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Volume 35 Issue 2
Mar.  2022
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Long-term Dynamics of Humus Layer Moisture in Monsoon Evergreen Broad-leaved Forest in Dinghushan

  • Corresponding author: LIU Xiao-dong, liuxd@scau.edu.cn
  • Received Date: 2021-07-25
    Accepted Date: 2021-12-18
  • Objective The humus layer moisture is one of the environmental characteristics of a forest. It is significant to explore the long-term dynamics of the humus layer moisture and the underlying regulatory mechanisms for understanding the water conservation function of a forest ecosystem and the changes of the forest environment. Method In this study, we focused on the zonal climax forest in lower subtropical China, i.e., the monsoon evergreen broad-leaved forest which was strictly protected in Dinghushan National Nature Reserve. Based on the unified standards for the construction and monitoring of forest plots by Chinese Ecosystem Research Network, long-term observation of humus layer moisture in the monsoon evergreen broad-leaved forest was conducted from 2005 to 2018. This study aims at exploring the long-term dynamic of humus layers moisture and its regulatory mechanisms. Result During 2005 to 2018, the average annual precipitation of Dinghushan was 1 975.6 mm, the average annual temperature was 22.3 ℃, the rainfall was abundant and the heat input was stable throughout the year, presenting warm wet summers and cool dry winters. The average standing mass of humus layer was 126.0 g·m−2, which was slightly higher in the dry seasons (130.3 g·m−2) than in the wet seasons (121.6 g·m−2). The average annual humus layer moisture was 89.5%. The mean humus layer moisture in wet seasons (103.2%) was obviously higher than that in dry seasons (75.8%), and the variation coefficient in the wet seasons (19.6%) was significantly lower than that in the dry seasons (35.9%). Compared with the change of soil water storage (0-15 cm) in the same period, the variation coefficient of humus layer moisture (30.1%) was significantly higher than that of the soil water storage (11.6%), indicating that humus layer was more sensitive to the change of external environment. Further analysis showed that the precipitation, air relative humidity, evaporation, vapor pressure deficit and soil water storage had profound effects on the dynamic of humus layer moisture, especially the air relative humidity in the wet seasons and the air relative humidity and precipitation in the dry seasons had the dominant effects, respectively. Conclusion The humus layer moisture in monsoon evergreen broad-leaved forest in Dinghushan is relatively high. It plays an important role in stabilizing the internal environment of forest ecosystem and indicating the change of regional environment.
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Long-term Dynamics of Humus Layer Moisture in Monsoon Evergreen Broad-leaved Forest in Dinghushan

    Corresponding author: LIU Xiao-dong, liuxd@scau.edu.cn
  • 1. College of Forestry and Landscape Architecture, South China Agricultural University, Guangzhou 510642, Guangdong, China
  • 2. South China Botanical Garden, Chinese Academy of Sciences, Guangzhou 510650, Guangdong, China
  • 3. Guangdong Provincial Key Laboratory of Silviculture, Protection and Utilization,Guangdong Academy of Forestry, Guangzhou 510520, Guangdong, China

Abstract:  Objective The humus layer moisture is one of the environmental characteristics of a forest. It is significant to explore the long-term dynamics of the humus layer moisture and the underlying regulatory mechanisms for understanding the water conservation function of a forest ecosystem and the changes of the forest environment. Method In this study, we focused on the zonal climax forest in lower subtropical China, i.e., the monsoon evergreen broad-leaved forest which was strictly protected in Dinghushan National Nature Reserve. Based on the unified standards for the construction and monitoring of forest plots by Chinese Ecosystem Research Network, long-term observation of humus layer moisture in the monsoon evergreen broad-leaved forest was conducted from 2005 to 2018. This study aims at exploring the long-term dynamic of humus layers moisture and its regulatory mechanisms. Result During 2005 to 2018, the average annual precipitation of Dinghushan was 1 975.6 mm, the average annual temperature was 22.3 ℃, the rainfall was abundant and the heat input was stable throughout the year, presenting warm wet summers and cool dry winters. The average standing mass of humus layer was 126.0 g·m−2, which was slightly higher in the dry seasons (130.3 g·m−2) than in the wet seasons (121.6 g·m−2). The average annual humus layer moisture was 89.5%. The mean humus layer moisture in wet seasons (103.2%) was obviously higher than that in dry seasons (75.8%), and the variation coefficient in the wet seasons (19.6%) was significantly lower than that in the dry seasons (35.9%). Compared with the change of soil water storage (0-15 cm) in the same period, the variation coefficient of humus layer moisture (30.1%) was significantly higher than that of the soil water storage (11.6%), indicating that humus layer was more sensitive to the change of external environment. Further analysis showed that the precipitation, air relative humidity, evaporation, vapor pressure deficit and soil water storage had profound effects on the dynamic of humus layer moisture, especially the air relative humidity in the wet seasons and the air relative humidity and precipitation in the dry seasons had the dominant effects, respectively. Conclusion The humus layer moisture in monsoon evergreen broad-leaved forest in Dinghushan is relatively high. It plays an important role in stabilizing the internal environment of forest ecosystem and indicating the change of regional environment.

