| There are approximately 3.44 million km~2 of karst area in China,which occupies approximately 1/3 of the total land area and mainly distributes in the southwest of China.The karst area is ecologically vulnerable.The phenomenon of"rocky desertification"occurs widely in Southwest China,causing serious ecological problems such as soil erosion,vegetation degradation,deterioration of regional water resources,and biodiversity loss.There is an urgent need of ecosystem restoration and vegetation reconstruction in the degraded karst lands.Karst drought,seasonal drought and occasional extreme drought occur in karst hilly habitats in Southwest China.Drought results in catastrophic hydraulic dysfunction and eventually plant death.Water is a key limiting factor for the ecological restoration in the rocky desertification areas.The degraded karst hill in the Tropical Forestry Experimental Center Chinese Academy of Forestry(ECTF)was restored in the 1980s and 1990s.At present,it has changed into a typical karst forest community with multiple layers of vertical structure and rich horizontal structure.The species richness is close to regional natural subtropical karst mountain monsoon rainforest.This forest plot provides an ideal research platform for exploring the drought adaptation of artificial karst forests under the humid to semi-humid monsoon conditions in the subtropical region.In this study,the growth rate and hydraulic properties of seven tree species in pure plantation stands on a karst hill were measured,including Excentrodendron tonkinense,Dalbergia odorifera,Zenia insignis,Diplodiscus trichospermus,Cornus wilsoniana,Erythrophleum fordii and Adenanthera microsperma.In addition,the sap flow density of the first five species was measured for three consecutive years.We analyzed the relationships between tree growth rate and hydraulic parameters,and explored the characteristics of sap flux density and influencing factors,forest water consumption and the water use per unit stem area at breast height.This thesis aimed to discuss two major issues:1)the annual water consumption and its main influencing factors of the plantation stands;2)the selection of afforestation tree species for karst vegetation restoration in terms of hydraulic characteristics,tree growth rate and water consumption,this paper discusses.The main findings are as follows:(1)There were significant differences in the long-term average height and DBH growth rates of tree species in karst plantation.There was no significant correlation between tree growth and efficiency parameters such as sapwood specific conductivity,leaf specific conductivity,wood density,Huber value,vessel density and hydraulic diameter.However,tree growth was significantly correlated with cavitation resistance and hydraulic safety margin;that is,tree species with higher hydraulic safety grew faster.The sapwood specific conductivity was not correlated with cavitation resistance,indicating that there was no trade-off between hydraulic efficiency and safety.The species of Zenia insignis and Erythrophleum fordii had high drought resistance and growth ability.(2)In the extreme drought period,the hydraulic safety margin of most tree species was significantly lower than that in the normal dry season.The hydraulic safety margin of Adenanthera microsperma and Excentrodendron tonkinense were-0.19 MPa and-1.64 MPa respectively,indicating a rather high risk of hydraulic failure.The comprehensive analysis of plant hydraulics characteristics can provide an important reference index for the selection of afforestation species in rocky desertification area.(3)The thermal diffusion method was used to measure sap flow of afforestation trees in karst hills.The original sap flow density formula of trees proposed by Grainer was corrected to improve the accuracy of sap flow density for the studied tree species.It was found that there was a significant positive correlation between sap flow density and the DBH of single trees in the karst plantation.The main factors affecting the diurnal variation of tree sap flow density were saturated water vapor pressure deficit and photosynthetically active radiation.Temperature and photosynthetically active radiation were the main factors affecting sap flow density on the monthly scale.(4)Based on sap flow measurements,stand survey and meteorological data,it was found that the annual water consumption of five karst mountain plantations:Excentrodendron tonkinense,Zenia insignis,Dalbergia odorifera,Cornus wilsoniana and Diplodiscus trichosperma,accounted for 22.45%,59.56%,10.50%,12.86 and 7.55%of the annual rainfall,respectively.In addition,the water consumption per unit basal area of karst plantation was not lower than that of non-karst forests,and the water consumption of forest was mainly related to per unit basal area the forest stand.To sum up,the vegetation restoration and ecological reconstruction in karst rocky desertification areas should be based on the site conditions,taking into account of the hydraulic characteristics of tree species,water consumption per unit basal area of stand,and tree grow ability.These important research data can provide theoretical basis for the management of forest species composition and density and optimization of the ecological and economic benefits. |
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