Elevated global temperatures and associated droughts have been implicated in widespread bark beetle-caused pine mortality in the western U.S. Many recent studies highlight the importance of retrospective growth and defense to assess pine resistance to bark beetle-attack. However, many of these studies have shown differences in the relative importance of defense and growth in predicting tree mortality. One possibility is that responses of growth and defense to climate and competition may differ among sites and species. Previous studies have shown that climate and competition suggest contrasting relationships with growth and defense measures. Here we compare the influence of climate and competition on growth and defense in three pine species (Pinus edulis, P. lambertiana, and P. ponderosa) and provide possible insights on future tree mortality.
We found that both climate and competition informed growth and defense models for each of the three species examined. However, the climate and competition factors related to growth and defense differed for some species. For instance, P. lambertiana growth was most strongly correlated to January temperature and lower intraspecific competition, while resin duct size was better associated with annual climatic water deficit and total competition. Similar contrasting factors were found in the other pine species examined, indicating that growth and defense measures are potentially uncoupled in their response to changing climate and variation in competitive environments. Our results highlight possible differential responses of growth and defense that may be related to the observed differences in previous studies on the relative importance of growth and defense in predicting tree mortality in these species.