The future of subalpine forests in the Southern Rocky Mountains: Trajectories for Pinus aristata genetic lineages

Abstract

Like many other high elevation alpine tree species, Rocky Mountain bristlecone pine (Pinus aristata Engelm.) may be particularly vulnerable to climate change. To evaluate its potential vulnerability to shifts in climate, we defined the suitable climate space for each of four genetic lineages of bristlecone pine and for other subalpine tree species in close proximity to bristlecone pine forests. Measuring changes in the suitable climate space for lineage groups is an important step beyond models that assume species are genetically homogenous. The suitable climate space for bristlecone pine in the year 2090 is projected to decline by 74% and the proportional distribution of suitable climate space for genetic lineages shifts toward those associated with warmer and wetter conditions. The 2090 climate space for bristlecone pine exhibits a bimodal distribution along an elevation gradient, presumably due to the persistence of the climate space in the Southern Rocky Mountains and exclusion at mid-elevations by conditions that favor the climate space of other species. These shifts have implications for changes in fire regimes, vulnerability to pest and pathogens, and altered carbon dynamics across the southern Rockies, which may reduce the likelihood of bristlecone pine trees achieving exceptional longevity in the future. The persistence and expansion of climate space for southern bristlecone pine genetic lineage groups in 2090 suggests that these sources may be the least vulnerable in the future. While these lineages may be more likely to persist and therefore present opportunities for proactive management (e.g., assisted migration) to maintain subalpine forest ecosystem services in a warmer world, our findings also imply heighted conservation concern for vulnerable northern lineages facing range contractions.

Estimating true density in large, alpine herbivores using Google Earth imagery

“…Increasing elk density in searchable areas with decreasing total forest cover (exponentially increasing abundance with openness) was consistent with winter preferences for graminoids (Christianson and Creel 2007) and open habitat (Creel et al 2005). Evidence for these preferences in summer suggests that upward shifts in tree line due to climate change (Malone et al 2018) could reduce elk abundance at high elevation. Our density estimate further suggests a potentially important ecological role of the dominant large herbivore in summer in Yellowstone's alpine ecosystems.…”

“…We included all terrain > 2750 m asl because marked Gallatin elk have been shown to move to these elevations in summer, but the summer population had never been systematically surveyed (Brazda 1953, Craighead et al 1972. At 2750 m asl, tree density begins to decline, and subalpine and alpine tundra and meadows become more common (Malone et al 2018). However, to ensure important areas were not excluded from our analysis, we expanded our initial search area by including all adjacent terrain between 2590 and 2750 m asl (an additional 92.4 km 2 ).…”

Interactions between white pine blister rust, bark beetles, and climate over time indicate vulnerabilities to limber pine health

“…Bark beetles can indiscriminately kill mature WPBR-resistant trees, reducing the favorable gene pool (Schwandt, 2006). Furthermore, evidence suggests that climates are becoming less hospitable for white pines (Rehfeldt et al, 2008;Larson, 2011;Malone et al, 2018;Millar et al, 2018), further exacerbating the ability for these species to recover. Knowledge of the current health status of limber pine and how stands are impacted by these damage agents over time will provide critical information to inform management and guide recovery efforts (Schoettle et al, 2022a).…”

Tracing the footprints of a moving hybrid zone under a demographic history of speciation with gene flow

“…The CPH makes key assumptions about the fundamental niche being similar across the geographical range of a species, and that climate optima are stable over time (Pironon et al, 2017;Sheth & Angert, 2018). Efforts to predict changes in range margins via ecological niche modelling (ENM) are now incorporating within-species variation in the fundamental niche and genetic structure (Ikeda et al, 2017;Malone, Schoettle, & Coop, 2018). Still, biotic factors such as competition, tolerance to insects and pathogens and hybridization with a closely related species are often neglected in predictive modelling (but see Engler, Rödder, Elle, Hochkirch, & Secondi, 2013;Pollock et al, 2014).…”