高山森林林窗对凋落叶分解的影响

吴庆贵, 吴福忠,谭　波,杨万勤,*,何　伟,倪祥银

1 四川农业大学, 生态林业研究所,林业生态工程省级重点实验室, 成都　611130 2 绵阳师范学院, 生态安全与保护四川省重点实验室, 绵阳　621000



高山森林林窗对凋落叶分解的影响

吴庆贵1,2, 吴福忠1,谭波1,杨万勤1,*,何伟1,倪祥银1

1 四川农业大学, 生态林业研究所,林业生态工程省级重点实验室, 成都611130 2 绵阳师范学院, 生态安全与保护四川省重点实验室, 绵阳621000

摘要: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。

关键词：林窗; 凋落物分解; 质量损失; 非生长季节; 针叶林; 高寒地区

林窗是川西高山森林生态系统普遍存在的干扰形式[1]。与郁闭林下相比，林窗改变了林内水热条件(光照、温度、降水)和分解者群落结构[2]，可能对凋落物分解过程具有强烈影响。在高寒森林生态系统，不同大小的林窗内冬季雪被厚度和持续时间的变异可能深刻作用于凋落物分解[3- 4]。一方面，林窗内雪被的绝热保温作用能使土壤微生物维持相对较高的活性[5]；另一方面，郁闭林冠下由于缺乏雪被覆盖，微生物活性受低温和冻结作用抑制[6]。因而，林窗可能促进冬季凋落叶分解，并且不同大小林窗对雪被厚度和冻融循环格局的改变也可能不同程度地作用于凋落叶分解[7- 8]。与此相反，尽管生长季节适宜的温湿度条件更有利于凋落物分解，但林窗内相对更高的光照强度和光辐射效率[9]可导致凋落物地表温度升高[10]，水分蒸发增加[11]，进而影响土壤微生物活性和土壤动物活动能力[12]。因此，相比于郁闭林下，林窗可能抑制生长季节凋落叶分解。这意味着，林窗对高寒森林生态系统不同季节凋落物的分解具有截然不同的影响，但迄今的研究一直缺乏相应的关注，这极大地限制了对高寒森林地下生态过程的认识。

川西高山森林在调节区域气候、涵养水源、保育生物多样性等方面具有突出的战略地位。前期研究表明，林窗是川西高山森林群落最主要的自然更新方式[13]，影响凋落物分解的生物和非生物因素与林窗密切相关[14]。因此，可以假设，林窗能显著影响川西高山森林凋落叶分解过程，在冬季促进凋落叶分解，在生长季节制约凋落叶分解；且阔叶和针叶凋落叶对林窗的响应不同。为验证上述假设，本研究采用凋落物分解袋法，以区域内代表性针叶树种岷江冷杉(Abiesfaxoniana)和阔叶树种红桦(Betulaalbo-sinensis)凋落叶为研究对象，研究林窗面积大小对针、阔树种凋落叶在非生长季节和生长季节分解的影响，以期为深入认识高寒森林生态系统物质循环过程提供科学依据。

1材料与方法

1.1研究区域与样地概况

研究区域位于四川省理县毕棚沟自然保护区(31°14′—31°19′ N，102°53′—102°57′ E， 2458—4619 m a.s.l.)，地处青藏高原东缘与四川盆地的过渡带。年平均气温2—4℃，最高气温23℃ (7月)，最低温度为-18℃ (1月)，年降水量约850 mm。该区域冬季具有明显的季节性雪被覆盖和冻融过程，雪被厚度达50 cm，冻融时间长达约120 d[3]。区域内典型植被为岷江冷杉(A.faxoniana) 、红桦(B.albo-sinensis)和川西云杉(Piceabalfouriana)。土壤为发育于坡积物上的雏形土，基本理化性质见杨玉莲等[15]。

表1　样地内不同林窗的面积、形成木、边界木和形成方式

图1　不同林窗下的日平均温度　Fig. 1　Daily means temperature in sampled gaps and the closed canopyLG: 大林窗large gap;IG: 中林窗intermediate gap; SG: 小林窗small gap; CC: 郁闭林下the closed canopy; BGS: 非生长季节beyond growing season; GS: 生长期季节growing season

