Hysteretic response of sap flow in Scots pine (Pinus sylvestris) to meteorological forcing in a humid low‐energy headwater catchment

Abstract

Transpiration mediated by physiology is driven by the meteorological variables.Understanding their relationships is crucial to advancing terrestrial ecohydrological studies. Here, we used sap flow measurements in Scots pine to examine the temporal responses of transpiration to temperature (Ta), vapour pressure deficit (VPD), and net radiation (Rn). Seasonal variations and the hysteresis between sap flux density (Js) and meteorological variables were investigated. The results show that VPD and Rn were dominant factors influencing hourly Js. Daily peaks of Js lagged behind peaks of Ta, VPD, and Rn, by~2-4 hr. Anticlockwise hysteresis loops between Js and Ta and VPD were observed, and the size of loops change with prevailing weather conditions (i.e., rainy or rainless) and seasons. Longer time lags were found in April and September than in other months. The hysteresis is partly related to the low energy, humid nature of the site which resulted in limited moisture stress to the trees, and thus the trees did not necessarily need a conservation mechanism to restrict water loss as proposed by other studies in semi-arid areas. Release and refill of trunk water storage could plausibly contribute to the hysteresis, although the effect may be entwined with atmospheric demand making it difficult to separate without sophisticated measurements. This study highlights the diversity in plant water relations across temporal scales and seasons and calls for more work across geographic locations and climates.

Time-Lag Effect Between Sap Flow and Environmental Factors of Larix principis-rupprechtii Mayr

“…Longer hysteresis often occurred with stomatal closure and a high VPD; therefore, the phenomenon that J s precedes VPD can also be seen as a self-protection mechanism to avoid excessive water loss by adjusting the time period between the peaks in J s and VPD [15]. In addition, on overcast and rainy days, J s could lag behind VPD; this phenomenon has also been reported in a humid low-energy headwater catchment in recent research [14]. A possible explanation for this scenario might be the rather weak influence of VPD on J s ; on rainy days, stomatal sensitivity has a poor relationship with VPD due to a sufficient water supply and relatively high humidity, and the influence of VPD on the stomata could diminish under this circumstance.…”

“…The hysteresis loop has long been a topic of great interest in a wide range of fields, including hydrology, ecology and plant physiology [10][11][12][13]. Time lag is another form of hysteresis that is defined as the time difference between peak values of J s and meteorological variables [14], the length and direction of a time lag can, in turn, reflect the magnitude of hysteresis [12], and the time lag between sap flux density (J s ) and environmental factors is often identified as a self-protection mechanism of plants to avoid dehydration [15]. Therefore, with increasingly frequent extreme weather events, investigating time lags is a continuous concern regarding plant and water relationships.…”

Assessing the Effects of Surface Conditions on Potential Evapotranspiration in a Humid Subtropical Region of China

“…In addition, partial correlation analysis was applied to quantify the effect of environmental variables on ET p at the annual and monthly scales, which is widely used for studying the linear relationship between two variables after excluding the effect of other independent factors (Fu et al, 2015). It should be noted that the response of ETp to some factors is non-linear, especially at a sub-daily scale because of the hysteresis between ETp and these factors (Zheng et al, 2014) which is more profound in wet periods than dry periods (Wang et al, 2019b); but at a longer time scale such as daily or monthly the relationships can be linearly approximized, at least for some of the environmental factors such as radiation and VPD if not for all (Liu et al, 2014;Xu et al, 2018;Wang et al, 2019b).…”

Exploring Environmental Factors That Drive Diel Variations in Tree Water Storage Using Wavelet Analysis

“…Water potential gradients between the subsurface and the atmosphere control transpiration rates; however, there are physiological mechanisms, such as the use of internal water storage, which trees can use to modify water potential gradients. Water stored internally within a tree is a critical reservoir to avoid consequences of dehydration (Lo Gullo and Salleo, 1992;Cermák et al, 2007;Matheny et al, 2015;Wang et al, 2019), buffer against hydraulic conductivity loss in the xylem and prevent cavitation, and maximize photosynthesis when transpiration exceeds soil water uptake (Zweifel et al, 2005;Verbeeck et al, 2007b). Water can be stored intracellularly within tree tissues or extracellularly under capillary forces in the void spaces between sapwood tissues (Holbrook, 1995).…”