Rethinking fire‐adapted species in an altered fire regime

Novel combinations of fire regime and forest type are emerging in areas affected by climate change, fire exclusion, and other stressors. Species interactions following wildfire in these areas are not well understood. In Sierra Nevada mixed‐conifer forests, large patches of stand‐replacing fire were once rare but are becoming increasingly prevalent and are quickly revegetated by native shrubs. There is uncertainty as to which tree species are best adapted to recover in the resulting post‐fire environments. We introduce a conceptual framework for understanding how the altered fire regime in the Sierra Nevada may affect species composition. We investigate an understudied link in this framework: how juvenile ponderosa pine (Pinus ponderosa) and white fir (Abies lowiana) growth and mortality rates are affected by shrub competition following stand‐replacing fire. We measured juvenile conifer growth in relation to shrub competition in five fire footprints ranging from 8 to 35 yr old and >400 ha in size. To test whether reductions in conifer growth may lead to increased mortality, we also evaluated how recent tree growth predicts mortality of similarly aged juvenile trees in nearby managed stands. We found that juvenile ponderosa pine growth was negatively associated with nearby shrub competition, but white fir growth was not. Both species grew slower preceding mortality, with a steeper relationship found in ponderosa pine. Across the range of shrub competition in this study, expected pine annual relative growth rate varied from 0.27 to 0.10, which corresponded to an eleven‐fold difference in annual probability of mortality (0.1–1.1%, respectively). These results show that ponderosa pine is sensitive to shrub competition following wildfire in terms of both growth and survival while white fir is less sensitive, presumably due to its high shade tolerance. Though pines are generally considered fire‐adapted, this study argues that post‐fire species interactions in a novel fire regime may exacerbate the already shifting species composition toward shade‐tolerant species, which are less well adapted to survive through future fires and to persist in future drier, warmer climates.