Causal effects of shelter forests and water factors on desertification control during 2000–2010 at the Horqin Sandy Land region, China

The Horqin Sandy Land（HSL）, the largest sandy land in the semi-arid agro-pastoral ecotone of Northeast China, has been subject to desertification during the past century. In response, and to control the desertification,government implemented the Three-North Shelter/Protective Forest Program, world＇s largest ecological reforestation/afforestation restoration program. The program began in1978 and will continue for 75 years until 2050. Understanding the dynamics of desertification and its driving forces is a precondition for controlling desertification.However, there is little evidence to directly link causal effects with desertification process（i.e., on the changing area of sandy land） because desertification is a complex process,that can be affected by vegetation（including vegetation cover and extent of shelter forests） and water factors such as precipitation, surface soil moisture, and evapotranspiration.The objectives of this study were to identify how influencing factors, especially shelter forests, affected desertification in HSL over a recent decade. We used Landsat TM imagery analysis and path analysis to identify the effects of spatiotemporal changes in water and vegetation parameters during2000–2010. Desertification was controlled during the study period, as indicated by a decrease in desert area at a rate of163.3 km2year-1and an increase in the area with reduced intensity or extent of desertification. Total vegetation cover in HSL increased by 10.6 % during the study period and this factor exerted the greatest direct and indirect effects on slowing desertification. The contribution of total vegetation cover to controlling desertification increased with the intensity of desertification. On slightly and extremely severe desertified areas, vegetation cover contributed 5 and 42 % of the desertification reduction, respectively. There were significant correlations between total vegetation cover and water conditions（i.e., evapotranspiration and precipitation）and the area of shelter forests（P ／ 0.0001）, in which water conditions and the existence of shelter forests contributed49.7 and 12.8 % to total vegetation cover, respectively. The area of shelter forests increased sharply due to program efforts, but only shrub forests had significant direct effects on reducing the area of desertification categorized as slightly desertified. The reason for the lack of direct effect of increased arbor forests（accounting for 95.3 % of the total increase in shelter forests） on reducing desertification might be that the selected arbor species were not suited to water conditions（low precipitation, high evapotranspiration） prevailing at HSL. The establishment of shelter forests aided control of desertification in the HSL region, but the effect was less than expected. Effective control of desertification in the HSL region or other similar sandy areas will require greater improvements in vegetation cover. In particular,shrub species should be selected for plantation with reference to their potential to survive and reproduce in the harsh climatic and weather conditions typical of desertified areas.

三北防护林体系建设工程区森林水源涵养格局变化研究

三北防护林体系水源涵养功能是三北地区生态环境状况的重要指示器,然而,三北防护林体系水源涵养研究仍较缺乏。动态评估三北防护林体系建设工程区(三北工程区)森林水源涵养功能及其影响因素,对科学认识、保护和调控三北防护林体系森林水源涵养,制定三北工程植被建设与保护决策具有重要意义。以三北工程区森林为研究对象,通过收集和分析相关数据,在植被分区的基础上,分析三北工程区森林水源涵养时空格局与变化特征,对比研究各区不同森林类型水源涵养功能差异,揭示各区森林水源涵养功能与地形及森林状况与质量的定量关系。结果表明:(1)三北工程区森林水源涵养功能持续增强,单位面积水源涵养量从1990年的73.92mm增加到2015年的75.14mm,空间格局呈东高西低、南高北低态势。(2)森林水源涵养功能在植被分区和森林类型间差异显著,森林植被区是三北工程区森林水源涵养的主体;针阔混交林是三北工程区水源涵养功能最强的森林类型。(3)三北工程区森林水源涵养受其地形、状况与质量的影响显著,除个别植被区外,各区森林水源涵养量随坡度、覆盖度和NPP增加而增大,随生物量增加而降低,这是区域植被适应及滥砍滥伐、毁林开垦、植被建设与保护等人为干扰共同作用的结果。因此,可通过调整与优化林分结构,调控区域森林水源涵养功能。

三北工程区降水量长时间序列与多尺度变化趋势检验及预测

三北防护林体系建设工程区(以下简称"三北工程区")早期的植被建设忽略了水资源承载力,对三北防护林的可持续维护产生了不利影响。为落实"以水定林草"的发展理念,在三个空间尺度上,基于1951—2018年降水量,采用Mann-Kendall非参数检验方法、自回归积分滑动平均模型(Autoregressive Integrated Moving Average Model,ARIMA)和地理信息系统空间分析等方法,开展全年、生长季和非生长季降水量多尺度变化趋势与未来30年预测研究,结果表明:"三北工程区"全年和生长季降水量呈增长趋势的面积百分比分别为73.64%和70.10%,主要分布在西北荒漠区;非生长季降水量呈增长趋势的面积比例达92.06%,除黄土高原南部和风沙区的少部分地区而外,均呈增长趋势。全年、生长季和非生长季降水量呈增长趋势且置信度为90%以上的面积百分比分别为45.43%、37.31%和36.79%。18个重点建设区的雷达统计图显示:生长季与全年降水量的变化趋势一致,由东向西,松辽平原等7个区域以不显著减少趋势为主,松嫩平原等7个区域以不显著增长趋势为主,西部的柴达木盆地等4个区域以显著性达到90%或95%的增长趋势为主;非生长季除晋陕峡谷、泾河渭河流域以非显著减少趋势为主而外,其他地区均以增长趋势为主。5个"重点县"的降水统计量UFk与其反序统计量UBk两条曲线出现交点,表明年降水量有突变发生,库尔勒市、磴口县、科尔沁左翼后旗UFk与UBk曲线多处出现交点,表明年降水量突变发生频繁。采用ARIMA预测得出未来30年的年降水量,计算得到未来30年间的年降水量变化数据,并绘制其空间分布图。本研究可为三北工程区开展基于水资源承载能力的林草资源优化配置提供基础数据,为发展"雨养林草植被"提供科学支撑。