The Effect of Drought Conditions on Vertical Resin Duct Production in Pinus ponderosa Across an Elevational Range

Abstract

Several research studies have investigated changes in ponderosa pine (Pinus ponderosa) resin duct production after drought, as well as the significant role that investment in resin defense plays in predicting tree mortality in response to drought and insect outbreak. However, there is less existing research on the long-term relationships between climate and adult ponderosa pine resin duct investment. I investigated the relationship between measures of annual precipitation, temperature and heat moisture index, and characteristics of vertical resin ducts produced by ponderosa pine at different elevations in Colorado from 1985 to 2013. Resin duct observations were performed on tree core samples originally collected for a previous study by Anderegg & Hille Ris Lambers (2015). In accordance with the methods of existing resin duct studies, I collected annual resin duct counts and the area of observed resin ducts (mm²). Total duct area per ring (average duct area multiplied by the number of ducts observed in a ring), and relative duct area investment (proportion of total duct area relative to area of an individual ring) were calculated as well. Consistent with existing research, findings demonstrated that duct production, duct area, and total duct area tended to decrease in response to drier conditions while relative duct area slightly increased. Additionally, resin duct characteristics followed the same trends in response to decreasing elevation. Despite these relationships, mean ring width was by far the most significant predictor of all resin duct observations, indicating a possible fixed relationship between resin duct production and ring size, which some studies have already suggested. These findings add to the existing literature on drought and resin duct production and have implications for forest conservation efforts, especially at low elevations, in an increasingly arid region of the Western United States.