| China's forestry has begun to shift from the long-term single-target management to future multi-functional management.Forest stands will serve as the basic spatial unit for such multi-functional management.The quantitative understanding and prediction of the spatio-temporal variation of stand structure index and their effects on the various forest services is the precondition of multifunctional management decision-making.In the Loess Plateau region of northwest China,where it is characterized by a arid climate,sparse vegetation,severe soil erosion and water shortage,fragile environment and strong forest-water conflict,there are outstanding and specific requirements on the multifunctional forest management.This study was carried out for the larch plantation in the Liupan Mountains in Ningxia,a very important water source area and also a major forest area for this region.Due to the excessive pursuit of forest area and timber stock and long-term non-management,the larch plantation here is too dense and has a single-layered structure,insufficient natural regeneration,poor stability,and significant water yield reduction,thus the multiple services/functions of forests cannot be fully used and the regional water supply safety is seriously threatened.In order to solve the above problems,the spatio-temporal variation of forest stand structure was analyzed using the data of 40 newly investigated temporary plots during the growing season of 2016and 2017 and 231 plots accumulated before in the small watershed of Xiangshuihe and surrounding areas;the eco-hydrological processes were monitored on 16 fixed plots on a typical slope.Based on the quantification and modeling of the effects of site conditions and stand structure on the main services/functions of forests(such as water yield,timber production,carbon sequestration,and plant diversity protection),the decision-making procedure for multifunctional forest management was developed,in which both the predominant service of water yield and other important services/functions are considered.The rational tree density range for multifunctional management was determined for stands on representative sites(elevation)and with different ages.All theses can serve as a theoretical basis and technical tool for multifunctional forest management.The main results are as follows:1.Spatio-temporal variation of main forest structure indexThe spatio-temporal variation of main stand structure index(forest canopy leaf area index(LAI),canopy density,stand mean tree height and DBH,etc)with forest age,stand density and site conditions(elevation)was analyzed.The mean tree height increases quickly with rising tree age before 30 years and thereafter gradually,decreases gradually with rising tree density but almost unchanged at the low density range,and increases first and then decreases with rising elevation with a peak at the elevation of around 2400 m.The stand mean DBH shows similar variation trend as the tree height.It increases gradually with rising forest age,e.g.,a large growth rate it still maintained at the age of 50years.The mean DBH decreases firstly rapidly with rising tree density,and then slowly after reaching 900 tree/hm~2.With rising elevation,the mean DBH increases firstly and then decreases,with a peak at the elevation of 2400 m.The forest canopy LAI increases firstly rapidly with rising tree density,but gradually after the density of 1100 tree/hm~2for reaching its maximum.The response to forest age and altitude showed a single-peak curve,reaching the peak at 30 years and at 2500 m.The canopy density is only affected by the tree age and stand density.It increases rapidly with rising forest age and density,and then gradually.When the stand density exceeds 1800tree/hm~2and the forest age is more than 45 years,it reaches its maximum and does not increase any more.Based on the comprehensive analysis above,the coupling models of stand structure index with main influencing factors were established,which lays a foundation for analyzing the variation of forest services/functions.2.Variation of timber production service and its managementWith rising tree density,the single-tree volume decreases gradually,but the stand timber volume increases rapidly,reaching the maximum at 1600 tree/hm~2,and thereafter decreases slowly.Both the single-tree volume and stand timber volume increases with rising forest age,and increases firstly and then decreases with rising elevation and reaching the peak at the elevation of 2500 m.Based on the tempo-spatial variation of mean tree height and DBH of forest stands,the prediction models were established.It was found that the maturity age of larch plantation is 50years,and both the mean single-tree volume and stand timber volume still maintain a rapid growth potential at the age of 60 years.Therefore,it is necessary to greatly extend the traditional short rotation period of 26 years or convert to rotational management to the close-to-nature and multifunctional management which is characterized by the cultivation of high-quality and large-diameter target trees.According to the difference of stand mean single-tree volume and stand timber volume with elevation,the timber production function in the Liupan Mountains region was zoned:the optimal elevation range of 2230-2700 m,the suitable elevation ranges of 2050-2230 m and 2700-2900 m,and the non-suitable elevation range of<2050m.3.Variation of understory vegetation protection service and its managementBased on the upper boundary line of the undergrowth vegetation characters against the stand canopy LAI,the response of understory vegetation to the change of stand canopy was analyzed.It was found that with rising LAI,the number of understory plant species increases at first and then decreases,and reaches its peak(34)at the LAI of 3.38.The coverage of shrub layer decreases slowly with rising LAI,but increases gradually after LAI of 1,and reaches its maximum when LAI was 3.25,and thereafter decreases gradually;the coverage of herb layer remains stable at high level before LAI reaches at 2.25,then decreases gradually and decreases rapidly after LAI is 3.1.With rising LAI,the biomass of undergrowth(both shrubs and grasses)remains stable before LAI is 2.0,and then decreases slowly in the LAI range of 2.0-3.0,and thereafter decreases rapidly.After giving a weight of 0.63 to the number of plant species,0.26 to the vegetation coverage and 0.11 to the biomass of undergrowth vegetation,the required tree