Controls on early post-fire woody plant colonization in riparian areas

Fire in riparian areas has the potential to influence the functions riparian vegetation provides to streams and aquatic biota. However, there is little information on the effects of fire on riparian areas. The objectives of the present study were to: (i) determine how fire severity interacts with riparian topographic setting, micro-environmental conditions, and pre-fire community composition to control post-fire regeneration; (ii) determine how riparian regeneration patterns and controls change during early succession; and (iii) determine how critical riparian functions are influenced by and recover after fire. Study locations included the Biscuit Fire in southwestern Oregon and the B&B Complex Fire in the Cascade Mountain Range of west-central Oregon, USA. We measured post-fire woody species regeneration, and measured factors such as fire severity, pre-fire species composition, and stream size as potential factors associated with post-fire regeneration patterns. At a relatively coarse spatial scale, patterns in post-fire colonization were influenced by elevation. At finer spatial scales, both conifer- and hardwood-dominated riparian plant communities were self-replacing, suggesting that each community type tends to occur in specific ecological settings. Abundant post-fire regeneration in riparian areas and the self-replacement of hardwood- and conifer-dominated communities indicate high resilience of these disturbance-adapted plant communities.     

Quantifying successional rates in western aspen woodlands: Current conditions,future predictions

Stands of quaking aspen (Populus tremuloides) rank among the most biologically diverse plant communities across the intermountain region of western North America. Marked declines of aspen have occurred in recent decades, likely due to a combination of effects from changes in fire regimes, herbivory, climate (e.g. drought), and interspecific competition with conifer species. However, it is poorly understood how the effects of these factors are manifested at a landscape scale over decadal time periods. Analysis of field data combined with topographic information collected across the 500,000 ha Owyhee Plateau in southwestern Idaho revealed that aspen in the area occur in three different biophysical settings; First, aspen stands exist at high altitudes on south-facing slopes where local conifer species are not likely to occur because of limiting temperature or precipitation levels under current climate conditions. In these areas aspen is the potential vegetation type rather than conifers. Second, aspen grow on anomalously wet microsites (e.g. near springs), and third, aspen grow within upland mixed aspen/conifer stands, which are experiencing rapid rates of conifer establishment. Based on a paired t-test (α = 0.05) we conclude that stands growing on wet microsites show significantly slower successional rates of conifer establishment relative to upland aspen stands. We developed a conceptual state-and-transition model for upland aspen/conifer stands occurring across a range of topographic positions. We then parameterized the model using extensive field data in the vegetation dynamics computer simulation model Vegetation Dynamics Development Tool (VDDT), and examined the current and future aspen distribution under varying fire regimes. Model results indicate that average fire return intervals of 50–70 years are desirable for maintenance of aspen in upland areas where conifers are present. Under the current fire regime in the area many upland aspen/conifer stands will likely be lost within 80–200 years. Thresholds for the effect of conifer encroachment and browsing on aspen regeneration identified through this research are similar to those described by others across the West. We therefore suggest that the results presented for the Owyhee Plateau are likely applicable to semi-arid aspen woodlands across the American West where succession to conifers is a cause of aspen decline.     

Improvement of direct tree seeding with cover crops in afforestation: Microclimate and resource availability induced by vegetation composition

The low cost of direct tree seeding makes it increasingly useful for afforestation. However, the success of the technique is often unpredictable due to a series of different adverse biotic and abiotic factors. A plant cover with low competitive ability towards resources could offer a natural alternative to the herbicides currently used to control weeds. The effects of cover plants on availability of resources and microclimate were studied in a field experiment in the central France during three years. An experiment design crossing three mixtures of tree seeds with four vegetation compositions (a bare soil, flora of the meadow, and two mixtures of cover plants) was established and repeated randomly in three blocks in a meadow. Vegetation composition, tree seedling emergence, light availability, soil water content, and temperatures under each treatment were measured. The vegetation composition of the meadow stayed stable, with grass species dominating. The cover plants sown disappeared rapidly after one or two years. Outcome of direct seeding of trees depended on the vegetation treatment. Bare soil gave the highest emergence rate while meadow vegetation gave the lowest, with the mixture of cover plants being intermediate. Light availability and soil water content were very low under the meadow vegetation but were very high on the bare soil, again with cover plant mixtures being intermediate. This study confirms that a bare soil obtained by herbicide is a secure way to ensure tree seedling establishment in afforestation by direct seeding. However, with the onus currently on reducing herbicide use, the results suggest that sowing a mixture of cover plants could be an acceptable alternative to herbicides. The cover plants could offer protection against frost, scorching temperatures, or the water run-off encountered on bare soil. However, there are also a number of unresolved issues that need be addressed before the cover plants technique can be recommended.     

