人为干扰对风水林群落林下木本植物组成和多样性的影响

华南地区的风水林是乡村聚落的一种特色林分, 具有守护村庄的象征意义。在过去的数百年中, 风水林在乡村的社会文化习俗的影响下而受到保护, 对当地的生物多样性保育有着重要作用。为揭示人为干扰对风水林的影响, 我们选择广东省东莞市大岭山镇同一林分起源的3个具有相似地形的风水林, 研究了在不同干扰强度下其林下木本植物种类组成和物种多样性。多响应置换过程(multi-response permutation procedures, MRPP)分析表明, 人为干扰显著改变了风水林林下木本植物组成(P = 0.001, A = 0.3886), 沿着干扰由弱至强的梯度呈现出中生性植物减少、阳生性植物递增的趋势。多样性指数变化趋势为重度干扰＞中度干扰＞轻度干扰, 但没有表现出统计学意义上的差异(P＞0.05)。随着干扰强度的增大, 3个风水林群落相互间的林下物种相似性降低, 物种替代率呈增加趋势。双向聚类分析较好地反映出林下物种因受不同人为干扰强度影响而表现出在空间分布上的差异。指示种分析进一步确定了不同干扰强度下具有显著指示值(IV ≥60)的指示种。综合分析表明, 人为干扰有利于阳性物种在风水林内定居生长, 并明显地改变了林下木本植物组成, 但未能引起物种多样性的显著差异。此外, 找出对人为干扰产生关键生态响应的林下指示种, 对增进风水林的生物多样性保育以及生态系统管理有着重要的理论意义和实践价值。     

林火干扰对北方针叶林林下植被的影响

林下植被在北方针叶林植被群落中的物种多样性最高, 且具有较高的生物量周转率和地上部分净初级生产力, 对北方针叶林生态系统功能起着重要作用。火干扰是决定北方针叶林林下植被结构与功能的一个重要景观过程。该文综述了火干扰是如何通过与地形、火前林冠组成的交互作用而影响环境资源和林下植被的。最近的研究表明: 林下植被能够影响火后树木更新苗的定植、重建速率及森林演替轨迹; 林下植被还会通过影响元素的生物地球化学过程(凋落物降解和养分循环)影响林下环境资源的数量与异质性。因此, 研究火后初期北方针叶林林下植被的动态变化, 对于物种多样性保护和森林管理具有重要意义。     

剔除灌草和添加翅荚决明对厚荚相思林土壤温室气体排放的影响

森林土壤是CO₂、CH₄和N₂O等温室气体的主要排放源.本研究采用静态箱/色谱分析技术,对中国科学院鹤山丘陵综合开放试验站内厚荚相思林土壤CO₂、CH₄和N₂O通量进行原位测定,研究剔除林下灌草和添加翅荚决明对土壤温室气体排放的影响.结果表明：厚荚相思林土壤CO₂通量在湿季维持较高水平,在旱季则明显降低.CH₄和N₂O在9-11月波动幅度较大,峰值出现在10月.在不同处理下,厚荚相思林土壤可能是CH₄的源也可能是CH₄的汇,而于CO₂和N₂O则是源.林下剔除灌草能显著增大土壤CO₂排放(P<0.05),而添加翅荚决明能加快土壤CH₄的排放(P<0.05).林下剔除灌草及添加翅荚决明两种处理都能够加大N₂O的排放通量.表层土壤温度、湿度、NO₃^--N和微生物生物量碳都是影响土壤温室气体排放的重要因子.     

