Canopy stomatal conductance characteristics of Pinus tabulaeformis and Acer truncatum and their responses to environmental factors in the mountain area of Beijing

Abstract

Aims Leaf stomata are channels for plants to exchange water vapor that affects transpiration and photosynthesis. However, leaf stomatal behaviors are affected by environmental factors and tree species. It is still unclear whether the responses of canopy stomatal conductance to environmental factors differ between tree species and whether the stomatal regulations on canopy transpiration change with different periods of the growing season. The objective of this study was to explore the relative contribution of environmental factors to canopy stomatal conductance and the regulation of leaf stomata on canopy transpiration for different tree species, which could provide references for further understanding the water use status of trees and forest management in mountain areas. Methods During the growing season of 2018, Pinus tabuliformis (58-year-old) and Acer truncatum (39-yearold) at Badaling National Forest Park in Beijing were selected. Sap flow was measured by using the thermal dissipation method. Environmental factors were also measured synchronously. Canopy stomatal conductance (Gs) was estimated by using the Penman-Monteith equation. Important findings (1) The daytime Gsof P. tabulaeformis and A. truncatum varied evidently at daily and monthly scales. From May to July, daily dynamic Gsof P. tabulaeformis and A. truncatum increased with vapor pressure deficit (VPD) and solar radiation (GR), in which the rising periods were longer than that during August and September. At the monthly scale, as VPD and GR decreased, soil moisture (VWC) increased, Gsgenerally increased from May to September. (2) VWC and VPD contributed to the major variation of the Gs, and then the GR, air temperature, and wind speed followed based on the boosted regression tree method. The relative contributions of VWC and VPD to Gswere 66.4% and 17.4% for P. tabulaeformis and 54.8% and 21.0% for A. truncatum, respectively. (3) The slopes between dGs/dlnVPD and the reference canopy stomatal conductance for both P. tabulaeformis and A. truncatum were significantly larger than 0.6, suggesting that their stomatal regulations were relatively strong. In summary, the response of stomata to environmental factors differs between tree species and different periods of the growing season. Under different soil water conditions, these two tree species could control transpiration through strict stomatal regulation to prevent excessive water loss.