SHORT-TERM RESPONSES OF ALPINE BUTTERCUPS TO EXPERIMENTAL MANIPULATIONS OF GROWING-SEASON LENGTH

We examined the role of growing season length in regulating percentage cover and seed mass of the alpine snow buttercup, Ranunculus adoneus, in a snowbed habitat. Season length varies from almost-equal-to 50 d in early-melting portions of the snowbed to 35 d in late-melting locations. Percentage cover and seed mass of snow buttercups decrease from early- to late-melting locations. Time available for growth was manipulated experimentally by altering the snowmelt schedule. Advancing snowmelt by 14 d in typically late-melting portions of the snowbed led to a threefold increase in percentage cover of snow buttercups (P < .025). However, delaying snowmelt to the same extent in typically early-melting locations had little influence on snow buttercups. These results indicate that time available for growth limits cover of snow buttercups in late-emerging vegetation, whereas other factors regulate cover of snow buttercups in early-emerging vegetation. Perturbation of snowmelt schedule also had significant effects on seed mass (P < .0055). These effects are probably mediated through changes in time available for seed maturation. Snow manipulation treatment accounted for 71% of the variation in seed maturation interval. Maturation interval, in turn, was correlated with seed mass (P < .01). Reducing or increasing the seed maturation interval by 12 d caused a 33% change in seed mass. These findings show strong direct effects of growing season length on plant performance at diverse life history stages.