Nonnative old-field species inhabit early season phenological niches and exhibit unique sensitivity to climate

Abstract

Native and nonnative plant species can exhibit differences in the timing of their reproductive phenology and their phenological sensitivity to climate. These contrasts may influence species' interactions and the invasion potential of nonnative species; however, a limited number of phenology studies expressly consider phenological mismatches among native and nonnative species over broad spatial or temporal scales. To fill this knowledge gap, we used two complementary approaches: First, we quantified the flowering phenology of native and nonnative plants at five old-field sites across a spatially extensive range of eastern North America. Second, we used herbarium records to compare the sensitivity of flowering and fruiting phenology to climate across a 114-yr time period in a subset of common old-field species in southwestern Pennsylvania. Across the study region, nonnatives reproduced substantially earlier in the growing season than natives, suggesting that nonnatives occupy a unique phenological niche (0.55 months earlier flowering across the North American study sites; 50.1 d earlier flowering and 17.5 d earlier fruiting in southwestern Pennsylvania). Both natives and nonnatives advanced their reproductive phenology between 1900 and 2014 but exhibited contrasting phenological sensitivity to climate factors. During the flowering stage of phenology, nonnatives were more sensitive to changes in precipitation than natives and generally delayed flowering in wetter years. Nonnative plants had greater sensitivity and advanced fruiting when the month preceding fruiting was warmer, while native plants had greater sensitivity and advanced fruiting when the three-month period preceding fruiting was warmer. Our findings suggest that nonnative old-field species occupy an earlier phenological niche relative to native species, which may facilitate their invasion into old-field communities. However, given the different sensitivities of native and nonnative plants to climate factors, present-day patterns of phenology are likely to shift with future climate changes, potentially leading to novel species interactions that may influence the outcomes of invasion.