Transpiration and canopy conductance dynamics of Pinus sylvestris var. mongolica in its natural range and in an introduced region in the sandy plains of Northern China

Determining changes in tree transpiration and its controlling mechanisms from regions of natural distribution to planted introduction are significantly important for afforestation and forest management. Here, transpiration and canopy conductance of Mongolian pine (Pinus sylvestris var. mongolica) in natural forest (MNF) of the natural distribution region and in plantation forest (MPF) and forest-grassland (MFG) of the planted introduction region were quantified by sap flow measurements and concurrent environmental observations. The results showed that the canopy transpiration per unit leaf area (EL) averaged 1.0, 1.4, and 1.7 mm d−1 for MNF, MPF, and MFG, respectively, indicating that the transpiration rate of trees significantly increased from natural to introduction regions due to higher evaporative demand and canopy conductance. However, the significantly lower EL in MPF than in MFG was due to the higher tree density at the old stand at the MPF site. The vapor pressure deficit (VPD) explained the slightly greater variability of daily EL than solar radiation for MPF, indicating that the transpiration of MPF was limited more by VPD than radiation. Canopy conductance (GL) averaged 104.6, 109.9 and 132.2 mmol m−2s−1 for MNF, MPF, and MFG, respectively. Moreover, GL significantly declined with VPD, but the reference canopy conductance was lower in MNF and MPF than in MFG, indicating that MNF and MPF had relatively lower stomatal conductance sensitivities to VPD. These findings suggested that plantation forest had a high transpiration rate but relatively loose stomatal regulation upon water loss; thus, it was more susceptible to dieback during extreme drought years.