FAST-GROWING JUVENILE PINYONS SUFFER GREATER HERBIVORY WHEN MATURE

Abstract

We examined the hypothesis that tree-ring analyses of juvenile growth traits could be used to predict herbivory during mature years. At Sunset Crater National Monument, Arizona, USA, some pinyon pines (Pinus edulis) are highly susceptible to the stem-boring moth, Dioryctria albovittella, and suffer chronic attack, while other adjacent trees are resistant and have remained relatively moth-free for years. Five major patterns emerged: (1) Susceptible trees suffer from 11 times more shoots killed per tree and produce <0.05 times as many cones as resistant trees. (2) This herbivore is largely restricted to reproductively mature trees that, at this site, are generally older than 46 yr. (3) Analysis of tree-ring data from resistant and susceptible trees shows that trees that eventually became susceptible grew more vigorously during their attack-free juvenile years than resistant trees (growth rings were 29% wider). However, when they matured and suffered chronic herbivory, susceptible trees produced growth rings 13% smaller than resistant trees. (4) Although previous studies showed that resistance was associated with higher oleoresin flow, we did not observe this pattern. Instead, we found that resistance was best correlated with slower juvenile growth rates. Current-year shoots of susceptible mature trees averaged 50% larger than those on resistant mature trees. (5) Tree-ring analyses supported the hypothesis of a trade-off between growth and resistance. This study shows that dendrochronology can be used as a tool to predict future herbivory and demonstrates that the fastest growing juvenile trees suffer the poorest performance decades later when they become moth-susceptible mature trees. Furthermore, it illustrates that traits associated with plant development can greatly affect herbivore distributions, and that herbivore selection pressures on juvenile trees can be very different than on mature trees.