Introduction

Bryophytes can dominate plant–atmosphere exchange surfaces in mesic to hydric Arctic, boreal, and temperate ecosystems and can contribute up to 50% of gross primary production (Goulden & Crill 1997; Bisbee et al. 2001; O'Connell et al. 2003a), although estimates in more dense forests are lower (Skre & Oechel 1979; Kolari et al. 2006). Soils in these systems store approximately one third of the world's reactive pool of soil carbon (McGuire et al. 1995) with a major contribution coming from bryophytes (Gorham 1991; O'Neill 2000; Turetsky 2003). Within these systems, the bryophyte layer also influences hydrology, nutrient uptake and cycling, and soil temperature.

In the boreal zone, significant research has been undertaken to determine how forest bryophytes affect carbon exchange and sequestration. These studies have focused on the influence of environmental forcing variables (e.g., temperature, light, and water availability) on the productivity and carbon dynamics of feathermoss (Pleurozium) and Sphagnum moss species, the two most dominant groups ecologically. This work has led to a better understanding of temporal variation in bryophyte function and has provided insights into how the performance of individual species varies across gradients of temperature, light intensity, and water availability (Skre & Oechel 1979; Trumbore & Harden 1997; Bisbee et al. 2001; O'Connell et al. 2003a, b; Heijmans et al. 2004; Kolari et al. 2006; Kulmala et al. 2008).

However, these studies have neglected the causes and consequences of intraspecific variation, which can be similar in magnitude to differences among species.