云南哀牢山中山湿性常绿阔叶林土壤氮矿化季节变化

为了揭示哀牢山中山湿性常绿阔叶林土壤有效氮的季节动态特征,我们用封顶埋管法对徐家坝地区典型的中山湿性常绿阔叶林进行了研究。结果表明:1)土壤有效氮含量季节变化为22.96～68.20mgN·kg-1,其中氨态氮的含量(10.89～47.85mgN·kg-1)大于硝态氮含量(1.48～31.74mgN·kg-1),是有效氮的主体;2)一年的净有效氮矿化总量为310.32mgN·kg-1·hm-2(土壤层0～15cm),有效氮总量和NO3N干湿季节变化极显著,NH4N的干湿季节变化不显著,NH4N在湿季末(200409)最高,干季末(200404)最低,NO3N湿季初(200405)最高,湿季末(200310)最低;3)净矿化速率、净氨化速率、净硝化速率的干湿季节变化均不显著,原因在于土壤内部的干湿季节变化平缓,且这种变化滞后于大气的降雨量变化。     

西双版纳地区公路两侧土地利用变化格局特征

根据西双版纳地区1976、1988和2003年三期LandsatMSS/TM/ETM影像解译的土地利用数据,运用Arc-G IS软件的缓冲区分析方法,分析该区内主干公路两侧土地利用变化的时空格局。结果如下:27 a间,与整个西双版纳州相比,主干公路两侧的土地利用强度更大、增长速度更快。公路两侧的土地利用转化主要表现为由林地向橡胶园和茶园转变,林地锐减,橡胶园迅速增加,林地是橡胶园的最主要来源。主要地类的面积在一定范围表现出与距公路距离的相关关系,距公路越近,林地面积百分比越小,橡胶园、建筑用地和水田面积百分比越大。土地利用变化也表现出明显的公路效应,距离公路越近,林地减少的速度、橡胶园增加的速度越快;随着距离的增加,林地向橡胶园的转化面积逐渐减小。土地利用变化的公路效应深度表现出不断扩展的趋势:1976年为公路两侧5 km,1988年为7 km,2003年达11 km。     

退耕还林政策实施的农户水土保持行为响应研究--以江西省丰城市梅林镇为例

农户水土保持行为与生态环境变化关系密切，分析农户水土保持行为响应机理对退耕还林政策的实施具有重要意义。通过对江西省丰城市梅林镇多元回归模型分析，结果表明：（1）农户水土保持行为与退耕还林实施力度、退耕地收入与退耕还林后补偿比重成正相关，与退耕补偿年限成负相关；（2）农户水土保持投入减少反映农户家庭收入结构发生变化：从农业为主的收入阶段向以非农业或农业与非农业收入为主的阶段转变；（3）退耕还林政策实施农户水土保持行为响应以效益引导型为主。据此，提出相关建议。     

婺源县森林景观游憩价值的经济评价

文章在简要分析森林游憩价值经济评价方法的基础上，采用旅行费用法（TCM）对婺源县森林景观游憩价值进行了经济评价。采用整群随机抽样的方法对研究区的游客进行半访谈式问卷调查，根据调查资料，将婺源县游客来源划分为15个出发区，建立了游林率-旅行费用回归模型。结果表明，旅行费用与游林率呈极显著的负相关，文化程度与游林率呈显著的正相关；在此基础上制定了游憩需求曲线，计算得出研究区2002年森林景观游憩经济价值为1．02亿元。     

Spatial and temporal rockfall activity in a forest stand in the Swiss Prealps—A dendrogeomorphological case study

