Alpine plants grow where they do because mountain summits are cold, icy, windy, snowy – but also sunlit, dry, and hot. Alpine plants are directly vulnerable to warming temperatures, and indirectly vulnerable to shifting ranges of other plant species. As the tree line moves upslope, the story goes, eventually forest growth will crowd out the summit specialists, many of which are rare. But a new study, reported in a June 2021 special issue of Northeastern Naturalist dedicated to climate change in the mountains of the Northeast, suggests that in the Adirondacks, rare alpine plants are maintaining their populations.
Timothy Howard, a scientist with the New York Natural Heritage Program, and Kayla White and Julia Goren of the Adirondack Mountain Club, along with many assistants, sampled hundreds of randomly selected locations across 15 Adirondack summits in 2006 to 2007, 2013, and 2018 to 2019, with a focus on 10 plants listed as rare by New York State.
Eight species showed either no change (northern bentgrass, Bigelow’s sedge, and northern lowbush blueberry) or significant increases (alpine sweetgrass, diapensia, mountain firmoss, Cutler’s alpine goldenrod, and deer’s hair club sedge) in density over time. Atmospheric deposition of nitrogen and climate warming don’t fully explain these increases, the researchers said. Many plants were associated with indicators of disturbance, such as exposure to weather systems or higher elevations. Their increased growth could be due to their own evolved tolerance for these tough conditions.
However, two species, bearberry willow and black crowberry, did have significant negative trends. These plants were found more often on lower, and possibly more protected, slopes within the alpine zone, wrote the research team, “just where we would expect higher rates of tree and shrub establishment.”
“The finding that only two species out of 10 showed a statistically significant decline in population size over the 13-year period suggests that, at least in this time window, many of the rare alpine plants in the Adirondacks are holding their own, and our hypothesis that rare species are slowly declining in abundance overall is not well supported by these data,” wrote Howard, White, and Goren. “Perhaps even more interesting is that we detected increases in population size for five of the 10 species. These increases suggest the conditions on the Adirondack summits are currently supporting positive population growth for at least some of the rare alpine plants growing there.”
These conditions include variability of the summit environment – relatively high spots and low spots, places exposed to more wind, and areas near trails or farther away. The species that increased are those that grow on slopes that face the prevailing westerly winds that pummel the Adirondack Mountains with harsh weather. The study provides another reason for hikers to take care to minimize their impact while hiking in this environment and, especially, appreciate the rare but tough alpine plants that continue to grow where they do in a rapidly changing climate.