Prescribed fire is commonly used to initiate redberry juniper (Juniperus pinchotti Sudw.) suppression, and herbivory by goats presents a potentially effective mechanism to prolong the efficacy of the reclamation treatment. Monoterpenes in redberry juniper leaves serve as a barrier to effective herbivory, but fire has the potential to reduce this barrier through reversion of aboveground growth to juvenile tissue. Traditional optimal defense theory predicts that because of the assumed fitness value of vegetative regrowth, plant secondary chemicals will be higher in this tissue than mature growth. This study was designed to measure the monoterpene concentration and composition from redberry juniper foliage sampled from 3 different ages of plant tissue. Prescribed fire was used to create 3- and 11-month regrowth juniper foliage, and mature growth of juniper was sampled as a control. Total monoterpene levels were lowest in the 3-month regrowth (P = 0.018). Monoterpene concentration and composition was similar for the 11-month and mature foliage. Concentration of terpinen-4-ol (P = 0.001) and alpha-terpineol (P = 0.007), identified as particularly aversive monoterpenoids to goats, was lowest in the 3-month regrowth but increased to levels found in mature leaves by 11 months of age. There was a trend in changes in composition of total oil as relative concentrations of monoterpene hydrocarbons (α-pinene, β-pinene/sabinene) decreased and monoterpene alcohols and oxygenated monoterpenes increased. These results identify a short period of time following a burn during which monoterpene levels in regrowth are low. This suggests a period of vulnerability in plant biochemical defenses that has the potential to be utilized by strategic herbivory by goats for more effective juniper management.
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Vol. 60 • No. 1