Canopy structure and below-canopy temperatures interact to shape seedling response to disturbance in a Rocky Mountain subalpine forest

Abstract

We examined the effects of two recent, high-severity disturbances on seed dispersal and conifer seedling establishment in a subalpine spruce-fir forest in the San Juan Mountains, Colorado. Our study area had undergone high forest mortality from a spruce beetle (Dendroctonus rufipennis) outbreak beginning ca. 2004, and a portion of the study area was additionally burned by the West Fork Complex wildfire in 2013. The aim of this study was to determine how seedling re-establishment patterns vary with seed availability, temperature, overstory composition, and understory cover across a topographically heterogeneous landscape with varying disturbance impacts. We established thirty study sites in the burned area and sixty sites in the unburned, beetle-affected area in order to capture a gradient of beetle outbreak severity. We assessed post-disturbance regeneration by conducting seedling counts in the summer of 2018, five years after the fire and more than a decade since the onset of the beetle outbreak. We determined seedling establishment after disturbances by using terminal bud scar counts to estimate seedling age. We measured Engelmann spruce (Picea engelmannii) seed dispersal and below-canopy temperatures over three years (2017–2019) using in situ seed traps and Logtag® temperature sensors at 2 m height. We assessed relationships between (1) seed availability and live overstory remaining after the spruce beetle outbreak, and (2) seedling abundance and all explanatory variables using hierarchical Bayesian models. Conifer seed and seedling counts were zero in most of the burned area, though there was abundant aspen regeneration at lower elevations. In unburned, beetle-affected forests, we found that the degree of spruce overstory mortality did not strongly affect seed availability but showed a strong effect on spruce seedling densities. Conifer seedling densities were also influenced by below-canopy temperatures, aspect, and understory litter and shrub cover. These results indicate that the West Fork Complex fire has potentially resulted in a long-term loss of conifer forest throughout much of the burn area where distance to a seed source is >100 m. High-severity spruce beetle outbreaks have also limited regeneration, although seedlings are still present in the majority of beetle-killed sites. Future patterns of re-establishment will also be strongly influenced by topography and future warming trends.