RESPONSES OF SNOWBED PLANT-SPECIES TO CHANGES IN GROWING-SEASON LENGTH

We assessed the role of growing-season length in regulating absolute and relative cover of six coexisting dominant plant species in an alpine snowbed habitat. To help explain disparity in species-specific responses to growing-season length, we examined the developmental phenology and distribution of each species in relation to natural snow depth variation. Season length varies from approximate to 50 d on early-melting edges of the snowbed to 35 d in the late-melting center, 100 m away. By experimentally altering snowpack, we uncoupled the relationship between spatial location and snowmelt schedule in three consecutive years, imposing the same early dates of snow release in a ''long growing-season'' treatment and the same late dates of snow release in a ''short growing-season'' treatment near the edge and center of the snowbed. Over the course of the experiment, growing-season length had significant effects on absolute and relative cover of the species studied (P < 0.025 and P < 0.005, respectively), and these effects were similar near both the edge and center of the snowbed. Yet, only for the snowbed specialist, Sibbaldia procumbens, were changes in absolute and relative cover under early and late snowmelt schedules predictable from the species' distribution along the historical snow depth gradient. S. procumbens increased in cover under a long growing-season and was more common in historically early-melting portions of the snowbed. Other species (e.g., Ranunculus adoneus, Artemisia scopulorum) were equally common in historically early- and late-melting locations within the snowbed, but showed discordant responses to experimentally imposed changes in snowmelt schedule. That the cover of many species under long- vs, short growing seasons was not predictable from their current distributional affinities in relation to snowmelt pattern likely reflects the disparity between the rates of processes exerting long-term control on species' abundances (colonization, soil development) and more immediate effects of growing-season length on plant growth. Consistent with this view, differences in developmental phenology better predicted species-specific responses to snowmelt schedule than distributional affinities. Species having leaf expansion schedules that are poorly synchronized with snowmelt typically had similar cover under early vs. late schedules of snow release (Geum rossii, Trifolium parryi, and Poa alpina). In contrast, species in which leaf expansion schedules are synchronized with snowmelt responded positively to early snow release (Ranunculus adoneus and Sibbaldia procumbens). We hypothesize that maintaining metabolic ''readiness'' under snowcover provides a mechanism for monopolizing nutrient flushes and competitor-free intervals at snowmelt, and exploiting occasional long intervals for growth in years of little snow accumulation, but incurs a respiratory cost that is manifest as reduced growth and vegetative cover when snowmelt is delayed. Our results suggest that interspecific differences in growth phenology of coexisting species will promote shifts in snowbed plant communities with climate change within generations.