摘要
本文以6年生杨树人工林为研究对象，按人工林不同间伐方式和间伐强度设计了4种间伐模式（对照、30％强度的下层伐、50％强度的下层伐、50％强度的机械伐），探讨了不同间伐模式对杨树人工林小气候、林分结构、植物多样性、养分循环及生产力的影响研究，以期通过间伐调控提高杨树人工林生产力并为杨树大径材人工林的培育提供理论依据。主要研究结果如下：
（1）不同间伐模式对杨树人工林小气候影响差异显著。50％强度的机械伐模式在夏季和秋季林内光合有效辐射强度均显著高于其他3个间伐处理，其他两个季节差异不显著。相同季节不同间伐处理林内空气温度除夏季外其他季节差异均不不显著，夏季机械伐处理的温度值显著高于其他3个间伐处理，均值达到21.25℃。相同季节不同间伐处理林内空气湿度差异不显著。各间伐模式土壤温湿度的差异仅仅出现在0~10cm土壤层次。
（2）对夏季不同间伐处理林分郁闭度进行分析，结果显示：不间伐处理（CK）林分郁闭度（78.6％）显著大于30％下层伐（64.3％），30％下层伐显著大于50％下层伐和机械伐（51.5％）；夏季不同间伐处理林分叶面积指数其结果与郁闭度变化基本一致，即：CK＞30％下层伐＞50％下层伐＞机械伐。
（3）杨树人工林间伐1年后的林下植被多样性指数和各物种重要值等数据分析表明，相同季节不同间伐处理林下植被Shannon-Wiener生物多样性均是机械伐处理的最高，但是在优势度指数上不间伐（CK）处理的最高，这说明在优势种上以CK处理的集中程度最高，机械伐最低。相同季节，机械伐和50%高强度下层伐林下植被生物量也显著大于其他两个处理。
（4）不同间伐模式下土壤有效氮的含量呈显著性差异，以机械伐和50％强度下层伐的间伐处理其微生物量氮（MBN）、水溶性有机氮（DON）、土壤无机氮含量均显著高于其他3个间伐处理。其中土壤无机氮以硝态氮为主要存在形式。间伐后土壤有效氮含量在不同土层中呈现明显的表聚性，即土壤有效氮含量在垂直分布上从下层土壤至上层土壤呈上升趋势。
（5）关于氮素矿化，在夏季和秋季，机械伐和50％强度伐下层伐土壤氮素净矿化量显著高于对照和30％强度下次伐，与对照相比，低强度间伐并不能促进土壤氮素的净矿化量增加。相同季节相同间伐模式的土壤氮素矿化量，基本是0～5cm土层的氮素净矿化量值显著高于5～10cm和10～20cm两个土壤层次，而后两者差异不显著。
（6）不同间伐模式间，土壤呼吸在春季变异最大，但是夏季却并不明显。间伐对微生物生物量碳的影响是一个缓慢的过程，在间伐后的6个月，不同间伐模式间，间伐措施仅仅影响了0～10cm层次土壤。50％强度的机械伐土壤微生物生物量碳含量显著高于其他3个间伐处理。
（7）生长季节，50%强度的下层伐和机械伐更能提升表层土壤（0～10cm）酶活性，间伐强度越强，土壤酶活性普遍也就越高。此外，低强度间伐（30%）能够促进生长季节中脲酶的活性，但是，低强度间伐（30%）与对照相比，夏季（7月）的蛋白酶和秋季（10月）的过氧化氢酶活性与对照并不存在显著差异
（8）对间伐一年后杨树人工林胸径生长量、材积生长量和生物量增加量分析表明机械伐和50%下层伐的单株胸径、材积年生长量和生物量增加量均显著大于不间伐ck和30%下层伐间伐处理。
（9）对林分间伐后氮素养分循环进行研究表明，不同间伐模式林分氮素年总供应量均大于林分年总需求量，说明该试验林分土壤作为氮素供应库足够满足现阶段林分生长的需求。其中机械伐的林分氮素年总供应量最大，而它的年总需求量最小。30％间伐伐林分氮素年总需求量最大，不间伐林分的年氮素总供应量最小。
In order to improve the quality of the poplar plantations and study the cultivation pattern ofbig-diameter wood, the6-year-old poplar plantations were selected to study thinning effects. Fourthinning treatments (contrast,30%intensity of the lower thinning,50%intensity of the lowerthinning and50%intensity of the mechanical thinning) were designed in this experiment. Theeffects of thinning treatments on poplar plantation microclimate, forest structure, plant diversity,nutrient cycling and poplar plantation productivity were investigated. The main findings are asfollows:
1. Poplar plantation microclimate was significantly affected by different thinning treatments. Insummer, Forest photosynthetic active radiation of50%intensity of the mechanical thinningwas apparently higher than the other three thinning treatments, while in other seasons, thisdifference was not significant. Forest air temperatures of different thinning treatments in thesame season, except for summer, were not significantly different. In summer, the airtemperature of mechanical thinning was significantly higher than the other three treatments,and the average temperature of mechanical thinning reach to21.25. The forest air moisturesof different treatments were not significantly different in the same season. For all types ofthinning treatments, the soil temperature difference and moisture difference only existed in0-10cm soil layer.
