Forest Gaps Inhibit Foliar Litter Pb and Cd Release in Winter and Inhibit Pb and Cd Accumulation in Growing Season in an Alpine Forest

Aims

The release of heavy metals (such as Pb and Cd) from foliar litter play an important role in element cycling in alpine forest ecosystems. Although natural forest gaps could play important roles in the release of heavy metals from foliar litter by affecting the snow cover during the winter and solar irradiation during the growing season, few studies have examined these potential roles. The objectives of this study were to document changes in Pb and Cd dynamics during litter decomposition in the center of gaps and under closed canopies and to investigate the factors that controlled these changes during the winter and growing seasons.

Methods

Senesced foliar litter from six dominant species, including Kangding willow (Salix paraplesia), Masters larch (Larix mastersiana), Mingjiang fir (Abies faxoniana), Alpine azalea (Rhododendron lapponicum), Red birch (Betula albosinensis) and Mourning cypress (Sabina saltuaria), was placed in litterbags and incubated between the gap center and closed canopy conditions in an alpine forest in the eastern region of the Tibetan Plateau. The litterbags were sampled at the snow formation stage, snow coverage stage, snow melt stage and during the growing season. The Pb and Cd concentrations in the sampled foliar litter were determined by acid digestion (HNO₃/HClO₄).

Important findings

Over one year of decomposition, Pb accumulation and Cd release from the foliar litter occurred, regardless of the foliar litter species. However, Pb and Cd were both released from the foliar litter during the winter and accumulated during the growing season. Compared with the gap center and the canopy gap edge, the extended gap edge and the closed canopy showed higher Pb and Cd release rates in winter and higher Pb and Cd accumulation rates during the growing season, respectively. Statistical analyses indicate that the dynamics of Pb were significantly influenced by frequent freeze–thaw cycles in winter and appropriate hydrothermal conditions during the growing season, the dynamics of Cd were strongly influenced by species and the presence of a forest gap at different decomposition stages. These results show that forest gaps could inhibit Pb and Cd release from foliar litter in the alpine forest of western Sichuan. In addition, a decrease in the snow depth in the winter warming scenario would promote the release of Pb during foliar litter decomposition. There exist some difference that may be influenced by litter quality, microenvironment and microtopography during litter decomposition.     

不同厚度雪被对高山森林6种凋落物分解过程中酸溶性和酸不溶性组分的影响

高山森林冬季不同厚度雪被格局可能通过影响凋落物的分解过程中酸溶性和酸不溶性组分特征,改变凋落物分解过程,但缺乏必要关注。采用凋落物分解袋法,研究了高山森林林窗中央至林下形成的天然雪被厚度梯度(厚型雪被、中型雪被、薄型雪被和无雪被)覆盖下,6种典型物种岷江冷杉(Abies faxoniana)、红桦(Betula albo-sinensis)、四川红杉(Larix mastersiana)、方枝柏(Sabina saltuaria)、康定柳(Salix paraplesia)和高山杜鹃(Rhododendron lapponicum)凋落物在不同关键时期(雪被形成期、雪被覆盖期和雪被融化期)的酸溶性组分和酸不溶性组分变化特征。经历一个冬季的分解后,6种凋落物酸溶性组分绝对含量呈降低趋势,除红桦外5种凋落物酸不溶性组分绝对含量呈增加趋势。不同厚度雪被显著影响雪被覆盖期和融化期凋落物酸不溶性和酸溶性组分绝对变化量;其中方枝柏、红桦和康定柳凋落物酸不溶性组分增加量在厚型雪被下显著高于其它雪被覆盖;而相对于阔叶凋落物酸溶性组分变化量在薄型雪被和无雪被梯度达到最大值,针叶凋落物酸溶性组分在厚型雪被下具有最大的变化量。一个冬季分解结束后,表征6种凋落物酸溶性和酸不溶性组分含量相对比例的LCI指数(Lignocellulose index)总体升高,雪被对LCI指数的影响主要表现在雪被覆盖期和融化期,且方枝柏、岷江冷杉和康定柳凋落物LCI在冬季分解后均在厚型雪被达到最高值。同时统计分析结果表明,物种极显著影响冬季不同阶段凋落物酸溶性和酸不溶性组分的变化。这些结果意味着气候变暖情景下,高山森林冬季雪被和冻融格局的改变将显著影响凋落物分解过程中酸溶性、酸不溶性组分以及LCI指数代表的抵抗性组分结构的变化,且影响趋势受到凋落物质量的调控。     

