Climatic Variability, Plant Phenology, and Northern Ungulates

Abstract

Models of climate change predict that global temperatures and precipitation will increase within the next century, with the most pronounced changes occurring in northern latitudes and during winter. A large-scale atmospheric phenomenon, the North Atlantic Oscillation (NAO), is a strong determinant of both interannual variation and decadal trends in temperatures and precipitation during winter in northern latitudes, and its recent persistence in one extreme phase may be a substantial component of increases in global temperatures. Hence, we investigated the influences of large-scale climatic variability on plant phenology and ungulate population ecology by incorporating the NAO in statistical analyses of previously published data on: (1) the timing of flowering by plants in Norway, and (2) phenotypic and demographic variation in populations of northern ungulates. We analyzed 137 time series on plant phenology for 13 species of plants in Norway spanning up to 50 yr (44 ± 0.5 yr, mean ± 1 se) and 39 time series on phenotypic and demographic traits of 7 species of northern ungulates from 16 populations in North America and northern Europe spanning up to 30 yr (18 ± 2.0 yr). Plant phenology was significantly related to the NAO in 97 time series (71% of the total), in which dynamics of the NAO explained, on average, between 9% and 28% of the interannual variation in flowering dates. Following increasingly warm, wet winters, most plant species (9 of 13 species) bloomed earlier by an average of 13.0 ± 0.8 d to 26.4 ± 1.8 d, (mean ± 1 se), one-third (4 of 11 species) bloomed longer by 13.4 ± 1.1 d to 18.8 ± 1.7 d, and there was an increase in spatial variability in timing of flowering across landscapes by five of six species. Woody plants displayed less sensitivity to climatic variability than did herbaceous species, and early-blooming plants were more strongly influenced by the NAO than were late-blooming plants. Ungulate phenotypic and demographic variables were significantly related to the NAO in 28 time series (72% of the total). Large-scale climatic variability influenced growth, development, fecundity, and demographic trends of all seven species of ungulates studied, and in some populations, the NAO acted together with density dependence. Individuals within mainland populations responded to winter warming with reduced body size and increased fecundity, whereas winter warming in maritime regions led to increased body size but reduced fecundity. Across sex and age classes, between 43% and 70% of the observed range in body mass among years was attributable to the dynamics of the NAO, and within cohorts of female red deer and reindeer, 47–70% of the observed range in fecundity was related to the NAO during the winter preceding cohort birth years. All but two populations of northern ungulates declined following increasingly warmer winters, and the NAO operated, in most instances, in concert with direct density dependence to limit populations. In comparison to the original analyses of these ungulate time series, in 10 of 19 series the NAO explained an equal or greater percentage of variation than that explained by local weather. These observations indicate that large-scale climatic variability has a detectable influence on the ecology of plants and animals in a variety of terrestrial ecosystems, and that the responses of plants to winter warming may, surprisingly, be more subtle than the responses of large herbivores.