| Litter decomposition, a fundamental process of carbon and nutrient cycling in terrestrial ecosystems, plays an important role in the primary productivity, community succession and ecosystem stability in forest. Forest gap has important impacts on environment factors (e.g. sunlight, temperature, and moisture) and the structure and distribution pattern of vegetation community, which could affects the process of litter decomposition on the forest floor, In the alpine/subalpine region, forest gap also could change the pattern of snow cover and freeze-thaw events during winter, and then affect the process of litter decomposition in winter. Unfortunately, few studies have focused on the effects of forest gap on litter decomposition in alpine/subalpine forest, which severely limits the understanding of the carbon and nutrient turnover patterns in alpine/subalpine forest ecosystem. To assess the effects of gap sizes on foliar litter decomposition, a field litterbag experiment was conducted at different altitudes of the alpine/subalpine forest at the east Tibetan Plateau. The objectives of this study were to explore:(1) whether the process of foliar litter decomposition is affected by gap formation, (2) how foliar litter decomposition responses to gap sizes and altitudes, (3) how foliar litter decomposition responses to different critical decomposing period, e.g. the onset of soil freezing period, soil freezing and thawing period, and the early, moderate and later growing season. The main findings are present below:(1) Forest gaps with density 14.67 per hm2 in this alpine forest were dominated by moderate and small gaps. About 63.64% of gap formation was caused by stem breakage in gap center. The expanded gap (EG) and canopy gap (CG) covered 12.60% and 23.05% of forest area, respectively. The natural disturbance frequency was 115.25 m2 hm-2 a-1 and 63.02 m2 hm-2 a-1 for EG and CG, respectively. The return interval of canopy gap was about 260.30 a. About 50.09% of gaps were formed by single gap maker, whose DBH and height ranged from 40 cm to 60 cm and from 25 m to 30 m, respectively. Every gap had 1.53 gap makers on average, with each gap maker forming 103.20 m2 EG and 56.43 m2 CG, respectively. The average DBH of gap border tree was 50.16 cm, which exhibited a curve distribution with a significant left peak. Moreover, power function relationships were observed between average DBH and height and the area of EG and CG.(2) The mass losses of birch and fir foliar litter over the two years were 43.69-51.06% and 39.00-47.66%, and the half decomposed times were 1.782-2.413 year and 2.041-2.768 year, respectively. In addition, mass losses of birch and fir foliar litter in non-growing season were 39.00-62.05% and 46.71-61.32% to two-year mass loss, respectively. As compared with closed canopy, the mass loss rates of birch foliar litter reduced by gap formation at the three altitudes, and significantly (P<0.05) decreased with the increasing gap size. In contrast, gap formation facilitated the mass losses of fir foliar litter at the altitudes of 3309 m and 3598 m, and significantly (P<0.05) increased with the increasing gap size, though suppressed the mass loss of fir foliar litter at the altitude of 2998 m. Moreover, gap formation increased the decomposed time of birch foliar litter, and inhabited a tendency of increased with the increasing gap size. In contrast, the decomposed time of fir foliar litter increased with the increasing gap size at the altitude of 2998 m, but a reverse trend was observed at the other two altitudes. Furthermore, gap size significantly (P<0.05) impacted mass loss rates of birch and fir foliar litter, especially during the first decomposing year. As compared with closed canopy, mass loss rates of birch foliar litter decreased due to gap formation at the three altitudes in the first year of decomposition but increased at the altitudes of 2998 m and 3309 m in the second year. Meanwhile, mass loss rates of fir foliar litter at the first year of decomposition were increased at the altitudes of 3309 m and 3598 m but reduced at the altitude of 2998 m, while various along altitudes during the second year by gap formation.(3) The remaining carbon of birch foliar litter significantly (P<0.05) increased by gap formation at the altitudes of 2998 m and 3598 m except for the altitude of 3309 m. However, the remaining carbon of fir foliar litter significantly (P<0.05) decreased by gap formation at the altitudes of 3309 m and 3598 m, but significantly (P<0.05) increased at the altitude of 2998 m. The effects of gap on the remaining nitrogen of birch foliar litter were reduced with the increase of altitude, but the effects on fir foliar litter showed the order as 3309 m> 3598 m> 2998 m. There were no significantly differences among altitudes for remaining phosphorus of both birch and fir foliar litter. For birch foliar litter, compared with closed canopy, the carbon release rates at 2998 and 3309 m at the first year, the nitrogen release rates at 2998 m and 3598 m at the first year and at 3309 m at the second year and the phosphorus release rates of birch foliar litter at all altitudes at the first year were significantly (P<0.05) reduced by gap formation. However, the carbon release rates at all altitudes at the second year, the nitrogen release rates at 2998 m and 3598 m at the second year and the phosphorus release rates of birch foliar litter at 2998 m and 3309 m at the second year were significantly (P<0.05) increased by gap formation. For fir foliar litter, the carbon release rates at 3309 m and 3598 m at the first year, the nitrogen release rates at 2998 m at the second year, at 3309 m at the first year and at 3598 m at the two years, the phosphorus release rates of fir foliar litter at 3309 m at the first year and the 3598 m at the second year were significantly (P<0.05) increased by gap formation. However, the nitrogen release rates at 3309 m at the second year, the phosphorus release rates of fir foliar litter at 2998 m and 3598 m at the first year were significantly (P<0.05) reduced by gap formation.