| The relationship between forest and water has been one of the hot research topics in ecology.Currently,plantation forests that occupy 140 million hectares globally increase by2～3 million hectares annually under overall forest cover declining,thus,playing an increasingly important role in ecosystem services.Pinus massoniana is one important pioneer tree species,which widely cultivated for restoration and reconstruction in subtropical low mountain and hilly region.However,over 60%P.massoniana plantations are monoculture plantations,which have a lower ability of nutrient recycling and resisting diseases,insect pests and fire.How to scientifically manage or transform the P.massoniana monoculture plantations to elevate the forest quality has been proposed a foundamental issue in managing P.massoniana plantations.Schima superba is a valuable tree species in subtropical evergreen broad-leaf forest for its precious timber and ecological firebreak,as well as stronger ecological defence.Previous studies suggested that mixed P.massoniana and S.superba plantation has advantages in enhancing forest primary productivity,improving soil fertile,resisting attacks from pests or diseases and reducing the occurance of forest fire,which would be conducive to maintain the regional ecological balance.However,how mixed P.massoniana and S.superba plantations use the limited water resources remains unclear.Therefore,three different types of plantation forests（P.massoniana and S.superba monoculture plantations and a mixed P.massoniana and S.superba plantation）in Huitong,Hunan Province were selected.By employing hydrogen-oxygen stable isotope techniques combined with biotic/abiotic factors（concluding canopy density,litter characteristics,soil physical structure,root distributions,soil water content,photosynthetic physiological characteristics,leaf water potential）and statistics models（HYSPLIT models,linear regression model,Mix SIAR model）,we systematically quantified the dynamic of hydrogen and oxygen stable isotope in different type of water in P.massoniana uptaking,source of utilizable water resource in forests,contributions of different magnitudes of precipitations to soil water in different soil layers and its driving factors,and the water use strategies of P.massoniana and S.superba,which could lay the foundation of quantitative study on water resource utilization in P.massoniana plantation ecosystem.The main findings were listed as followed:（1）The local meteoric water line in Huitong,Hunan Province was established asδD=（7.45±0.17）δ18O+（10.10±1.25）（R2=0.93,p<0.01）.Its slope was slightly lower than meteoric water line in China and the global meteoric line.The D and 18O values of precipitation samples at this field station were closely coupled with local meteorological conditions and dominant moisture sources.The 18O and D contents were depleted during summer monsoon seasons while enriched during winter monsoon seasons.During the summer monsoon and post-monsoon seasons,the precipitation in this area came mainly from the Bay of Bengal,the South China Sea,and the West Pacific at three different types of air pressure,therefore the d-excess value of the precipitation was close to the global average.The lower intercept of the meteoric water line and the higher d-excess value for the precipitation during the winter monsoon seasons resulted from moisture from remote westerly air masses,degenerated tropical marine air masses from the Bay of Bengal,inland moisture in the pre-monsoon period,and local environmental factors.（2）Our results revealed that the contribution of rainfall to soil water（CRSW）of both P.massoniana monoculture plantation and mixed plantation was greater than that of S.superba monoculture plantation following three rainfall events.No significant difference in the CRSW was found between the P.massoniana plantation and the mixed plantation within 9 days following the light rainfall event（precipitation≤10 mm）.However,the CRSW was significantly higher in the mixed plantation than in the two monoculture plantations within 11days following the moderate（10 mm≤precipitation≤25 mm）and heavy rainfall events（precipitation>25 mm）,suggesting that the soils of the mixed plantation had higher water-holding capacity.The most important factors affecting the CRSW were canopy openness and root biomass after the light rainfall event,whereas root biomass and litter characteristics acted as the dominant regulating factors after the moderate rainfall event.Under the heavy rainfall event,dry weight and saturation moisture capacity within the semi-decomposed litter layer influenced the CRSW the most.Thus,mixed plantations in humid subtropical regions have a significantly higher capacity to intercept and maintain heavy rainfall than their respective monoculture plantations and thus can enhance their adaptation to extreme rainfall events and following frequent droughts.（3）Our results revealed that both P.massoniana and S.superba exhibited plasticity in sources of water uptake between the monoculture plantation and mixed plantation.The mixed afforestation significantly enhanced the water uptake proportion from shallow soil layers for P.massoniana and that from deep soil layers and shallow groundwater for S.superba.In mixed plantation,P.massoniana and S.superba exhibited hydrological niche partitioning.Compared with S.superba,P.massoniana in the mixed plantation had a lower water uptake proportion from deep soil layers and underground water,while a higher water uptake from shallow soil layers,especially after heavy rainfall.The plasticity in sources of water uptake of the two species were associated with variations in plant functional traits（e.g.,plant physiological traits and phenotypic traits）.When these two tree species were mixed,P.massoniana enhanced to uptake water from shallow soil layers by regulating stomatal and root growth to maintain the transpiration,while S.superba enhanced to uptake water from deep soil layers and underground water by reducing the leaf water potential yet with a lower net photosynthetic rate.（4）When exploring the moisture conversion based on linear regression model,we found significant positive relationships in hydrogen and oxygen stable isotopes between precipitation and surface water,shallow groundwater,soil water,plant water,suggesting that precipitation was the dominant water sources of surface water,shallow groundwater,soil water and plant water in this region.Further results showed that the hydrogen and oxygen isotopes of stream was similar with that of shallow underground water,suggesting a moisture conversion between them.In addition,soil water could be supplied by shallow underground water,as well as precipitation.Another most important finding was that the direct water source of S.superba and P.massoniana is soil water and shallow underground water.Overall,compared with monoculture plantations,the S.superba and P.massoniana mixed plantations have stronger ability to intercept heavy rainfall and more superior water uptake patterns,which help them had advantages in resisting extreme climate changes.Study well promoted our understanding of water resource utilization in P.massoniana plantations,while providing scientific case for P.massoniana plantations management,forest quality elevation,water conservation improvement. |