Forest Gaps Alter the Total Phenol Dynamics in Decomposing Litter in an Alpine Fir Forest

The total phenol content in decomposing litter not only acts as a crucial litter quality indicator, but is also closely related to litter humification due to its tight absorption to clay particles. However, limited attention has been focused on the total phenol dynamics in foliar litter in relation to forest gaps. Here, the foliar litter of six representative tree species was incubated on the forest floor from the gap center to the closed canopy of an alpine Minjiang fir (Abies faxoniana) forest in the upper reaches of the Yangtze River and eastern Tibetan Plateau. The dynamics of total phenol concentration in the incubated litter was measured from November 2012 to October 2014. Over two-year incubation, 78.22% to 94.06% of total phenols were lost from the foliar litter, but 52.08% to 86.41% of this occurred in the first year. Forest gaps accelerated the loss of total phenols in the foliar litter in the winter, although they inhibited the loss of total phenols during the growing season in the first year. In comparison with the effects of forest gaps, the variations of litter quality among different species were much stronger on the dynamics of total phenols in the second year. Overall, the loss of total phenols in the foliar litter was slightly higher in both the canopy gap and the expanded gap than in the gap center and under the closed canopy. The results suggest that the predicted decline in snow cover resulting from winter warming or vanishing gaps caused by forest regeneration will retard the loss of total phenol content in the foliar litter of alpine forest ecosystems, especially in the first decomposition year.     

Effects of forest gap on hemicellulose dynamics during foliar litter decomposition in an subalpine forest

 亚高山森林林窗可能通过改变冬季雪被格局和生长季水热环境影响林窗内凋落物中半纤维素的分解动态, 但目前对此还缺乏研究。采用凋落物分解袋法, 以亚高山森林5种典型物种岷江冷杉(Abies faxoniana)、红桦(Betula albosinensis)、四川红杉(Larix mastersiana)、方枝柏(Sabina saltuaria)和高山杜鹃(Rhododendron lapponicum)凋落物为研究对象, 研究雪被形成期、雪被覆盖期、雪被融化期和生长季节从林窗中心、林冠林窗、扩展林窗到郁闭林下物种凋落物的半纤维素变化特征。经历一年分解后, 5种凋落物的半纤维素均呈现净累积现象。针、阔叶凋落物半纤维素分别在雪被覆盖期和融化期表现出相对较高的损失率。在雪被覆盖期和融化期, 凋落物半纤维素在林窗中心和林冠林窗具有相对较高的损失率; 而在生长季节, 林窗中心呈现相对较低的凋落物半纤维素累积率。统计分析结果表明凋落物分解过程中半纤维素损失率与环境因子和凋落物质量因子均显著相关。这些结果表明亚高山森林林窗对凋落物分解过程中半纤维素损失率具有显著影响, 分别促进了半纤维素在冬季的损失以及抑制了半纤维素在生长季节的累积, 意味着亚高山森林林窗的形成有利于凋落物半纤维素的降解。     

亚高山森林林窗大小对凋落叶木质素降解的影响

木质素降解是认识高寒森林凋落物分解过程的关键环节,可能受到林窗大小及其在不同季节水热环境的影响。采用分解袋法,研究了川西亚高山森林不同面积大小林窗下红桦(Betula albo-sinensis)和岷江冷杉(Abies faxoniana)凋落叶在初冻期、深冻期、融化期、生长季节初期、生长季节中期和生长季节后期的木质素分解动态特征。研究结果表明,采样时间和林窗面积大小对两种凋落叶的木质素降解均有显著影响。经历1a分解,红桦凋落叶的木质素降解了21.53%—27.65%,而岷江冷杉凋落叶的木质素富集了7.95%—19.40%。较大林窗促进了冬季岷江冷杉凋落叶和生长季节红桦凋落叶木质素的降解,抑制了冬季红桦凋落叶木质素的降解;而生长季节岷江冷杉凋落叶木质素富集速率则为林下大林窗中林窗小林窗。逐步回归分析表明,凋落叶木质素的降解过程在冬季主要受到负积温和土壤冻融循环次数的影响(木质素结构的物理破碎),而在生长季节则主要受到平均温度和正积温的影响(木质素的生物降解)。可见,川西亚高山森林木质素降解受林窗格局变化的显著影响,且林窗大小对凋落叶木质素降解的影响与物种和分解时期有关。     

