Subalpine seed dispersal capacity: understanding the role of disturbance adjacency in the context of post-fire recovery

Abstract

There is a growing concern about the ability of forest systems to recover following recent large, high-severity wildfires. While such fires are a natural part of subalpine forests in the Southern Rockies, a warming climate and the interaction between wildfires and recent bark beetle outbreaks may be challenging forest resilience (the ability to return to pre-fire conditions). Seed production and dispersal – a critical process that mediates landscape recovery – is not yet well understood for systems having been impacted by beetle-kill, but is expected to decline. Therefore, the purpose of this research is to identify how a bark beetle outbreak in adjacent, unburned subalpine forest surrounding a high-severity burn patch shapes seed dispersal following a wildfire. Specifically, this study aims to understand how spruce beetle-kill and forest characteristics may influence seed production of Picea engelmannii (Engelmann spruce) in the unburned edge of wildfire and subsequent seed dispersal into a burned patch. In the summer of 2022, I installed 275 seed traps and quantified seed dispersal into high severity burn patches of the 2020 Cameron Peak Fire. I fit a series of Bayesian statistical models to estimate the role that spruce beetle severity and other characteristics of the unburned forest edge explained Engelmann spruce seed production and dispersal distance. Site beetle severity ranged from 6% beetle related mortality to 92% beetle related mortality. However, I found no evidence that this mortality had any significant influence on the quantity of seed collected or the distance seeds dispersed. Seeds were collected at all trap installation distances of every site burn patch, with roughly 3.3 Engelmann spruce seed per trap. There was no statistical difference between the average seed collected at each trap installation distance, meaning that the average quantity of seed collected at 15m from the unburned edge was not statistically different than the average seed collected at 100m from the unburned edge across sites. These findings suggest that spruce beetle kill does not necessarily have a limiting effect on seed production and seed dispersal as would be expected. In years of high seed production for Englemann spruce, forest characteristics which may commonly limit seed dispersal (e.g. tree height, slope, tree age, etc.) may have less of a limiting effect.