摘要：
Aims Recognizing ungulate browsing thresholds between viable and declining aspen (Populus tremuloides Michx.) stands are critical to ensuring long-term persistence of this biologically important plant community. Studies have shown declines in vigor and regeneration when as few as 30% of current annual twigs are browsed while other studies have shown higher limits. Although the effects of ungulate herbivory are of concern in aspen forests, few studies have assessed browsing effects following wildfire and few criteria exist for determining potential effects of ungulate browsing on aspen forests following wildfire. We evaluated the effects of ungulate abundance and foraging intensity on regenerating aspen 1 to 6 years post-fire and assessed the use of abundance and foraging intensity indicators in predicting impacts to regenerating aspen. Rocky Mountain elk (Cervus elaphus nelson) was the primary ungulate in the study area.
Methods The study area was located within the 17 500 ha Cerro Grande Fire burn area in the Jemez Mountains, New Mexico, USA. We used percent aspen twigs browsed and pellet-group counts to evaluate relationships between these indicators and aspen patch structure (height, size). We collected data in randomly generated 3 × 33 m plots 5–6 years post-fire. We also established 4 fenced exclosures (25 × 55 m and 3.3 m in height) with paired unfenced plots 1-year post-fire to monitor aspen regeneration. Each spring, we recorded percent browsed twigs from the previous fall through the early spring period which coincided with the highest ungulate use period within the study area. We assessed associations between percent twigs browsed and pellet-group density and patch size and height of aspen using Spearman's correlation coefficients.
Important findings Mean percent twigs browsed and ungulate pellet-group density across the burn area was ≤31% and 1 pellet-group 100 sq m-1, respectively. Patch size and height decreased with increasing browsing and pellet-group density 5–6 years post-fire. However, mean aspen heights were approaching or exceeding a minimum browsing level of 2 m and, therefore, ungulate browsing did not appear sufficient to cause significant impacts to aspen across the burn area. We observed a positive correlation between pellet-group density and twig browsing suggesting that one or both measures could be used to assess potential effects of browsing on regenerating aspen following fire.
