Seedling Survival in a Variable Environment: Connecting Physiology Across Life Cycle Stages

Abstract

Seedling establishment is a crucial phase when plants are particularly vulnerable to environmental stress. Nonetheless, seedling traits and survival are rarely studied in functional ecology. Using five common species in a well-studied guild of desert annuals, I measured seedling survival and functional traits in response to drought under three temperature regimes in growth chambers that spanned the range of temperatures seedlings normally experience during the germination season. I compared seedling longevity and growth rates to a suite of previously measured plant traits to determine how seedling responses to temperature and drought correspond to physiology at other life cycle stages, as well as to long-term patterns of seedling survival in the field. Generally, seedlings of all species survived longer in cooler temperatures. However, some seedlings had limited early mortality prior to drought conditions, especially in colder temperatures. Temperature-dependent patterns of survival were strongly related to traits expressed at different points in the life cycle, including seed size, germination and, to a lesser extent, growth traits, indicating coordinated functional strategies. Species with larger seeds, faster germination and higher water-use efficiency survived drought longer, especially in the colder temperature. Seedling survival in the growth chamber was correlated with seedling survival rates in the field. Neither relative growth rate nor specific leaf area plasticity under experimental drought conditions connected meaningfully to other functional traits and further research is needed to determine how seedling growth under stress relates to these traits. This study is the first to show that temperature-dependent seedling survival under drought conditions covaries with functional strategies across the lifecycle and predicts natural patterns of establishment in the field.