Biophysical Regulation Of Transpiration In Typical Plantations In Northern China

Forest transpiration,as one of the main components of evapotranspiration in terrestrial ecosystems,plays an important role in the global water cycle.However,the transpiration patterns and their regulation mechanisms in various types of forests in different environments are quite different.Especially in water-stressed regions,the relationship between forest water use and their biophysical control under different water conditions is still unclear.This study selected the common tree species in North China as the research object,including Pinus tabulaeformis Carr.in urban and mountainous areas,Pinus sylvestris var.mongolica in the urban area,Picea meyeri in the urban area,and Populus×euramericana’74/76’in the suburb area,and we continuously measured their canopy transpiration by the thermal diffusion method,and the soil moisture and meteorological factors of each forest land were monitored synchronously.The results showed:（1）Canopy transpiration（E_c）of each plantation varied in time scale.In terms of daily dynamics,the E_c of Pinus tabulaeformis,Pinus sylvestris,and suburban Populus euramericana plantations began to rise at a similar time,which was earlier than Picea meyeri and mountainous Pinus tabulaeformis plantations.The peak of E_c for urban Pinus tabulaeformis and Pinus sylvestris plantations was earlier.During the growing season of May and June,the E_c of Pinus tabulaeformis,Pinus sylvestris in urban areas,and Pinus tabulaeformis plantations in mountainous areas were significantly impacted by soil moisture shortage than Picea meyeri and Populus euramericana.During the same period of the growing season,the average E_c of Pinus tabulaeformis,Pinus sylvestris,Picea meyeri in urban areas,Populus euramericana in suburban areas,and Pinus tabulaeformis plantations in mountainous areas were 0.98±0.03 mm d^–1,0.65±0.02 mm d^–1,0.26±0.01 mm d^–1,0.59±0.01 mm d^–1,and 0.53±0.02 mm d^–1,respectively.（2）Soil moisture had relatively little effect on the E_c of Picea meyeri plantation and Populus euramericana plantation,but it had a significant driving effect on the E_c of urban Pinus tabulaeformis plantation and Pinus sylvestris plantation.When the soil moisture was relatively sufficient（i.e.the relative extractable water REW≥0.4）,the E_c of Picea meyeri plantation was not affected by（R_s）,but vapor pressure deficit（VPD）and R_s had a significant effect on the E_c of other plantations.In addition,air temperature had a significant effect on the E_c of plantations in the urban environment,while wind speed had a significant effect on the E_c of Populus euramericana plantation in suburban areas and Pinus tabulaeformis plantation in mountainous areas.Overall,as soil drought progressed,the effects of meteorological factors on the E_c of each plantation showed a downward trend during droughts,except for Picea meyeri plantation.（3）Canopy conductance（g_c）of each plantation was mainly impacted by soil and atmospheric water conditions under different environmental conditions,and VPD showed significant inhibitory effects on g_c.Additionally,soil moisture level could affect the response of g_c to VPD.Except for Picea meyeri plantation and Populus euramericana plantation,the sensitivity of g_c to VPD for other plantations increased with the increase of soil moisture.Stomatal regulations were relatively strong for Pinus tabulaeformis in the urban area,Populus euramericana in the suburban area,and Pinus tabulaeformis in the mountainous area,and their water use pattern was more inclined to isohydric behavior,while the stomatal regulation on transpiration for Pinus sylvestris and Picea meyeri was weak,and their water use was more inclined to anisohydric behavior.（4）The transpiration of urban Pinus tabulaeformis was controlled by the synergistic control of branch and leaf water potential,the whole-tree hydraulic conductance（K）,and leaf stomata.The decrease of K can directly or through reducing branch and leaf water potential to induce stomatal closure during droughts,which in turn led to a decrease in g_c and transpiration.This study systematically explored the variations of transpirational water consumption in several typical plantations under different environmental backgrounds in North China and highlighted the influence of urban environmental conditions on tree water use.Considering the differences in environmental impact and tree species characteristics comprehensively,we revealed the biophysical control of canopy transpiration for each plantation under different environmental conditions,which improved the understanding of the regulation mechanism of water use in different plantations.