Transpiration of Anatolian black pine and sessile oak forest stands in a sub-humid region of Turkey

Transpiration is a key component of the watershed water budget. Therefore, determining the transpiration of forest stands with different characteristics have been of interest for watershed hydrology and forest management practices in a wide range of environments. The objectives of this study were to compare transpiration of Anatolian black pine (Pinus nigra Arn. subsp. pallasiana (Lamb.) Holmboe) and sessile oak (Quercus petraea (Matt.) Liebl.) tree stands and to model transpiration based on the measured climatic factors. Stand transpirations were calculated from sap flow measurements made by the trunk heat balance method. We conducted an exploratory factor analysis (PCR) to detect affecting meteorological factors of stand transpiration, and we developed linear regression equations to predict transpiration of pine and oak stands. Mean daily and yearly canopy transpiration (Ec) were 1.05 mm day-1 and 378.3 mm year-1 for the pine stand and 3.52 mm day-1 and 801.7 mm year-1 for the oak stand. There was a highly positive correlation between daily stand transpiration and wind speed, global radiation, air vapour pressure deficit and air temperature, but a negative correlation with relative humidity for both stands. Soil water potential had little effect on stand transpiration. The model equations accounted for 81% of the variations in transpiration for the pine stand and 85% for the oak stand. Therefore, the transpiration of forest stands should be considered for effective vegetation management practices, as model equations to estimate the transpiration of pine and oak stands in the region.

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

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.

Environmental Control on Transpiration: A Case Study of a Desert Ecosystem in Northwest China

Arid and semi-arid ecosystems represent a crucial but poorly understood component of the global water cycle. Taking a desert ecosystem as a case study, we measured sap flow in three dominant shrub species and concurrent environmental variables over two mean growing seasons. Commercially available gauges (Flow32 meters) based on the constant power stem heat balance (SHB) method were used. Stem-level sap flow rates were scaled up to stand level to estimate stand transpiration using the species-specific frequency distribution of stem diameter. We found that variations in stand transpiration were closely related to changes in solar radiation (Rs), air temperature (T), and vapor pressure deficit (VPD) at the hourly scale. Three factors together explained 84% and 77% variations in hourly stand transpiration in 2014 and 2015, respectively, with Rs being the primary driving force. We observed a threshold control of VPD (~2 kPa) on stand transpiration in two-year study periods, suggesting a strong stomatal regulation of transpiration under high evaporative demand conditions. Clockwise hysteresis loops between diurnal transpiration and T and VPD were observed and exhibited seasonal variations. Both the time lags and refill and release of stem water storage from nocturnal sap flow were possible causes for the hysteresis. These findings improve the understanding of environmental control on water flux of the arid and semi-arid ecosystems and have important implications for diurnal hydrology modelling.

Estimation of water use of Pinus tabulaeformis Carr. in Loess Plateau of Northwest China

Tree transpiration plays a determining role in the water balance of forest stands and in seepage water yields from forested catchments, especially in arid and semiarid regions where climatic conditions are dry with severe water shortage, forestry development is limited by water availability. To clarify the response of water use to climatic conditions, sap flow was monitored by heat pulse velocity method from May to September, 2014, in a 40–year–old Pinus tabulaeformis Carr. plantation forest stands in the semiarid Loess Plateau region of Northwest China. We extrapolated the measurements of water use by individual plants to determine the area–averaged transpiration of the woodlands. The method used for the extrapolation assumes that the transpiration of a tree was proportional to its sapwood area. Stand transpiration was mainly controlled by photosynthetically active radiation and vapor pressure deficit, whereas soil moisture had more influence on monthly change in stand transpiration. The mean sap flow rates for individual P. tabulaeformis trees ranged from 9 to 54 L d−1. During the study period, the mean daily stand transpiration was 1.9 mm day−1 (maximum 2.9 and minimum 0.8 mm day−1) and total stand transpiration from May to September was 294.1 mm, representing 76% of the incoming precipitation over this period. Similar results were found when comparing transpiration estimated with sap flow measurements to the Penman–Monteith method (relative error: 16%), indicating that the scaling procedure can be used to provide reliable estimates of stand transpiration. These results suggested that P. tabulaeformis is highly effective at utilizing scarce water resources in semiarid environments.