Strategizing conifer seed collection for California reforestation under climate change

Background/Question/Methods

: Tree mortality is escalating globally due to stressors associated with anthropogenic climate change. California forests have been especially impacted by changing wildfire regimes, drought, and insect outbreaks. This has created new challenges and opportunities for reforestation in an era of climate change. Traditionally, conifer seeds were selected from the same spatial zone (i.e., seed zone) as the area being replanted. However, there is increasing need to replant with seeds that are adapted to future climate conditions. One species which state and federal nursery managers have highlighted as a top priority for seed collection is Ponderosa pine (Pinus ponderosa). We mapped the species distribution of Ponderosa pine across 18 national forests in California and defined the baseline climate envelope in relation to current species range. We applied a vegetation climate exposure analysis using two global climate models (MIROC ESM and CNRM CM) to evaluate where the species is projected to occupy currently marginal or non-analog conditions in 2040-2069. Then, we examined the accessibility of vulnerable locations in relation to roads and cross-checked forest conditions using high-resolution imagery of recent wildfire impacts. We also calculated the proportion of the species extent within each national forest that is projected to be climatically marginal.

Results/Conclusions

: Our results revealed that 9.0% of Pinus ponderosa range on national forest system units is expected to fall under climatically marginal or non-analog conditions by 2040-2069 under climate model MIROC ESM and 9.2% under model CNRM CM. However, vulnerability varies greatly across national forests and between the two model projections. Ponderosa pine is projected to occupy marginal or non-analog conditions across most of its range in the four southernmost California national forests under MIROC ESM: Angeles (99.4%), Cleveland (99.9%), Los Padres (90.6%), and San Bernardino (79.0%). This projection is lower under CNRM CM: Angeles (87.0%), Cleveland (81.4%), Los Padres (71.5%), and San Bernardino (42.4%). By identifying locations where Ponderosa pine may already be adapted to climatic extremes and are also vulnerable to future climate stress, our results can help forest managers to prioritize areas for cone crop surveys before valuable seed resources are lost. Furthermore, seed resources from southern forests may be beneficial for replanting patches of forests farther north that are projected to experience warmer and drier climatic conditions in the future. This demonstrated approach for strategizing seed collection and replanting can be applied to other tree species and other geographic areas to support forest resilience to climate change.