雪被斑块对高山森林两种灌木凋落叶质量损失的影响

高山/亚高山森林灌木层植物凋落物的分解对于系统物质循环等过程具有重要意义, 并可能受到冬季不同厚度雪被斑块下冻融格局的影响。该文采用凋落物分解袋法, 研究了高山森林典型灌层植物华西箭竹(Fargesia nitida)和康定柳(Salix paraplesia)凋落物在沿林窗-林下形成的冬季雪被厚度梯度(厚型雪被斑块、较厚型雪被斑块、中型雪被斑块、薄型雪被斑块、无雪被斑块)上在第一年不同关键时期(冻结初期、冻结期、融化期、生长季节初期和生长季节后期)的质量损失特征。在整个冻融季节, 华西箭竹和康定柳凋落叶的平均质量损失分别占全年的(48.78 ± 2.35)%和(46.60 ± 5.02)%。冻融季节雪被覆盖斑块下凋落叶的失重率表现出厚型雪被斑块大于薄型雪被斑块的趋势,而生长季节无雪被斑块的失重率明显较高。尽管如此, 华西箭竹凋落物第一年分解表现出随冬季雪被厚度增加而增加的趋势, 但康定柳凋落物第一年失重率以薄型雪被斑块最高, 而无雪被斑块最低。同时, 相关分析表明冻融季节凋落叶的失重率与平均温度和负积温呈极显著正相关, 生长季节凋落叶的失重率与所调查的温度因子并无显著相关关系, 但全年凋落物失重率与平均温度和正/负积温均显著相关。这些结果清晰地表明, 未来冬季变暖情境下高山森林冬季雪被格局的改变将显著影响灌层植物凋落物分解, 影响趋势随着物种的差异具有明显差异。     

雪被覆盖对高山森林凋落物分解过程中水溶性和有机溶性组分含量的影响

季节性雪被可能通过冻结、淋溶以及冻融循环等对高山森林凋落物水溶性和有机溶性组分含量产生影响.本文采用凋落物分解袋法,以川西高山森林典型乔木（四川红杉、岷江冷杉、红桦、方枝柏）和灌木（高山杜鹃、康定柳）凋落物为研究对象,研究了雪被覆盖不同时期（雪被形成期、雪被覆盖期和雪被融化期）和雪被厚度（厚型雪被、中型雪被、薄型雪被和无雪被）下凋落物水溶性和有机溶性组分含量的动态变化特征.结果表明： 在一个冬季的分解过程中,6种凋落物水溶性组分含量在雪被形成期和融化期降低而雪被覆盖期增加,但除高山杜鹃凋落物有机溶性组分含量在雪被覆盖期增加外,其他5种凋落物有机溶性组分含量在整个冬季呈降低趋势.相对于凋落物有机溶性组分含量,不同厚度雪被斑块对凋落物水溶性组分含量变化的影响更大,且主要表现在雪被形成期和雪被覆盖期.相对于其他雪被斑块,薄型雪被斑块更加显著地促进了高山柳和高山杜鹃凋落物水溶性组分含量降低,但显著抑制了方枝柏凋落物水溶性组分含量降低,而其他凋落物水溶性组分含量变化在不同斑块间无显著差异.冬季高山森林雪被对凋落物水溶性和有机溶性组分含量的影响主要受控于凋落物质量.     

