Germination responses to winter warm spells and warming vary widely among woody plants in a temperate forest

Abstract

Winter underpins key ecological processes, such as dormancy loss and seedling emergence. Enhanced warm spells, together with warming are occurring and will continue in the future. The consequences of these climate phenomena on germination were investigated among co‐occurring woody plants, whose seeds are bird‐dispersed in autumn and require cold stratification for spring emergence. Seeds from nine common southeastern USA plants were collected in autumn. We verified that seeds of the study species required cold stratification for dormancy loss. We then examined the following aspects in the laboratory or field: effect of warm spells during cold stratification on germination, effect of a warm spell during winter on seed survival and germination phenology, and effect of warming from autumn dispersal through winter dormancy loss on timing of germination. While no consistent effects of warm spells were found in the laboratory on quantity of germination, warm spells advanced spring field germination for several species. Some species germinated during cold stratification and during warm spells, especially extreme spells, in the laboratory. In the field, about half of Lonicera maackii seedlings that emerged with a warm spell died by late winter. With warming from autumn through spring, laboratory germination shifted from spring to predominately autumn for some species. With precocious germination during warm spells or germination phenology shifts, two scenarios are possible. Seedlings may die during winter, reducing the size of the soil seed bank and number of emergents, or they would survive in warmer winters, which would give them a competitive advantage over spring‐emerging seedlings.

Current and Predicted Future Winter Warm Spells Would Affect Douglas Fir (Pseudotsuga menziesii (Mirb.), Franco) Seeds in the Early Stage of Germination More Than in the Late Stage of Germination

“…After 63 days from the start of incubation, the germination capacity of seeds from each population (except two cases) was no longer dependent on the occurrence of a warm spell, both in terms of its duration and the time during stratification when it occurred. This type of response was consistent with what was documented for non-native species by Flanigan et al [10]. Those authors analyzed the effects of winter warm spells and warming on germination of selected native and non-native species of woody plants in middle Tennessee (USA).…”

“…Hence, germination occurs during late winter or early spring, thereby allowing seedlings to grow in a period before a hot and dry summer [9]. The consequences of winter warm spells in connection with the forecasted increasing temperatures during the dormancy break and germination periods have not been widely described in the literature, especially for trees [1,10]. Faster and enhanced germination of seeds from exotic (non-native) species in comparison to the native ones indicates that seeds of the former feature possess weak dormancy and could react more strongly to winter warm spells [10][11][12][13].…”

Seasonal Variability of Trends in Regional Hot and Warm Temperature Extremes in Europe

“…Late winter and early spring warm spells accelerate plant growth and consequently affect the timing of the onset of phenological seasons [22][23][24][25][26]. On the other hand, premature vegetation growth increases the vulnerability of plants to late spring frost spells [23,27,28].…”

“…Late winter and early spring warm spells accelerate plant growth and consequently affect the timing of the onset of phenological seasons [22][23][24][25][26]. On the other hand, premature vegetation growth increases the vulnerability of plants to late spring frost spells [23,27,28]. Interestingly, research by Siegmund et al [25] shows that phenological processes in spring are also affected by warm extremes in the preceding autumn.…”

Forest understorey communities respond strongly to light in interaction with forest structure, but not to microclimate warming

“…Moreover, in our study, the warming treatment was only applied during the growing season and some plants might respond to growing season length or warming outside the growing season for dormancy breaking or seed germination (Walck et al ., 2011; Orru et al ., 2012; Flanigan et al ., 2020; Newton et al ., 2020), affecting species’ reproduction and the community composition over a longer time period. Additionally, experimental warming inherently reduces soil moisture (Ettinger et al ., 2019), as in our experiment (Fig.…”