依据野外实测的蒸腾速率对几种沙地灌木水分平衡的初步研究.pdf

植物生态学报1997,21(3)208225Acta Phytoecologica Sinica依据野外实测的蒸腾速率对几种沙地灌木水分平衡的初步研究?董学军张新时杨宝珍(中国科学院植物研究所,北京100093)摘要本文旨在将毛乌素沙地植被建设的水分平衡与半固定沙丘持续发展原则应用于治沙造林的实践中。毛乌素沙地是一个灌木“王国”,然而沙地灌丛植被的发育常常受到水分亏缺的严重制约。为此,根据水分平衡的原则与方法确立适宜的植物种植密度,对沙地植被的经营管理具有重要的指导意义。在水分平衡研究中,蒸散是最难估计的一项。本文提供了一种根据叶面积指数的季节变化与蒸腾速率的观测资料计算蒸腾耗水量的方法,并根据沙地水分平衡的要求估算了几种优势灌木的适宜种植密度。结果表明,毛乌素沙地灌丛生态系统的蒸发散主要来自植物蒸腾作用;在所研究的植物当中,除沙地柏(Sabinavulgaris)可以形成很大的密度外(因其强的蒸腾控制能力),其它灌木的适宜种植密度应控制在使沙丘处于半固定状态的水平上。关键词毛乌素沙地灌木蒸腾作用水分平衡治沙种植密度A PRELIMINARY STUDY ON THE WATER BALANCEFOR SOME SANDLAND SHRUBS BASED ONTRANSPIRATION MEASUREMENTS IN FIELD CONDITIONSDong Xuejun,Zhang Xinshi and Yang Baozhen(Institute of Botany,Chinese Academy of Sciences,Beijing100093)AbstractThis paper aims at applying the principles of water balance and the semi-fixeddunes for the establishment of the product-protective systems in Maowusu Sandland(ZhangXinshi,1994;1996)to the practices of afforestation and control of desertification.As waterserves as a limiting factor for the shrubs ecosystems in Chinas Maowusu Sandland,one of theshrubs kingdomsin the temperate arid zones of the world,it is very important to estimatethe appropriate planting densities with the aid of the principles and methodologies of waterbalance for the rational management and sustainable development for the areas vegetation.Inthis,evapo-transpiration(ET)is the most difficult to be determined accurately.This paperproposes a method for the estimation of seasonal transpiration water loss based on field mea-?本文于1996-10-09收稿,1997-01-06收到修改稿。T his paper is supported by National Science Foundation of China,and is part of the doctorate of the first author.We thank Dr.Guoke,Mr.Liu zhimao,M r.Alatengbao and Mr.Zhao Yuxing for helping conduct field measure-ment of transpiration.sured data of transpiration rate,considering the typical pattern of change of above-groundgreen biomass during the typical growing seasons from mid-April to mid-October.The evapo-ration from sand surface during the growing seasons is estimated by using a semi-empiricalmodel and taking the monthly mean temperatures at sand surface as the input.Finally,the ap-propriate planting densities for the major shrubs are discussed as constrained by the waterbalance equation.According to the results obtained,the evapo-transpiration of the shrubs e-cosystems of Maowusu Sandland comes mainly from plants rather than the soil.As a result,the appropriate planting densities for the dominant shrubs ought to be controlled at such a lev-el that the dunes are semi-fixed by the plants,with the exception of Sabina vulgaris standshaving a high density owing to its powerful capability of transpiration control.Key words Maowusu Sandland,Shrubs,Transpiration,Water balance,Control of de-sertification,Planting densityThe ecological problems of the Maowusu Sandland have received tremendous recognitionfrom the world-wide academic circles in the past decades(Petrov,1966;1967;Department ofGeography of Beijing University,1983;Dong Guangrong et al.,1988).It is only in recentyears,however,that a set of comprehensive principles on the rational management forthe shrubs-dominated sandland vegetation has been proposed with an appropriate eco-logical lucidity(Zhang Xinshi,1994;1996),in which the most crucial ones are the princi-ples of water balance and the semi-fixed dunes which emphasize that the appropriatedune vegetation coverage in this area,considering water and heat balance,ought to benear such a level that the dunes are semi-fixed by plants.This hypothesis,satisfying thefrequently occurred intuition of many ecological workers,is established quantitatively,however,based on the overall vegetation-climate relations in this area(Zhang Xinshi,1994;1996).In order for this research result to be used in the practices of afforestationand desertification control,some small scale research efforts are required to estimate thesuitable planting densities for the specific shrubs vegetation in light of water balance.This can be best fulfilled by assuming an observation oriented approach,which reliesheavily on the field measured data of transpiration for the major shrubs species in thisarea.There are two considerations in assuming this approach.First,in all the terms in thewater balance equation,evapo-transpiration(ET)has long been regarded as the most dif-ficult one to determine accurately(Monteith,1975;Shuttleworth&Wallace,1985;Shut-tleworth,1993).The reason for this lies not only in the fact that the appropriate micro-meteorological observations,especially the ones describing both the vertical and horizon-tal variations of the parameters are,at present,mainly lacking at local sites,but also inthe reality that evapo-transpiration itself has long been a complicated process.So,the ap-plication of the approaches based on the well-known surface resistanceparadigm,pro-posed by Penman and improved by others(Monteith,1975;Shuttleworth&Wallace,1985;Shuttleworth,1993),is difficult in a complicated ecological