Although seedling mortality can be a primary determinant of tree populations and resulting communities, it remains unclear whether large-scale patterns of seedling mortality are a function of seasonal changes in the environment or local variation in vegetation. Moreover, because field sampling is often constrained to a single time period, it is unclear whether our ability to discern broad-scale patterns in seedling mortality is also obscured by temporal variation in seedling mortality. We investigated the mortality of transplanted seedlings in response to changes in plant community composition along a 120 km latitudinal gradient of mixed hardwood forest. The objectives of this study were to compare seedling mortality 1) between summer and autumn, and 2) among different vegetation assemblages.
In June 2015, we conducted canopy and herb layer surveys at ten sites in northern Wisconsin. We transplanted 66 two-month-old seedlings of five common tree species (Abies balsamea, Acer rubrum, Acer saccharum, Tsuga canadensis, Ulmus americana) at each site in July. Seedling mortality was monitored at two time periods: at the end of summer (September) and autumn (November/December). We used multiple linear regression to determine the response of seedling mortality to seedling species, canopy composition, and herb layer composition.
Autumn seedling mortality was significantly greater (21.1%) than summer seedling mortality (3.5%). Total seedling mortality varied among species (range: 7.7%, Ulmus americana to 44.6%, Tsuga canadensis). Additionally, total seedling mortality was negatively correlated with basal area of canopy Acer rubrum, but positively correlated with percent cover of Acer saccharum seedlings.
This study demonstrates that seedling mortality shows strong seasonal dynamics, which are not captured by summer observations alone. Therefore, vegetation surveys should be conducted later in the year to account for temporal variation in seedling mortality. The significant negative relationship between total seedling mortality and canopy Acer rubrum basal area was driven by particularly low autumn seedling mortality, suggesting that differences in canopy phenology (e.g. leaf senescence), may be important drivers of seedling mortality. Additionally, the significant positive relationship between total seedling mortality and percent cover of Acer saccharum seedlings suggests that A. saccharum either is a strong competitor with other woody seedlings, or can tolerate a wider range of abiotic conditions. We will continue to quantify seasonal dynamics of seedling mortality in this system by tracking 2015-2016 winter seedling mortality. Specifically, we will manipulate snow depth and herbivore access to identify mechanisms of winter seedling mortality.