Environmental Response And Biophysical Control Over Transpiration By Trees/Stands

Change of rainfall pattern induced by climate change has potential impact over water cycle and vegetation cover. Meanwhile, the influence of trees in regional water balance has long been debatable. Therefore, thorough understanding of the environmental response and biophysical control over tree water use will be conducive to accurate evaluation and prediction of the component quota in water balance. Such knowledge will also guide the practices in eco-restoration. Our study adopted thermal dissipation probes to monitor tree transpiration of four landscape tree species in Dalian, namely Cedrus deodara, Zelkova schneideriana, Euonymus bungeanus, Metasequoia glyptostroboides; the poplar plantatio (Populus×euramericana cv."74/76") in suburb-Beijing, and the Robinia pseudoacacia-Pinus tabulaeformis stand in Shanxi province to test differences derived from living environment and species and further the current understanding of stand water use. The simultaneous environmental factors were also observed. The specific objectives are:(1) to quantify transpiration of the stands and the species under different environment;(2) to compare transpiration patterns of varied time scale in contrasting environmental conditions;(3) to compare the environmental responses and coping strategy to drought stress by different species;(4) to analyze the differences of stomatal control over transpiration and modeled species sensitivity to VPD by species;(5) to compare transpiration responses to environmental factors under urban and natural conditions.The results showed that:(1) the monthly transpiration of urban landscape tree cluster ranged from16.92to60.01mm. This value was22to76mm for suburb poplar plantation in Beijing and9to15.46mm for Robinia-Pinus stand in Shanxi.(2) stand/tree canopy transpiration showed differed statistical relationship with tree size. Transpiration of urban landscape trees has exponential relationship with DBH. The species factor is significant. Transpiration of trees of different species in the same DBH class showed5time differences. The changing pattern of canopy transpiration of poplar plantation with DBH was the same as that of sapwood area with DBH, but no significant relationship with DBH.was observed. Canopy transpiration of Robinia-Pinus stand was negatively related with DBH.(3) the sub-daily transpiration pattern were similar among species and sampled trees, but the magnitude was subjected to species and tree size. On daily scale, the canopy transpiration was well coupled with the atmosphere and not influenced by soil water condition. Therefore, trees exerted effective stomatal control over transpiration. The nocturnal sap flux was used for both transpiration and repletion for daytime water depletion of trunk water reservoir. It accounted7-39%of total daily transpiration. The time lag between VPD and transpiration was rather influenced daily average VPD than by soil water conditions. During the whole growing season, the stand/tree transpiration was kept at a stable level. Except for Zelkova schneideriana and Robinia pseudoacacia, transpiration did not show apparent decline as the growing season approached to the end. Increase of total rainfall did not trigger necessary increase of transpiration; therefore, the percentage of transpiration in rainfall fluctuated among years and months. This ratio was large under conditions with small rainfall amount.(4) influence of solar radiation over transpiration was concentrated on inducing stomatal opening before it reached1.8MJ m-2h-1. Transpiration showed saturated relationship with solar radiation. The transpiration of landscape trees leveled off as VPD increased. Similar relationship was followed by poplar plantation but this relationship stratified by soil water conditions. Robinia-Pinus stand could not maintain the maximum transpiration under high VPD. On daily scale, the coupling status was relatively weak in the morning, the highest decoupling coeffecient (Ω) can be0.35. But the coupling improved as VPD increased with Ω around0.1. Therefore, environmental control over transpiration from solar radiation concentrated in the morning and switched to VPD in the afternoon. The analysis indicated that besides of rainfall amount, the frequency of rainfall timing influenced transpiration of corresponding month. The effect of soil water condition over transpiration appeared when being ranked.(5) canopy conductance exhibited logarithm decrease as VPD increased under contrasting environmental conditions. The well coupled canopy transpiration to atmosphere suggested that trees exerted effective biophysical control over transpiration. The sensitivity of canopy conductance to VPD and reference Gc varied under different soil water and solar radiation conditions, but their ratio kept stably around0.6. Post-rainfall transpiration recovery was influenced by VPD rather than by soil water conditions. However, soil water affected time duration to attain maximum transpiration.Based on the results, the study arrives to the conclusion that:species observe the same isohydraulic control over transpiration, but species differences can not be obscured. Trees under urban conditions tend to have stronger transpiration coupling with the atmosphere. Post-rainfall transpiration recovery results indicate that changes of the rainfall characteristics (scale and frequency) will lead to variation of vegetation cover under natural conditions. For species under management, the application of irrigation should be conducted directly into deep layers. The universally observed0.6ratio suggests that the canopy transpiration can be estimated reliably based on the accurate canopy conductance measurement under1kPa and concurrent meteorological data.