COS 296-4 - Climate wants to kill them and humans want to save them: What’s the regeneration future of pondersa pine forests?

Abstract: As climate changes in coming decades, ponderosa pine forest persistence will be increasingly dictated by their ability to regenerate naturally. Sustained regeneration failure has been predicted for forests of the southwestern US (SWUS) even in absence of stand-replacing wildfire, but regeneration in undisturbed forests has been studied infrequently. It is therefore unclear to what degree ponderosa pine forests are failing to regenerate naturally, and what the drivers of regeneration-associated forest declines could be. We also lack insight on the physical processes shaping regeneration, especially across diverse climatic conditions and forest stand characteristics. If regeneration failures are occurring, it is unclear how human management interventions could counteract them.

We characterized 77 managed and unmanaged ponderosa pine sites in 7 SWUS locations, documented regeneration occurring over the past ∼20 years, and utilized gridded meteorological estimates and water balance modeling to determine the climate and environmental conditions associated with regeneration failure. Our work provides 3 data-driven insights on regeneration and management. First, climate-driven regeneration failure in ponderosa pine forests is already widespread across the SWUS and has great potential to expand in the future. Second, forest management – especially understory burning – supports regeneration success, both by improving climate-driven environmental conditions and by reducing forest biomass and the cover of competing tree species. Finally, the climate context of management is important – a successful regeneration response is delayed by ~10 years post-management, and although low precipitation during the most recent two decades has been especially unfavorable, subsiding regional drought conditions could offer a window for successful management interventions. To counteract regeneration-associated forest declines, management interventions will need to influence climate-driven environmental change by adjusting forest characteristics at local scales, will need to expand the scope and scale of tree planting efforts to undisturbed forests, and will also need anticipate the climate-driven conditions that forests will experience following management. Management actions have the potential to support ponderosa pine regeneration through both climatic and non-climatic pathways, but will require planning, assessment, and revision through time to be effective.