  • 森林枯落物层作为森林调节水源的第二层,包括林木及林下植被掉落下来的茎、叶、花、果实、树皮和枯死的植物残体等,能够有效对林冠层截留后的降雨进行再分配,从而有效减少地表径流,增强土壤的抗冲能力[1-3]。枯落物层较强的保水能力对土壤水分蒸发有较好抑制作用,同时对森林水源涵养有促进作用。

    枯落物层自然含水量是森林生态系统水文循环中的重要分量之一,对森林生态系统地表界面的土壤蒸发、水分下渗、产流等水文过程起着关键作用[4-5],故中国生态系统研究网络(Chinese Ecosystem Research Network, CERN)将枯落物层含水量列为森林生态系统试验站长期定位观测的水文要素之一,从而为我国主要类型生态系统水文过程的时间动态研究提供坚实基础。

    目前,相关研究主要集中在枯落物的持水能力方面[6-11],这对揭示不同气候区典型森林类型枯落物层的生态水文效应及其功能评估奠定了重要基础,但对自然状态下森林枯落物层含水量的研究关注较少。以往学者研究表明,枯落物层自然含水量受自身理化性质及与气候有关的环境因子的影响[12],其中,气候因子主要包括温度、降雨量、相对湿度、降雨天数、风速和蒸发等[13-16]。然而,这些研究都只是基于短期时间内含水量的动态变化与相关因素的研究。枯落物层含水量长期动态研究的缺乏限制了对森林生态系统水源涵养功能的理论认知与评估工作。

    鼎湖山季风常绿阔叶林是北回归线附近保存较为完好的地带性顶极植被类型,是当前相同纬度带上最具特色和研究价值的植被之一[17]。本文基于鼎湖山森林生态系统定位研究站季风常绿阔叶林永久样地2005—2018年枯落物层自然含水量定期观测的数据,探究该地带性顶极森林群落地表枯落物层自然含水量的时间动态,并结合气象要素、土壤水分要素等的长期观测揭示影响枯落物层含水量的主要因子。

    • 研究地点为鼎湖山国家自然保护区(112°30′39″~112°33′41″ E,23°09′21″~23°11′30″ N),位于广东省肇庆市,总面积11.33 km2。该区属亚热带季风气候,日照长,太阳辐射丰富,年均气温21.0 ℃,最热月7月,最冷月1月。年均降水量约1 860.0 mm[18]。受季风气候影响,干湿季明显,4—9月为湿季,10月至翌年3月为干季。土壤由富含腐殖质的赤红壤、黄壤和山地灌丛草甸土构成,土壤pH值为4~4.9[19]

