Snowmelt timing alters the phenology but not the performance of an understory spring ephemeral plant

Abstract

Spatial and temporal variation in fine-scale environmental conditions can influence local population abundance and structure ecological communities. In temperate deciduous forests, some herbaceous plant species, including some spring ephemerals, locally cluster around the bases of canopy trees. Despite the conspicuousness of this pattern, no experimental tests have been conducted to discriminate among hypothesized mechanisms. We tested whether earlier spring snowmelt in rings around the base of deciduous trees (‘thaw circles’) was responsible for improved plant performance and greater local abundance via advanced spring phenology and longer growing seasons (the time between emergence and either senescence or canopy closure). Individuals of Dutchman's breeches (Dicentra cucullaria), a common spring ephemeral plant, were transplanted into a fully factorial field experiment of manipulated snowmelt timing (early vs. late) and tree proximity (near vs. far) in a temperate deciduous forest (Québec, Canada). Spring phenology and plant performance (peak leaf-bearing stem abundance and total leaf area) were tracked for 2 years. Advanced spring emergence with early snowmelt, in both naturally occurring thaw circles and where snow was experimentally removed, lengthened growing seasons by 10%–19%, a meaningful extension for spring-flowering herbs with a short window of activity. Surprisingly, Dutchman's breeches transplanted near trees experienced lengthened growing seasons as well, even after accounting for snowmelt timing and stem abundance. However, leaf area and stem abundance increased only with close tree proximity, with no effects of snowmelt timing on these aspects of performance. Synthesis. Growing season length was disconnected from Dutchman's breeches' performance, with early snowmelt likely not the proximate cause of greater local abundance of this spring ephemeral herb around the base of deciduous trees. Instead, other factors associated with tree proximity, including edaphic conditions and microclimate buffering, may play a primary role in Dutchman's breeches' local distribution. Experimental manipulation of fine-scale environmental conditions and resource availability will prove increasingly informative for understanding population dynamics and community structuring in an era of rapid global change.