基于前期研究，在海拔3598m 原始岷江冷杉林内设置1 hm2样地(坡向NE 38°，坡度24°)。乔木层岷江冷杉占据绝对优势，其次是红桦，伴生少量野樱桃(P.tatsienensis)；灌木主要有高山杜鹃(Rhododendrondelavayi)、三颗针(Berberissargentiana)和绢毛蔷薇(Rosasericea)等；草本以铁线莲(Clematis)、苔草(Carex)和羊茅(Festuca)等为主，林窗内草本盖度约85%。按照林窗面积大小在样地内分别选取大林窗(large gap, LG)、中林窗(intermediate gap, IG)和小林窗(small gap, SG)各3个。同时，在岷江冷杉郁闭林下(the closed canopy, CC)设置3个10 m×10 m的样方作为对照。然后调查林窗的基本性质 (表1)，并进行整理和清除工作。

1.2凋落物分解试验

2011年秋季搜集岷江冷杉和红桦新鲜凋落叶，室温下自然风干，称取10 g风干凋落叶装入分解袋(大小为20 cm×20 cm，贴地面孔径0.50 mm，上表面孔径1 mm)，于2011年11月埋设于样地，分解试验为期1a，共埋设分解袋1440袋=每次取样 (10) × 林窗 (4) × 3 (重复) × 2 (物种) × 6次。同时，采用纽扣式温度记录器(iButton DS1923-F5, Maxim/Dallas Semiconductor, Sunnyvale, USA)记录空气和各林窗下凋落袋内温度，设定每2 h记录1次温度数据。此外，采用钢尺测量各海拔林窗及林下雪被厚度。各林窗温度和雪被变化见图1、图2和表2。

根据前期监测，按高山植物的物候期分非生长季节(冬季)和生长季节进行样品采集，具体为：非生长季节的初冻期(12月27日，onset of soil freezing period, OF)、深冻期(3月7日，soil deep freezing period, DF)、融化期(4月27日，soil thawing period, TP)；生长季节的初期(6月16日，early growth period, EP)、中期(8月25日mid-growing period, MP)和后期(10月29日，later growth period, LP)。每次采样时随机从每个样方捡回10袋凋落叶，去除杂物后，于65℃烘干至恒重并称量，计算凋落叶失重率。2种凋落叶的初始化学特征见表3。

图2　不同大小林窗和郁闭林下的雪被厚度Fig. 2　Snow depths in forest gaps and the closed canopy

林窗类型Typesofgap初冻期Onsetofsoilfreezingperiod深冻期Soildeepfreezingperiod融化期Soilthawingperiod生长初期Earlygrowthperiod生长中期Mid-growingperiod生长后期Latergrowthperiod大林窗Largegap-0.56±0.16a-1.39±0.19a2.98±0.41a9.78±1.04a12.51±0.41a7.94±0.74a中林窗Intermediategap-1.60±0.13b-2.12±0.29b2.16±0.36b9.40±1.08a12.34±0.54ab5.68±0.51b小林窗Smallgap-1.98±0.15c-2.13±0.33b1.95±0.34b6.16±0.96b11.63±0.44b5.66±0.63b郁闭林下Closedcanopy-2.40±0.12d-3.14±0.45c0.59±0.14c6.04±0.53b10.13±0.55c4.37±0.55c

不同小写字母表示差异显著(独立样本t检验,P< 0.05)

表3　红桦和岷江冷杉凋落叶初始质量特征(平均值±标准偏差, n=5)

1.3数据处理与统计分析

半分解时间(T50%)、95%分解时间(T95%)、质量损失率和质量损失速率[16- 17]按如下公式计算：

50%分解时间=-ln(1-0.50)/(k)

95%分解时间=-ln(1-0.95)/(k)

质量损失率Lt(%)=100× (Mt-M0)/ M0

各关键时期每30d质量损失速率Vt(%/d)= 30 ×(Mt-1-Mt)/ M0×100/DΔt

各关键时期质量损失占全年比率Rt(%) = 100× (Mt-1-Mt) /(M0-M6)

式中，Mt为t时刻凋落叶干质量(g)，M0为初始干重(g)，(Mt-1-Mt)为相邻采样时间凋落叶残留量差(t=1，2，3,…,6)，DΔt为相邻采样时间间隔天数。

用单因素方差分析(one-way ANOVA)检验凋落叶日均温、质量损失率、损失速率在不同林窗间的差异显著性；用Pearson相关分析检验日均温、冻融循环次数与质量损失速率之间的相关关系。显著性水平设为P= 0.05，数值以平均值±标准误(mean ± SE)表示。数据分析采用SPSS 20.0 (IBM SPSS Statistics Inc., Chicago, IL, USA)软件包进行。