Factors influencing the formation of unburned forest islands within the perimeter of a large forest fire

Large forest fires have recently increased in frequency and severity in many ecosystems. Due to the heterogeneity in fuels, weather and topography, these large fires tend to form unburned islands of vegetation. This study focuses on a large forest fire that occurred in north-eastern Spain in 1998, which left large areas of unburned vegetation within its perimeter. Based on a satellite post-fire severity map we searched for the relative influence of biotic and abiotic factors leading to unburned island formation. We divided the area of the fire into individual units we called “slopes” which were meant to separate the differential microclimatic effects of contrasted aspects. The number of unburned islands and their areas were related to 12 variables that influence their formation (i.e. land cover composition, aspect, steepness, forest structure, two landscape indices and weather variables). We hypothesized that unburned vegetation islands would concentrate on northern aspects, in less flammable forests (i.e. broadleaf species) and higher fragmentation to interrupt the advance of fire. While north and western aspects did have a higher presence of unburned vegetation islands, our study suggests greater presence of islands in slopes that are larger (i.e. more continuous areas with relatively homogeneous aspect), with greater proportions of forest cover, with higher wood volumes and with lower proportions of broadleaf species. Climate also played a role, with relative humidity and wind speed positively and negatively correlated to island formation, respectively. Unburned vegetation was more frequent on slopes with lower diversity of land covers and higher dominance of one land cover in the slope. Since slopes with only one land cover (i.e. forests) had more islands than slopes with multiple cover types, we infer that under severe meteorological conditions, fragmented forests can be more affected by wind and by water stress, thus burning more readily than forests that are protected from this edge phenomenon. These results would reinforce forest management strategies that avoid linear features (fire-lines and fire-breaks), to enhance fuel treatments that focus on areas and minimize fragmentation.     

1981～2000年中国陆地生态系统NPP时空变化特征分析

利用GLO-PEM模型估算了1981~2000年中国陆地生态系统植被净第一性生产力（NPP）。结果表明：1981~2000年中国总NPP呈现增加趋势,从1981年的2.82 PgC/a增加到2000年的3.13 PgC/a,年均增加0.0155PgC/a,20 a间NPP的平均值为2.98 PgC/a,约占全球NPP总量的4.7%~5.0%。NPP积累主要发生在4~10月,春季、夏季、秋季、冬季NPP量分别占全年总量的15.68%,54.54%,25.50%,4.2%。其中夏季NPP增长最快,占全年NPP增长的64.14%。全国NPP年均值从西北向东南方向呈逐步增加趋势,不同植被类型中,常绿阔叶林年均NPP最高,达745.68 gC/m2,除农业植被外各种植被类型的月均NPP最大值都出现在7月,最小值都出现在1月。经向剖线NPP变化规律不明显。纬向40°N线是南北分界线,往南随着纬度的降低,剖线平均NPP逐渐升高,往北随着纬度的增加,剖线平均NPP逐步变大。纬向剖线NPP均值最高的是20°N线,异质性最高的是30°N线。     

黑龙江沿边县市植被动态变化及其对经济因子的响应

[目的]研究黑龙江沿边县市植被动态变化及其对经济因子的响应。[方法]利用1998～2007年黑龙江沿边地区SPOT Vegetation每旬合成的归一化植被指数（NDVI）数据集,研究了近10年黑龙江沿边县市植被动态变化特征,并分析了植被变化对经济因子的响应。[结果]1998～2007年黑龙江沿边地区总体植被覆盖率高,且植被变化基本向好的趋势发展;除呼玛县部分区域植被呈退化状态外,近10年黑龙江沿边地区植被整体状况处于基本保持与改善状态,植被状况基本保持不变和有轻微改善的地区分别占71.00%和26.81%。同时,1998～2007年植被指数与国内生产总值均有不同程度的增长,但两者间增长趋势并不完全一致;MNDVI与职工平均工资呈显著正相关（P〈0.01）,而与耕地面积呈显著负相关（P〈0.01）,且这种负相关关系具有延迟性,即第一年的耕地面积减少将对第二年、第三年的MNDVI产生持续的影响。[结论]该研究为黑龙江界河流域经济发展、生态环境保护等政策制定提供了科学依据。     

