Effects Of Snow Patches On Litter Decomposition In The Alpine Forest

The snow cover caused by canopy gap in alpine forest may play an important role in litter decomposition, further change the material cycling and other key ecosystem processes. However, little information has been available on the effects of snow patches on litter decomposition in alpine forest. Therefore, in order to deeply understand alpine forest litter decomposition process, a field litterbag experiment was conducted in Eastern Tibetan plateau, China. Four widely distributed species as fir (Abies faxoniana), larch (Larix mastersiana), cypress (Sabina saltuaria) and birch (Betula albo-sinensis) foliar litter were selected. Five kinds of snow patch (thick snow cover; thicker snow cover; medium snow cover; thin snow cover; no snow cover) with different snow depths in winter were chose to simulate the changing process of snow cover from outside to inside canopy gap in the scenario of climate warming. Mass loss rate, C, N and P release rate, and cellulose, lignin and total polyphenols loss rate together with the changes of litter quality were investigated. The objectives were to reveal the affecting mechanisms of snow cover on litter decomposition, and to provide efficient information to understand the key ecological processes such as material cycling in alpine forest and their feedbacks to the ongoing climate change.(1) After two years decomposition, mass loss rates of foliar litter were33.98%-39.55%for fir,46.49%-48.22%for birch,42.30%-44.93%for larch, and40.34%-43.84%for cypress, respectively. Snow cover significantly affected the mass loss rate of four species foliar litter, but different species displayed different responses to snow cover. The k values from Olson decomposition model for three needle litters (fir, larch and cypress) were the highest under thick snow cover and lowest under no snow cover compared with other snow cover treatments. However, the k value for broad-leaved species as birch showed the order as:no snow cover> thin snow cover > thicker snow cover> thick snow cover> medium snow cover. Relative to other critical periods, late growing season first year (LGS1) showed the highest mass loss rate regardless of species. The lowest mass loss rates of larch and cypress needle litter were respectively observed under thicker snow cover and under thin snow cover,, but which in larch and birch were under medium snow cover in thawing stage of second year (TS2).(2) After two years decomposition, the results indicated that snow cover did significantly affect on litter quality, but different species displayed different responses to snow cover in different critical periods. Snow cover significantly affected C concentration of birch during litter decomposition, but the effects for fir, larch and cypress were not significant in late decomposition. Nitrogen concentration and cellulose content under different snow covers showed significant differences from the first thawing stage. Phosphorus concentration and L/N under different snow cover showed significant differences in the whole two years decomposition. In addition, cellulose content, total polyphenols concentration and C/N showed significant responses to snow cover from the first deep freezing period (DF1). Except for birch, snow cover also had significantly effects on total polyphenols concentration of other three species litter. The contents of C, N, P, cellulose, total polyphenols, and C/N, C/P and N/P showed a decline trend, but lignin content, C/N, L/N, total polyphenols/N and (total polyphenols+L)/N showed a rising trend as foliar litter decomposition proceeded.(3) After two-year decomposition, the litter under different snow covers showed that its C release rates were46.45%-50.62%for fir,48.78%-50.97%for larch,45.29%-48.78%for cypress and52.72%-56.93%for birch; N release rates were41.29%-48.60%for fir,48.87%-53.57%for larch,44.34%-47.37%for cypress and46.58%-51.81%for birch; and P release rates were45.48%-49.84%%for fir,48.95%-51.32%for larch,36.95%-41.58%for cypress and47.33%-49.79%for birch. Different snow covers had significant impact on the release characteristics of C, N and P of four species’litters, and the C, N, P release dynamic of different species showed obvious differences under different snow covers. Relative to the other critical periods, for C release rate fir litter under different snow covers had maximum value in LGS1and minimum value in the onset of freezing period of first year (OF1); larch had maximum value in DF1and minimum value in DF2; cypress had maximum value in EGS1and minimum value in LGS2; and birch had maximum value in DF1, minimum value in TS2. For N release rate, both fir and larch litter under different snow covers had maximum values in OF1and minimum values in EGS1; cypress had maximum value in EGS1and minimum value in EGS2; and birch had maximum value in DF1and minimum value in TS2. For P release rate, fir litter under different snow covers had maximum value in LGS1and minimum value in EGS2; larch had maximum value in DF1and minimum value in EGS2; cypress had maximum value in OF1and minimum value in EGS2; and birch had maximum value in DF1and minimum value in TS2.(4) In the two-year decomposition, the cellulose of litter of four species under different snow covers all had the release to a big extent, the Cellulose loss rates were37.30%-43.03%for fir,40.75%-43.22%for larch,49.75%-52.65%for cypress and49.95%-51.64%for birch, respectively. Meanwhile, the Lignin loss rates were29.08%-32.25%for fir,33.26%-38.67%for larch,37.61%-43.70%for cypress and40.70%-47.06%for birch. Different snow covers significantly affected the loss characteristics of the cellulose which showed that all four species had maximum values in DF1and minimum values in OF2. Different snow covers significantly affected the loss characteristics of lignin as well, for three coniferous litters (fir, larch and cypress) the lignin loss rates were maximum in DF1and minimum in EGS2, while for birch the lignin loss rate was maximum in DF1and minimum in TS2.(5) In the two-year decomposition, the litter polyphenols under different snow covers had the release to a larger degree. The release rates of total polyphenols were62.82%-72.49%for fir,60.55%-71.00%for larch,59.48%-72.46%for cypress and76.48%-88.23%for birch. Different snow covers significantly affected the release characteristics of polyphenols of four species litter, the release rates of total polyphenols of four species litter were all maximum in OF1and minimum in TS2.In summary, snow covers with different thickness in winter significantly affected foliar litter decomposition in the alpine forest, but the responses were regulated by litter quality and the thickness of the snow cover. Both broad-leave and needle litters decomposition increased with the increase of snow thickness in the first decomposition year, but which in the second decomposition year were only observed during coniferous litters decomposition. Relative to other critical periods, thick snow cover significantly promoted litter decomposition in winter, although no snow cover significantly promoted litter decomposition in early growing season. As winter snow depth reduced, litter C release rate, N release rate, P release rate, cellulose loss rate, lignin loss rate, total polyphenols release rate, C/N and L/N decreased regardless of species. Snow cover also changed litter quality (such as C/N and L/N). After two-year decomposition, the results indicated that snow cover thickness reduction would slow down litter decomposition in the alpine forest in the scenario of climate warming. These results can provide efficient scientific data to understand the effects of climate change on material recycling and other related key ecological processes.