Effects of snow-load and shading by vascular plants on the vertical growth of hummocks formed by Sphagnum papillosum in a mire of northern Japan

Abstract

The growth of hummock Sphagnum species in bogs depends on hydrological and climatic conditions, with different hummock morphologies being found across geographical regions. We investigated how plant shade and winter snow-load regulate the growth pattern and height of Sphagnum papillosum hummocks in a cool-temperate bog in Japan. Hummocks were subjected to four treatments across 2 years (1 hummock per treatment per year): no treatment (control), snow-load-free (S), vascular plant trimming (T), and snow-load-free and vascular plant trimming in combination (S&T). Further, the effects of vascular plant cover and snow -load on the vertical growth and height of hummocks were examined. Annual growth rates of S. papillosum were higher for the control (34–41 mm) than S&T treatment, and were intermediate for separate S and T treatments. In contrast to vascular plant-growing hummocks, vascular plant-trimmed hummocks showed a negative correlation between water-table depth (measured from the capitulum to the water-table) and Sphagnum growth. Hence, in summer, shading by vascular plants may prevent desiccation and facilitate the growth of Sphagnum. Snow-loaded hummocks were weighed down by 3–11 cm. After snowmelt, the shoots continued to grow within the water-table depth range that allowed growth. Hence, heavy winter snow-loads may depress the surfaces of hummocks closer to the water-table, which stimulates Sphagnum growth, resulting in the recovery of hummock height. Thus, the water stress caused by summer desiccation is critical in regulating the upper limit of hummock height in bogs subject to dry summer conditions and heavy winter snowfall.