川西南山地植被对2009/2010年冬干的响应研究

利用气象资料和卫星遥感资料,初步分析了2009/2010年冬季及2010年初春3月川西南山地植被对2009/2010年冬干的响应。结果表明,季降水评估的气象干旱为中到特旱,气象干旱强度突出;冬季平均温度偏高约2.0℃,冬季平均相对湿度偏少约17.0%。冬季及初春3月土壤相对湿度资料显示气象干旱发生发展趋势为冬季12、1月为轻旱,2～3月发展维持为中旱;冬季及初春3月卫星遥感监测月最大植被指数近5年安宁河平原区域植被指数逐月增加,川西南山地其余区域逐月减小,川西南山地中部及西部林区植被指数最高;植被指数差值分布显示,冬季到初春3月川西南山地2009/2010年冬干由南至北发展,总体上川西南山地南部区域以及安宁河平原区域受旱影响较为显著。     

Comparing water-vegetative indices for rice (Oryza sativa L.)-wheat (Triticum aestivum L.) drought assessment

Drought indices (DI) are an useful tool for assessing different sectarian droughts. Standardized Precipitation Index (SPI) has been used worldwide to assess/monitor the onset, active phase, cessation and severity of drought. Normalized Difference Vegetation Index (NDVI) provides a comprehensive vegetation dynamics, which directly linked with rainfall received in a particular region. Indo-Gangetic Region (IGR), providing employment and livelihood to tens of millions of rural families directly or indirectly and rice (Oryza sativa L.)-wheat (Triticum aestivum L.) (RW) system of the Indo-Gangetic Plains (IGP) contributes 80% of the total cereal production and is critical to food security of the region. This study tries to verify the applicability of water-vegetative indices viz., SPI, Rainfall Index (RI) and NDVI for drought assessment of rice-wheat system productivity over IGR-India. The relationship between monsoon rainfall and NDVI shows that at around 1100 mm rainfall, the NDVI reached saturation point and no further significant increase in NDVI with increase of rainfall is noticed. Even though, there was a positive correlation of seasonal monsoon rainfall and average NDVI, conflicting results are noticed between monthly distribution of rainfall and monthly anomaly of NDVI over IGR States. It is noticed that June dif NDVI (actual NDVI-mean NDVI) contributes more to rice productivity followed by July. However, the combined effect of June, July and August, explains 15% of the variation of Kharif Rice Productivity Index (KRPI). As far as wheat is concerned, statistically significant relation was found between Wheat Productivity Index (WPI) and anomaly NDVI during December-March. This explains 35% of the variability in WPI.     

露营活动对山地草甸植被的影响——以武功山景区为例

为了明确露营活动对山地草甸植被的影响程度,运用典型样地法对武功山景区露营基地的草甸植被进行了调查与分析。结果表明:露营活动对山地草甸植被造成了较为明显的影响,而且不同区域的草甸植被,因承载的活动不同,露营活动强度也不同,其干扰程度也存在差异。受露营活动干扰后,草甸植被的外观特征变化较大,盖度减少率(CR)、高度减少率(HR)、植物区系组成变异度(FD)以及干扰指数(IVI)均明显增加;同时,草甸植被的多样性也受到了一定的影响,山地草甸的多样性指数(H')明显增加,均匀度指数(E)也略有增加,优势度指数(C)则明显降低。     

棉花生长初期灌溉信息遥感提取与校正

为提高生长初期低覆盖度作物长势的遥感监测精度,需要消除灌溉引起的土壤水分背景变化对归一化差值植被指数(NDVI)的影响。为了实现棉花生长初期灌溉信息提取与校正,提高棉花作物长势监测与产量预判精度,本文以美国加利福尼亚州San Joaquin Valley的2个棉花地块为研究区,选取棉花生长初期灌溉过程中的遥感影像,构建两种灌溉信息提取方法(分阶段阈值法和灌溉线提取法),确定最优灌溉像元提取方法;比较分析灌溉与未灌溉情况下棉花的NDVI与归一化差值水分指数(NDWI)以及土壤调节植被指数的关系,提取含有灌溉信息的像元,并对NDVI进行校正,消除灌溉对NDVI的影响。研究结果表明:在棉花生长初期,灌溉与未灌溉像元NDVI变化率达12%,差异较显著;灌溉与否的棉花NDVI与NDWI间均存在极显著的线性关系,决定系数在0.80以上;利用灌溉线方法提取灌溉信息与分阶段阈值相比精度更高,精度达88%以上;校正后线性回归模型精度达0.95,灌溉校正效果明显,灌溉与未灌溉像元的NDVI差异减小至2%。本研究通过对含有灌溉信息像元NDVI值的校正,去除灌溉对NDVI造成的影响,反映了真实的植被信息,可实现对作物生长初期长势的准确遥感监测,为遥感定量监测提供便利。