Spatial Distribution,Structural Characteristics And Evapotranspiration Of Qinghai Spruce Forests In The Qilian Mountains,Northwest China

Qilian mountain,as the headstream of Heihe,Shiyang river,Shule river,is located in the arid zone of northwest China.The forest ecosystem plays an important role in various forest service functions such as water supply,hydrology regulation,soil retention etc.Qinghai spruce?Picea crassifolia?forest is the main constructive specie,accounting for 79.6%of the total area of Qilian Mountain,has a pivotal role on hydrologic cycle and regulation of Qilian mountains,and other ecological service functions.Due to the history of human activities,and continuously strengthened effects of climate change,the Qinghai spruce forest confronts the issues of area reducing,the quality degradation,function weakened,which needs efficient protection,recovery and reconstruction and management.This research based on the collected data and supplemently measured data.These data were collated to form a set of ecological hydrological parameters of Qinghai spruce forests in the Qilian Mountains,which has been submitted to the Cold and Arid Regions Scientific Data Center for free download by researchers.After in-depth analysis of these data,the main results are shown as follows.?1?Spatial distribution of Qinghai spruce forests in a typical watershed.Elevation and slope aspect are the key factors determining the distribution of Qinghai spruce forests.The potential core distribution area of denser forests?the relative forest coverage>0.3?and potential distribution area?the relative forest coverage>0?including sparse forests are between the axes of elevation?2635.5–3302.5 and 2603.4–3325.8 m a.s.l.?and slope aspect?-74.4–61.2°and-162.6–147.1°deviated from north?.The corresponding threshold of mean annual air temperature at the upper elevation boundary is-2.59 and-2.73?,while the threshold of mean annual precipitation at the lower elevation boundary is 378.1 and 372.3 mm,respectively,in Dayekou watershed.Using these thresholds and the elevation gradients of climatic factors,the shifting of elevation boundaries under climate change scenarios can be predicted,if the temperature rises by 0.45?,the upper elevation boundary will increase by 77.6 m,if the precipitation increases by 20 mm,the lower elevation boundary of the potential and potential core distribution area will shifted down by 108.4 m and 109.5 m,respectivaly.However,the forest distribution is also limited by a soil thickness of?40 cm and by slope position of lower-,lower-and middle-,and entire-slope within the elevation ranges of<2800,2800 to 2900,and>2900m a.s.l.respectively.When adding the considered factors as the altitude,slope direction,soil thickness and slope positions successively,the influence of the prediction accuracy of the spatial distribution of Qinghai spruce forest were 55.8%,71.5%,73.9%,71.5%,showed that both considering the influence of climate and terrain factors will greatly improve the prediction of forest distribution area in dryland mountains.?2?Growth characteristics of Qinghai spruce forests in Qilian Mountains.Qinghai Spruce is a slow-growing tree species,the individual tree height and DBH growth began to accelerate after 10 years,matured at about 80 and 100 years,respectively,and the growth of timber volume tended to stabilize after about 120 years.Tree height decreased with increasing density,but mainly in the density range of 500-2000 tree/ha.The response of DBH to tree density was more sensitive than tree height,decreased more drastically with increasing density.The stand volume of Qinghai spruce forest was shown to increase with the increasing forest density.The changing rate of tree height,DBH and stand volume all became slowly when the density higher than 2000 tree/ha.The variations of tree height and DBH with elevation were greatly,and the maximum were both at the elevation range of 2700-3100 m.The maximum of stand volume occurred at the elevation of 2900 m.The maximum values of tree height,DBH and stand volume were all found on shady slopes?-45°to 45°?.Tree height,DBH and stand volume basically remained stably when the slopes below 40°,40°,and 35°,but decreased sharply after these thresholds.By multiplying the function of height,DBH,and volume of Qinghai spruce forests responsing to single factor,coupling models responding to forest age,density,elevation,and slope aspect were established.Changing of height,DBH,and volume of Qinghai spruce forests responding to forest age,density,elevation,and slope aspect can be predicted by using these coupling models,which can provide a quantitative decision support tool for the rational management of the Qinghai spruce forests in our study area.?3?Biomass and vertical structure characteristics of Qinghai spruce forests in Qilian Mountains.The proportions of the organ biomass of individual tree and Qinghai spruce forest were basically consistent,the trunk and root generally accounted for 75%;the branches and bark were both approximately 10%;the leaves were approximately 5%.The proportion of rootlet?<1cm?accounted for root biomass increased with age,with an average of 12.20%,while others varied little with age,middle roots?1³cm?accounting for 19.29%,thick roots?3⁵cm?and collar?