Forest Inventory and Analysis (FIA) data show significant declines of aspen in the Interior West relative to historic highs. It is hypothesized that aspen losses are primarily driven by competitive interactions as conifer abundance increases under longer fire return intervals. However, few studies have tested this hypothesis. We conducted a field study 8 years after a 30,000 hectare fire on the Dixie National Forest in Utah. The burned overstory trees are still standing and can be identified as aspen or conifer. Along 66 transects in the burn zone we measured tree type (aspen or conifer) and density of the former stand, slope and aspect, soil texture and chemistry, and herbivore activity as independent variables and aspen regeneration height and density as dependent variables. In a follow up field study in unburned aspen-conifer forests we examined how conifer abundance and associated reductions in light availability and soil fertility impacted aspen regeneration success.
Results/Conclusions
In the first study conifer abundance had the greatest impact on aspen regeneration success among all the independent variables examined. Conifer abundance was negatively correlated (R2 = -0.51) with aspen regeneration density following fire. Aspen regeneration decreased linearly from 60,000 stem/ha in former pure aspen stands to less than 10,000 stems/hectare in stands that were composed of <10% aspen and >90% conifer. The second study indicated that reductions in light and soil fertility which occurs as conifer abundance increases in aspen stands drastically reduces photosynthesis, biomass, height growth and defense chemistry production in regenerating aspen stems. Results from these studies provide guidelines for management intervention (i.e. prescribed burns) where sustainability of aspen in the landscape is desired.
