Growth and chemical responses of balsam fir saplings released from intense browsing pressure in the boreal forests of western Newfoundland, Canada

Abstract

For ungulates, balsam fir (Abies balsamea (L.) Mill.) is thought to be a low quality forage due to high levels of indigestible fiber and secondary metabolites. Regardless, intensive browsing of fir saplings by overabundant moose (Alces alces L.) populations has led to the failure of fir-dominated forests in Newfoundland, Canada to regenerate following disturbance. The construction of moose exclosures in Gros Morne National Park (GMNP), western Newfoundland, presented an opportunity to compare levels of secondary metabolites (e.g. phenols and tannins), thought to act as anti-herbivore allelochemicals in balsam fir, under extremes of moose browsing pressure. We examined browsing damage to fir saplings and compared levels of carbon (C), nitrogen (N), total phenols and condensed tannins in current-year foliage outside and inside of eight moose exclosures. Outside of exclosures, 74% of fir saplings showed apical browsing while 97% were browsed laterally. At five exclosures, exposed saplings had significantly greater total phenol concentrations than inside exclosures. Such trends for condensed tannins were observed at only two exclosures, both locations having relatively high moose density and browsing damage. Unlike condensed tannins, total phenols were responsive to different patterns and levels of browsing. Foliar phenol concentration was negatively related to canopy closure, and to height and diameter of saplings, and positively related to the foliar C level. Foliar tannin level was unrelated to canopy or growth characteristics but positively related to the foliar N concentration. Moose browsing affected the defense chemistry of balsam fir; however, phenols and tannins responded differently, this likely being influenced by time of year. Foliar nitrogen levels were greater for inside saplings at only the two exclosures having highest moose density and browsing damage. Our data suggest that balsam fir employs constitutive chemical defense; however, this investment appears to be ineffective in deterring moose browsing.