2. The canopy densities of contrast thinning treatment was significantly higher than30%intensity of the lower thinning, the canopy of which was then significantly higher than50%intensity of the lower thinning and50%intensity of the mechanical thinning; In summer,forest leaf area index corresponded to change of canopy densities.
3. One year later, Understory vegetation plant diversity and importance value of different plantsshowed that for the same season and different thinning treatments, mechanical thinning hadthe highest Shannon-Wiener biodiversity, while contrast treatment had the highest dominanceindex, which showed that dominant species was most concentrated in contrast treatment, leastin mechanical treatment. And in the same season, biomass of50%intensity of the lowerthinning and50%intensity of the mechanical thinning was significantly larger than that ofthe other two treatments.
4. Under different thinning treatments, the contents of available nitrogen were significantlydifferent. The contents of inorganic nitrogen、DON (soil dissolved organic nitrogen) andMBN (soil microbial biomass nitrogen) of50%intensity of the lower thinning and50%intensity of the mechanical thinning were significantly higher than that of the other twotreatments. The surface gathering of the available nitrogen was found for all the treatments,which meant that the contents of available nitrogen increased along with the depth of soil,from the bottom soil to the surface soil.
5. In summer and autumn, the net soil mineralized nitrogen contents of50%intensity of the lower thinning and50%intensity of the mechanical thinning were significantly higher thanthat of contrast and30%intensity of the lower thinning. Compared to the contrast thinning,low intensity of thinning didn’t promote the increase of net soil mineralized nitrogen. In theother two seasons, the net soil mineralized nitrogen contents of different thinning treatmentshad no significant difference. Under the same season and the same thinning treatment,commonly the net soil mineralized nitrogen contents of0-5cm layer were significantly higherthan that of5-10cm layer and10-20cm layer, the difference of which two was not significant.
6. Under different thinning treatments, the soil respiration had significant difference only inspring. It is a long and slow process for thinning to affect microbial biomass carbon. Duringthe six months after thinning, between different thinning treatments did the thinningtreatments only had influence on0-10cm layer. The contents of microbial biomass carbon of50%intensity of the mechanical thinning were significantly higher than that of the other threethinning treatments.
7. In growing season (spring and autumn), compared to contrast and low intensity of thinning,high intensity of thinning(50%) and mechanical thinning were more efficient in improvingenzyme activity of0-10cm soil layer. Commonly, the higher the intensity of thinning themore active those soil enzymes would be. Then when it came to the same thinning treatments,the enzymes activity, especially protease, had no significant difference between different soillayers. Low intensity of thinning promoted the activity of urease in growing season. Andprotease activity (summer) and catalase activity (autumn) of low intensity of thinning (30%)had no significant difference with contrast group.
8. One year after the thinning individual plant DBH increment, quantity increment, in thetreatments with50%intensity of the lower thinning and50%intensity of the mechanicalthinning were all significantly higher than that of contrast and30%intensity of the lowerthinning.
9. The research on nitrogen nutrient cycling of plantation after thinning showed that underdifferent thinning treatments, annual forest nitrogen supply was always larger than forestnitrogen demand, which showed that this experimental forest could meet the need of forestgrowth at the present stage. And the annual nitrogen supply of mechanical thinning was thebiggest while the annual demand of it was the least; the annual nitrogen demand of30%intensity thinning was the biggest while the annual nitrogen supply of contrast was the least.
引文
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