Associate Professor University of California Merced Merced, California, United States
Abstract: Because of the long generation times of trees, lags in ecological or evolutionary responses to rapid climate change may have negative impacts on forests. Potential management options to alleviate these lags include assisted migration or gene flow. However, the degree of local adaptation or the climatic transfer distance that is feasible given current climate conditions is often unknown. Moreover, factors such as competition from local vegetation may limit establishment success.
We selected 13-14 source populations of ponderosa (Pinus ponderosa), Jeffrey (P. jeffreyi), and western white pine (P. monticola) to represent the full range of climatic conditions occupied by each species within California. Seedlings were planted in 2017 at four sites within Sequoia National Park and have been monitored twice yearly for survival and growth. In 2020 this was replicated for 3 and 4 sites, respectively, in Sierra and Tahoe National Forests. Climate transfer distances were calculated based on 1981-2010 averages. On average, planting site January minimum temperatures increased 2.4 Cº from 1951-80 to 2011-2021, July maximums increased 1.1 Cº; annual precipitation on average decreased by 17 cm, and Climatic Water Deficit (CWD) increased by 7 cm.
For all three species, growth was higher at lower-elevation sites with longer growing seasons. Multivariate models show that survival was maximized for seed sources transferred to sites with a long-term average July maximums 4.4 Cº lower and January minimums 2.98 Cº lower than their origin. Growth was maximized for transfers to 1.6 Cº cooler January minimums. Transfer to areas with lower CWD than the source favored both survival and growth. This, along with data from other planting experiments, suggests that many conifer populations in the Sierra Nevada are already locally maladapted and might benefit from assisted gene flow (to support existing local populations of a given species that managers wish to preserve) or assisted migration (to facilitate range shifts into areas that are becoming suitable).
