The effects of fire severity on tolerance and resistance of aspen against ungulate herbivory

Background/Question/Methods

Ungulate herbivory can strongly influence post-fire regeneration success in forest systems, but plant-herbivore interaction in relation to fire severity are much less understood.  Resistance and tolerance denote two general strategies that plant employs to withstand disturbance damage.  Does fire severity alter patterns of plant tolerance and resistance strategies to herbivory?  After a fire that scorched over 44,000 acres in Twitchell Canyon, Utah, we examined the effects of fire severity on browsing patterns and aspen defense responses.  We hypothesize that fire severity is positively correlated with increased tolerance and resistance to elk herbivory, Regenerating aspen suckers were quantified across high, moderate, and low severity burn sites for the following characteristics: height, density, browse%, foliar nutrition and foliar defense chemistry.

Results/Conclusions

Mean aspen shoot counts in high burn sites were 257 per plot, compared with 117 and 148 per plot in moderate and low burn sites respectively.  Additionally, percentage browsed decreased from 24.6% in low burn severity sites to 7.1% in moderate burn sites and 11.4% in high burn sites.  These changes may be tied to differences in chemical defense compound productions of regenerating aspen shoots.  This study indicates that aspen tolerance to elk herbivory varies depending on fire severity, and that low severity fires can reduce aspen regeneration potentials.  The overall results suggest that, under high browsing pressure, aspen stands may have better regeneration potential following high severity fires.