      在鼎湖山国家自然保护区严格保护下的区域地带性顶极群落类型——季风常绿阔叶林,主要分布在保护区核心区的三宝峰,海拔为100~490 m。研究样地面积约2 500 m2,土壤类型为水化赤红壤,土层厚50~80 cm,局部地区土层厚100 cm以上,林龄400年以上,郁闭度大于0.95。该样地植物种类繁多,群落结构复杂,分为乔、灌、草三层,其中,乔木层又分为三个亚层,第一个亚层以数量不多的锥栗(Castanea henryi (Skan)Rehd. et Wils.)和木荷(Schima superba Gardn. et Champ.)为优势种;第二层以黄果厚壳桂(Cryptocarya concinna Hance)、厚壳桂(Cryptocarya chinensis (Hance)Hemsl.)和肖蒲桃(Syzygium acuminatissimum (Blume)Candolle.)等为优势种;第三亚层以云南银柴(Aporosa yunnanensis (Pax et Hoffm.)Metc.)为优势种。

    2.   研究方法
    • 本文涉及的气象因子包括大气温度、降雨量、空气相对湿度、蒸发量(数据均来源于鼎湖山自动气象观测站2005—2018年的自动监测)及饱和水汽压差。大气温度、空气相对湿度分别用温度传感器(HMP45D,Vaisala公司,芬兰)和湿度传感器(HMP45D,Vaisala公司,芬兰)记录,降雨量采用雨量器(SM1-1型,上海气象仪器厂有限公司,上海)进行人工观测,蒸发量采用小型蒸发皿(ADM7,中环天仪(天津)气象仪器有限公司,天津)人工观测,饱和水汽压差(VPD)的计算公式如下:

        式中:T为大气温度,RH为空气相对湿度。

    • 在季风常绿阔叶林样地内冠层结构比较均匀的下方,按照一定的距离间隔随机设置3个1 m × 1 m的样方,每月中旬在没有降雨的情况下采集1次样品。取样时,将样方内所有的枯枝落叶都收集起来,去除夹杂的土壤后用透明不透气的封口胶袋包好并记录样方面积和位置信息。样品采集完成后带回实验室称质量并记录鲜质量(ma/g),然后用烘箱在105 ℃下将样品持续烘干至质量恒定,记录样品干质量(mb/g)。枯落物层含水量WL(%)计算公式如下:

    • 观测期内,用CERN统一配置的中子仪(CNC503B,北京超能科技公司,北京)定期测量样地内表层土壤(0~15 cm)的土壤含水量。土壤体积含水量VWC(%)和观测剖面的土层土壤储水量SWS(mm)的计算公式为:

        式中:R是土壤中的中子计数率;Rw是水体中的中子计数率;mc为常数,取值分别为12.272和−1.2683;h为土层厚度。

    • 本文以2005—2018年为时间节点,所收集到的数据均用Excel 2016及SPSS 24.0处理,对所得的数据求取平均值、标准差及变异系数;同时对枯落物层含水量与所选取的影响因子做相关性分析,在P < 0.05水平上检验相关系数的显著性,相关系数在0.8~1.0为极强相关,0.6~0.8为强相关,0.4~0.6为中等程度相关,0.2~0.4为弱相关,低于0.2为极弱相关[20]。另外,为了避免多元线性回归中存在的共线性和个别自变量贡献率较小的问题,进一步采取多元逐步回归分析法建立枯落物层自然含水量与影响因子的模型。本文作图均采用Origin 2017完成。

    3.   结果与分析
    • 研究期内,鼎湖山的年降雨量的变化范围为1 423.1~2 869.2 mm,多年平均值为1 975.6 mm。年降雨量的平均相对变率(最大年值/多年平均值)和降雨变率(最大年值/最小年值)分别为1.5和2.0,变异系数为20.0%。大气温度的多年平均值为22.3 ℃,变异系数为1.2%。空气相对湿度的多年平均值为77.1%,其变异系数为5.5%。年蒸发量变化范围为992.8~1 338.1 mm,多年平均值为1 117.3 mm,变异系数为8.7%。饱和水汽压差的多年平均值为0.58 kPa,变异系数为29.4%。整体上,本地区降雨量丰富,热量输入相对稳定,干湿分明且雨热同期特征明显(图1)。

      Figure 1.  Monthly dynamics of meteorological factors in Dinghushan from 2005 to 2018

