High severity burn increases post-fire resistance and resilience of aspen forest against ungulate herbivory

Background/Question/Methods . Fire and herbivory are two important disturbances that shape the development of forest communities.  Herbivory is known to negatively impact post-fire forest recovery, but how plant-herbivore interactions change in relation to fire severity, which is highly variable in forest landscapes, is much less understood.  Successful forest regeneration depends on the ability of plants to withstand browse pressure.  Plants employ two general strategies – resistance and tolerance – to withstand such pressure.  Does fire severity alter patterns of plant tolerance and resistance to herbivory?  In this study, we examined browse patterns, growth, functional responses and defense of regenerating aspen (Populus tremuloides) to differences in fire severity (high, moderate, low, unburned).  Using 30 plots that spanned across a fire severity mosaic in Fishlake National Forest, Utah, USA, we tested the following hypotheses: (i) post-fire aspen regeneration vigor increases with fire severity; (ii) high burn severity has positive effects on water relations, nutrient acquisition, carbon metabolism and growth rate of aspen suckers; and (iii) high severity burn conditions enhance the production of phytochemical defense compounds (phenolic glycosides and condensed tannins) resulting in better defended aspen suckers.

Results/Conclusions . The percentage of browsed stems in moderate and high severity burn plots averaged 14% and 17% respectively, which was significantly lower than 35% in low severity and unburned plots.   Aspen suckering density was approximately 150% higher in moderate and high severity burn plots than in low severity and unburned plots.  Saplings were 25% taller in high severity burn plots than unburned and moderate burn plots, but showed no significant difference compared with low severity burn plots.  Phenolic glycosides concentration showed no significant change in unburned and low severity fire conditions but increased 79% and 139% in moderate and high severity burn environments over the summer. Condensed tannins of aspen suckers in high severity burn plots increased nearly six-fold over the summer, and was more than double the concentration level of other burn groups.  Higher burn severity contributed to lower foliar phosphorus and had no effect on total nitrogen.  Photosynthetic rates increased with burn severity in early summer with smaller effects in late summer.  Burn severity showed little influence on water potential.  Our results suggest that increasing burn severity may lead to greater resistance and tolerance to herbivory by influencing defense chemistry and growth response in plants, which may provide opportunities for more successful recruitment process and forest regeneration in areas with high ungulate pressure.