| The proportion of cutting/forestation area is ususally adopted to quantify vegetation changes instudies of eco-hydrological influences of forest ecosystems. This may rational to some extentin quantifying vegetation changes, the change of vegetation in quality and spatial patternhowever, can not be mirrored. Leaf area index (LAI) can not only describe quantitative andqualitative characteristics of vegetation, but also exhibits its efficacy mapping spatial pattern aswell as temporal dynamic of vegetation. Sub-alpine dark coniferous forest in western Sichuanprovince, which is one of major natural vegetation types in the upper reaches of Yangtse River,plays a crucial role in waterhead protection, soil and water conservation and ecologicalequilibrium maintaining. Several key issues were addressed in this paper based on fieldobservation and experiment at both catchment scale and at stand scale, firstly, the quantitativeanalysis of vegetation structure and spatial pattern of Minjiang fir forest Minjiang fir (Abiesfaxoniana) stand, particularly the spatial and temporal pattern in terms of LAI; secondly,ecohydrological functions including canopy interception, soil moisture variability andevapotranspiration; thirdly, the spatio-temporal distribution of wetness index in the catchmentwas simulated by using TOPOG model, and the relationship between wetness index andvegetation, terrains, soil and climate; lastly, responses of these above-mentionedeco-hydrological functions to vegetation changes were therefore evaluated. The objective ofthis paper is to reveal the interactions between sub-alpine dark coniferous forest and water andilluminate the mechanism in regulating stream flows. The results showed as follows:The dominant vegetation in the catchment is Minjiang fir natural forest (occupies 88% of thecatchment area) and the secondary species are mainly Rhododendron przewalskii shrubs,(occupies about 12% of the catchment area). Minjiang fir forest can be classified into thefollowing four stand types based on difference in their understories: moss-bamboo-fir,grass-bamboo-fir, moss-rhododendron-fir and grass-rhododendron-fir. Single peak curve ofLAI seasonal change pattern occurred in sub-alpine dark coniferous forest during growingseason, with the peak value of 5.82±1.32 in the mid-August. Four stands showed significantdifferences in their LAI values (p＜0.001), with the mean LAI value of 5.44±0.83. The highestLAI occurred in the moss-bamboo-fir, followed by the grass-bamboo-fir and thegrass-rhododendron-fir, the lowest LAI value was in the moss-rhododendron-fir. Withincreasing altitude, LAI increased slightly at the beginning and followed by a dramaticallydecrease with the increasing elevation when exceeded 3000 m. LAIs at northeast-aspect standswas larger than those at southwest-aspect, the difference of LAIs between the two aspectswere signifcant (p＜0.05). The coefficient of variability (CV) of LAI at whole catchment scale is varies from 19.79% to 22.80%, this value was approximate twice in amount compared withstand scale (10.02～12.22%). CV varied with altitudinal gradients, the highest CV (23.09%)was occured at the elevation of 3800 m, while the lowest (7.64%) was at 3500 m. The CV atnortheast-aspect (16.58%) was higher than that of southwest-aspect (13.44%).Furry and moderate rains are the majority in this area and with abundant rainy days. Monthlyinterception rate of bamboo-Minjiang fir old-growth ranged from 33% to 72%, with theaverage of 48%. In growing season, there was a linear or powerful or exponential relationshipbetween rainfall and interception volume and a negatively exponential relationship betweenrainfall and interception rate when the amount rainfall was less than 10mm. The meanmaximum canopy interception by the vegetation in the catchment of 1.44km~2 in area was1.74mm and the marked differences among the five communities occurred in the followingsequence: moss-bamboo-fir＞grass-bamboo-fir＞moss-rhododendron-fir＞grass-rhododendron-fir＞rhododendron shrub. The simulated value of canopy interception rate,maximum canopy interception rate and addition interception rate of the vegetation in thecatchment were 39.24%, 25.36% and 13.88%, respectively. Simulation of the