Effects of Forest Gaps on Abies faxoniana Rehd. Leaf Litter Mass Loss and Carbon Release along an Elevation Gradient in a Subalpine Forest

Abstract

Changes in the microenvironment induced by forest gaps may affect litter decomposition, yet it is unclear how the gap effects respond to altitudinal and seasonal differences. Here, a four-year litterbag decomposition experiment along an elevation gradient (3000, 3300, 3600 m) was conducted in an Abies faxoniana Rehd. subalpine forest of southwestern China, to assess the potential seasonal effects of forest gaps (large: ≈250 m2, middle: ≈125 m2, small: ≈40 m2 vs. closed canopy) on litter mass loss and carbon release at different elevations. We found that the A. faxoniana litter mass loss and carbon release reached 50~53 and 58~64% after four years of decomposition, respectively. Non-growing seasons (November to April) had a greater decline than the growing seasons (May to October). Litter in the forest gaps exhibited significantly higher mass loss than that under the closed canopy, and the decomposition constant (k) exhibited a gradually declining trend from large gaps, middle gaps, small gaps to closed canopy. Moreover, more significant differences of gap on both carbon content and release were observed at the 3600 m site than the other two elevations. Our findings indicate that (i) a rather high mass loss and carbon release during the decomposition of A. faxoniana litter was observed at high elevations of the subalpine forest subjected to low temperatures in the non-growing seasons and (ii) there were stimulative effects of forest gaps on litter mass loss and carbon release in early decomposition, especially in the non-growing seasons, driven by fewer freeze–thaw cycles when compared to the closed canopy, which diminished at the end of the experiment. The results will provide crucial ecological data for further understanding how opening gaps as a main regeneration method would induce changes in carbon cycling in subalpine forest ecosystems.