Antiherbivore chemicals in emerging and previous-year foliage of balsam fir saplings responding to prior simulated browsing

Abstract

Herbivory alters plant chemistry thereby affecting subsequent palatability for the same or different herbivore species. Balsam fir is an important food for various insect and mammalian herbivores that consume tissues at different times of year. In eastern Canada, intense selective browsing of this tree species by ungulates has impeded natural regeneration of fir-dominated forests. Our objective was to determine whether late-season (simulated) browsing of apical and lateral branches affected the defense chemistry of emerging foliage of balsam fir saplings in the following season. At each of five locations in the vicinity of Gros Morne National Park, western Newfoundland, four browsing treatments were imposed in the late growing season on healthy fir saplings; apical, lateral, apical + lateral, and no browsing. In the following season, newly expanded and previous-year needles were collected shortly after bud break and analyzed for total phenols and condensed tannins. Browsing treatments had no effect on levels of these defense chemicals in either needle age class indicating the lack of an induced chemical response or preferential defense of the apical versus lateral shoots. New foliage appeared to be better defended than previous-year foliage. Neither phenol nor tannin levels were related to canopy closure or height and diameter of saplings; however, foliar phenol concentration was positively related to soil pH for both age classes of fir needle indicating a possible relationship between soil characteristics and plant secondary metabolites. Levels of both phenols and tannins in current-year needles were positively related to those of previous-year needles, whereas the concentration of phenols and tannins were negatively related for previous-year needles only. Our results support the premise that the main antiherbivore strategy of balsam fir is tolerance of intense browsing.