冬季雪被对青藏高原东缘高海拔森林凋落叶P元素释放的影响

雪被是影响高海拔森林凋落物分解的重要生态因子,其是否影响到生长季节与非生长季节凋落物中的P元素释放,尚未量化。为了量化季节性雪被对高海拔森林凋落物分解过程中P元素释放的影响,于2010年10月至2012年10月间,在青藏高原东缘川西高海拔森林不同厚度冬季雪被斑块下,设置凋落物分解袋实验。检测该地区代表性树种岷江冷杉(Abies faxoniana)、红桦(Betula albo-sinensis)、四川红杉(Larix mastersiana)和方枝柏(Sabina saltuaria)凋落叶在雪被覆盖不同关键时期(雪被形成前期、完全覆盖期和消融期)以及生长季节的P元素动态。结果表明,凋落物质量与雪被厚度均显著影响了P元素的释放过程。雪被覆盖时期凋落物P元素释放率表现为有雪被覆盖大于无雪被覆盖,而生长季节中除岷江冷杉外的其他3种凋落物P元素释放率均为无雪被覆盖下最大。相对于无雪被覆盖斑块,冬季雪被的存在提供了保护绝缘层,促进凋落物P元素释放,提高了各物种冬季P元素释放贡献率。这些结果表明,全球变化情景下的雪被减少可能减缓高海拔森林凋落物P元素的释放过程,改变森林土壤P元素水平。所以在研究高寒、高海拔地区全球气候变化下生态系统功能的工作中,应注重雪被这一异质性环境因子对生态系统功能的影响。     

高山森林林窗对凋落叶分解的影响

林窗对降水和光照等环境条件的再分配以及分解者群落的影响可能深刻作用于森林凋落物分解过程,但有关高山森林林窗大小对凋落物分解的影响尚无研究报道。采用凋落物分解袋法,研究了川西高山森林不同大小林窗对非生长季节和生长季节红桦(Betula albo-sinensis)和岷江冷杉(Abies faxoniana)凋落叶质量损失的影响。结果显示,经过1a的分解,不同生境下红桦和岷江冷杉凋落叶分别分解了27.25%—30.12%和27.04%—27.96%,其中非生长季节占53.83%—60.18%和50.23%—59.09%。林窗对红桦和岷江冷杉凋落叶质量损失的影响因物种不同而呈现季节差异。总体上,林窗加快了岷江冷杉凋落叶的分解而延缓了红桦凋落叶的分解。与郁闭林下相比,林窗显著增加了2种凋落叶非生长季节的质量损失速率,显著降低了生长季节2种凋落叶的质量损失速率;2种凋落叶质量损失速率在非生长季节随林窗面积增大而加快,在生长季节随林窗面积增大而减慢。林窗显著影响了初冻期、深冻期和融化期岷江冷杉凋落叶的质量损失率,但对红桦凋落叶质量损失率影响不显著。可见,高山森林凋落物分解过程受到林窗的显著影响,并且阔叶和针叶凋落叶在非生长季节和生长季节对林窗的响应具有明显差异。     

雪被斑块对川西高山森林凋落叶N和P释放的影响

采用凋落物网袋法,研究冬季不同关键时期雪被斑块对川西高山森林6种代表性树种凋落物分解过程中N和P释放的影响.结果表明: 整个雪被覆盖季节凋落物N表现为富集,P表现为释放,且雪被融化期P释放速率最大.厚型和中型雪被斑块下凋落物P释放速率大于薄型和无雪被斑块,而薄型和无雪被斑块下凋落物的N释放速率明显较高.6种凋落物N释放率和释放速率与日均温呈显著负相关;除岷江冷杉外,其他树种凋落物P释放率和释放速率与日均温表现为正相关.气候变暖情景下冬季雪被覆盖的减小将促进高山森林冬季凋落物分解过程中N释放,抑制P释放.     

