Effects of long-term drought on conifers across Northern California

Wallis Robinson and Gabriel J. Roletti, Forestry & Wildland Resources, Humboldt State University, Arcata, CA, Rosemary L. Sherriff, Geography Department and Forest, Watershed, Wildland Science Graduate Program, Humboldt State University, Arcata, CA, Lucy P. Kerhoulas, Forestry and Wildland Resources, Humboldt State University, Arcata, CA

Background/Question/Methods
Numerous studies indicate that competition, species, and climate are important predictors of drought response, but few studies have considered the collective influence of these factors across a wide range of conifer species at a regional scale. We used 13C discrimination rates (Δ13C) in annual tree rings to assess the interplay of species, habitat, climate, and competition on tree response prior to and during the unprecedented 2012 to 2016 drought period for six conifer species (Abies magnifica var. shastensis, Picea breweriana, Picea sitchensis, Pinus lambertiana, Pinus monticola, and Tsuga heterophylla) throughout northern California forests. In total, 270 dominant or co-dominant focal trees were cored across 45 Calflora locations (5 trees per species/site; 2 species co-occurred at 9 sites). At each site, competition (within 10m of each focal tree) and tree- and stand-level data were also collected. Linear mixed-effects (LME) models were used to evaluate the effects of species, habitat, climate, competition, and focal tree characteristics on annual Δ13C. A two-way analysis of variance of the LME models was used to test for species- and habitat-level differences in drought resistance (Drought Δ13C/Pre-Drought Δ13C).
Results/Conclusions
Water stress (Δ13C) in montane species was driven by drought severity (Palmer Drought Severity Index) and snowpack, whereas in coastal conifers it was driven by drought severity and interspecific competition, with no interaction between competition and climate. Coastal species (P. sitchensis and T. heterophylla), which are both at the southern extent of their North American ranges, notably decreased Δ13C during drought, suggesting increased stomatal regulation to maintain favorable water status. Although these coastal species employed a wide range of discrimination rates, they may be forced to function within a much narrower range of stomatal conductance, possibly resulting in decreased vigor, if severe droughts become more common. Drought resistance declined with each successive drought year in coastal species while in montane species it was lowest in the third drought year (2014) but relatively stable across all other drought years. Regionally endemic P. breweriana responded similarly to other montane species during drought, but decreased Δ13C more in drier environments compared to wetter environments, suggesting that P. breweriana may not be an effective competitor in dry environments. Thus, range may present a challenge for current populations of this endemic conifer under future climate projections.