Forage availability for white-tailed deer following silvicultural treatments in hardwood forests

Closed-canopy upland hardwood stands often lack diverse understory structure and composition, limiting available nutrition for white-tailed deer (Odocoileus virginianus) as well as nesting and foraging structure for other wildlife. Various regeneration methods can positively influence understory development; however, non-commercial strategies are needed to improve available nutrition in many stands, as some contain timber that is not ready to harvest and others are owned by landowners who are not interested in harvesting timber. Applications of herbicide and prescribed fire have improved availability of food and cover for deer and other wildlife in pine (Pinus spp.) systems. However, this strategy has not been evaluated in hardwood systems. To evaluate the influence of fire and herbicide treatments on available deer forage in upland hardwood systems, we measured forage availability and calculated nutritional carrying capacity (NCC) at 14% crude protein mixed diet, following 7 silvicultural treatments, including controls, in 4 mixed upland hardwood stands July–September 2007 and 2008. We compared NCC among forest treatments and within 4 paired warm-season forage food plots to evaluate the usefulness of food plots in areas where forests are managed. Nutritional carrying capacity estimates (deer days/ha) were greatest following canopy reduction with prescribed fire treatments in both years. Understory herbicide application did not affect species composition or NCC 1 year or 2 years post-treatment. Production of forage plantings exceeded that of forest treatments both years with the exception of early-maturing soybeans and retention cut with fire 2 years post-treatment. We encourage land managers to use canopy reducing treatments and low-intensity prescribed fire to increase available nutrition and improve available cover where needed in upland hardwood systems. In areas where deer density may limit understory development, high-quality forage food plots may be used to buffer browsing while strategies to reduce deer density and stimulate the forest understory are implemented. © 2011 The Wildlife Society.     

Don’t miss the forest for the trees! Evidence for vertical differences in the response of plant diversity to disturbance in a tropical rain forest

Ecological studies in tropical rain forests traditionally focus on trees above a threshold diameter at breast height (dbh), since ignoring plant species of the other structural compartments is believed to be an acceptable tradeoff between exhaustiveness and effectiveness. However, the consequences of missing species below a threshold dbh value have been largely neglected so far. We evaluated whether the response of species diversity of ≥10-cm dbh trees was similar to the response of other structural ensembles (namely treelets, saplings, and terricolous herbs) in a lowland tropical rain forest, to three disturbance regimes: natural gap dynamics (control), and selective logging with and without additional thinning. We studied forest vegetation composition and diversity in a 20-yr replicated field experiment comprising nine 1 ha permanent plots established in a semi-deciduous rain forest of the Congo Basin and equally distributed among the three treatments. Once corrected by stem density, species richness was similar between logged (20 years since logging) and untouched old-growth forest stands with respect of trees, but higher with respect of treelets. As disturbance intensity increased, species richness increased within sapling layers but decreased within herb layers, while species spatial turnover (beta diversity) increased in both cases. Regarding the parameters of the partitioned rarefaction curves and relative abundance distribution curves, no correlation was found between trees and any of the other structural compartments. Whilst tree and treelet species composition was similar among treatments, the understories still reflected past disturbance intensity, with a strong response of the sapling and herb layers. These results show that ecological studies based solely on tree layers (dbh ≥ 10 cm) are misleading because their response to disturbance cannot be used as a surrogate for the response of other structural ensembles. Long-lasting effects of anthropogenic disturbance on the sapling bank and the herb layer may durably influence the long-term forest dynamics. Since overstory but not understory plant communities have recovered from human disturbances 20 years after silvicultural operations, African tropical rain forest ecosystems may not be as resilient to selective logging as previously thought.     

马尾松人工林林分密度对林下植被及土壤性质的影响

采用样方调查及取样分析方法,研究广西大青山南亚热带不同密度马尾松人工林林下植被及土壤特征.结果表明:马尾松人工林自然发育14 a后,林分密度从1050 株·hm^-2增加到1800株·hm^-2,林下灌木层物种多样性指数增高.密度为1800 株·hm^-2时,多样性指数最高.当林分密度继续增大到2250 株·hm^-2,灌木层物种多样性指数却呈降低趋势.林下草本物种多样性指数对林分密度变化的响应不敏感;在林分密度影响下,林下灌木生物量与灌木层物种多样性指数变化规律一致.随着林分密度的增大,林下草本生物量呈降低趋势;不同密度马尾松人工林土壤理化特性差异显著(P<0.05).除全K、速效K和速效P外,土壤表层(0～20 cm)养分含量与林下灌木层物种多样性指数变化趋势一致.不同密度林地速效K和速效P含量变化波动较大.密度为2100 株·hm^-2的林分土壤 全P含量较高.中密度(1800 株·hm^-2)林地的土壤持水量和孔隙度均较高,土壤容重较低.     

