate variability) and vulnerability to mortality from environmental stress, there is a pressing need to understand if and when biotic interactions may influence the climate responses of overstory trees. This information is critical in order to accurately predict the effects of climate change on forest ecosystems and understand how management actions, such as altering stand density and structure, could modify responses to climate change. Using a combination of correlation analyses and mixed-effects models, we analyzed patterns of tree-ring variability of Douglas-fir (Pseudotsuga menziesii) from 10 biophysically similar, yet well-distributed sites in northeastern Washington and assessed the combined influences of competition and annual climate variability on radial growth. Based on dendroecological theory and the dominant conceptual models predicting the relative importance of competition in structuring plant communities across environmental gradients, we predicted that (1) trees experiencing high levels of competition would be less climate sensitive (i.e. the growth of trees with more neighbors would be less coupled to climate variability); and (2) competition-related differences in climate sensitivity would be less pronounced in dry years relative to wet years.

Results/Conclusions

In years when soil moisture was below the long-term average, growth of all trees was tightly coupled to climate but trees with fewer neighbors were slightly more responsive to climate variability (i.e. competition has a slightly negative effect on climate sensitivity in dry years). However, in wet years, the effect of competition was reversed and much more pronounced: whereas low-competition trees showed no response to climate when soil moisture was above the long-term average, growth of high-competition remained tightly coupled to climate variability and was almost 40% greater in the wettest years relative to years with average climatic conditions. Notably, we found no relationship between competition and tree responses to extreme drought conditions – all trees exhibited a nearly 30% reduction in radial growth during drought years regardless of their competitive status. Our results suggest that differences in hydraulic architecture – associated with long-term, plastic responses to competitive stress – may significantly modify water relations and the effect of climate on growth.