Water-holding capacity in forest litter of a seasonal tropical rainforest and a rubber plantation of Xishuangbanna in southwest China

XIONG Zhuang; YE Wen; ZHANG Shubin; CHEN Hui

doi:10.11833/j.issn.2095-0756.2018.06.008

Volume 35 Issue 6

Nov. 2018

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XIONG Zhuang, YE Wen, ZHANG Shubin, CHEN Hui. Water-holding capacity in forest litter of a seasonal tropical rainforest and a rubber plantation of Xishuangbanna in southwest China[J]. Journal of Zhejiang A&F University, 2018, 35(6): 1054-1061. doi: 10.11833/j.issn.2095-0756.2018.06.008

Citation:

XIONG Zhuang, YE Wen, ZHANG Shubin, CHEN Hui. Water-holding capacity in forest litter of a seasonal tropical rainforest and a rubber plantation of Xishuangbanna in southwest China[J]. Journal of Zhejiang A&F University, 2018, 35(6): 1054-1061. doi: 10.11833/j.issn.2095-0756.2018.06.008

Water-holding capacity in forest litter of a seasonal tropical rainforest and a rubber plantation of Xishuangbanna in southwest China

doi: 10.11833/j.issn.2095-0756.2018.06.008

1.
Geography College, Southwest Forestry University, Kunming 650224, Yunnan, China
2.
Key Laboratory of Tropical Forest Ecology, Chinese Academy of Sciences, Mengla 666303, Yunnan, China

Received Date: 2017-11-20
Rev Recd Date: 2018-04-25
Publish Date: 2018-12-20

Abstract

In order to assess the differences in water-holding capacity of the litter between the seasonal tropical rainforest (STRF) and rubber plantation (RP), the water-holding characteristics were measured in the litter of two forests in Xishuangbanna of southwest China. The litter storage, water-holding rate, water-absorption rate, and water-holding capacity of litter between these two forests were compared using independent t-tests. Results showed that the total litter mass in RP (3.79 ±0.34) t·hm^-2 was significantly higher than STRF (2.19 ±0.14) t·hm^-2 (independent t-test, P=0.012). The maximum water-holding capacity for litter was significantly higher in RP (12.50 t·hm^-2) than in STRF (5.53 t·hm^-2) (independent t-test, P=0.000). Moreover, the efficient maximum water-holding capacity of RP litter (10.63 t·hm^-2) was significantly higher than STRF (4.71 t·hm^-2) (independent t-test, P=0.000). These results suggested that litter quantity and its water-holding capacity were superior in RP compared to STRF, which could indicate the greater water-holding capacity in litter of RP over STRF even though a rubber forest canopy has a higher evapotranspiration rate than a seasonal tropical rainforest.
- forest ecology,
- seasonal tropical rainforest,
- rubber plantation,
- litter,
- water-holding capacity,
- Xishuangbanna

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森林凋落物是森林生态系统的重要组成部分，在物种多样性保持、物质循环、土壤结构改良、涵养水源等方面发挥着重要作用^[1-2]。凋落物增加了土壤表面的粗糙度，使水流蜿蜒曲折，阻滞径流产生的速度，增加了水分向土壤渗透的时间。有研究发现：有凋落物覆盖的林地产生径流的时间是无凋落物覆盖的荒地的1.8~7.7倍^[3-6]。凋落物层通常结构疏松，覆盖在土壤表面，可防止雨滴对土壤的滴溅侵蚀^[7]，并能阻挡阳光对土壤的直接曝晒，调节土壤的温度，当土壤含水量为1/2田间持水量时，土壤表面5 cm厚的凋落物就能比裸地减少27.1%的日平均蒸发量，并且土壤含水量越高，这种抑制土壤水分蒸发的能力越高^[8-9]。因此，森林凋落物在森林生态系统的水土保持和水分循环过程中发挥着重要作用。热带森林是地球陆地上物种最丰富、结构最复杂的生态系统，在全球碳循环、生物多样性，以及气候调节等方面发挥着重要的作用，同时，热带森林也是遭受干扰最严重的植被类型之一^[10]。三叶橡胶树Hevea brasiliensis原产亚马逊河流域的热带雨林，由于经济发展的需要和受经济利益的驱使，东南亚、南美、非洲的很多地区，成片的热带森林被砍伐后改种橡胶，土地利用发生了显著变化。从20世纪70年代开始，西双版纳地区广泛种植橡胶树，大量侵占海拔800 m以下的热带雨林区，甚至在个别地方已经侵占到了海拔1 200 m的山地，低海拔地区橡胶林几乎占据了所有能植胶区，原有的热带季节雨林、山地雨林面积锐减，破碎化严重。据统计，在约30 a里热带森林面积锐减了14万hm²，西双版纳地区橡胶林的种植面积已经超过该地区土地面积的20%^[11-15]。许多学者评估了大规模橡胶种植对生态环境的影响。SONG等^[14]研究发现：橡胶林林冠温度全年都高于热带雨林。TAN等^[16]发现橡胶树的蒸腾作用强烈，对水的消耗要高于热带雨林，土壤水分不足以支撑地表径流的产生，导致在干季时橡胶林下水资源短缺。还有学者在橡胶林的物种多样性^[17]、生物量^[18]、生态水文^[4]、土壤养分元素及固碳^[12]等方面做了大量研究，但在大量种植橡胶、砍伐热带森林对当地的生态环境、土壤、水文等产生影响的过程中，凋落物所起到的作用并不清楚，橡胶林和热带季节雨林的凋落物在持水性能上有何差异，以及橡胶林取代热带森林后凋落物的水源涵养能力如何变化等也都需要进行相关的验证。本研究选取了西双版纳地区热带季节雨林和人工橡胶林样地，测定了2种森林凋落物不同分解层的地表现存量及持水特性，揭示了热带季节雨林被橡胶林替代后凋落物的持水特性和水源涵养能力的变化，探讨了大规模种植橡胶对生态环境的影响，以期为推动环境友好型胶园的示范栽植，以及退化热带季节雨林的恢复提供科学依据。