    • 在年尺度上,季风常绿阔叶林的枯落物层鲜质量平均值为230.0 g∙m−2,变化范围为186.1~263.5 g·m−2,变异系数为11.9%;干质量的平均值为126.0 g∙m−2,变化范围为75.6~178.8 g∙m−2,变异系数为27.4%。年尺度上季风常绿阔叶林枯落物层自然含水量的多年均值为89.5%,变化范围为59.7%~131.7%,变异系数为20.0%。在年内格局中,枯落物层自然含水量的最大值和最小值分别出现在6月和12月,其值分别为129.6%和53.0%,变异系数为30.1%(图2)。

      Figure 2.  Monthly dynamics of standing mass of humus layer and humus layer moisture in the monsoon evergreen broad-leaved forest from 2015 to 2018

      图3a可知:枯落物层自然含水量在干季的平均值为75.8%,湿季为103.2%,湿季明显高于干季;干季变异系数为35.9%,湿季变异系数为19.6%,干季显著高于湿季。枯落物层现存量(干质量)的平均值表现为干季(130.3 g∙m−2)> 湿季(121.6 g∙m−2),变异系数分别为6.4%和5.0%。湿季的降雨量(1 496.0 mm)远大于干季(479.6 mm),湿季的变异系数(67.3%)显著高于干季(27.6%)(图3b)。表层(0~15 cm)土壤储水量湿季的平均值(56.0 mm)显著高于干季(45.7 mm),干季的变异系数较高(7.3%),湿季的变异系数为3.4%。在降雨季节分配极不均的鼎湖山地区,表层土壤和地表枯落物层均呈现出调蓄降水的重要水文生态功能,枯落物层与土壤层的自然含水量呈现出相同的季节变化格局。

      Figure 3.  Seasonal dynamic of standing mass of humus layer, humus layer moisture, precipitation, and soil water storage in the 0-15 cm soil layer)

    • 图4表明:湿季期间降雨量(P < 0.01)和空气相对湿度(P < 0.001)与枯落物层含水量均呈极显著正相关,表层土壤储水量(P < 0.05)与其呈显著正相关;其中,降雨量和土壤储水量为弱相关,空气相对湿度为中等程度相关;蒸发量(P < 0.01)和饱和水汽压差(P < 0.001)均与枯落物层含水量呈极显著负相关,相关程度分别为弱相关和中等程度相关;而大气温度与枯落物层自然含水量的相关不显著(P > 0.05)。干季期间,降雨量、空气相对湿度和土壤储水量均与枯落物自然含水量呈极显著正相关(P < 0.001),均为中等程度的相关;蒸发量和饱和水汽压差均与枯落物层自然含水量呈极显著负相关(P < 0.001),均为中等程度的相关;而大气温度与枯落物层自然含水量呈不显著负相关(P > 0.05)。

      Figure 4.  Relationship between humus layer moisture and environmental factors

      为深入了解枯落物层自然含水量与各环境因子的关系,对枯落物层含水量与各因子进行多元逐步回归分析(表1),结果表明:在湿季,枯落物层自然含水量主要受空气相对湿度影响(P < 0.001),空气相对湿度能单独解释枯落物层含水量26%的变异。在干季,空气相对湿度和降雨量能共同解释35%的变异,表明干季期间空气相对湿度和降雨量共同主导着森林地表枯落物层的自然含水量变化(P < 0.001)。

      季节
      Season
      回归方程
      Regression equation
      R2P
      湿季 Wet seasony=−292.3 + 4.8RH0.260.000
      干季 Dry seasony=−111.0 + 2.4RH + 0.2P0.350.000
       注Note:RH为空气相对湿度(Air relative humidity),P为降雨量(Precipitation)。

      Table 1.  Multivariate stepwise regression analysis between humus layer moisture and environmental factors

    4.   讨论
    • 森林的气候调节作用是生态系统的基本功能之一,也是开展其他结构与功能研究的基础,气候反过来又会影响森林生态系统的结构与功能[21-22]。研究表明,枯落物现存量取决于凋落物输入量、分解速率及积累年限[8, 23],同时受气候影响,其中,温度和湿度的作用最突出[24]。官丽莉等[17]研究表明,鼎湖山季风常绿阔叶林有明显的凋落节律现象,凋落物量在湿季(4—9月)明显高于干季。本研究表明,地带性顶级群落季风常绿阔叶林的枯落物层现存量鲜质量的年均值为230.0 g∙m−2,干质量的年均值为126.0 g∙m−2,且鲜质量与干质量在干季的平均值均大于湿季。这是由于季风常绿阔叶林内植物种类丰富、冠层结构复杂,叶面积较大,能够较好的抑制水分蒸发,湿季降水和热量输入充足,形成特殊的湿热环境,使得土壤中的动物和微生物较活跃,从而加快了湿季枯落物的分解速度。因此,枯落物现存量在干季高于湿季。