situation,such as the2093期董学军等:依据野外实测的蒸腾速率对几种沙地灌木水分平衡的初步研究one in the dune-dominated Maowusu Sandland,when sufficient profile-observations arelacking.Fortunately,the direct leaf-level measurement of transpiration has been maderather easy with the widespread use of porometric method and it is indeed a shortcut toestimate directly the transpiration water loss if the canopies under consideration aresparse in character.Then,secondly,why does the sparsity of the typical shrubs canopiesmake it feasible for this approach,i.e.,using the field measured data of transpirationrate in the estimation of the ET,to be used.Because the light relations of different leaveswithin the typical shrubs canopies are relatively similar with one another compared withthose of the typical broad-leaved canopies.When both the direct sunlight,penetratingthrough the relatively wider spacing among the tiny leaves and reflected sunlight fromthe sand surface come to reach the leaf surfaces,the change in transpiration rate withinthe canopies may be relatively smaller than that for the typical broad-leaved canopies.This will advocate the use of leave-level measurements to estimate transpiration fromplant stands.In China,there have been fruitful research conducted at similar situations and fo-cused also on the study of transpiration from some psammophytes(Liu Yingxin,1963;Huang Yinxiao and Lin Shunhua,1974;Liao Rutang and Zhang Wenjun,1992).Howev-er,it is surprising that the calculated transpiration from sandland plant stands exceededthe available rainfall input significantly(Liu Yingxin,1963).T his situation may partly becaused by using the peak green biomass as a mean value for the biomass contributing totranspiration during the whole growing period.In addition,the transpiration measure-ment itself for similar plants and in similar areas,as reported by different workers,havebeen shown to differ considerably(Liu Yingxin,1963;Liao Rutang and Zhang Wenjun,1992;Dong Xuejun et al.,1994;Yang Baozhen et al.,1994;Guo Ke et al.,1996).As aresult,more accurate measurement of transpiration and a better estimation of transpira-tion water loss from plant stands are to be made in order for the assessment of waterbalance to be more rational in Maowusu Sandland.In this paper,we first propose amethod for the estimation of the seasonal transpiration water loss based on field mea-sured data of transpiration rate,considering the typical pattern of change of above-ground green biomass during the typical growing seasons from mid-April to mid-Octo-ber.T he evaporation from sand surface during the growing seasons is estimated by usinga semi-empirical model and taking the monthly mean temperatures at sand surface as in-put.Finally,the appropriate planting densities for the major shrubs are discussed as con-strained by the water balance equation.1MATERIALS AND METHODS1.1Research site and its scientific importanceAll the measurements were carried out at the typical substrates,i.e.,the aeoliansand dunes accumulated on the Cretaceous and Jurassic rocks,in Yi Jin Huo Luo Bannerand Wu Shen Banner,Yi ke Zhao League,Inner-mongolia,the typical area of Maowusu210植物生态学报21卷Sandland,during the period from 1989 to 1992.The prevailing climate here is of the tem-perate grassland type.However,owing to the extensive covering of sand and remarkableinfluences by landscape heterogeneity,the substrates here are diversified greatly,creat-ing the highly varied micro-habitats(Chen Zhongxin and Xie Haisheng,1994)suitablefor the survival of a wide spectrum of plant and animal species,among which the sandyshrubs species are of highly ecological and economic importance.T he research site lo-cates at the southeastern part of the Ordos ecotone,where the potential responses of thenatural ecosystems to climate changes have been considered to be very sensitive(ZhangXinshi,1994),and consequently,an understanding of the characteristics of water flux inthe sand-shrubs system will be helpful to uncover the response patterns of this areasnatural vegetation to possible climate changes.In forestry practices,the accurate estima-tion of transpiration water flux from natural vegetation can be helpful in the design ofoptimal planting densities for the artificial and semi-artificial vegetation.1.2Water balance of the sand duneThe water balance equation for a typical soil layer can be written asP=R+D+E+T+W2(1)in which P is precipitation,R is surface runoff,D is deep layer seepage,T transpirationwater loss,E is evaporation from soil surface and W2is the storage term by soil layers.W2can be negligible if the time period considered is longer than one month(Hillel,1980).In Maowusu Sandland,the surface runoff occurs almost always at soft and hardridges,for it is unlikely to have significant runoff on typical dunes(Yang Baozhen et al.,1994).As a result,Eq.