Physiological Ecology of Peatland Bryophytes

Abstract

Bryophytes, notably mosses of the genus Sphagnum, are significant and essential primary producers in peatlands. Peatland bryophytes face specific physical conditions; they are exposed to direct sunlight, but due to their permanent hydration they do not escape by drying as typical xeric bryophytes of open habitats. Being desiccation avoiders they are actually sensitive to drought. During photosynthesis, hydration increases the diffusion resistance to CO2, which can be supplied also from respiration in the underlying peat. The distance to the water table affects the degree of hydration, but also influences nutrient availability as mineral nutrients can be carried in capillary water. Consequently, gradients of nutrient and water availability are related in peatlands and their variation in addition to light maintains bryophyte species diversity in peatlands. Habitats with low stress intensity, typically forested peatlands and wet microhabitats of open bogs and fens, host mosses with competitive life strategies, characterized by high rates of photosynthesis, growth and production. In contrast, mosses inhabiting sun-exposed, nutrient poor microhabitats, typically hummocks, must cope with low water availability and photodamage. Their stress-tolerance/avoidance strategy is reflected by slow photosynthetic and growth rates, and allocation to water holding tissues. In this chapter, I review the effects of ecologically relevant (stress) factors affecting photosynthesis and growth, especially in Sphagnum. Potential consequences of global climate change are also discussed. I mention how the non-uniform experimental conditions used in photosynthetic gas exchange measurements may affect the diffusion resistance to CO2 and consequent estimates of photosynthesis and evaporation. Suggestions for further research are proposed.