高山林线交错带高山杜鹃的凋落物分解

凋落物分解是维持生态系统生产力、养分循环、土壤有机质形成的关键生态过程。高山林线交错带是陆地生态系统中对气候变化响应的敏感区域。季节变化和海拔梯度上的植被类型差异可能会影响该区域凋落物的分解,进而对高山生态系统的碳氮循环产生重要影响。采用凋落物分解袋的方法,研究了川西高山林线交错带优势种高山杜鹃(Rhododendron lapponicum)凋落叶在雪被期和生长季的分解特征。结果显示:(1)季节变化和植被类型对高山杜鹃凋落物的分解均具有显著影响(P0.05),凋落叶的质量损失主要发生在生长季且在高山林线最大,暗针叶林中雪被期的质量损失略高于生长季,但差异不显著;(2)林线交错带上高山杜鹃凋落叶分解缓慢,一年干物质失重率为9.62%,拟合分解系数k为0.145;(3)高山杜鹃凋落叶的质量变化主要体现在纤维素降解显著且集中在雪被期,木质素无明显降解,在高山林线上C/N、C/P、木质素/N变化幅度较小且C、N、P的释放表现得稳定而持续。结果表明,季节性雪被对林线交错带内高山杜鹃分解的影响不仅局限在雪被期内,雪被融化期间频繁的冻融作用和雪融水淋洗作用可能会促进高山杜鹃凋落物在生长季初期的分解。总的来看,在气候变暖的情景下,雪被的缩减、生长季的延长和高山杜鹃群落的扩张可能加速高山林线交错带高山杜鹃凋落物的分解。     

马尾松人工林林窗大小对两种凋落叶难降解物质含量的影响

林窗通过改变森林微环境及土壤环境而影响凋落物难降解物质的降解, 目前关于人工林林窗对凋落物分解过程中难降解物质影响的研究较少。该文采用凋落物分解袋法, 以马尾松(Pinus massoniana)人工林人工砍伐形成的7个不同面积的林窗(G1: 100 m²、G2: 225 m²、G3: 400 m²、G4: 625 m²、G5: 900 m²、G6: 1225 m²、G7: 1600 m²)为研究对象, 以林下为对照, 研究了林窗大小对两种乡土树种——樟(Cinnamomum camphora)和红椿(Toona ciliata)凋落叶分解过程中难降解物质(木质素、纤维素、总酚、缩合单宁)含量的影响。结果表明: 1)林窗大小对林窗中心红椿凋落叶缩合单宁、总酚、木质素的含量有显著影响, 对其纤维素含量和樟凋落叶中4种难降解物质含量均无显著影响。随着林窗面积的增大, 红椿凋落叶中除纤维素含量外的其余3种难降解物质含量, 中小型林窗(G1-G5, G1: 100 m², G2: 225 m², G3:400 m², G4: 625 m², G5: 900 m²)低于大型林窗(G6、G7, G6: 1225 m², G7: 1600 m²)。2)林窗不同位置, 只有红椿凋落叶中缩合单宁含量林窗中心显著低于边缘, 其余难降解物质含量和樟凋落叶中4种难降解物质含量均无显著差异。樟凋落叶的木质素含量在G3林窗显著低于林下; 红椿凋落叶除纤维素含量外的其余难降解物质含量, 中小型林窗从林窗中心到边缘均显著低于林下。3)随着分解时间的延长, 两种凋落叶都表现出缩合单宁、纤维素含量降低, 木质素含量升高, 总酚含量先升高后降低的变化趋势。研究结果表明: 中小型林窗(100-900 m²)较大面积林窗干扰更有利于凋落叶中难降解物质的降解, 而林窗内的环境异质性应该是凋落物中难分解物质分解动态的主要调控因子, 并且这种效应依赖于初始凋落物质量。     

Effects of forest gap size on litter recalcitrant components of two tree species in Pinus massoniana plantations

Aims The objective of the study was to evaluate the dynamics of recalcitrant components during foliar litter decomposition at different forest gap size in Pinus massoniana plantation in the low hilly land, Sichuan basin. Methods The experiment was set up by thinning P. massoniana plantation to establish seven different gap sizes (G1: 100 m², G2: 225 m², G3: 400 m², G4: 625 m², G5: 900 m², G6: 1225 m², G7: 1600 m²). The contents of four recalcitrant components (condensed tannins, total phenol, lignin, cellulose) in foliar litter of two native species (Cinnamomum camphora and Toona ciliata) placed in litterbags at different locations in the forest gaps were evaluated. The litterbags placed under closed canopy were used as the control. Litterbags with air-dried leaves of C. camphora and T. ciliata were placed at center, edge of the gap and under the closed canopy in November 2013, and collected in December 2013, February 2014, May 2014 and August of 2014 for lab analysis. Important findings The results showed that: 1) Forest gap size had significant effect on the content of condensed tannins, total phenol and lignin for T. ciliata in gap center. However, the forest gap size had no significant effect on the content of recalcitrant components in the litters of C. camphora and cellulose content of T. ciliata. With the increase of gap size, except for cellulose content, the other three recalcitrant components content in small and medium sized gaps (G1-G5) were significant lower than in large gaps (G6, G7). 2) The condensed tannis content of T. ciliata at the gap center were significant lower than at the gap edge. The lignin contents at gap center of G3 was significant reduced in the C. camphora litter. The condensed tannins, total phenol, and lignin contents of T. ciliata litter in small and medium gaps significantly decreased. 3) The contents of the four recalcitrant components in both species’ foliar litter changed with time. The contents of condensed tannins and cellulose decreased and the content of lignin increased significantly with time, however, the total phenol content increased initially, and then decreased. Therefore, small and medium sized gaps (100-900 m²) could be the optimal gap sizes to promote the degradation of litter recalcitrant components for two native species in P. massoniana plantations.     

