2008特大冰冻灾害后大明山常绿阔叶林林冠结构动态

林冠结构是研究森林生态系统众多关键生态功能和过程的重要参数,常绿阔叶林是亚热带林区具有代表性的森林类型,对其林冠结构及动态特征的研究还很不深入。在广西大明山中山区选择了一个斜坡水平长200 m、宽160 m的典型坡面,在整个坡面建立了80个20 m×20 m的样地,将样地均匀分为5个坡段,每个坡段包含16个连续的样地,在2009—2012年的生长季测定了林冠高度(CH)、林冠体积(CV)、林冠覆盖度(CC)、林冠上/下冠盖比(HLr)和林冠叶面积指数(LAI),分析了各林冠结构指标的坡位及年际动态,揭示了亚热带常绿阔叶林的林冠结构特征及短期动态规律。研究结果表明,大明山常绿阔叶林林冠结构的一般特征是:平均CH(12.09±0.05)m,平均CV(2642.51±278.33)m~3(每400 m~2样地),平均CC(59.90±3.29)%,平均HLr2.48±0.23,平均LAI 2.00±0.06。大明山常绿阔叶林的林冠结构存在多层性,上层林冠覆盖度平均为42.20%,中层为30.35%,下层为18.05%。大明山常绿阔叶林的林冠结构存在坡面和年际差异,坡面变异系数为CV(29.84%—55.89%)HLr(32.90%—53.52%)LAI(22.48%—43.89%)CC(16.61%—25.74%)CH(8.26%—12.77%);年际变异系数为HLr(47.33%—57.00%)CV(39.70%—49.06%)LAI(21.58%—48.13%)CC(20.35%—24.15%)CH(9.19%—12.59%),表明CH有较强的稳定性。林冠LAI存在明显的坡面尺度效应,即向下顺坡每滑动100 m冠层LAI升高0.34。坡位对CH、HLr有显著(P=0.022)和极显著(P0.001)影响;年份对HLr有显著影响(P=0.013),对CV和CC有极显著影响(P0.001);坡位×年份对CV和LAI的交互作用显著(P=0.016,P=0.017)。回归分析发现树冠面积与林木胸径呈极显著的线性关系。此研究结果表明大明山常绿阔叶林冠层高度较低、林冠体积较小、林冠覆盖度不高、上/下冠盖比和叶面积指数偏小,这与研究区域的海拔较高(934—1223 m),土层浅薄(30—45 cm)以及经常受到冰冻灾害(特别是2008年的特大冰冻灾害)的影响有关,是山地常绿阔叶林树冠结构与山地环境条件长期适应的结果。

Short-term dynamics of canopy structure of evergreen broadleaved forest after a freezing disaster in 2008 in Damingshan, Southern China

Canopy structure and dynamics are critical components in the functioning and key ecological processes of forest ecosystems. Evergreen broadleaved forests are the representative community type in the subtropics, but the characteristic of canopy structure of this forest is poorly understood. In the present study, 80 permanent plots (20 m×20 m for each plot) on a typical slope (200 m×160 m) in the mid-mountain region of Damingshan Mountain were built and equally divided into 5 groups on the slope (16 continuous plots on each slope segment). In order to reveal the feature and short-term dynamic regulation of the canopy structure in this evergreen broadleaved forest, we investigated and analyzed the slope effects and annual dynamics of canopy structure indexes, including canopy height (CH), canopy volume (CV), canopy cover (CC), ratio of high to low cover (HLr) and leaf area index (LAI) during the growing seasons from 2009 to 2012. Mean CH, CV, CC, HLr, and LAI of this evergreen broadleaved forest averaged (12.09±0.05) m, (2642.51±278.33) m³ (in each plot), (59.90±3.29)%, 2.48±0.23, and 2.00±0.06, respectively. The canopy structure was multilayered with CC averaging 42.20% in the upper layer, 30.35% in the middle layer, and 18.05% in the lower layers. Meanwhile, the canopy structure showed differences between slopes and growth years. For different slopes, the coefficient of variation of the index ranked as follows:CV (29.84%-55.89%) > HLr (32.90%-53.52%) > LAI (22.48%-43.89%) > CC (16.61%-25.74%) > CH (8.26%-12.77%). For different growth years, the coefficient of variation of the index ranked as HLr (47.33%-57.00%) > CV (39.70%-49.06%) > LAI (21.58%-48.13%) > CC (20.35%-24.15%) > CH (9.19%-12.59%), showing that CH had relatively strong stability. We found significant effects of slope scale on LAI with an elevation of 0.34 when sliding down 100 m in the downslope direction. The results of two-way ANOVAs showed that slope position had significant and extremely significant effects on CH (P=0.022) and HLr (P < 0.001), respectively, while year of growth had significant effects on HLr (P=0.013) and extremely significant effects on CV and CC (both P < 0.001). The interaction effects of slope×year on CV and LAI were significant (P=0.016 and P=0.017, respectively). Correlation analysis showed that there was extremely significant positive correlation between DBH and canopy area. Our results indicated that the canopy structure of evergreen broadleaved forests on Damingshan Mountain have the characteristics of relatively lower CH, CC, HLr, and LAI, and smaller CV. This could be attributed to the relatively higher elevation (934-1223 m), shallow soil (30-45 cm), and the frequency of freezing disturbances, especially the severe ice storm damage in 2008 on Damingshan Mountain. This could also be the result of the long-term adaptation of canopy structure in montane evergreen broadleaved forest to environmental conditions.