COS 262-3 - The Effects of Water Availability and Nutrient Addition on Resin Duct Production in Loblolly Pine (Pinus taeda)

Abstract: Resin production in Pinus spp. is an important defense against bark beetles. We determined the impacts of water availability (ambient vs 30% reduction) and nutrient availability (fertilized and non-fertilized) on resin duct production in planted stands of Pinus taeda in southeastern Oklahoma. We collected ten cores per plot from a factorial combination of plots with and without throughfall exclusion established year 5 and with and without fertilization applied years 5 and 10 (n=4). We counted resin ducts and measured resin duct area in rings representing ages 6 to 12 using ImageJ (U.S. National Institute of Health). The number of ducts and duct area decreased with tree age. Ring width increased (p=0.0008) with fertilization and decreased (p< 0.0001) with throughfall exclusion for years with below average precipitation. Duct count per ring followed the similar trends as ring growth. The size of individual ducts decreased (p< 0.05) with throughfall exclusion by 8%, from 0.042 mm² to 0.038 mm². The combination of larger individual duct size and larger growth rings resulted in the fertilized trees with ambient precipitation having the largest total duct area per ring (p=0.043). Throughfall exclusion increased (p=0.027) duct density, i.e., ducts per mm^-2 of ring area, from 0.442 to 0.412 ducts mm^-2, but there was an interaction (p=0.005) between throughfall exclusion and fertilization because duct density in the control treatment was less than the other treatments. Fertilization increased (p=0.002) relative duct area, i.e., duct area per ring area, by 8% (1.61% to 1.74%) and the fertilized treatment with ambient precipitation was largest (1.80%). Overall, reduced water availability increased duct density, but decreased duct size and ring growth while greater nutrient availability increased ring growth, duct count per ring, and duct relative area. The net effect was that greater resource availability was associated with more ducts per year, larger ducts, and more duct area per ring area. Therefore, drought and low nutrient availability may reduce the defenses of P. taeda against bark beetles while management to reduce stand density and increase fertility may increase defenses.