Warming during maternal generations delays offspring germination in native and nonnative species

As environmental conditions shift due to global warming and other human-caused environmental changes, plastic responses in phenological traits like germination or flowering time may become increasingly important. While phenological plasticity is a common response to global warming, with many populations exhibiting earlier germination or flowering in warmer years, warming may also result in transgenerational plasticity, especially on early life stages. In other words, seeds produced by mothers inhabiting warmer environments may germinate faster (or slower) than seeds produced by mothers inhabiting ambient or cooler environments. Here, we use seeds collected from a field warming experiment to examine how germination and early growth differ in response to ambient vs. warmed (+3°C) temperatures experienced by both maternal and offspring generations. Because nonnative species are often more phenotypically plastic than native species and because a variety of life history traits and environmental factors affect the evolution of both within and transgenerational plasticity, we include multiple invasive and native plant species in our study. On average, warming experienced during maternal generations delayed germination by ~0.2 days/°C, although species varied in the magnitude of response. In contrast, warming during the offspring generation tended to advance germination by ~0.1 days/°C. Nonnative species demonstrated higher germination success than native species, but we detected no differences in germination timing between native and nonnative species or that native and nonnative species differed in either within- or transgenerational plasticity, although species (independent of native status) did exhibit differing degrees of within- and transgenerational plasticity in germination timing and early growth. This study suggests that temperatures experienced by maternal plants can influence their offspring's germination phenology, potentially even more so than temperatures experienced in the offspring's immediate environment.