The Variation in Water Consumption by Transpiration of Qinghai Spruce among Canopy Layers in the Qilian Mountains, Northwestern China

Yanfang Wan,Xiande Liu,Yipeng Yu,Xiao Wang,Pengtao Yu,Bin Wang,Yanhui Wang,Shunli Wang,Zebin Liu,Wei Xiong

doi:10.3390/f11080845

Abstract

It is important for integrated forest-water management to develop a better understanding of the variation of tree transpiration among different canopy layers in the forests and its response to soil moisture and weather conditions. The results will provide insights into water consumption by trees occupying different social positions of the forests. In the present study, an experiment was conducted in the Qilian Mountains, northwest China, and 13 trees, i.e., 4–5 trees from each one of dominant (the relative tree height (HR) > 1.65), subdominant (1.25 < HR ≤ 1.65) and intermediate-suppressed (HR ≤ 1.25) layers) were chosen as sample trees in a pure Qinghai spruce (Picea crassifolia Kom.) forest stand. The sap flux density of sample trees, soil moisture of main root zone (0 to 60 cm) and meteorological conditions in open field were observed simultaneously from July to October of 2015 and 2016. The results showed that (1) The mean daily stand transpiration for the study period in 2015 and 2016 (July–October), was 0.408 and 0.313 mm·day−1, and the cumulative stand transpiration was 54.84 and 40.97 mm, accounting for 24.14% (227.2 mm) and 16.39% (249.9 mm) of the total precipitation over the same periods, respectively. (2) The transpiration varied greatly among canopy layers, and the transpiration of the dominant and codominant layers was the main contributors to the stand transpiration, contributing 86.05% and 81.28% of the stand transpiration, respectively, in 2015 and 2016. (3) The stand transpiration was strongly affected by potential evapotranspiration (PET) and volumetric soil moisture (VSM). However, the transpiration of trees from the dominant and codominant layers was more sensitive to PET changes and that from the intermediate-suppressed layer was more susceptible to soil drought. This implied that in dry period, such as in drought events, the dominant and codominant trees would transpire more water, while the intermediate-suppressed trees almost stopped transpiration. These remind us that the canopy structure was the essential factor affecting single-tree and forest transpiration in the dryland areas.

Highlights

Transpiration is an important component of water consumption from vegetated ecosystems worldwide and usually accounts for a large proportion of water consumption [1,2,3,4,5], especially for forest ecosystems in the water-limited arid regions such as northwest China [5,6]
The larger trees were larger than their neighbors, which occupied a favorable social position in the forest, and transpired more
The transpiration was sensitive to variations in potential evapotranspiration (PET) and volumetric soil moisture content (VSM) in the Qinghai spruce forest

Summary

Introduction

Transpiration is an important component of water consumption from vegetated ecosystems worldwide and usually accounts for a large proportion of water consumption [1,2,3,4,5], especially for forest ecosystems in the water-limited arid regions such as northwest China [5,6]. The effects of these differences on forest transpiration was not given sufficient attention and forest transpiration was usually estimated by multiplying the mean sap flux density of a certain number of sample trees by the stand sapwood area [4,5,16]. This lead the situation that the variation of tree transpiration among canopy layers, i.e., among social positions in the forest, is still unclear

Methods

Results

Discussion

Conclusion