2结果与分析

2.1质量损失率

分解1a后，红桦和的岷江冷杉凋落叶质量损失率分别为27.25—30.12%和27.04—27.96% (图3)。2种凋落叶半分解和95%分解时间分别为2.015—2.413 a、2.110—2.557 a和8.708—10.428 a、9.121—11.053 a，红桦凋落叶分解周期随林窗面积增加而增加，岷江冷杉凋落叶则相反(表4)。与郁闭林下相比，林窗形成显著(P<0.05)影响非生长季节(初冻期、深冻期和融化期)岷江冷杉凋落叶的质量损失率，且林窗面积越大其质量损失率越高；而对红桦凋落叶质量损失率无显著影响(P>0.05)。

图3　不同大小林窗和郁闭林下的凋落叶质量损失Fig. 3　Mass losses of foliar litter in forest gaps and the closed canopy

物种Species林窗类型Typesofgap回归方程Regressionmodels分解系数kDecompositionconstantk相关系数R2CorrelationcoefficientR2半分解时间/aTimeofhalfdecomposition95%分解时间/aTimeof95%decomposition红桦大林窗y=96.476e-0.287t0.2870.9872.41310.428Betulaalbo-sinensis中林窗y=97.412e-0.306t0.3060.9942.2669.794小林窗y=96.927e-0.321t0.3210.9912.1579.324郁闭林下y=97.503e-0.344t0.3440.9952.0158.708岷江冷杉大林窗y=97.494e-0.328t0.3280.9922.1109.121Abiesfaxoniana中林窗y=97.684e-0.307t0.3070.9932.2569.751小林窗y=97.182e-0.283t0.2830.9862.44910.585郁闭林下y=97.286e-0.271t0.2710.9802.55711.053

2.2质量损失速率

2种凋落叶分解速率总体上呈现逐渐减慢再增加的趋势(图4)。林窗形成显著(P<0.05)影响了深冻期、融化期和生长后期红桦凋落叶的质量损失速率，显著增加了融化期质量损失速率，且随林窗面积增大而增加。对岷江冷杉而言，林窗形成显著(P<0.05)影响了初冻期、融化期和生长初期凋落叶的质量损失速率。林窗形成后显著(P<0.05)增加了初冻期、深冻期和融化期岷江冷杉凋落叶的质量损失速率，且质量损失速率随林窗面积增大而增加；显著降低了减小降低了生长季节后期岷江冷杉凋落叶的质量损失速率，且质量损失速率随林窗面积增大而降低。

图4　不同大小林窗和郁闭林下的凋落叶分解速率Fig. 4　Mass loss rates of foliar litter in forest gaps and the closed canopy

由图5可得，林窗显著(P<0.05)影响了2种凋落叶第1年的质量损失速率。林窗形成后显著降低了2种凋落叶第1年生长季节质量损失速率，且林窗面积越大质量损失速率越低；显著增加了红桦凋落叶非生长季节和岷江冷杉凋落叶非生长季节以及第1年的质量损失速率，且林窗面积越大质量损失速率越高。总体上，2种凋落叶质量损失速率在第1年非生长季节显著(P<0.05)高于生长季节。

图5　冬季和生长季节凋落叶分解速率Fig. 5　Mass loss rates of foliar litters in forest gaps and the closed canopy in winter and the growing season

2.3各关键时期的质量损失比率

由图6可得，2种凋落叶各关键时期质量损失所占比例为第1年深冻期>生长中期>初冻期>生长初期>生长后期>融化期。4种不同面积林窗内(从大林窗、中林窗、小林窗到林下)红桦和岷江冷杉凋落叶第1年非生长季节质量损失分别占第1年的60.18%、54.46%、55.34%、53.83%和59.09%、54.37%、52.22%、50.23%，非生长季节显著(P<0.05)高于生长季节。

图6　各关键时期质量损失在全年分解过程中所占的比率Fig. 6　Ratios of mass losses at each critical stage compared to the entire decomposition year

3讨论

与假设基本一致，川西高山森林林窗对凋落叶分解有显著影响：促进了冬季凋落叶分解，抑制生长季节的凋落叶分解，尤其是对岷江冷杉凋落叶分解过程作用更为明显。一方面，林窗面积越大，冬季形成雪被的厚度也越厚，林窗内雪被覆盖的时间也越长(图2)，雪被的保温作用能持续促进凋落叶分解。并且，大林窗内的凋落叶在雪被融化期可接收更多雪融水和降水的淋洗作用，促进凋落叶内可溶解性组分的流失。另一方面，在生长季节，由于缺乏林冠对光照的过滤和拦截作用，林窗内的太阳辐射显著高于郁闭林下，且这种作用随林窗面积增加和增强[18]。这促进了林窗内凋落叶水分的蒸发，不利于土壤无脊椎动物和微生物对凋落物分解的贡献，进而抑制凋落叶分解。这表明林窗形成对微环境的改变能深刻影响高寒森林地下生态系统的物质循环。