1. 研究地区与方法

1.1. 研究区概况

云南省西双版纳傣族自治州(21°09'~22°33'N，99°58'~101°50'E)地处北亚热带北缘，山地面积约占全州总面积的95%，盆地面积约占5%，热带森林茂密，河谷众多，土壤以砖红壤和赤红壤为主，属北热带季风气候区，年平均气温为21.5 ℃，最热月(6月)平均气温为25.5 ℃，最冷月(1月)平均气温为14.8 ℃，终年无霜，干湿季分明，年平均降水量为1 557 mm，雨季为5-10月，降水量约占全年的87%，干季为11-4月，降水量约占全年的13%，年径流量为764 mm，年日照时数为1 828 h，干燥度为1.01^[19]。

1.2. 样地基本概况

实验样地位于云南省西双版纳州勐腊县勐仑镇。这里大面积橡胶林与热带季节雨林交错融合，小区域内气象条件相同。所选择的热带季节雨林样地(21°55'7"N，101°16'22"E)面积为50 m × 50 m，海拔570 m，属于该地区典型的湿性热带季节雨林群系组；所选择的橡胶林样地(21°54'37"N，101°16'27"E)距离热带季节雨林样地约0.5 km，面积为50 m × 50 m，海拔560 m，植株行距5 m，株距2 m，为典型的人工橡胶种植模式(表 1)。2块样地均为砖红壤，群落特征^[20-24]具体见表 1。

植被类型	土壤颗粒级含量/%		w_养分/(g·kg^-1)				群落层	高度/m	盖度/%	主要植物
植被类型	0.002~0.020 mm	0.02~2.00 mm	总有机碳	总氮	总磷	总钾	群落层	高度/m	盖度/%	主要植物
热带季节雨林	23.04	57.43	16.70	1.74	0.27	8.10	乔木Ⅰ层	> 40	30	绒毛番龙眼Pometia tomentosa
										千果榄仁Terminalia myriocarpa
							乔木Ⅱ层	20~40	80	梭果玉蕊Barringtonia macrostachya
										绒毛紫薇Lagerstroemia tomentosa
							乔木Ⅲ层	3~20	50	大叶红光树Knema linifolia
										窄序崖豆树Millettia laptobotrya
							灌木层	1~3	25	细腺萼木Mycetia gracilis
										细罗伞Ardisia affinis
							草本层	< 1		山壳骨Pseudoranthemum malaccense
										莠竹Microstegium ciliatum
橡胶林	36.84	31.88	18.40	1.99	0.36	15.1	乔木层	15	85	三叶橡胶树Hevea brasiliensis
							草本层	< 1		飞机草Eupatorium odorafum
										画眉草Eragrostis pilosa
										马唐Digitaria sanguinalis