Rockfall is a major threat to settlements and transportation routes in large parts of the Alps. While protective forest stands in many locations undoubtedly reduce rockfall risk, little is known about the exact frequency and spatial distribution of rockfall activity in a given place or about how these parameters can be assessed. Therefore, the objective of the present study was to reconstruct rockfall events with dendrogeomorphological methods and to analyse the spatial and temporal rockfall activity in a subalpine forest stand. The study site is located in the transit zone of frequently passing, rather small rockfall fragments (mean diameter of 10 to 20 cm). In all, 33 stem discs from previously felled Picea abies trees found at the foot of Schwarzenberg in Diemtigtal (Swiss Prealps) were sampled, and a total number of 301 rockfall events were dated to between A.D. 1724 and 2002.Results showed that the spatial distribution of rockfall changed slightly with time, and that rockfall activity increased considerably over the last century. In contrast, rockfall magnitude presumably remained on a comparable level. The seasonal occurrence of rockfall showed a clear peak during the dormant season of trees, most probably in early spring. Furthermore, on a 10-year moving average basis, rockfall rates were positively correlated with mean annual as well as summer and winter temperatures. This means that higher temperatures resulted in increased rockfall activity. On the other hand, no correlation with annual or seasonal precipitation totals was revealed. Overall, this study provides an appropriate method for the detailed assessment of spatial and temporal variations in rockfall activity in a given place.     

Late Holocene development of a floodplain along a small meandering stream, northern Québec, Canada

This paper describes the activity of a small meandering stream and the development of its floodplain during the last 4600 years (calendar years BP) in the northern boreal zone of Québec. Three trenches were excavated across the floodplain's full width and permitted the interpretation of morphosedimentary units in relation to modern analogs. Chronological controls within trenches was provided by the dendrochronological and radiocarbon dating of buried tree trunks. From 4600 to 2900 cal. BP and from 1000 to 120 cal. BP, the channel migrated and constructed its floodplain at very slow rates, mostly because of low flow velocities, vegetated streambanks and the cohesive texture of marine sediments reworked by the channel. Vertical accretion rates were extremely variable on the floodplain, with high rates proximal to the modern channel and low rates over distal (also older) portions of the floodplain. Following a major channel shift (meander cut-off or avulsion) around 2900 cal. BP, channel migration appears to have been constrained to a narrow zone adjacent to the modern channel. Within this constrained zone, the migrating channel has reworked its own sediments leading to a marked unconformity between 2900–1000 cal. BP. It is thought that underlying marine sediments protrusions, and perhaps the forested banks, protected older alluvial sediments from being eroded during the last three millenniums. Our study shows that small boreal floodplains may contain, in a very small area, abundant and diversified archives of their evolution.     

Impact of forest fires on regional air quality in Southeast Asia

Biomass burning, the burning of living and dead vegetation for land-clearing and land-use change, has been in practice in many tropical countries such as Brazil, Indonesia, Nigeria, and Mexico. Intense forest fires can also ignite subsurface organic soil components （e.g. peat）, which can continue to smolder long after the original surface fires are put out. Combustion of vegetation and peat has only recently been recognized as a major source of atmospheric pollution. The immediate effect of burning is the production and release of gases and particles into the atmosphere, in quantities that in some cases can be significant not only locally and regionally but also on a global scale. Vegetation fire emissions account for roughly half of the atmospheric constituents of hydrocarbon, carbon monoxide, nitrogen oxides, and precursors of tropospheric ozone and may be the second largest source of anthropogenic aerosols after the production of sulfates from SO2. As a result, there has been considerable interest in studying the contribution of forest fires to the production of air pollutants. In recent years, Southeast Asian countries have been affected repeatedly by episodes of smoke haze from forest fires in Indonesia. During 1997-1998, the region was again shrouded in a haze worsened by prolonged drought linked to the E1 Nino weather phenomenon. Satellite pictures confirmed that air pollutants, originating from fires covering an estimated 800000 hectares of southern Kalimantan, Sumatra, and Java, have been transported by southwesterly monsoon winds across neighbouring Malaysia and Singapore to as far as Thailand and the Philippines. The impact of these extensive fires on the health of the residents and ecology in the affected areas was substantial, costly, and possibly long lasting. Most of the research on the environmental aspects of biomass burning has been directed towards the tropics in Africa and America, specifically to the tropical rain forests of Brazil and to the savannas of Africa and South America.     