在高寒森林生态系统中，尽管冬季低温不利于土壤动物[19]和微生物[17]对凋落叶的降解，但季节性雪被[3]和冻融循环[14,16]能显著作用于冬季凋落叶分解。已有的研究表明，雪被融化的淋洗作用和冻融循环的物理破碎[3, 14, 16]都能显著促进凋落叶分解。同时，雪被覆盖环境中存活的土壤动物[20- 21]和微生物活性[5]也能促进冬季凋落叶分解。本研究中，红桦和岷江冷杉冬季凋落叶分解占全年分解的比例分别为53.83%—60.18%和50.23%—59.09%，高于生长季节(图6)。这说明冬季凋落叶分解对高山森林物质循环具有十分重要的作用[17]。另一方面，林窗形成能改变雪被环境、土壤冻融格局[22]和耐寒微生物的活性[23]，直接或间接影响冬季凋落叶分解[12]。本研究中，林窗形成显著提高了非生长季节2种凋落叶质量损失速率。地表凋落叶日均温度和土壤冻融循环与深冻期等关键时期质量损失速率显著相关(表5),这表明林窗形成改变了林窗内冬季环境条件，对川西高山森林冬季凋落叶分解产生了重要影响[2, 24]。

表5日均温度(T)和冻融循环次数(F)与凋落叶各关键时期质量损失速率的相关性分析

Table 5Correlation analyses between daily mean temperature and frequencies of freeze-thaw cycle and mass loss rates of foliar litters at critical decomposition period

类别Types物种Species初冻期Onsetofsoilfreezingperiod深冻期Soildeepfreezingperiod融化期Soilthawingperiod生长初期Earlygrowthperiod生长中期Mid-growingperiod生长后期Latergrowthperiod日均温(T)红桦Betulaalbo-sinensis-0.2010.607**0.687**-0.139-0.2360.077Averagetemperature岷江冷杉Abiesfaxoniana0.691**0.422*-0.2300.564**-0.182-0.007冻融循环(F)红桦Betulaalbo-sinensis-0.1730.1510.355*0.079-0.085Frequencyofsoilfreeze-thawcy-cle岷江冷杉Abiesfaxoniana0.581**0.587**0.650**0.280-0.023

*P<0.05; **P<0.01

林窗对凋落物分解的影响常常随林窗面积大小而发生变化，但已有的研究结果具有不确定性[24- 28]。有研究者发现，大林窗中凋落物的质量损失速率比小林窗和林下低[25- 26]，也有研究者认为林窗面积大小对凋落物分解影响不显著[27- 28]，甚至在大林窗中凋落物分解更快[29]。本研究发现，在1a分解过程中，2种凋落叶在生长季节的质量损失速率随林窗面积增加而减小，在非生长季节的岷江冷杉凋落叶以及融化期的红桦凋落叶的质量损失速率随林窗面积增加而增加，而初冻期和深冻期红桦凋落叶质量损失速率在不同面积林窗内差异不显著(中林窗除外)。如前所述，不同面积林窗由于冬季雪被覆盖以及生长季节水分蒸发量差异等限制着分解者群落的定着，直接影响林内凋落叶的分解过程。研究表明，凋落物质量(quality)是植物残体自身分解的关键因子[30]，控制着中小尺度上凋落物的分解过程[31]。本研究中红桦和岷江冷杉分别是阔叶和针叶凋落叶，其基质质量存在显著差异(表3)，因此对环境变化的响应过程和时间也存在不同(图3)。综上所述，川西高山森林林窗面积大小对凋落叶分解产生显著影响，且阔叶和针叶凋落叶在非生长季节与生长季节对林窗的响应存在明显差异。

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基金项目:国家自然科学基金项目(31170423, 31270498); 国家“十二五”科技支撑计划(2011BAC09B05); 四川省杰出青年学术与技术带头人培育项目(2012JQ0008, 2012JQ0059); 中国博士后科学基金(2012T50782); 四川省教育厅青年基金(2013SZB0553, 13TD0015); 生态安全与保护四川省重点实验室开放基金项目(ESP201309, ESP1404)

