Transpiration and canopy stomatal conductance dynamics of Mongolian pine plantations in semiarid deserts, Northern China

Mongolian pine (Pinus sylvestris var. mongolica) plantations have ecological significance for desertification control and degraded land restoration in drought-induced regions. To date, knowledge about the dynamics of transpiration and canopy stomatal conductance (gs) of Mongolian pine in the semiarid deserts in Northern China is quite limited. Thus, better understanding its physical response to environmental factors and exploring the mechanisms of forest transpiration can offer a theoretical basis for a reasonable tree planting program in semi-arid regions. In this study, transpiration and gs changes in Mongolian pine plantations for a mature forest (MMPP), half-mature forest (HMPP), and young forest (YMPP) were obtained using sap flow observations, while simultaneously monitoring the atmospheric and soil moisture contents. The results showed that the canopy transpiration per unit leaf area (EL) averaged 0.97 mm d–1, 0.60 mm d–1, and 0.45 mm d–1 in MMPP, HMPP, and YMPP, respectively, and the EL of Mongolian pine could be attributed to evaporative demand, soil moisture status, and gs. EL was obviously affected by air temperature, photosynthetic active radiation (PAR), and vapor pressure deficit (VPD) with the highest determined coefficient. The low values measured for the dimensionless coefficient (Ω = 0.041, 0.15, and 0.18) indicated that the canopy and the atmosphere were highly coupled. gs had more control over EL, and thus gs was more limited by VPD than that PAR. The average gs was 105.27, 105.26, and 99.44 mmol m–2 s–1 for MMPP, HMPP, and YMPP, respectively; Both MMPP and YMPP had lower sensitivity for stomatal regulation than HMPP, and the maximum gs was found in trees with both small and large tree diameters at breast height. Therefore, MMPP and YMPP were suspected to vary based on environmental conditions and were more susceptible to decline under mega drought conditions.