Fire severity alters plant regeneration patterns and defense against herbivores in mixed aspen forests

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Fire and herbivory are primary disturbances that often overlap and strongly influence plant community development, but it is unclear how herbivory changes in relation to variability in burn severity. With climate change expected to alter fire regimes globally there is a critical need to understand how heterogeneity in post-fire habitat conditions modifies plant–herbivore interactions. We examined herbivory patterns, growth responses and defense chemistry expression (phenolic glycoside, condensed tannins) of regenerating aspen Populus tremuloides that experienced variable burn severity in the 2010 Twitchell Canyon Fire, Utah, USA. Browse damage was approximately 60% lower in moderate and high burn severity plots compared to low severity and unburned plots. Aspen regeneration density was 2.3 and 3.1 fold greater in high and moderate severity burn plots than in low severity and unburned plots. High burn severity stimulated photosynthesis, vertical growth and biomass accumulation. Defense chemistry expression responded dynamically over time depending on burn severity. From June to August, phenolic glycoside concentrations showed no significant change in unburned and low severity fire conditions but increased 79% and 139% in moderate and high severity burn environments. By the end of summer, condensed tannins increased six-fold in high severity burn plots, with increases of 50% or less in the lower burn severity plots. Deer activity, as defined by pellet counts, was inversely related to fire severity and positively related to browse damage. Elk and cattle activity showed no significant relationship with browse activity. Greater light availability in higher severity burn environments appears to enhance tolerance and resistance of aspen against herbivory by increasing growth potential and defense chemistry expression of aspen. These results suggest that burn severity influences plant–herbivore interactions through bottom–up and top–down mechanisms, and that higher fire severity increases post-disturbance vegetation recruitment potential by increasing resilience to herbivory.

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