Effects of Extended Growing Season on Seasonal Growth of Sphagnum Species

The global increase in surface air temperature, mainly attributed to greenhouse forcing, is amplified in arctic regions because of feedbacks resulting from the retreat of sea ice and snow cover and the earlier snowmelt. A growing literature describes the effects of earlier snowmelt on vascular plants, but the response of Sphagnum species remains largely overlooked. This deficiency is mostly related to the poor understanding of its physiology and particularly its growth processes. However, peat moss communities play a critical role in carbon and water cycles in arctic tundra. To advance understanding of the effects of earlier snowmelt on growth of peat moss, we conducted a four-year-long snow removal experiment. The main goals of this experiment were: (1) to describe the seasonal dynamics of Sphagnum growth; and (2) to quantify the impact of earlier snowmelt on Sphagnum growth patterns. We hypothesized that the advantage of a lengthened snow-free season might be counteracted by photo-inhibition, frost damage, and greater water stress.

This study was conducted in a moist dwarf-shrub tundra, typical of the Alaskan Arctic. In half of the studied plots, early snowmelt was simulated by careful removal of snow cover approximately two and a half weeks before natural melt-out, from 1999 to 2002. Every ten days throughout the growing seasons, the vertical growth of 48 individuals was measured using cranked wire technique.

A significant synchronism of the seasonal dynamic of growth revealed a common determinism between Sphagnum individuals, without any effect of the treatment. The high interannual variability of this seasonal pattern suggested the large importance of environment compared to ontogeny in the determinism of growth. Overall, Sphagnum experiencing an earlier snowmelt grew slower compared to the controls. The correlations between height growth and climate parameters suggested that a lengthening of the growing season might be disadvantageous due to frost damage and lower water availability more than photo-inhibition. This decline of moss community may induce a decrease of the carbon sink capacity of these ecosystems and a decrease in soil insulation against moisture and temperature variations. A closer study of the response of Sphagnum growth to microclimate is needed to identify potential compensatory effects of other environmental parameters.