Formation mechanisms and remediation techniques for low-efficiency artificial shelter forests on the Chinese Loess Plateau

The construction of artificial shelter forests(ASFs)has resulted in substantial ecological,economic,and societal benefits to the Chinese Loess Plateau(CLP).However,the health and benefits of ASFs are being increasingly threatened by the formation of low-efficiency artificial shelter forests(LEASFs).In this study,LEASFs are systematically analyzed in terms of their status,formation mechanisms,and developmental obstacles.The key restoration techniques and schemes were summarized to improve the quality and efficiency of LEASFs.LEASFs are formed by relatively complex mechanisms,but they arise mainly due to poor habitat conditions,improper tree species selections,mismatch between stands and habitat,extensive forest management measures,and human interferences.The restoration and improvement of LEASFs are hindered by water deficits,mismatch between stands and habitat,single management purpose,and low efficiency.LEASFs are becoming more complex due to their wide range,the challenges associated with their restoration,and insufficient technological measures for their restoration.The key techniques of the quality and efficiency improvement of LEASFs include basic forest tending methods,near-natural restoration,multifunction-oriented improvement,and systematic restoration.An understanding on the formation mechanisms of LEASFs and a scientific approach toward their restoration are urgently needed and critical for the ecological protection and high-quality development of LEASFs on the CLP.Based on these analyses,we recommend strengthening the monitoring and supervision of LEASFs,considering the bearing capacity of regional water resources,implementing multiple restoration techniques,promoting multifunction-oriented ecological development,and exploring new management concepts to achieve the sustainable development of ASFs on the CLP.

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.

Path Analysis on Environmental Factors Controlling Runoff and Sediment Yields in Shelter Forests in Three Gorges Reservoir Region

Effects of environmental factors such as climate,topography,vegetation and soil in shelter forests in Three Gorges Reservoir Region on runoff and sediment yields were monitored to identify dominant environmental factors controlling runoff and sediment yields in 15 runoff plots in study area by soil sampling,laboratory analysis,stepwise regression analysis and path analysis,and to establish the main control environmental factors that affect runoff and sediment yields. The results showed that soil bulk density,herbaceous cover,slope,and canopy density were the significant factors controlling runoff,and the direct path coefficient of each factor was ranked as canopy closure(-0. 628) > litter thickness(-0. 547) > bulk density( 0. 509) > altitude( 0. 289). The indirect path coefficient was ranked as soil bulk density( 0. 354) >litter thickness(-0. 169) > altitude( 0. 126) > canopy closure(-0. 104). Therefore,canopy closure and litter thickness mainly had direct effects on runoff,while soil bulk density mainly had indirect effects through their contributions to other factors. Herbaceous cover,litter thickness,slope,canopy density,and altitude were the significant factors controlling sediment yields. The direct path coefficient of each factor was ranked as herbaceous cover(-0. 815) > litter thickness(-0. 777) > canopy closure(-0. 624) > slope( 0. 620). The indirect path coefficient was ranked as slope( 0. 272) > litter thickness(-0. 131) > canopy closure(-0. 097) > herbaceous cover(-0. 084). Therefore,herbaceous cover and litter thickness mainly had direct effects on sediment yields,while slope mainly had indirect effects through their contributions to other factors. All the selected environmental factors jointly explained 85. 5% and 78. 3% of runoff and sediment yield variability,respectively. However,there were large values of remaining path coefficients of other factors influencing runoff and sediment yields,which indicated that some important factors are not included and should be taken into account.

Nutrient retranslocation strategies associated with dieback of Pinus species in a semiarid sandy region of Northeast China

In the semiarid sandy region of Northeast China,Mongolian pine(Pinus sylvestris var.mongolica)suffers dieback after the age of 35,while Japanese red pine(Pinus densiflora)and Chinese pine(Pinus tabuliformis)stay healthy.Foliar nutrient retranslocation reflects the nutrient conservation and utilization mechanism of plants in response to their habitats.However,the nutrient retranslocation strategies employed by three Pinus tree species to cope with nutrient limitations remain largely unknown.For this study,we investigated the seasonal variations in nitrogen(N)and phosphorus(P)concentrations of Mongolian pine,Japanese red pine,and Chinese pine plantations in terms of the green needles of all ages,senesced needles,and soil.Further,the N retranslocation efficiency(NRE),and P retranslocation efficiency(PRE),and correlations between the N:P ratios of needles and soil were analyzed.The results showed that,except for the spring NRE in 1-year-old needles of Mongolian pine,the spring NRE and PRE in 1-and 2-year-old needles of the three tree species were greater than zero.The autumn PRE was higher than zero for Mongolian pine,but lower than zero for Japanese red pine and Chinese pine.Among the three Pinus species,Mongolian pine showed greater spring PRE in 2-year-old needles,and PRE from 1-to 2-yearold needles,and from 2-year-old needles to litter.However,Japanese red pine had higher P concentrations and lower N:P ratios in senesced needles,while greater PRE was found in Chinese pine litter.Significant relationships between the N:P ratios were found in the current year and 1-year-old needles and soil in the Mongolian pine plantation,while there was an insignificant relationship between the N:P ratios of the needles and soil in the Chinese pine plantation.Thus,for Mongolian pine,the removal of P from needles in autumn,and higher P translocation from older needles under P-deficient soil may have contributed to the tree dieback.In contrast,Japanese red pine and Chinese pine stored P in their needles during autumn.Japanese red pine returned more P to the soil via litter,while Chinese pine maintained N:P homeostasis and increased P withdrawal prior to needle abscission.