The effect of water table decline on plant biomass and species composition in the Zoige peatland: A four-year in situ field experiment

The Zoige peatland is the largest alpine peatland in the world, and it is suffering the threat of water table decline. Plant productivity and species composition are important to ecosystem carbon sequestration and soil carbon input in peatlands. We examined the responses of plant community composition and biomass accumulation to water table decline to better understanding the responses of this peatland to environmental changes. A four-year in situ field experiment was conducted involving three treatments: deep, shallow, and control water tables, which were achieved by experimental drainage with 50 cm, 20 cm, and 0 cm deep ditches, respectively. Experimental drainage decreased the annual mean height of water table by ca. 12 cm and 15 cm (relative to the control) in the shallow and deep water table treatments, respectively, over the four years. The response of aboveground plant biomass (APB) to water table decline declined in the first year, remained unchanged in the second year and increased during the third and fourth years. However, water table decline had a non-significant effect on belowground plant biomass. This duration-dependent response of APB can be attributed to the changes in community species composition during the study years. Specifically, the negative effect of water table decline in the first year was due to the significant decrease in APB of hygrophytes (sedges and rushes). In the second year, although water table decline significantly increased APB of mesophytes (grasses and forbs), this increase was offset by the decrease in APB of hygrophytes, leading to a neutral effect. In both the third and fourth years, the extent of the increase in APB of mesophytes (typically the forb species Anemone trullifolia var. linearis) was greater than that of the decrease in APB of hygrophytes, leading to a positive effect. Our results indicate that short-term decline of the water table may increase the primary productivity by shifting dominant species of hygrophytes to mesophytes in the Zogie peatland.