canopyinterception model was poor at daily scale but it is better at the overall growing season scale.The mean relative error of daily canopy interception was 33～35% and that over the wholegrowing season was 9～14%. In addition, the litter and moss layer had a higher water holdingcapacity (WHC) due to moss plant and more litter accumulation, the mean maximum waterstorage was 10.5mm, the highest was up to 20mm.Sub-alpine dark coniferous forest ecosystem showed a deep soil layer, a low bulk density, ahigh soil porosity, a fast infiltration velocity and a high soil water holding capacity. The meanvalues of these parameters were respectively 80～120cm, 0.96g/cm~3, 65%(total porosity)and11% (noncapillary), 2.63 mm/min(saturated soil infiltration rate), 414.3mm(0～40cm soil waterstorage)and 51.4mm(0～40cm noncapillary water storage). Interestingly, these parametersshowed slightly differences among different communities while significant differences amongdifferent soil depths. From July to October, the mean volumetric soil moisture content in thecatchment varied from 0.59 to 0.66. The highest soil moisture content occurred in thesubsurface soil layer while the lowest soil moisture content in below 45cm soil depth. Withascending of altitude, increasing trend was firstly found in soil moisture content (SMC),however, when altitude exceeded 3700m, SMC showed inversely decreasing trend. Comparedwith southwest-aspect slope, northeast-aspect slope showed higher SMC in 0～30cm soil depthwhile lower SMC in soil depth below 30cm. Soil moisture of the lower slopes were higherthan those of upper slopes. The coefficient of variability (CV) of soil moisture at catchmentscale was 0.22～0.41. The CV increased with increasing SMC while decreased with increasingsoil depth. The CV at northeast-aspect was higher than that at southwest-aspect, at lower slope higher than that at middle slope and upper slope.Annual actual evapotranspiration (ETa) in sub-alpine dark coniferous forest ecosystem wascaculated by using modified Priestley-Taylor formula and Rithchie (1972) method, was350.8mm, growing season, the seasons when this study were conducted, was found accountedfor 55% of the annual ETa. Significant difference was found in ETa of different communities(p＜0.001), the highest ETa occurred in the grass-bamboo-fir (422.0mm), followed by themoss-rhododendron-fir(363.1mm), the grass-bamboo-fir(355.6mm)and the moss-bamboo-fir(323.4mm), the lowest LAI value was in the rhododendron shrubs (248.4mm). Total solarradiation is the predorminant factor of evapotranspiration. ETa was relatively lower thanexpected (about 35% of rainfall), this may due largely to a fact that more rainy and foggy daysand less sunlight hours and solar radiation in growing season in this area.Wetness index at the bottom of the catchment was higher than that at hill slope, at the middleelevation higher than that at the lower elevation and higher elevation, and at northeast-aspecthigher than that at southwest-aspect. The highest wetness index occurred in the rhododendronfir stands, followed by the bamboo-fir stands, the lowest wetness index was in rhododendronshrubs. Moreover, wetness index increased with the increases of precipitation, solar radiationand vegetation density.Sub-alpine dark coniferous forest ecosystem showed strong capacity of rainfall interceptionand soil water storage, with a sum of 89.5mm, including 86% of soil, 12% of litter and mossand only 2% of canopy. Different sub-alpine dark conifer forest types showed significantdifferences in their canopy interception, litter and moss interception and soil water content andevapotranspiration. The decreasing sequence of the total capacities are as follows:moss-bamboo-Minjiang fir, grass-bamboo-Minjiang fir, moss-rhododendron-Minjiang fir,grass-rhododendron-Minjiang fir. As expected, strong linear relationship was found betweenleaf area index (LAI) and maximum canopy interception. With the increasing vegetationcoverage and LAI, maximum canopy interception and evapotranspiration increase whereas soilmoisture decrease. When vegetation coverage is higher than 70% or LAI value greater than 4.8,the increase in evapotranspiration is not evident. |
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