The effects of a half century of warming and fire exclusion on montane forests of the Klamath Mountains, California, USA

Abstract

Climate warming and altered disturbance regimes are changing forest composition and structure worldwide. Given that species often exhibit individualistic responses to change, making predictions about the cumulative effects of multiple stressors across environmental gradients is challenging, especially in diverse communities. For example, warming temperatures are predicted to drive species upslope, whereas fire exclusion promotes the expansion of species at lower elevations where fire was historically frequent. We resampled 148 vegetation plots to assess 46 years (1969–2015) of species and community‐level response to warming and fire exclusion in a topographically complex landscape in the Klamath Mountains, California, USA, a diverse region that served as a climate refugia throughout the Holocene. We compared cover and assessed change in the elevational distributions of 12 conifer species at different life stages (i.e., seedlings, saplings, canopy). We observed consistent but non‐significant shifts upward in elevation for eight species, and a significant shift upward for one species, all of which were far less than expectations based on recent warming. Six species declined in total cover and another five declined in at least one life stage, whereas the drought‐ and fire‐intolerant Abies concolor increased by 30.7%. The largest declines were at lower elevations in drought‐tolerant, early‐seral species (Pinus lambertiana and Pinus ponderosa) and at higher elevations for the shade‐tolerant Abies magnifica var. shastensis and the regionally rare Abies lasiocarpa. Regionally rare (Picea engelmannii) and endemic (Picea breweriana) species had reductions in early life stages, portending future declines. Multivariate analyses revealed a high degree of inertia with a minor, but significant, shift in composition and a slight decrease in species turnover along the elevation gradient driven by the expansion of A. concolor. Our results indicate that most species are declining, especially at lower and mid‐elevations where fire exclusion has increased the cover of shade‐tolerant species and reduced the recruitment for fire‐adapted species. Collectively, declines in most species, insufficient upward movement to track warming, reductions in drought‐ and fire‐tolerant early‐seral species, and an increase in a single, shade‐tolerant species will leave these communities maladapted to projected climate scenarios and questions the potential for future climate refugia in this region.

More than 17,000 tree species are at risk from rapid global change

“…Threats are distributed unequally worldwide creating hotspots of highly exposed species in areas where threats are most severe 11 , 21 , 22 , 58 . For example, the gradual reduction in burned area over the last two decades is a global phenomenon 59 that creates shifts in inter-specific competition 60 , 61 , increases the risk of megafires due to biomass built-up from promoted succession 62 , 63 , and halts reproduction or regeneration in fire-dependent species 64 , 65 . We find hotspots of highly exposed species in areas with extreme changes in burned area (Sub-Saharan Africa, Australia) 59 .…”

Fire sparks upslope range shifts of North Cascades plant species

“…Although the preponderance of upward and poleward movement is consistent with species tracking thermal niches (i.e., community “thermophilization”; De Frenne, Rodríguez‐Sánchez, et al, 2013), there is considerable heterogeneity in responses among species (Angert et al, 2011; MacLean & Beissinger, 2017; Zu et al, 2023). For example, some species have shifted ranges uphill much farther and faster than others (Wolf et al, 2016), others have shifted downhill (Crimmins et al, 2011; Lenoir et al, 2010; Tingley et al, 2009), and many species have not shifted significantly at all (Jules et al, 2022; Zhu et al, 2011). Some heterogeneity in rates of shifts can be explained by intrinsic differences among species (e.g., in dispersal ability, life history, or sensitivity to biotic interactions; MacLean & Beissinger, 2017; Zu et al, 2023) and by extrinsic environmental differences (e.g., in rates of climate change or buffering by microclimatic refugia, favorable habitats created by differences in microclimates across the landscape; Hannah et al, 2014).…”

Bioclimatic Characterisation of Specific Native Californian Pinales and Their Future Suitability under Climate Change

“…Plants and some tree species in particular may not be able to adapt quickly enough to changing conditions [ 33 , 34 ]. Drought has been the cause of forest mortality events in the state of California, often in combination with other abiotic and biotic factors [ 34 , 35 , 36 , 37 , 38 ]. Furthermore, vulnerability to drought is of fundamental importance in determining the geographic distribution of individual species and communities.…”

“…Plants 2023, 12,1966 2 of 38 One of the major causes of vegetation loss in California is urban development. As the state's population has grown, more land has been devoted to residential, commercial, and industrial uses.…”