PS 25-90
Interactive effects of bryophyte species and climate warming on tree seedling establishment
Arctic and Alpine tree line expansions induced by climate warming are dependent on tree seedling establishment above the tree line. This involves interactions between tree seedlings and bryophytes, because bryophytes dominate the tundra above the tree line. Bryophytes control a range of abiotic factors important for seedling establishment including soil moisture and nutrients. However, bryophyte species are diverse in their ecological attributes and their response to climate change and could therefore potentially have contrasting effects on tree seedlings.
We grew tree seedlings of Betula pubescens and Pinus sylvestris (species that form the tree line or that occur at the sub tree line level in Fennoscandia, respectively) in cores containing one of eight bryophytes species or bryophyte-free control soil. The bryophyte cores were collected above the tree line close to Abisko National Park in subarctic Sweden. The experiment was run in climate-controlled chambers at tree line temperature (~7°C) and five degrees warmer than that temperature to represent projected climate warming. Seedling establishment was expressed as biomass upon harvest after 4 months (one growing season).
We hypothesized that tree seedling biomass would differ between bryophyte species but that these differences would be influenced by warming treatment. We also hypothesized that these differences between moss species could be explained by bryophyte traits that relate to moisture.
Results/Conclusions
Seedling biomass depended on bryophyte species (p<0.0001) and was generally higher in bryophytes than in control soil. Warming increased seedling biomass (p<0.0001) but the total increase depended on bryophyte species (i.e., bryophyte species x warming interaction; p=0. 02117). Seedlings of both tree species responded most to warming when grown in the pan boreal and subarctic bryophyte species Hylocomium splendens. Seedling biomass was positively correlated to the bryophytes water loosing rates (a trait describing bryophytes ability to hold water).
Our results suggest that the impact of individual bryophyte species on seedling establishment might change under a warmer climate and that bryophyte traits related to soil moisture are important for seedling success. To understand interactive effects between climate and mosses is important for understanding mechanisms explaining tree line dynamics and to make accurate predictions of future tree line expansions.