Spatial and temporal variation in resin defense and growth investment across three subsections of Pinus from West Texas mountains

Oleoresin (resin) is the predominant defense used by pine trees in response to herbivory, especially bark beetles (Coleoptera: Curculionidae: Scolytinae). The density of the ducts by which resin is stored and transported throughout the tree predicts both bark beetle attack rates and success from bark beetles in Pinus species. Although relative growth and defense investment is known to change with resource availability, environmental conditions, and ontogeny, the relative influence of these factors in relative resin duct investment still lacks understanding. To address this issue, I investigated the relative influence of environmental variables (Palmer modified drought index [PMDI], basal area factor [BAF], elevation, and mountain range), ontogeny (age), growth (ring width), and ecological strategies (taxonomic subsection) on resin duct density. A 12 mm diameter tree core was collected from 159 trees belonging to five species (Pinus arizonica var. stormiae, P. cembroides, P. edulis, P. ponderosa var. scopulorum, and P. strobiformis) representing three subsections (Cembroides, Ponderosae and Strobus) from both subgenera of Pinus (Pinus and Strobus) of which annual ring width and resin duct density were calculated. Linear mixed effects modeling methods were used to quantify differences for the two tree ring response variables. As expected, tree growth is sensitive to drought and across individuals through time. Sensitivity varied by subsection with Strobus demonstrating the greatest growth rate sensitivity to drought. Resin duct density shows both growth rate (ring width), ontogenetic effects, and differences in average resin duct density by subsection: (1) Annual resin duct density decreases with tree age, and the rate by which it decreased was strongest in Strobus trees; (2) young trees (< 10 years) tend to invest less in resin duct in high growth years and therefore resin duct density is negatively associated with ring width in younger trees. As trees age, this relationship changes and in older trees, resin duct investment is either proportional to ring width (Ponderosae and Cembroides) or, in the case of Strobus, older trees invest disproportionally more in resin ducts in high growth years. (3) After accounting for environmental and age effects, Cembroides trees produced greater annual resin duct density than Ponderosae and Strobus. My results suggest that resin duct density does not directly respond to environmental conditions, but is directly responsive to age (negative) and indirectly responsive through the interaction of age and ring width (negative response in younger years, positive response in older years). The effect of age on resin duct density has often been overlooked, but should be included in dendroecological analyses involving resin duct density. Additionally, the responses to resin duct density varied by subsection, suggesting that the overall ecological strategies associated with each taxonomic subsection significantly influence their relative investment in defense.