High-elevation conifer forests occupy environments near or beyond the physiological tolerances of other tree species, and their extreme longevity and slow growth have allowed for their persistence through significant climatic changes. Unprecedented climate change coupled with threats from mountain pine beetle, white pine blister rust, and changes in fire activity now threaten the resilience of these endemic forests. To improve our understanding of climate and disturbance effects on Great Basin high-elevation forests dominated by limber pine (Pinus flexilis), bristlecone pine (Pinus longaeva), and whitebark pine (Pinus albicaulis), we asked: a) how does disturbance interact with water availability to influence the regenerating community?, b) to what extent is the regenerating community dominated by surviving advanced regeneration versus post-disturbance recruitment?, and c) what are the potential implications of species-specific responses to disturbance across climatic gradients? We evaluated natural regeneration in 70 sites varying in climatic and disturbance characteristics across eastern California and the Great Basin. Sites were either undisturbed or affected by one or more disturbance agent. We sampled community composition in addition to quantifying disturbance history and potential seed availability. We used structural equation models to evaluate direct and indirect effects of abiotic and biotic drivers on the regenerating community.
Results/Conclusions
Extensive beetle-induced mortality occurred across the study area, as well as several recent wildfires and white pine blister rust outbreaks (mean canopy mortality = 60%). Natural regeneration was highly variable, with abundant regeneration of whitebark pine across varying disturbance and climatic conditions but significantly lower limber and bristlecone pine regeneration. All species showed reduced regeneration with greater understory competition. Sandy soils and decreased summer temperature favored whitebark pine regeneration, while limber pine regeneration increased with increasing summer temperature and canopy cover, and decreasing slope and pH. While increased spring snowpack and summer temperature favored bristlecone pine regeneration in undisturbed sites, these drivers had strong negative effects in burned sites.
Our findings highlight the complex drivers of natural regeneration in high-elevation pine forests of eastern California and the Great Basin. Water availability, through increased snowpack or soil characteristics, is an important driver of regeneration, and water stress is expected to increase under projected future conditions. Bird dispersal may buffer the effects of disturbance by overcoming local seed limitations, however forest mortality may exacerbate microclimate conditions, leading to increasingly rare opportunities for establishment. Recent mortality throughout the study area necessitates continued monitoring of natural regeneration and implications for forest persistence.