收稿日期:2014- 10- 19; 网络出版日期:2015- 05- 20

*通讯作者

Corresponding author.E-mail: scyangwq@163.com

DOI:10.5846/stxb201410192049

Effects of gap sizes on foliar litter decomposition in alpine forests

WU Qingui1, 2, WU Fuzhong1, TAN Bo1, YANG Wanqin1,*, HE Wei1, NI Xiangyin1

1Long-termResearchStationofAlpineForestEcosystem,KeyLaboratoryofEcologicalForestryEngineering,InstituteofEcology&Forestry,SichuanAgriculturalUniversity,Chengdu611130,China2EcologicalSecurityandProtectionKeyLaboratoryofSichuanProvince,Mianyang621000,China

Abstract：Forest gaps are one of the most normal interferences in primary forest ecosystems, and they play an important role in not only aboveground processes but also belowground processes. However, many studies have paid more attention to the effects of forest gaps on aboveground processes. Increasing evidence has demonstrated that forest gaps have a strong effect on mass loss and carbon and nutrient release in litter by redistributing precipitation, light, and other environmental factors. At high latitudes and altitudes, litter decomposition due to forest gaps may be more complex because of seasonal snow cover and freeze-thaw cycles. Theoretically, interception of rainfall and snowfall in the winter by the canopy and canopy shading may cause the hydrothermal dynamics on the forest floor to vary with respect to gap sizes and critical periods. Currently, little information is available on the effects of gap sizes on litter decomposition at different critical periods. In order to understand the effects of forest gaps on litter decomposition in forest ecosystems at high altitudes, litterbags with red birch (Betula albosinensis) and Minjiang fir (Abies faxoniana) foliar litter were incubated on the forest floor in small, medium, and large gaps and the adjacent closed canopy from November 21, 2011 to October 29, 2012. Then, the litterbags were sampled at the periods of onset of soil freezing (OF), soil deep-freezing (DF), soil thawing (TP), early growth (EP), mid-growth (MP), and late growth (LP), and the mass loss was calculated. After a year of litter decomposition, red birch and Minjiang foliar litter lost 27.25—30.12% and 27.04—27.96% mass, respectively. The mass loss of red birch and Minjiang fir foliar litter in the non-growing season accounted for 53.8—60.18% and 50.23—59.09% of the total mass loss, respectively. Gap sizes have strong effects on the mass loss of Minjiang fir and red birch foliar litter, depending on tree species and sampling periods. Forest gaps accelerated the litter decomposition of Minjiang fir, but slowed down the litter decomposition of red birch. Regardless of tree species, forest gaps significantly accelerated the rates of mass loss of foliar litter in the non-growing season (P < 0.05), but significantly lowered them in the growing season (P < 0.05). The rates of mass loss of foliar litter increased with gap sizes in the non-growing season, but decreased with gap sizes in the growing season. Moreover, the rates of mass loss of Minjiang fir foliar litter in the periods of OF, DF, and TP increased significantly with gap sizes (P < 0.05), but the effects on red birch foliar litter were not significant (P > 0.05). In addition, the contribution of litter mass loss at different critical periods to a year of mass loss was in the order of DF > MP > OF > EP > LP > TP. Mass loss of red birch foliar litter in the non-growing season accounted for 60.18%, 54.46%, 55.34%, and 53.83% of the one-year mass loss in large gaps, middle gaps, small gaps, and closed canopy, respectively; similarly, the contributions of mass loss of Minjiang fir foliar litter to the one-year mass loss in the non-growing season were 59.09%, 54.37%, 52.22%, and 50.23%, respectively. These results suggest that forest gaps accelerated the mass loss of foliar litter in the snow-cover season, but slowed down the mass loss in the growing season. The responses of litter decomposition to gap sizes in the snow-cover season were different from those in the growing season, implying that gap sizes could have strong effects on litter decomposition at different critical periods in the coniferous forest ecosystem of alpine regions.

Key Words:forest gap; litter decomposition; mass loss; non-growing season; coniferous forest; alpine region

吴庆贵, 吴福忠,谭波,杨万勤,何伟,倪祥银.高山森林林窗对凋落叶分解的影响.生态学报,2016,36(12):3537- 3545.

Wu Q G, Wu F Z, Tan B, Yang W Q, He W, Ni X Y.Effects of gap sizes on foliar litter decomposition in alpine forests.Acta Ecologica Sinica,2016,36(12):3537- 3545.