High forest stand densities can lead to severe competition on resources (e.g., water, light and nutrients) among trees , which causes stress on tree growth and health, and consequently limits forest carbon sequestration and water use . Thinning, as a conventional practice to reduce competition, exerts considerable effects on forest carbon and water cycles . The key hydrological processes under thinning treatments has been studied for decades, but recently gained prominent attention and been adopted as a mitigation of drought in forest ecosystems [4-8]. The effects of thinning on transpiration (T) and evapotranspiration (ET) at the forest stand level, and T, ET and water yield at the watershed level are elaborated below.
Transpiration of thinned stands was less than unthinned counterpart due to decreased basal area (BA) and canopy leaf area [4, 8-13]. The decrease of stand transpiration of the thinned stand was smaller than the reduction of leaf area index (LAI) [9, 10], because of the enhanced transpiration of individual trees in the thinned stand following thinning treatments. Such increase of the remaining trees can be partly explained by the higher leaf water potential due to open canopy exposure, and higher soil water availability caused by decreased canopy interception . However, soil water moisture was not always promoted by thinning treatments. For example, Lopushinsky (1975) reported that thinned logdepole pine underwent slight moisture stress in thinned plots than in unthinned plots, which was due to increased evaporation because of higher exposure to sunlight in thinned plots . Zhu et al (2017) found that thinning increased water content of deep soil, while reduced that of surface soil, resulting from modified soil infiltration under thinning .
Although arguments on the effect of thinning on soil moisture exist, the above effects of thinning on transpiration of the individual tree and the stand in the short term were found quite consistent among publication regardless of tree species, thinning intensity, and stand condition. In a long run, the discrepancy on transpiration between thinned and unthinned sites diminishes over time, and transpiration of thinned stand may even exceed that of unthinned stand . Nevertheless, exceptions were reported. For instance, Simonin et al. (2007) observed a higher stand transpiration in the thinned stand during a severe drought, suggesting that higher transpiration of individual trees in the thinned stand during drought overcompensated the loss of LAI . And Black et al. (1980) found that with understory presents, the transpiration of individual trees in the thinned stands was similar to that of the nearby unthinned stand without understory .
Understory plants further complicates the response of stand evapotranspiration (SET) to thinning treatments, as understory evapotranspiration was increased by thinning due to greater canopy openness that allows more solar radiation reaches the forest floor. In a forest stand without understory, thinning initiated the regrowth of weeds and promoted bare soil evaporation . When understory was present, thinning increased the herbaceous cover, leading to a surge of understory transpiration which could even be three times more than that in the nearby unthinned stand . But this effect obscured under extreme drought when transpiration and plant growth were suppressed as a result of stomatal closure . Nonetheless, the increasing understory evapotranspiration  counterbalances the decreasing overstory transpiration, resulting in SET unaffected  or decreased [7, 10, 17-19] under thinning treatments. No observation of SET increase has been report to author’s knowledge. Furthermore, the differences of SET between thinned and unthinned stand could not be detected after four or five years after thinning [7, 19], which suggests that such effect was transient, and highly depends on the thinning intensity , understory growth and environmental variables.
The response of watershed-scale transpiration and evapotranspiration (WET) which are derived from site measurements, concords with stand-scale observations under thinning treatment. WET, which decreases with decreasing forest cover (or with increasing thinning intensity) [21-27] because its biggest contributor (i.e., forest transpiration) was largely reduced , leads to an increase in the water yield [22, 24, 25, 27-33], regardless the type of thinning treatment (e.g. strip thinning, uniform thinning or patch cutting). A detailed study concluded that strip thinning resulted in highest increase in water yield, followed by uniform thinning and patch cutting , but this conclusion may be watershed-depended. Notably that the increase in water yield are revertible when trees regrowth to the pre-disturbance level with increasing canopy interception and WET [27, 34], though the recovery period spans from 9 to more than 34 years [31, 34].
Increasing water yield usually leads to the increasing in the quantity and duration of streamflow, as well as base flow  and peak flow , but is not a guarantee of increase, as streamflow is highly subject to soil water storage capacity and thinning intensity [35, 36]. The magnitude of streamflow increment is various (some result are summarized in the table below) under similar thinning treatments, suggesting that watershed topographic properties (e.g. slope, area and soil characteristics) and rainfall amount [22, 25, 36, 37] play an important role in regulating the surface runoff.
Table 1. Some research on the effect of thinning on streamflow
|Research||Citation||Thinning treatment intensity||Streamflow increase|
|Stoneman (1986)||||66% reduction of forest density||86mm|
|Ruprecht et al. (1991)||||reduce crown cover from 60 to 14%||260 mm|
|Lesch and Scott (1997)||||22-46% reduction of density||19-99 nm|
|Johnson and Kovner (1956)||||53% of the basal area||55 mm|
|Baker (1986)||||31–68% of the basal area||25–46.3 mm|
|Dung et al. (2012)||||Reduce 43.2% of the basal area||240.7 mm|
|Hawthrone et al. (2013)||||Reduce 33-54% basal area||~36% increase|
|Saksa et al. (2017)||||Reduce LAI from 9.9 to 9.1||130-140mm|
As climate change projects a continuous global warming, and an increase in the frequency of hot extremes at the global scale (IPCC Climate Change 2014 Synthesis Report), hydrology-oriented thinning may serve as a temporary makeshift to tackle with drought. Over a longer period, forest LAI recovery and understory growth enhanced by thinning may decrease water yield , which further exacerbates the water crisis. Thus, decisions on adopting thinning treatments should be made with cautions.
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