Temperate tree species are expected to expand their distribution into the boreal forest in response to climate change. However, it is becoming increasingly clear that many species will experience significant setbacks in their migration due to a series of unfavourable conditions impacting their recruitment success, and thus their ability to colonize new locations. We quantify the relative influence of a series of factors important for tree seedling recruitment at range margins: dispersal, substrate favourability, and the influence of the local hetero-specific species canopy composition. We hypothesized that boreal trees are responsible for priority effects that influence the establishment of temperate tree species seedlings. To do so, we analyzed two recruitment stages (first year seedlings and older seedlings) for seven tree species; Abies balsamea (ABBA), Acer rubrum (ACRU), Acer saccharum (ACSA), Betula papyrifera (BEPA), Betula alleghaniensis (BEAL), Populus tremuloides (POTR), and Fagus grandifolia (FAGR) commonly found within the temperate-boreal ecotone forests of northeastern North America. Overall, we found that boreal canopy trees influence the distribution of substrates, more specifically the occurrence of needle cover and decayed wood in recruitment plots. This association between canopy and substrate led to highly unfavourable substrates that affected the seedling densities of all temperate tree species. In addition, we found that seedling dispersion was highly localized, where mean dispersal distance of all trees occurred in close proximity of parent trees. Ultimately, we found evidence that priority effects imposed by resident boreal trees are magnified as a result of unfavourable substrates and limited MDD (mean dispersal distance) of trees within these ecosystems, which together promise to cause significant lags in temperate tree species migration into the boreal forest in the future.
We predict that
(1) Boreal trees will influence the spatial distribution of substrates within a stand
which would then
(2) Influence the seedling density of temperate tree species.
(3) Seedling dispersal would be limited to areas in close proximities of parent trees and thus magnify priority effects imposed by boreal tree species.
The study was conducted at three permanent sample plots which were established as part of the QUICCFOR network (QUantifying and mapping the Impact of Climate Change on FORest productivity of Eastern North America).