Influences Of Stand Density And Age On Canopy Transpiration,Nocturnal Sap Flow,and Their Biophysical Controls

Forest transpiration is the main component of terrestrial ecosystem evapotranspiration,and nocturnal sap flow accounts for a considerable amount of plant total water uptake.While forest management(e.g.,thinning and reforestation)alters the water and carbon budget of forest ecosystems,it also modifies the adaptation and resistance of forest ecosystems to environmental stress(e.g.,soil droughts).However,it is still unknown that how thinning influences forest transpiration and its biophysical controls in a long time scale,and how about the biophysical controls on nocturnal sap flow for plantations with different ages after reforestation;additionally,in the context of global climate change,how more frequent and intense soil droughts modify the above biophysical controls also remain unclear.Therefore,in a semi-arid rocky mountainous area of northern China,we measured growing season sap flow of two widely distributed afforestation species,Pinus tabuliformis and Acer truncatum,in monoculture forest stands with different stand densities and ages,which generated by a thinning practice three decades ago and a reforestation years ago,respectively.Our findings showed:(1)the relationship between diameter at breast height and sapwood was density independent;the high diameter at breast height and sapwood area at the tree level,as well as their annual growth rates,resulted the highest stand sapwood area in less dense stands rather than in the densest stand;(2)similar to the performance short-term after thinning,three decades after thinning,canopy transpiration in sparse stands demonstrated higher sensitivity to climatic fluctuations and drought depressions than dense ones;stand density had no effect on the isohydric strategy of Pinus tabuliformis and Acer truncatum;(3)contrary to the positive relationship between stand canopy transpiration and stand density soon after thinning,sparse stands exhibited higher growing season canopy transpiration than dense stands three decades later;in the dry year,the density difference in canopy transpiration was more pronounced than the wet year;(4)there was a convergence in nocturnal sap flow between two species belonged to different plant functional types(i.e.,Pinus tabuliformis and Acer truncatum),and the proportion of nocturnal sap flow to the total sap flow across species and ages is also convergent(12.2-15.0%);the significant larger diameters at breast height of older stands was the essential reason that nocturnal sap flow were significantly higher for middle-aged stands than for young stands,and it explained the main variation of nocturnal sap flow;(5)nighttime vapor pressure deficit influence on nocturnal sap flow of young stands was soil moisture dependent;nighttime wind speed indirectly controlled nocturnal sap flow in young stands through enhancing nighttime vapor pressure deficit and directly promoted nocturnal sap flow in middleaged stands with relatively low tree densities;for each species,both increased and decreased soil water content were able to promote nocturnal sap flow in stands with relatively dry soils,which might be due to enhanced nighttime water recharge for capacitance refilling and for avoiding hydraulic failure under prolonged water stress,respectively;(6)total effects of three widely concerned environmental factors(i.e.,vapor pressure deficit,wind speed,and soil water content)explained less than 55% of the nocturnal sap flow variations.This research highlights that stand density was not an appropriate indicator of thinned forest transpiration over long time scales and built a conceptual model of changes in thinned forest canopy transpiration over time,which would be helpful to improve sustainable forest management strategies.Our findings also help to improve the existing vapor pressure deficit based method for partitioning nighttime transpiration and water recharge and suggest the importance of incorporating nocturnal sap flow into large-scale models.