Table 1. Community characteristics of two study plots

1.3. 凋落物的收集

中国科学院西双版纳热带雨林生态系统定位研究站在生长旺季的9月，均收集凋落物地表现存量，在每个样地选取5个点，设定0.584 m²的圆形收集面积，收集框内土壤以上的全部凋落物，从而得出热带季节雨林和橡胶林凋落物地表现存量(t·hm^-2)。将收集到的凋落物按照分解程度分为未分解(新鲜凋落物，质地坚硬，外部无分解痕迹)、半分解(已有分解痕迹，但外形仍完整，未达腐朽状态)和已分解(颜色变黑，外表破碎，已达腐朽状态)3种^[25]，分别在80 ℃烘干至恒量，然后称量并记录，并计算不同分解程度凋落物所占的比例(%)。本研究采用了2014-2016年收集的地表凋落物现存量。

1.4. 持水率、持水速率和持水量的测定

采用浸泡法测定凋落物各分解层的持水量和持水速率。在实验室内将各分解层的凋落物分别装入尼龙网袋，扎紧封口并编号，放入烘箱在80 ℃下48 h后称量。然后浸入水中，分别在0.5, 1, 2, 4, 6, 8, 10, 12, 24 h后捞起并悬挂静置，至不再滴水时迅速称量，所得凋落物的湿质量减去干质量便是不同时间凋落物的持水量(g)；持水量与干质量的比值是不同时间凋落物的持水率(%)^[26]；持水量与浸水时间的比值是不同时间凋落物的持水速率(g·h^-1)；经试验测定凋落物在浸水24 h后的持水量基本不再增加，故将浸水24 h的持水量认为该凋落物的最大持水量，但是在现实情况中，很少会出现地表凋落物完全浸水24 h的情况，所以将最大持水量乘以0.85的系数^[2]，作为有效最大持水量。根据1.3测量的凋落物地表现存量，就可以计算样地内的最大持水量和有效最大持水量(t·hm^-2)。

1.5. 统计分析

采用软件SPSS 16.0进行数据统计分析，利用独立样本t检验比较2种植被类型的凋落物及其持水特性的差异，显著水平为α=0.05，并利用软件SigmaPlot 10.0绘制图形。

3. 讨论和结论

3.1. 2种植被类型凋落物现存量的比较

本研究发现：橡胶林的凋落物现存量为(3.79±0.34) t·hm^-2，显著高于热带季节雨林的(2.19±0.14) t·hm^-2(P＜0.05，表 2)。三叶橡胶树原产亚马逊河流域的热带雨林，作为经济植物被引种到中国热带和东南亚地区。西双版纳处于北热带的边缘地区，有明显的干湿季，冬季还有短暂的低温^[19]。三叶橡胶树木质部具有较大的导管，叶片和枝条的抗干旱能力都比本地同科的植物低，因此在每年的2月底存在一个明显的换叶期^[28]。本研究采样时间为9月，此时处于西双版纳地区的雨季中期，橡胶树还处于生长期，落叶较少，地表凋落物多为上一个干季积存下来的，在该地区高温高湿的气候条件下，经过半年时间的分解，大部分都已达分解状态，从本研究中测得橡胶林凋落物的已分解层占到88.98%的比例也可以得到印证(表 2)。热带季节雨林结构较复杂，树种组成丰富，大部分植物种为常绿植物^[29]，没有集中落叶期。同时，热带季节雨林层次多，上层乔木产生的凋落物部分被阻挡、截留在下层乔木的树冠、树枝上。可见，橡胶树自身的生理特征和橡胶林的群落特征与热带季节雨林具有较大差异，决定了橡胶林比热带季节雨林具有更高的凋落物产量和现存量，为凋落物持水能力的发挥提供重要的基底。

3.2. 2种植被类型凋落物持水能力比较

森林凋落物的最大持水能力能客观地反映最大截留量和持水能力，这一指标常常用于评估其潜在蓄水能力^[30]。本研究发现：橡胶林凋落物的未分解层、半分解层和已分解层的最大持水率(245.69%，306.19%，335.01%)均高于热带季节雨林(211.30%，226.76%，267.36%)(表 5)，而且橡胶林凋落物的现存量也高于热带季节雨林(表 2)，特别是凋落物中持水率最高的已分解层，在橡胶林占总凋落物的88.98%，在热带季节雨林占总凋落物的63.29%(表 2)，橡胶林已分解层的数量是热带季节雨林的2.4倍，致使在凋落物总最大持水量里，橡胶林已分解层的最大持水量达(11.29 ± 0.72) t·hm^-2，是热带季节雨林已分解层最大持水量的3倍多，所以也就导致了橡胶林凋落物的最大持水量和有效最大持水量，均显著高于热带季节雨林(P＜0.05)(表 5)。因此，橡胶林的地表凋落物相比热带季节雨林具有更好的降雨拦截能力。TAN等^[16]研究发现：橡胶林的生长比热带季节雨林需要更多的水，如同“抽水泵”一般从土壤中索取更多的水资源，导致在干季时，橡胶林下水资源短缺、地表径流消失。本研究发现：橡胶林的凋落物无论是在数量上还是在持水能力上都优于热带季节雨林，从森林水源涵养能力角度来看，橡胶林凋落物具有较好的生态持水功能。