季节性冻融期间土壤动物对高山草甸两种凋落叶失重的贡献

季节性冻融期间高山草甸凋落叶的分解可为生长季节植物生长提供必要的养分,对于维持生态系统物质循环和养分平衡具有重要作用。然而,土壤动物对凋落叶分解是否具有明显的贡献仍然缺乏一致认识。因此,以高山草甸代表性植物黄花亚菊(Ajania nubigena)和黑褐苔草(Carex atrofusca)凋落叶为研究对象,采用不同孔径凋落叶袋排除土壤动物的方法,研究冬季不同冻融时期(冻结前期、冻结期和融化期)土壤动物对凋落叶失重的贡献。整个季节性冻融期间土壤动物对黄花亚菊和黑褐苔草两种凋落叶失重率的作用分别为12.07%和4.03%,总贡献率分别为46.39%和24.14%。土壤动物对两种凋落叶失重率的作用均在融化期最大,而土壤动物对黄花亚菊凋落叶失重率的作用在冻结初期最小,土壤动物对黑褐苔草凋落叶失重率的作用在冻结期最小。整个季节性冻融期,土壤动物对凋落叶失重率的作用和贡献率与正积温和凋落叶初始C、N浓度和C/N比均呈显著的正相关关系。因此,季节性冻融期间土壤动物对高山草甸凋落叶分解具有明显的贡献,但这些过程受冻融格局和凋落叶初始质量的调控。     

雪被期川西高山林线交错带两种地被物凋落物分解与土壤动物多样性

以针叶林代表性地被植物锦丝藓和高山冷蕨为研究对象,采用凋落物分解网袋法,研究了高山林线交错带(暗针叶林-林线-高山草甸)的锦丝藓植物残体、高山冷蕨凋落叶及混合凋落物经过一个雪被期分解后的质量损失与土壤动物群落结构特征.结果表明: 雪被期林线交错带上两种地被物的质量损失率在高山草甸最大,锦丝藓表现更为显著,两种凋落物混合促进了分解过程且在林线上表现尤为显著.在交错带凋落物中共获取土壤动物968头,隶属于5纲10目35科,优势类群以弹尾目和蜱螨目为主.在林线上凋落物中获得的土壤动物个体数和类群数高于高山草甸和暗针叶林.典范对应分析(CCA)表明,土壤动物类群与雪被期平均温度关系最为密切,特有物种如等翅目和地蜈蚣目仅在暗针叶林出现,半翅目和啮目仅在高山草甸出现.地被物种类对土壤动物多样性的影响在暗针叶林和高山草甸大于林线.多元回归分析表明,日平均温度和雪被厚度能够解释凋落物质量损失率变异的30.8%,而土壤动物因子能解释质量损失率变异的8.3%,它们共同解释质量损失率变异的34.1%.雪被是影响高山两种地被物凋落物分解的最关键因子.     

