Effects of Experimental Warming on Plant Reproductive Phenology in a Subalpine Meadow

Increasing “greenhouse” gases are predicted to warm the earth by several degrees Celsius during the coming century. At high elevations one likely result is a longer snow-free season, which will affect plant growth and reproduction. We studied flowering and fruiting of 10 angiosperm species in a subalpine meadow over 4 yr, focusing on plant responses to warming by overhead heaters. The 10 species reproduced in a predictable sequence during 3–4 mo between spring snowmelt and fall frosts. Experimental warming advanced the date of snowmelt by almost 1 wk on average, relative to controls, and similarly advanced the mean timing of plant reproduction. This phenological shift was entirely explained by earlier snowmelt in the case of six plant species that flowered early in the season, whereas four later-flowering species apparently responded to other cues. Experimental warming had no detectable effect on the duration of flowering and fruiting, even though natural conditions of early snowmelt were associated with longer duration and greater overlap of reproduction of sequentially flowering species. Fruit set was greater in warmed plots for most species, but this effect was not significant for any species individually. We conclude that global warming will cause immediate phenological shifts in plant communities at high elevations, mediated largely through changes in timing of snowmelt. Shifts on longer time scales are also likely as plant fitnesses, population dynamics, and community structure respond to altered phenology of species relative to one another and to animal mutualists and enemies. However, the small spatial scale of experiments such as ours and the inability to perfectly mimic all elements of climate change limit our ability to predict these longer term changes. A promising future direction is to combine experiments with study of natural phenological variation on landscape and larger scales.