1,000-year record of landslide dams at Halden Creek, northeastern British Columbia

Large, rapid, low-gradient landslides are common in clay-rich glacial sediments in northeastern British Columbia. Many of the landslides create upstream impoundments that may persist for years in small watersheds in the region. We have documented such events in the Halden Creek watershed, 60 km southeast of Fort Nelson. The events are recorded geologically in two ways. First, trees are drowned in lakes dammed by the landslides and subsequently buried by deltaic sediments, where they are protected from decay. Bank erosion later exhumes the drowned trees. Second, landslide deposits with entrained wood are exposed along stream banks. We have reconstructed the recent history of landslide damming at Halden Creek by performing radiocarbon dating on exhumed trees and wood in and beneath landslide deposits at 13 sites in the watershed. Drowned trees range in age from 169±59 to 274±49 ¹⁴C year bp. Wood in and below landslide deposits yielded radiocarbon ages ranging from modern to 965±49 ¹⁴C year bp.     

Optimization strategies for sediment reduction practices on roads in steep,forested terrain

Many forested steeplands in the western United States display a legacy of disturbances due to timber harvest, mining or wildfires, for example. Such disturbances have caused accelerated hillslope erosion, leading to increased sedimentation in fish‐bearing streams. Several restoration techniques have been implemented to address these problems in mountain catchments, many of which involve the removal of abandoned roads and re‐establishing drainage networks across road prisms. With limited restoration funds to be applied across large catchments, land managers are faced with deciding which areas and problems should be treated first, and by which technique, in order to design the most effective and cost‐effective sediment reduction strategy. Currently most restoration is conducted on a site‐specific scale according to uniform treatment policies. To create catchment‐scale policies for restoration, we developed two optimization models – dynamic programming and genetic algorithms – to determine the most cost‐effective treatment level for roads and stream crossings in a pilot study basin with approximately 700 road segments and crossings. These models considered the trade‐offs between the cost and effectiveness of different restoration strategies to minimize the predicted erosion from all forest roads within a catchment, while meeting a specified budget constraint. The optimal sediment reduction strategies developed by these models performed much better than two strategies of uniform erosion control which are commonly applied to road erosion problems by land managers, with sediment savings increased by an additional 48 to 80 per cent. These optimization models can be used to formulate the most cost‐effective restoration policy for sediment reduction on a catchment scale. Thus, cost savings can be applied to further restoration work within the catchment. Nevertheless, the models are based on erosion rates measured on past restoration sites, and need to be updated as additional monitoring studies evaluate long‐term basin response to erosion control treatments. Copyright © 2006 John Wiley & Sons, Ltd.     

Assessing burn severity and comparing soil water repellency,Hayman Fire,Colorado

An important element of evaluating a large wildfire is to assess its effects on the soil in order to predict the potential watershed response. After the 55 000 ha Hayman Fire on the Colorado Front Range, 24 soil and vegetation variables were measured to determine the key variables that could be used for a rapid field assessment of burn severity. The percentage of exposed mineral soil and litter cover proved to be the best predictors of burn severity in this environment. Two burn severity classifications, one from a statistical classification tree and the other a Burned Area Emergency Response (BAER) burn severity map, were compared with measured ‘ground truth’ burn severity at 183 plots and were 56% and 69% accurate, respectively. This study also compared water repellency measurements made with the water drop penetration time (WDPT) test and a mini‐disk infiltrometer (MDI) test. At the soil surface, the moderate and highly burned sites had the strongest water repellency, yet were not significantly different from each other. Areas burned at moderate severity had 1·5 times more plots that were strongly water repellent at the surface than the areas burned at high severity. However, the high severity plots most likely had a deeper water repellent layer that was not detected with our surface tests. The WDPT and MDI values had an overall correlation of r = ?0·64(p < 0·0001) and appeared to be compatible methods for assessing soil water repellency in the field. Both tests represent point measurements of a soil characteristic that has large spatial variability; hence, results from both tests reflect that variability, accounting for much of the remaining variance. The MDI is easier to use, takes about 1 min to assess a strongly water repellent soil and provides two indicators of water repellency: the time to start of infiltration and a relative infiltration rate. Copyright © 2005 John Wiley & Sons, Ltd.