Loss of total phenols from leaf litter of two shrub species: dual responses to alpine forest gap disturbance during winter and the growing season

高山森林林窗能够通过调控水热动态进而控制林下生态系统过程。因此,本研究旨在通过两种典型灌木[华西箭竹(Fargeisia nitida (Mitford) Keng f.)和康定柳(Salix paraplesia Schneid.)]凋落叶分解,探索林窗干扰对分解过程中总酚释放的影响。我们在一个典型的岷江冷杉林(Abies faxoniana Rehd.)样地,通过设置不同林窗位置(林窗中心南侧、林窗中心北侧、林缘、扩展林缘和林下)进行了凋落物分解实验;并在两年的不同关键时期：雪被形成期、雪被覆盖期、雪被融化期、生长季节初期和生长季节后期,对两种灌木凋落叶分解袋进行采样,并测试其总酚释放率和微生物丰富度(真菌和细菌)。实验结束后,我们发现,雪被厚度、冻融循环以及真菌细菌比对凋落叶的总酚释放具有显著影响。在两年的雪被形成期、雪被覆盖期、雪被融化期和生长季节后期,真菌和细菌丰富度表现出从林窗中心到林下逐渐降低的趋势,而在两年的生长季节初期,其表现出从林窗中心到林下逐渐增加的趋势。而从不同林窗位置来比较,两种灌木凋落叶的总酚释放率仅在第一年各关键时期表现出同真菌细菌丰富度类似的规律。此外,在第一年的冬季,以及从整个第一年和整个两年分解时间段来看,林窗中心位置两种凋落叶的总酚释放率皆较其他林窗位置高。这些研究结果表明,高山森林林窗的形成能够促进凋落叶总酚的释放,尽管其释放对林窗干扰的响应在不同关键时间具有不同的表现。总之,在高寒生物区,林窗的更替消失以及在气候变化背景下冬季变暖导致的雪被覆盖厚度和持续时间的减少会降低凋落叶总酚的释放率。     

季节性雪被对高山森林凋落物分解的影响

季节性雪被可能对高山森林凋落物分解产生重要影响, 但一直没有深入的研究。该文采用凋落物分解袋法, 于2010-2012年雪被覆盖下几个关键时期(冻结初期、深冻期和融化期)以及生长季节, 研究了川西高山森林代表性树种岷江冷杉(Abies faxoniana)、红桦(Betula albosinensis)、四川红杉(Larix mastersiana)和方枝柏(Sabina saltuaria)凋落叶在不同厚度冬季雪被下的分解动态。经过两年的分解, 不同雪被覆盖下岷江冷杉凋落物分解率为33.98%-39.55%, 红桦为46.49%-48.22%, 四川红杉为42.30%-44.93%, 方枝柏为40.34%-43.84%。相对于无雪被覆盖环境, 厚型雪被覆盖均小幅提高了4种凋落物两年的失重率(1.57%-5.57%)。3个针叶树种(岷江冷杉、四川红杉和方枝柏) Olson凋落物分解系数k均以厚型雪被覆盖最大, 薄型雪被覆盖最小, 而阔叶树种红桦分解系数k则表现为无雪被>薄型雪被>较厚型雪被>厚型雪被>中型雪被。尽管在第二年生长季中雪被对红桦凋落物分解的促进作用不明显, 但雪被覆盖明显促进了两年各个关键时期岷江冷杉、四川红杉和方枝柏凋落物的分解。第一年雪被期凋落物分解对当年分解总量的贡献达42.5%-65.5%, 季节性雪被变化明显改变了凋落物冬季分解格局, 对深冻期凋落物分解过程影响尤为显著。综上所述, 当前气候变化情景下冬季雪被的减少可能减缓该区森林凋落物分解过程, 但相对于易分解的阔叶凋落物, 针叶凋落物的响应特征可能更为强烈。     

马尾松人工林林窗边缘效应对樟和红椿凋落叶难降解物质分解的影响

采用凋落物分解袋, 以四川低山丘陵区马尾松人工林人工砍伐形成的7个不同面积的林窗边缘(100、225、400、625、900、1225、1600 m²)为研究对象, 以林下为对照, 研究了2种乡土树种——樟和红椿凋落叶难降解物质(木质素、纤维素、总酚、缩合单宁)在不同大小林窗边缘的降解动态特征.结果表明:马尾松人工林林下与不同大小林窗边缘相比较,红椿凋落叶中除纤维素外,其余难降解物质的降解率以及樟凋落叶木质素降解率均显著高于林下.在全年分解过程中, 2种凋落叶4种难降解物质的降解率总体均呈现持续上升的趋势.其中,缩合单宁降解最快,其次是总酚和纤维素,而木质素降解最慢.随林窗面积的增大, 红椿凋落叶除纤维素外,其余难降解物质在中型林窗边缘(400、625 m²)具有相对较高的降解率,而樟凋落叶的木质素在625 m²林窗边缘时也表现出较高的降解率.在凋落叶分解过程中,难降解物质降解率与凋落叶袋内温度和凋落物质量均呈显著相关.中型林窗(400～625 m²)对凋落物分解过程中难降解物质的降解具有更显著的边缘效应, 而这种边缘效应与物种有一定关联.     

