Canopy Transpiration and Stomatal Conductance Dynamics of Ulmus pumila L. and Caragana korshinskii Kom. Plantations on the Bashang Plateau, China

Abstract

Constructing protective forests to control water and soil erosion is an effective measure to address land degradation in the Bashang Plateau of North China, but forest dieback has occurred frequently due to severe water deficits in recent decades. However, transpiration dynamics and their biophysical control factors under various soil water contents for different forest functional types are still unknown. Here, canopy transpiration and stomatal conductance of a 38-year-old Ulmus pumila L. and a 20-year-old Caragana korshinskii Kom. were quantified using the sap flow method, while simultaneously monitoring the meteorological and soil water content. The results showed that canopy transpiration averaged 0.55 ± 0.34 mm d−1 and 0.66 ± 0.32 mm d−1 for U. pumila, and was 0.74 ± 0.26 mm d−1 and 0.77 ± 0.24 mm d−1 for C. korshinskii in 2020 and 2021, respectively. The sensitivity of canopy transpiration to vapor pressure deficit (VPD) decreased as soil water stress increased for both species, indicating that the transpiration process is significantly affected by soil drought. Additionally, canopy stomatal conductance averaged 1.03 ± 0.91 mm s−1 and 1.34 ± 1.22 mm s−1 for U. pumila, and was 1.46 ± 0.90 mm s−1 and 1.51 ± 1.06 mm s−1 for C. korshinskii in 2020 and 2021, respectively. The low values of the decoupling coefficient (Ω) showed that canopy and atmosphere were well coupled for both species. Stomatal sensitivity to VPD decreased with decreasing soil water content, indicating that both U. pumila and C. korshinskii maintained a water-saving strategy under the stressed water conditions. Our results enable better understanding of transpiration dynamics and water-use strategies of different forest functional types in the Bashang Plateau, which will provide important insights for planted forests management and ecosystem stability under future climate changes.

Nocturnal Water Use Partitioning and Its Environmental and Stomatal Control Mechanism in Caragana korshinskii Kom in a Semi-Arid Region of Northern China

“…However, the E n of C. korshinskii was much lower than that reported in plant species in the past. This may be related to the smaller daily transpiration of C. korshinskii plantations and the lower soil water content (Figure 3c) on the Bashang Plateau [31]. The E n :E values in this study were 14% and 13% in 2020 and 2021, respectively, which were similar to the values reported for P. tabuliformis and A. truncatum [22].…”

“…The higher value of R e :E n in the water-limited environment indicated a greater reliance of plants on the water stored through stem refilling, which could help forests to maintain hydraulic support under higher transpiration demand [47,48]. This might be why canopy transpiration in C. korshinskii remained steady during the middle and end of the growing season (Figure listed in Zhang et al [31]). Moreover, compared with that occurring in 2021, a relatively higher R e :E n (50.24%) value was observed in 2020, influenced by the relatively lower REW (Figures 3c and 5).…”

“…The C. korshinskii plantation is about 20 years old, with an average height of 179.28 ± 30.74 cm and stem basal diameter at stem base (10-15 cm above the ground) of 2.38 cm. The soil is mainly sandy loam with a high sand content of 52.89% and low clay content of 7.86% [31].…”

“…Additionally, for the homogenous and dense plantation stands, the influences of natural thermal gradients on sap flow measurements could be assumed to be negligible [22,33]. The detail procedure for measuring sap flow velocity can be found in [31]. The sap flow velocity (SF, mL cm −2 min −1 ) could be calculated from the measured temperature difference at 30 min intervals (CR1000, Campbell Scientific Inc., Logan, UT, USA) according to Granier [34]:…”

“…Since the distribution of the C. korshinskii plantation was relative uniform and the canopy was not closed (the largest value of leaf area indices was 0.43 m 2 m −2 ), the canopy was well-coupled to the atmosphere [31], The canopy conductance can be calculated by the simplified Penman-Monteith equation [39] using the following formula:…”