(1)can be reduced toT=P-D-E(2)by which the transpiration water loss(consumption)by plants could be obtained.1.3Estimation of evaporation from sand surfaceAlthough it has been widely recognized that the sandy soils can powerfully limit thesurface evaporation when soils are sufficiently dry(Davis,1974;Hillel,1980),it is not atall redundant to pay the surface evaporation a specific consideration in the research ofwater balance.We adopt the method developed by Kobayashi T etsuo(Kobayashi T et-suo,et al.,1992)for calculating the water vapor flux in the dry sand layer:q=0.0629?exp(0.0629T0-36.92)f(?)-f(?)/1-exp(-0.0629?)(3)in whichf(?)=(1/9)?9-(18.9/8)?8+21.40?7-105.18?6+299.14?5-472.64?4+311.90?3q is evaporation rate(cm/s),?is the rate of change in temperature with the change ofsoil depth,T0is the temperature at sand surface,f(?)is a function of soil wetness,?isthe volumetric water content at the interface between the wet and dry layer,?0is the sur-face water content of the sands,and?is the depth of the dry surface layer(cm).If?as-sumes a value of 2%2.5%,then f(?)can be approximated to f(?)=130;and whenthe temperature change within the dry surface layer is far less then 16k,i.e.,?16k,Eq.(2)can be reduced to2113期董学军等:依据野外实测的蒸腾速率对几种沙地灌木水分平衡的初步研究 q=exp(0.0629T0-36.92)(130-f(?0)/?(4)We use Eq.(4)to calculate the evaporation from sand surface,because both conditions,?=2%2.5%and?16k,can generally be satisfied at the typical climatic condi-tions in Maowusu Sandland(Kobayashi Tetsuo,et al.,1992).Fig.1Dependence of evaporation from the dune surface on thedepth of the dry surfacelayer,calculated with the monthly means of the dune surface temperature in MaowusuSandland from April to October in 1991.Each curve is labelled with a number from 1 to7,which stands for October,April,September,May,June,July and August,respectively1.Oct.2.April3.Sep.4.May5.June6.July7.Aug.1.4Measurement of transpirtion rate for individual leaves or leafy twigsAll the measurements of transpiration were made in field condition on some fre-quently occured plants,especially shrubs,in this area,by a mechanic balance(LI Bo,etal.,1964;Guo Ke,et al.,1996),a LI-1600 Steady State porometer(Shulze,et al.,1980)and a LI-6000 photosynthesis Analyzing System(Zheng Hailei,et al.,1992),respective-ly.On each day of measurement,data were collected every two hours from 8 to 20oclock for the top,middle and lower layers of the canopies,with 510 replications foreach layer.In order to keep a direct comparison with other workers results,the unitused for Li-Cor instruments is in?g?cm-1?s-1,whereas that for quick-weighting methodis in mg?g-1?min-1(see table 2 for details).If the transpiring period for each day is setto 12 hours,and the actual leaf area indices(measured as the maximum of shadowed leaf212植物生态学报21卷area)are considered,then,daily transpiration water consumption(DTWC),T(L,t),forthe LI-cor instruments,can be expressed asT(L,t)=36 10-3 L ni=1(siti)(5)where T(L,t)is for DT WC(mm day-1),L is leaf area index(m2/m2),si,the mean oftranspirtion rate of the ith measurement(?g?cm-1?s-1),n,the number of measurementfor the whole day,tiis the time interval between measurements(hr).Daily transpira-tion water consumption by unit LAI can be expressed as T(t)=T(L,t)/L.For the quick-weighting method,DT WC,T(L,t)isT(L,t)=60 10-6 L ni=1(?siti)(6)where T(L,t)is DTWC(mm?d-1),?is the leaf area of the unit leaf fresh weight,and siis the mean of transpiration rate of the ith measurement(mg?g-1?min-1).Other sym-bols are the same as in Eq.(5).1.5Estimation of LAI for the shrubs standsWithin typical stands of A rtemisia ordosica at soft ridge,A.ordosica at dune andCaragana intermedia at dune,ten healthy twigs for each stand were selected,and thechanges of their lengths,from July to September were measured in 15-day intervals.T helengths of the twigs can be related to corresponding leaf area(Dong Xuejun,et al.,1993).By relating the averaged twig length for the early Sep.with the biomass data ofthe same period,we obtain the leaf area index(LAI)for this time(early Sep.),which ispresumably considered as the maximum of the green biomass of the growing season.T heLAIs for other periods are obtained by extrapolating from the observed data,with theassumption that the accumulation of green biomass begins from April 15.So the changein LAI with time(day)is shown in Fig.2.1.6Estimation of transpiration water consumption by plants during the growing sea-sonsThe transpiration water consumption by plants during a time interval from t0to t1can be expressed asT=t1t0T(t)L(t)dt(7)in which T(t)stands for daily transpiration water consumption by unit LAI at time t(day)(mm?day-1),and L(t)represents the LAI at time t(day).The pattern of change of T(t)during the growing season may be very complicated(Liu Yingxin,1963),however,the following treatment,we believe,can be tolerable:T(t)first remains at a constant value,T0,averaged through the total measurements duringthe period from April 15 to Sep.1;then it goes down linearly until reaching zero on Oct.15,i.e.2133期董学军等:依据野外实测的蒸腾速率对几种沙地灌木水分平衡的初步研究T(t)=T00 t 135T(t)=at+b135 t 180T(t)=0t=180(8)Let L(t)change according to the trends in Fig.2 from April 15 to Sep.1,