density ranges at different elevations and ages were determined for considering the protection requirements of undergrowth plants.4.Variation of forest carbon sequestration service and its managementThe variation of carbon pool composition and their total amount with elevation,tree density and age was studied.The mean carbon density(t/ha)and its percentage of all plots with different age and elevation is 33.97(12.3%)for tree layer,8.93(4.0%)the understory vegetation layer,9.0(3.3%)for the humus layer,and 196.4(80.3%)for the root-zone layer(0-100cm soil layer),respectively.The vegetation carbon density of tree layer increases rapidly with rising tree density,reaching its peak at the density of 2000 trees/ha,and then decreases slowly;but increases gradually with rising tree age,and increases first and then decreases with rising elevation.The carbon density of soil layer decreases rapidly with rising tree age firstly,due to the disturbance of soil during clear cutting afforestation and the forestland exposure during the young stand period,reaching the lowest values at the ages of 22,20,18,20 years at the elevations of 1800,2100,2400,and 2700 m,and thereafter recovers gradually.At the elevation of 1800,2100,2400,2700 m,the required tree ages for recovering to the pre-afforestation level are 48,27,24,29 years at the elevations of 1800,2100,2400.2700 m,and thereafter the carbon density begins to increase gradually and to be stabilized slowly.The variation trend of total carbon density of forest ecosystems with the main influencing factors is basically the same as that of soil carbon density.From the point of view of maintaining and improving the carbon pool of soil and the whole carbon sequestration function of forests,the continuous covering of forest land should be pursued in afforestation process and forest management periods,so as to prevent afforestation after clear cutting and excessive cutting.5.Variation of water yield from forest stand and its managementBased on the eco-hydrological monitoring results of fixed sample plots and the water balance data of sample plots accumulated for many years,the coupling model of stand evapotranspiration and its components response in growing season were established for reflecting their responses to main influencing factors.The variation of water yield from forestland in the growing season with tree density,elevation and forest age in different precipitation years was analyzed by using the difference between precipitation and evapotranspiration based on the principle of long-term water balance.The growing season water yield increases with rising precipitation and decreases near linearly with rising LAI.When the LAI exceeds some threshold values,it will produce negative water yield because the evapotranspiration is higher than the precipitation,that means that the forest/vegetation must consume soil water or the input water from upper slope to survive,and only when LAI is below these threshold values the net water yield can be formed.In order to maintain or increase the water yield from forestland,it is necessary to properly reduce the tree density and forest canopy LAI,to ensure the safety of water supply as the predominant service/function of forests and the restriction condition of multifunctional forest management,but to avoid an excessive reduction of tree density resulting in excessive reduction of other main services/functions.According to the potential of timer production and other services/functions at different sites and the effects of stand structure on water yield,the tree density should be controlled in a reasonable range in order to improve the overall value of forest ecosystem services.6.Multifunctional forest management decision-making and typical casesThe procedure of trade-off based decision-making for multifunctional(density)management at different site(elevation)with different tree ages is proposed,based on the variation regulation of each single service with changing elevation,tree density and age.The first step is to define the preconditions clearly.i.e.,firstly to determine the basic tree density range which is corresponding to the canopy density of 0.6-0.8 for guaranteeing the stand stability;secondly is to determine the importance order of the dominant and other forest services,based on the ecosystem services supply potential of forest and their management requirements;And then an optimal tree density range should be determined for each single service by defining this density range can assure that more than 90%of this service can be realized,or a suitable tree density range can be determined for each single service by defining more than 80%of this service can be realized if it is difficult to meet the needs of optimal tree density.The second step is the trade-off decision-making of multifunctional forest management.The overlapping tree density range among the basic tree density range and all the optimal(or suitable,if the optimal tree density ranges do not apply)tree density ranges of all single services can be determined and used as the multifunctional management density range.If such overlapping tree density does not exist,the non-dominant services can be no more fully considered,or be properly sacrificed,for determining the management tree density ranges.According to the principles that the management tree density range of older stands needs to be less than or equal to the tree density of younger stands,and the strength of each thinning should be not more than 20%and a time span of at least 2-3 years is required between two thinning,the calculated multifunctional management tree density range can be properly adjusted,so that these tree density ranges are more reasonable and feasible.Using the above-mentioned trade-off decision-making procedure,the optimal(or suitable)density ranges for multifunctional management were determined at different ages for some typical sites,such as the low-elevation(1800 m)dry site which is not suitable for timber production,the relatively humid site with middle-low elevation(2100 m)and the wet-cold site with high elevation(2400m)both of which are suitable for timber production,and the optimal site with middle elevation(2700 m)which is optimal for timber production.Then the corresponding dominant and other main services under such multifunctional management densities are evaluated in comparison with the traditional high-density stands. |
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