亚热带地区阔叶林与杉木林土壤活性有机质比较

通过对亚热带3个地区地带性阔叶林和杉木林土壤活性有机质的比较,分析森林类型变化及杉木连栽对土壤有机碳和养分含量的影响.结果表明：地带性阔叶林转变为杉木林后,土壤总有机碳含量下降27.8%～52.1%、腐殖酸碳下降32.2%～52.8%、胡敏酸下降36.4%～59.0%、富里酸下降29.7%～50.0%;杉木连栽也使土壤总有机碳和腐殖质含量下降.森林类型改变和杉木连栽对土壤活性有机质的影响更明显.杉木林取代阔叶林后,土壤微生物生物量碳、微生物生物量氮、可溶性有机碳和可溶性有机氮含量的最大降幅分别为61.8%、38.2%、43.3%和69.0%;与第1代杉木林相比,第2代杉木林土壤微生物生物量碳、微生物生物量氮、可溶性有机碳和可溶性有机氮含量的最大降幅分别为34.7%、29.3%、30.4%、18.4%.经相关性分析,除冷水浸提有机氮外,土壤活性有机质与养分含量之间具有密切的相关关系.     

不同肥力条件下黑土及其有机无机复合体的腐殖质组成

采用Kumada法研究了不同肥力黑土及其复合体的腐殖质组成.结果表明,黑土培肥后,粉粒和细砂复合体含量增多;土壤及各级复合体游离态和结合态腐殖质含量得到不同程度提高,但游离态腐殖质的腐殖化度降低.这种作用在粉粒和细砂复合体中表现得更为明显.随复合体粒径增大,复合体有机碳含量、腐殖质提取率、游离态腐殖质含量及游离态胡敏酸的腐殖化度等均有降低趋势.胡敏酸分类结果表明,高肥土壤游离态胡敏酸均为R_p型,低肥则为R_p型和B型.随复合体粒径增大,游离态胡敏酸类型变化序列主要为:A型(粘粒)→B型(粉粒)→R_p型(细砂);无论粒径大小,结合态胡敏酸大多为A型.土壤培肥后,可使全土及各级复合体的游离态腐殖质的腐殖化度降低,进而会发生型变.在黑土中,游离态胡敏酸的型变主要体现在粉粒复合体中,结合态胡敏酸的型变主要体现在细砂复合体中.     

Litter decomposition: a Russian matriochka doll

Litter is decomposed in a sequential process. In a concerted action animals and microorganisms break down complex organic matter to mineral products. Higher animals fragment and partially solubilize plant material. Subsequently, microorganisms (protozoa, fungi and bacteria) further degrade the organic matter to end products that cannot be metabolized further under the prevailing environmental conditions. During the process of decomposition some parts of the organic substrate and the excess energy are used to form new biomass. Some free organic intermediates may interact chemically to form relatively recalcitrant organic matter, such as humic substances. The degree of mineralization depends strongly on the type of organic matter in the litter and the physical and chemical conditions of the environment.     

Viscosimetric studies on molecular weight fractions of fulvic and humic acids of aquatic,terrestrial and microbial origin

Summary Viscosities were investigated of solutions of fulvic and humic acid molecular weight fractions of aquatic, terrestrial and microbial origin. Aquatic fulvic and humic acid molecules were, at pH 7, more voluminous than other types of humic compounds of similar molecular weight. It would appear that in low molecular weight non-aquatic humic matter, more inter- than intra-molecular bonding is present, with increasing molecular weight the bonding becomes more intra-molecular. Differences between average molecular weight values as obtained by an ultrafiltration method (Amicon) and by viscosimetry ranged from –18.7 to 18.5%. The largest deviations were in the low molecular weight range (<5,500 daltons). Higher molecular weight humics (in particular humic acids) appeared to have a more elongated structure than lower molecular weight material (in particular fulvic acids). Indications were obtained that on hydration humic moleculars become more elongated.