Abstract
AbstractClimate‐mediated disturbances outside the range of historical variability can have severe consequences on vital, post‐disturbance regeneration processes. High‐elevation forests of the Rocky Mountains that are dominated by Engelmann spruce (Picea engelmannii) and subalpine fir (Abies lasiocarpa) are expected to be sensitive to climate change. Additionally, these forests have experienced recent epidemic spruce beetle (Dendroctonus rufipennis) outbreaks that have often resulted in >95% mortality of overstory Engelmann spruce. Therefore, the future distribution of Engelmann spruce forests depends largely on natural regeneration processes. We examined Engelmann spruce seedlings across gradients in soil moisture and stand structural conditions 20 yr post‐disturbance on the Markagunt Plateau in southern Utah. All Engelmann spruce seedlings were mapped, measured, and aged, and aspects of stand structure and the microclimate were measured. The goal of our research was to infer processes affecting Engelmann spruce establishment by determining if patterns of advance regeneration that established before the outbreak (~60% of individuals) differed from seedlings that established during and immediately following the outbreak (combined into one group, ~40% of individuals). A generalized linear multi‐model approach identified that the density of advance regeneration (seedlings/saplings) was negatively influenced by historical competition with overstory trees. In contrast, post‐outbreak regeneration was related to microclimate conditions, including positive relationships with climatic moisture deficit and July soil water content. All seedlings were not significantly clustered around Engelmann spruce snags; however, there was evidence of facilitation of post‐outbreak seedlings by pre‐outbreak seedlings at higher elevation sites with lower moisture deficit. Together, these findings suggest post‐outbreak seedlings were not moisture‐limited at lower elevations but instead encouraged by higher evapotranspiration. Moreover, facilitation at higher elevations likely resulted from how pre‐outbreak seedlings modify snowpack and associated seedbed environments. Our study provides insight for managing Engelmann spruce after a beetle outbreak. In these forests, pre‐ and post‐outbreak regeneration can increase resilience to climate–disturbance interactions, but are patchy and structured at different scales. Therefore, the presence of advance regeneration and the likelihood of post‐outbreak seedlings depend on local environment (soil moisture and stand structure) and could be taken into account to most effectively plan post‐disturbance planting activities.
Highlights
Climate change is expected to directly alter the geographic distributions of tree species by affecting physiological processes that govern establishment, growth, and mortality, and indirectly by increasing the frequency and severity of disturbances (Dale et al 2001, Rehfeldt et al 2006, Lenoir et al 2008, Turner 2010, Walter et al 2017)
The advance regeneration model included a positive relationship of seedling/sapling density to percent understory cover (Fig. 2c), and Soil water content (SWC) on Julian day 265 (September 22nd; Fig. 2d) which marks the transition from rain to snow on the Markagunt Plateau (Appendix S1: Fig. S3)
Post-outbreak forest structure consisted of a combination of seedlings that established prior to and following the disturbance
Summary
Climate change is expected to directly alter the geographic distributions of tree species by affecting physiological processes that govern establishment, growth, and mortality, and indirectly by increasing the frequency and severity of disturbances (Dale et al 2001, Rehfeldt et al 2006, Lenoir et al 2008, Turner 2010, Walter et al 2017). Competition with associated non-host trees can limit post-outbreak regeneration (Veblen et al 1991, DeRose and Long 2007, 2010, Dodson et al 2014, Temperli et al 2015). In addition to these biotic effects, the removal of closed, continuous canopies connects the macroclimate to the microclimate affecting understory environmental conditions (Carlson and Groot 1997, Gray et al 2002, Lazarus et al 2018). While increases in resources following disturbance may boost growth and survival of established seedlings, low seed production, competition, and the relatively severe microclimate may discourage the establishment of new seedlings and increase mortality rates of existing seedlings (De Frenne et al 2013, Frey et al 2016)
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