    • 枯落物层自然含水量是林地内部环境特征的一种表现,受林地外界环境条件的制约[25-26]。闫俊华等[27]通过对鼎湖山3 种演替群落凋落物自然含水量的短期同步测定,揭示了演替顶极群落季风常绿阔叶林地表枯落物与演替早期、中期的马尾松人工林和马尾松针阔叶混交林相比有较好的保水能力。本研究基于长期观测进一步证实了这一点,枯落物层自然含水量的年均值达89.5%。这是因为季风常绿阔叶林林内空气湿度与土壤水分含量均较高[19, 28],故在两界面中间的枯落物层也具有较高含水量。从季节动态而言,枯落物层自然含水量与土壤储水量的同期动态相一致,且枯落物层自然含水量的变异系数高于同时期土壤层的变异系数。这表明枯落物层不仅具有截蓄降水、防止土壤溅蚀、抑制土壤蒸发、增加土壤入渗等方面的重要水文生态功能,同时对外界环境的变化具有更为敏感的指示作用。

      降雨量增大,枯落物层含水量也会随之增加[29]。本研究表明,季风常绿阔叶林枯落物层的自然含水量与降雨量之间无论在干季还是湿季均呈显著正相关,这与张大明等[30]的研究结果一致。另外,大气温度和空气相对湿度也是影响枯落物层含水量的重要因素[31-33]。研究结果显示,空气相对湿度与枯落物层含水量无论在干季还是湿季均呈显著正相关;而大气温度与枯落物层含水量无论在干季还是湿季均不显著相关,这与张增信等[34]在研究苏南丘陵森林枯落物层含水量与温度之间关系的结果一致。温度升高会加快地表枯落物中水分的蒸发,从而直接影响枯落物层的自然含水量。本研究揭示了枯落物层含水量与饱和水汽压差、蒸发量在干季与湿季间的显著负相关关系。此外,土壤层可以与枯落物层进行水分交换,土壤储水量越大,枯落物含水量越高[35-36]。枯落物层含水量与土壤储水量之间无论在干季还是湿季均呈显著正相关,且干季相关性强于湿季(干季R2 = 0.209,湿季R2 = 0.077),这可能是因为湿季降雨量多,导致土壤与枯落物层含水量均处于较高状态,二者之间可以进行水分交换的空间较小;相反,干季的降雨量较少,二者之间进行水分交换的空间较大,故干季的相关性更强。

      整体上,地带性顶级常绿阔叶林的枯落物层自然含水量较高,且受到以空气相对湿度为主的环境因子的深刻影响。作为森林涵养水源作用的第二层,对保障森林生态系统的水源涵养功能具有重要意义。

    5.   结论
    • 对鼎湖山国家自然保护区内严格保护下的季风常绿阔叶林的地表枯落物层自然含水量及外界环境因子的长期观测表明,在降水丰富、变率大的鼎湖山地区,季风常绿阔叶林地表枯落物层自然含水量始终处于一个较高的水平。与同期表层土壤储水量的变化相比,枯落物层的水分状况能更好的指示与响应外界环境的变化,反映了枯落物层在抑制土壤层水分蒸发等方面的重要作用,从而促进林地的水源涵养功能。此外,枯落物层自然含水量的季节动态受降雨量、空气相对湿度、饱和水汽压差、蒸发量和土壤储水量等外界环境因子的显著影响,尤其表现为干、湿季期间的空气相对湿度及干季期间的降水量对其变化的深刻影响。整体上,本研究表明了森林地表枯落物层在稳定生态系统内部环境以及指示外界环境变化中的重要功能,未来应予以更多关注。

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