>5cm?accounted for 68.21%.The biomass of Qinghai spruce forest increased with the increasing of forest density and canopy density,but it changed slowly after density larger than 1500 tree/ha or canopy density larger than 0.8.The tree height estimating model considering DBH,basal area at breast height,mean dominant height and densities of Qinghai sprucew was the best,but the fitting accuracy varied under different elevation,slope locations,and densities.The equation of estimating LAI baesd on the SLA and leaf biomass of Qinghai spruce forests was determined,which can be used to predict the changes of LAI.The coverage of understory shrub layer in Qinghai spruce forest was less than 10%,and the species richness was low,both of them firstly increased and then decreased with canopy density,reached the maximum at canopy density of 0.5.The coverage of understory grass layer was higher than shrub layer,but the biomass was lower,reached the maximum when canopy density was about 0.5.The coverage and biomass of moss layer increased slowly with the increasing of canopy density,and the growth rate was significantly slower after canopy density larger than 0.7.The thickness and biomass of the litter layer increased with the increasing of canopy density,and the growth rate slowed down significantly when the canopy density was higher than 0.7.The quantitative relationships between the structural characteristics were established by using the bourndary line method,which facilitates the conversion between structural characteristics.?4?Evapotranspiration of Qinghai spruce forests in a typical watershed.The canopy interception of Qinghai spruce forest increased with the increasing amount of single rainfall,and it became increasingly saturated due to the maximum capacity of the canopy interception.While the interception rate decreased with increasing amount of single rainfall,and is generally higher than 50%when the single rainfall is<5 mm,and is generally lower than 10%when the single rainfall is>30 mm,and the annual interception rate is generally 25-35%when the annual precipitation is among 300-600 mm.Models that considering the canopy density or the maximum interception capacity determined by the LAI,have a high simulation accuracy for both single rainfall and annual precipitation.In Pailugou watershed,the annual average transpiration of Qinghai spruce forest was 0.45 mm·d^-1,the maximum transpiration of2002-2008 and 2015 appeared at 205^th day(2.28 mm·d^-1)and 207^th day(2.13 mm·d^-1),respectively.The annual average understory evapotranspiration of Qinghai spruce forest was0.71 mm d^-1,and the maximum value of 2014 and 2015 were at 198^th day(2.76 mm d^-1)and210^th day(2.25 mm d^-1),respectively.?5?Spatial distribution of water yield of Qinghai spruce forests in a typical watershed.The forest transpiration increased parabolically with the increasing of potential evapotranspiration?PET?,and showed an exponential increase with increasing of REW and LAI.The understory evapotranspiration gradually increased with the increasing of PET and REW exponentially,and firstly decreased rapidly then gradually decreased exponentially with the increasing of LAI.By using the canopy intercept model of Qinghai spruce forest and coupling models of the forest transpiration,understory evapotranspiration response to LAI,PET and REW,the water yield depth and spatial distribution of Qinghai spruce forest in Dayekou watershed in 2008 were estimated based on the water balance.It was found that the water yield depth of Qinghai spruce forest generally increased with the elevation and the inclination of the shady slopes,and decreased as the forest growth condition became better.This study determined the spatial distribution of Qinghai spruce forest under the combined influence of climatic characteristics?temperature,precipitation?and site environment?altitude,aspect,thickness of soil layer,slope position?in the Qilian Mountains,and the threshold and quantitative relationship between the main factors.And established coupling models of Qinghai spruce foest stand structures responing to age,density,and site factors.And established coupling models of Qinghai spruce forest evapotranspiration responsing to multi-factors,based on the quantitative analysis of forest evapotranspiration components?canopy interception,forest transpiration,and understory evapotranspiration?in response to potential evapotranspiration,soil moisture,canopy structure and other factors.Using these results,it can guide the determination of suitable locations of the Qinghai spruce forests in the study area and its response to climate change,dynamically predict the change characteristics of stand structure and growth of Qinghai spruce forests,and predict the temporal and spatial changes of evapotranspiration and water production under climate change and operating management.These results can provide certain theoretical and technical guidance for the restoration,management and quality improvement of Qinghai spruce forest in Qilian Mountains,also have reference value in research methods for similar studies in other regions.