Plant growth, survival, and reproduction is strongly dependent upon the ability to respond to environmental cues and appropriately budget a finite amount of resources. Among conifers, investment in oleoresin (resin) production (a predominant defense mechanism) reduces the likelihood of mortality by herbivores, particularly bark beetles (Coleoptera: Curculionidae: Scolytinae). Yet trees can also allocate their resources to growth or reproduction, each of which have their own fitness benefits. In this study, we sampled 44 pinyon pine (Pinus edulis) trees at eight sites that span an elevation gradient in southwestern Colorado to assess how weather and site productivity influence resource allocation. The objectives of this study were to: 1.) Quantify the relationship between tree defenses (resin duct density and area), growth (both primary shoot growth and basal area growth), and reproduction (seed cone production); 2.) Identify the weather variables that are correlated with allocation to these different components; and 3.) Determine whether site productivity and resource availability influences trade-offs between growth, defense, and reproduction allocation.
Results/Conclusions
There were strong and positive associations between primary shoot growth and seed cone production as well as basal area growth and primary shoot growth, suggesting that during years of favorable weather trees strongly invest in both growth and reproduction. Notably, there was also a strong positive association between basal area growth and our measurement of tree defenses (resin duct production), but the strength of this relationship between growth and defenses varied by physiographic gradients that influence environmental stress. There was a weak relationship between cone production and basal area growth, suggesting a potential resource allocation trade-off that may be driven by different weather cues among this species. We had hypothesized that trees that produced more cones would have lower growth rates or be less defended, yet despite large differences in reproduction and growth rates across the 44 sampled trees, we found no evidence supporting this hypothesis. Taken together, these results suggest that landscape physiography and weather jointly determine resource allocation to growth, defense, and reproduction, but that trade-offs between resource allocation are not readily apparent.