另外，相比人类干扰小的热带季节雨林，橡胶林人为活动较多，所以在收集橡胶林凋落物的时候，有选择地避开人类割胶活动踩踏严重的地方。而且有研究发现：橡胶林下凋落物对土壤含水量只在降水偏少的干季有影响^[3]，西双版纳地区的割胶时间(4-11月)与雨季(5-10月)高度重合，所以虽然橡胶林人为活动多，但是对土壤含水量的影响却不大。

3.3. 结论

橡胶林的凋落物地表现存量为(3.79 ± 0.34) t·hm^-2，显著高于热带季节雨林的(2.19 ± 0.14) t·hm^-2 (P＜0.05)；2种植被类型凋落物的持水率随着浸泡时间的延长不断增加，浸水时间2 h是一个重要节点，凋落物在浸水2 h内持水率增长迅速，之后变缓。已分解层的凋落物的持水率最高，在整个凋落物的持水能力里发挥着主要作用；橡胶林凋落物的最大持水量(12.50 t·hm^-2)显著高于热带季节雨林(5.53 t·hm^-2)(P＜0.05)；橡胶林和热带季节雨林凋落物的有效最大持水量为分别为10.63和4.71 t·hm^-2，所以橡胶林具有更高的有效最大持水量(P＜0.05)。

总之，橡胶林凋落物在储量和持水能力方面均显著优于热带季节雨林。在中国西南热带地区环境友好型胶园的构建过程中，应该加强对凋落物的管理，减少人为清除地表凋落物的措施，减少降水对土壤的直接侵蚀作用，降低地表径流，从而提高橡胶林的水土保持和水源涵养功能。

4. 致谢

中国科学院西双版纳热带雨林生态系统定位研究站为本研究提供了凋落物地表现存量数据，并在野外采样过程中给予了大力支持，在此表示诚挚的谢意！

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植被类型	凋落物现存量/(t·hm^-2)	未分解层		半分解层		已分解层
植被类型	凋落物现存量/(t·hm^-2)	数量/(t·hm^-2)	比例/%	数量/(t·hm^-2)	比例/%	数量/(t·hm^-2)	比例/%
热带季节雨林	2.19 ± 0.14	0.17 ± 0.01	7.54 ± 0.30	0.63 ± 0.06	28.55 ± 2.65	1.40 ± 0.06	63.92 ± 2.74
橡胶林	3.79 ± 0.34	0.13 ± 0.03	3.48 ± 0.90	0.29 ± 0.03	7.53 ± 0.86	3.37 ± 0.02	88.98 ± 0.54
P	0.012	0.371	0.003	0.000	0.000	0.000	0.000

植被类型	凋落物层	拟合方程	R²
	未分解层	U=143.968+23.510 lnt	0.970^*
热带季节雨林	半分解层	U=163.256+20.046 lnt	0.953^*
	已分解层	U=215.250+15.617 lnt	0.924^*
	未分解层	U=142.356+33.321 lnt	0.978^*
橡胶林	半分解层	U=260.729+14.317 lnt	0.965^*
	已分解层	U=289.764+14.830 lnt	0.966^*
说明：^*表示P＜0.001

植被类型	凋落物层	拟合方程	R²
	未分解层	V=11.678 t^-0.843	0.999^*
热带季节雨林	半分解层	V=14.647 t^-0.892	0.999^*
	已分解层	V=22.077 t^-0.928	0.999^*
	未分解层	V=7.294 t^-0.820	0.999^*
橡胶林	半分解层	V=21.439 t^-0.936	0.999^*
	已分解层	V=25.174 t^-0.943	0.989^*
说明：^*表示P＜0.001

Water-holding capacity in forest litter of a seasonal tropical rainforest and a rubber plantation of Xishuangbanna in southwest China

doi: 10.11833/j.issn.2095-0756.2018.06.008