Biotic filtering of northern temperate tree seedling emergence in beyond‐range field experiments

Abstract

Abstract Changing climatic regimes are causing increasing temperatures, changing precipitation patterns, and are subsequently expected to impact the spatial distribution of species. The distribution of plants at the scale of continents broadly conforms to the climatological regimes at play; however, in scaling down to the regional and landscape context, the influence of climate becomes confounded by complex and interacting biotic and abiotic factors. These factors have often been cited as important variables in determining the locations of biome overlap, including the boreal forest–temperate forest ecotone (BTE). The BTE exists as a broad latitudinal swath between the boreal and temperate forest biomes in the eastern half of North America. While the impact of non‐climatic factors on the location of the BTE has been investigated within the ecotone, few studies focus on how these same factors are shaping the colonization of the southern boreal forest by northern temperate tree species as part of a potential northward shift of the BTE. The effects of seed and seedling herbivory, substrate composition, microclimate, and canopy composition on seedling emergence of four northern temperate tree species were investigated in a beyond‐range seeding experiment spanning the southern portion of the island of Newfoundland, Canada. To represent a range of temperate tree reproductive strategies, we examined two small‐seeded tree species, Betula alleghaniensis and Thuja occidentalis, and two tree species with large seeds relative to typical boreal forest species, Acer saccharum and Fraxinus nigra. The results of this experiment show a reproductive strategy‐dependent emergence response; large seeds and seedlings appear more vulnerable to predation, while small seeds are more vulnerable to smothering by leaf litter. Canopies with greater representation of deciduous species posed a significant barrier to small seeds that produce small seedlings, while the food‐rich reward of the larger seeds made for attractive targets to granivorous vertebrates. This work represents a rare glimpse at the challenges northern temperate tree species will face if they are to colonize the southern boreal forest and track changing climates as is broadly expected.

Keywords

• boreal temperate ecotone
• canopy composition
• ecological filter
• ecotone
• emergence
• exclosure