亚高山森林林窗对凋落物分解过程中半纤维素动态的影响

亚高山森林林窗可能通过改变冬季雪被格局和生长季水热环境影响林窗内凋落物中半纤维素的分解动态, 但目前对此还缺乏研究。采用凋落物分解袋法, 以亚高山森林5种典型物种岷江冷杉(Abies faxoniana)、红桦(Betula albosinensis)、四川红杉(Larix mastersiana)、方枝柏(Sabina saltuaria)和高山杜鹃(Rhododendron lapponicum)凋落物为研究对象, 研究雪被形成期、雪被覆盖期、雪被融化期和生长季节从林窗中心、林冠林窗、扩展林窗到郁闭林下物种凋落物的半纤维素变化特征。经历一年分解后, 5种凋落物的半纤维素均呈现净累积现象。针、阔叶凋落物半纤维素分别在雪被覆盖期和融化期表现出相对较高的损失率。在雪被覆盖期和融化期, 凋落物半纤维素在林窗中心和林冠林窗具有相对较高的损失率; 而在生长季节, 林窗中心呈现相对较低的凋落物半纤维素累积率。统计分析结果表明凋落物分解过程中半纤维素损失率与环境因子和凋落物质量因子均显著相关。这些结果表明亚高山森林林窗对凋落物分解过程中半纤维素损失率具有显著影响, 分别促进了半纤维素在冬季的损失以及抑制了半纤维素在生长季节的累积, 意味着亚高山森林林窗的形成有利于凋落物半纤维素的降解。     

Initial Stages of Tundra Shrub Litter Decomposition May Be Accelerated by Deeper Winter Snow But Slowed Down by Spring Warming

The Arctic climate is projected to change during the coming century, with expected higher air temperatures and increased winter snowfall. These climatic changes might alter litter decomposition rates, which in turn could affect carbon (C) and nitrogen (N) cycling rates in tundra ecosystems. However, little is known of seasonal climate change effects on plant litter decomposition rates and N dynamics, hampering predictions of future arctic vegetation composition and the tundra C balance. We tested the effects of snow addition (snow fences), warming (open top chambers), and shrub removal (clipping), using a full-factorial experiment, on mass loss and N dynamics of two shrub tissue types with contrasting quality: deciduous shrub leaf litter (Salix glauca) and evergreen shrub shoots (Cassiope tetragona). We performed a 10.5-month decomposition experiment in a low-arctic shrub tundra heath in West-Greenland. Field incubations started in late fall, with harvests made after 249, 273, and 319 days of field incubation during early spring, summer and fall of the next year, respectively. We observed a positive effect of deeper snow on winter mass loss which is considered a result of observed higher soil winter temperatures and corresponding increased winter microbial litter decomposition in deep-snow plots. In contrast, warming reduced litter mass loss during spring, possibly because the dry spring conditions might have dried out the litter layer and thereby limited microbial litter decomposition. Shrub removal had a small positive effect on litter mass loss for C. tetragona during summer, but not for S. glauca. Nitrogen dynamics in decomposing leaves and shoots were not affected by the treatments but did show differences in temporal patterns between tissue types: there was a net immobilization of N by C. tetragona shoots after the winter incubation, while S. glauca leaf N-pools were unaltered over time. Our results support the widely hypothesized positive linkage between winter snow depth and litter decomposition rates in tundra ecosystems, but our results do not reveal changes in N dynamics during initial decomposition stages. Our study also shows contrasting impacts of spring warming and snow addition on shrub decomposition rates that might have important consequences for plant community composition and vegetation-climate feedbacks in rapidly changing tundra ecosystems.