(4) The remaining lignin content of birch foliar litter decreased 35.44%-54.35%, 32.87%-42.74% and 12.15%-33.97% at the altitudes of 2998 m,3309 m and 3598 m, respectively, and-0.14%-12.83%,3.48%-24.46% and 2.46%-7.37% for fir foliar litter, respectively. As compared with closed canopy, remaining lignin contents were higher and lower under large gap for birch and fir foliar litter, respectively, regardless of altitudes. Highest lignin release rates of birch foliar litter at the altitude of 2998 m were observed at the first growing season, while that at the altitudes of 3309 m and 3598 m were at the first non-growing season. Meanwhile, the lignin content of fir foliar litter net accumulated at the altitudes of 2998 m and 3309 m at the second growing season, but net released at the altitude of 3598 m at the second non-growing season. Furthermore, cellulose release rates reduced 69.30%-71.91%,63.85%-68.86% and 65.75%-69.04% at the altitudes of 2998 m,3309 m and 3598 m for birch foliar litter, respectively, and 57.94%-62.69%,59.63%-69.72% and 57.58%-64.82% for fir foliar litter, respectively. Compared with closed canopy, gap formation facilitated cellulose release rate of birch foliar litter at 2998m, while suppressed it for fir foliar litter. In contrast, gap formation suppressed cellulose release rates of birch foliar litter at 3309m and 3598m, but facilitated it for fir foliar litter at the other two altitudes. Even so, highest cellulose release rates of both birch and fir foliar litter at the three altitudes were observed at the first non-growing season.(5) The C/N, lignin/N and lignin/cellulose were significantly (P<0.05) affected by gap formation. Gap formation significantly reduced C/N of birch foliar litter at the altitudes of both 2998 m and 3309 m in the non-growing season of the two years and at the 3598 m in the growing season of the second year, while increased the C/N of birth foliar litter at the 2998 m in the later growing season of the two years, at 3309 m in the moderate growing season of the two years and at 3598 m in the onset of freezing and deep freezing of the first year. However, the C/N ratios of fir foliar litter reduced by gap formation at the altitudes of both 2998 m and 3309 m at the deep freezing period of the first year, and at the altitude of 3598 m at the onset of freezing and thawing period of the two years, but increased at the altitudes of both 2998 m and 3309 m at the onset of freezing period of the two years, and at the altitude of 3598 m at the early growing season of the second year. For birch foliar litter, the lignin/N ratios of birch foliar litter increased by gap formation at the altitudes of both 2998 m and 3309 m at the onset of freezing period and early growing season of the first year, at the altitude of 3598 m at the deep freezing and thawing period, but reduced at the altitudes of both 2998 m and 3309 m at the onset of freezing and deep freezing period, at the altitude of 3598 m at the periods from first early growing season to second early freezing period. The lignin/N ratios of fir foliar litter were increased by gap formation at three altitudes at the deep freezing period of the second year. Meanwhile, the lignin/ cellulose ratios of birch foliar litter increased by gap formation at the altitude of both 2998 m and 3309 m at the onset of freezing period, early and moderate growing season, at the altitude of 3598 m at the periods from the first thawing period to the second deep freezing period, but reduced at the altitudes of both 2998 m and 3309 m at the later growing season of the second year, at the altitude of 3598 m at the periods from second thawing period the moderate growing season. However, the lignin/cellulose ratios of fir foliar litter increased by gap formation at the altitudes of both 2998 m and 3309 m at the thawing period of the first year, at the altitude of 3598 m at the periods from the deep freezing sage to the early growing season of the first year, but reduced at the altitudes of both 2998 m and 3309 m at the early growing season, and at the altitude of 3598 m at the moderate growing season of the two years.(6) The mass loss rates of both birch and fir foliar litter were significantly (P<0.05) related to the temperature at the first deep freezing period and the number of freeze-thaw cycles at the first thawing period. Moreover, the carbon release rates of both birch and fir foliar litter were significantly (P<0.05) related to temperature at the altitude of 3598 m at the deep freezing period of the first year, at the altitude of 3309 m at the deep freezing period of the second year, and significantly (P<0.05) related to the number of freeze-thaw cycles at the altitudes of both 3309 m and 3598 m at the deep freezing period of the first year. Meanwhile, the nitrogen release rates of birch foliar litter at the altitude of 2998 m and fir foliar litter at the altitude of 3309 m were significantly (P<0.05) related to temperature, and the mass loss rates of birch foliar litter at the altitude of 3309 m and fir foliar litter at the altitude of 3598 m were significantly (P<0.05) related to the number of freeze-thaw cycles. In addition, the phosphorus release rates of both birch and fir foliar litter were significantly (P<0.05) related to temperature and the number of freeze-thaw cycles at non-growing season. However, the correlations between lignin, cellulose release rates of both birch and fir foliar litter and daily mean temperature and the number of freeze-thaw cycles were different at different altitudes.In conclusion, gap formation significantly affected the sunlight, temperature and snow cover conditions over the winter and growing season in various altitudes in the subalpine/alpine forest, and thus has obvious affected the mass loss, nutrient release and litter quality during the onset of soil freezing, soil deep and thawing period and the growing season. In addition, these affects were sensitive response to climate, altitude and gap size. |
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