Future changes in climate are expected to have important implications for tree recruitment within mixed deciduous-evergreen forests. These changes are perhaps most apparent at the southern boundaries of North American conifer species where climatic shifts have ostensibly already driven changes in tree recruitment. In particular, tree recruitment limitation imposed by variation in the strength of species interactions (e.g., attack by soil pathogens), novel climatic conditions (e.g., increased climatic variation), and the possible interaction between these two factors may influence forest recruitment dynamics by altering tree seed germination dynamics. Using soil collected from 12 sites spanning a 225 km latitudinal gradient in northern Wisconsin and seeds of two pine species (Pinus strobus, P. resinosa), we tested the hypotheses that 1) fungal pathogens residing in soils from the southern edge of mixed deciduous-evergreen forest zones display strong latitudinal variation in their severity of attack on conifer seeds and 2) that particular consequences of winter climate change (i.e., increased soil freeze-thaw cycling) exacerbate fungal pathogen attack on conifer seeds. Our factorial experiment manipulated freeze-thaw cycles (control vs. 1 freeze-thaw cycle vs. 4 freeze-thaw cycles) and fungal pathogen presence (via fungicide addition) and tracked germination of both Pinus species for 2 months.
Results/Conclusions
Germination for both Pinus species was greater in soils collected at northern sites than soils collected at southern sites. Fungicide application consistently resulted in greater Pinus strobus germination, but the effect of freeze-thaw stress on P. strobus germination differed depending upon the latitude of soil collection. In soils collected at lower latitudes, experimental exposure to greater freeze-thaw frequency resulted in the least P. strobus germination. Both freeze-thaw treatments similarly limited P. strobus germination relative to controls in soils from median latitudes, but in soils from high latitude sites the lowest germination was observed following the one freeze-thaw treatment and germination did not differ between controls and four freeze-thaw cycles treatment. Pinus resinosa germination was not influenced by soil freeze-thaw stress, but the positive effect of fungicide application was greater in soils collected at higher latitude sites (+18.1%) than those collected at median (+14.1%) or lower (+12.0%) latitude sites. Our results demonstrate that soil pathogen attack and freeze-thaw stress could generate unique barriers to Pinus spp. recruitment along latitudinal gradients located at the southern edge of mixed deciduous-evergreen forest zones, and suggest these factors may be important obstacles to tree recruitment within the northerly retreating distribution of North American conifers.