Seedling Establishment in Alpine Buttercups under Experimental Manipulations of Growing-Season Length

Abstract

We tested effects of growing-season length and site fertility on early seedling growth and seedling establishment in a snowbed population of Ranunculus adoneus. On average, growing-season length decreases by 30% from shallow to deeper parts of the snowbed. Past work has shown that plant cover and soil fertility decline with delayed snowmelt date along this steep habitat gradient. We experimentally uncoupled the relationship between snowmelt schedule and site fertility by adding snow to plots at naturally shallow snow depths and removing snow prematurely from plots with naturally deeper snow in the summers of 1991–1994. Seeds of R. adoneus were transplanted into manipulated plots and adjacent control plots in 1990 and 1991. Seedling shoot size and survival were monitored over the course of the experiment. Production of true (adult) leaves by newly emerging seedlings was significantly reduced by the delayed snowmelt schedule. Seedlings failing to produce true leaves during their first summer were at a strong survival disadvantage over the winter months. Leaf length for seedlings that produced true leaves was not affected by snowmelt or location on the snow depth gradient; nor did leaf length affect overwinter survival. We defined seedling establishment as the successful transition from seed to seedling of age two years. At this age, all seedlings have produced true leaves. Effects of growing-season length on seedling establishment rate were pronounced in inner portions of the snowbed. There, average establishment success increased from <1% under natural conditions to 5% with experimental extension of the growing season. In contrast, establishment rate in plots near the snowbed edge tended to increase when snowmelt was experimentally delayed. Possibly, delaying snowmelt in relatively fertile outer plots enhanced establishment by synchronizing seed germination with warmer mid-season soil temperatures in the snowbed environment. Regrowth of other plant species into planting microsites occurred more extensively in early snowmelt plots than in late snowmelt plots but had little effect on seedling establishment. Pooled across the entire cohort of germinating seedlings, emergence, survival over the first winter, and survival during the second growing season contributed significantly to variation in establishment rate (R2 = 37%, 7%, and 32%, respectively) but were poorly intercorrelated (r = −0.15). These findings show that sites favorable at any one stage in establishment do not remain favorable at other stages. We conclude that the relationship between growing-season length and seedling establishment success in snow buttercups depends on other aspects of microsite quality. In nutrient-poor habitats, short growing seasons play a major role in restricting establishment. However, in more fertile sites, the timing of snowmelt and its associated effects on the quality of the microclimate for seed germination may be more critical than the period of time from snowmelt until the onset of winter. For snow buttercup seedlings much of establishment is stochastic, in that microsites conducive to emergence do not ensure survival over subsequent phases of the life cycle.