How does thinning affect key hydrological processes?

High forest stand densities can lead to severe competition on resources (e.g., water, light and nutrients) among trees [1], which causes stress on tree growth and health, and consequently limits forest carbon sequestration and water use [2]. Thinning, as a conventional practice to reduce competition, exerts considerable effects on forest carbon and water cycles [6]. The key hydrological processes under thinning treatments has been studied for decades, but recently gained prominent attention[3] 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 [5]. 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 [11]. 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 [12].

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 [13]. 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 [5]. 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 [14].

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 [15]. 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 [5]. But this effect obscured under extreme drought when transpiration and plant growth were suppressed as a result of stomatal closure [15]. Nonetheless, the increasing understory evapotranspiration [16] counterbalances the decreasing overstory transpiration, resulting in SET unaffected [15] 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 [20], 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 [28], 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 [27], 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 [15] and peak flow [26], 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) [30] 66% reduction of forest density 86mm
Ruprecht et al. (1991) [24] reduce crown cover from 60 to 14% 260 mm
Lesch and Scott (1997) [31] 22-46% reduction of density 19-99 nm
Johnson and Kovner (1956) [32] 53% of the basal area 55 mm
Baker (1986) [33] 31–68% of the basal area 25–46.3 mm
Dung et al. (2012) [25] Reduce 43.2% of the basal area 240.7 mm
Hawthrone et al. (2013) [27] Reduce 33-54% basal area ~36% increase
Saksa et al. (2017) [22] 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 [27], which further exacerbates the water crisis. Thus, decisions on adopting thinning treatments should be made with cautions.

 

Reference:

  1. Brix, H. and A.K. Mitchell, Thinning and nitrogen fertilization effects on soil and tree water stress in a Douglas-fir stand. Canadian Journal of Forest Research, 1986. 16(6): p. 1334-1338.
  2. Louda, S.M. and S.K. Collinge, Plant resistance to insect herbivores: a field test of the environmental stress hypothesis. Ecology, 1992. 73(1): p. 153-169.
  3. del Río, M., et al., A review of thinning effects on Scots pine stands: From growth and yield to new challenges under global change. 2017, 2017. 26(2).
  4. Smit, G.N. and N.F.G. Rethman, The influence of tree thinning on the soil water in a semi-arid savanna of southern Africa. Journal of Arid Environments, 2000. 44(1): p. 41-59.
  5. Simonin, K., et al., The influence of thinning on components of stand water balance in a ponderosa pine forest stand during and after extreme drought. Agricultural and Forest Meteorology, 2007. 143(3): p. 266-276.
  6. Tang, J., et al., Forest thinning and soil respiration in a ponderosa pine plantation in the Sierra Nevada. Tree Physiology, 2005. 25(1): p. 57-66.
  7. Dore, S., et al., Recovery of ponderosa pine ecosystem carbon and water fluxes from thinning and stand-replacing fire. Global Change Biology, 2012. 18(10): p. 3171-3185.
  8. Fernandes, T.J.G., et al., Simultaneous assessment, through sap flow and stable isotopes, of water use efficiency (WUE) in thinned pines shows improvement in growth, tree-climate sensitivity and WUE, but not in WUEi. Forest Ecology and Management, 2016. 361(Supplement C): p. 298-308.
  9. Bréda, N., A. Granier, and G. Aussenac, Effects of thinning on soil and tree water relations, transpiration and growth in an oak forest (Quercus petraea (Matt.) Liebl.). Tree Physiology, 1995. 15(5): p. 295-306.
  10. Wang, Y., et al., Water-Yield Reduction After Afforestation and Related Processes in the Semiarid Liupan Mountains, Northwest China1. JAWRA Journal of the American Water Resources Association, 2008. 44(5): p. 1086-1097.
  11. Donner, B.L. and S.W. Running, Water Stress Response After Thinning Pinus contorts Stands in Montana. Forest Science, 1986. 32(3): p. 614-625.
  12. Zhu, X., et al., Temporal variability in soil moisture after thinning in semi-arid Picea crassifolia plantations in northwestern China. Forest Ecology and Management, 2017. 401(Supplement C): p. 273-285.
  13. Lagergren, F., et al., Thinning effects on pine-spruce forest transpiration in central Sweden. Forest Ecology and Management, 2008. 255(7): p. 2312-2323.
  14. Black, T.A., U. Nnyamah, and C.S. Tan, TRANSPIRATION RATE OF DOUGLAS FIR TREES IN THINNED AND UNTHINNED STANDS. Canadian Journal of Soil Science, 1980. 60(4): p. 625-631.
  15. Skubel, R.A., et al., Short-term selective thinning effects on hydraulic functionality of a temperate pine forest in eastern Canada. Ecohydrology, 2017. 10(1): p. e1780-n/a.
  16. Sun, X., et al., The effect of strip thinning on forest floor evaporation in a Japanese cypress plantation. Agricultural and Forest Meteorology, 2016. 216(Supplement C): p. 48-57.
  17. Moreaux, V., et al., Paired comparison of water, energy and carbon exchanges over two young maritime pine stands (Pinus pinaster Ait.): effects of thinning and weeding in the early stage of tree growth. Tree Physiology, 2011. 31(9): p. 903-921.
  18. Goodell, B.C., Watershed-management aspects of thinned young lodgepole pine stands. Journal of Forestry, 1952. 50: p. 374-378.
  19. Aussenac, G. and A. Granier, Effects of thinning on water stress and growth in Douglas-fir. Canadian Journal of Forest Research, 1988. 18(1): p. 100-105.
  20. Anderson, H.W., M.D. Hoover, and K.G. Reinhart, Forests and water: effects of forest management on floods, sedimentation, and water supply. 1976.
  21. Biederman, J.A., et al., Increased evaporation following widespread tree mortality limits streamflow response. Water Resources Research, 2014. 50(7): p. 5395-5409.
  22. Saksa, P.C., et al., Forest thinning impacts on the water balance of Sierra Nevada mixed-conifer headwater basins. Water Resources Research, 2017. 53(7): p. 5364-5381.
  23. Bosch, J.M. and J.D. Hewlett, A review of catchment experiments to determine the effect of vegetation changes on water yield and evapotranspiration. Journal of Hydrology, 1982. 55(1): p. 3-23.
  24. Ruprecht, J.K., et al., Early hydrological response to intense forest thinning in southwestern Australia. Journal of Hydrology, 1991. 127(1): p. 261-277.
  25. Dung, B.X., et al., Runoff responses to forest thinning at plot and catchment scales in a headwater catchment draining Japanese cypress forest. Journal of Hydrology, 2012. 444(Supplement C): p. 51-62.
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  27. Hawthorne, S.N.D., et al., The long term effects of thinning treatments on vegetation structure and water yield. Forest Ecology and Management, 2013. 310(Supplement C): p. 983-993.
  28. Ruprecht, J.K. and G.L. Stoneman, Water yield issues in the jarrah forest of south-western Australia. Journal of Hydrology, 1993. 150(2): p. 369-391.
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  30. Stoneman, G. Thinning a Small Jarrah Forest Catchment: Steamflow and Groundwater Response After 2 Years. in Hydrology and Water Resources Symposium 1986: River Basin Management; Preprints of Papers. 1986. Institution of Engineers, Australia.
  31. Lesch, W. and D.F. Scott, The response in water yield to the thinning of Pinus radiata, Pinus patula and Eucalyptus grandis plantations. Forest Ecology and Management, 1997. 99(3): p. 295-307.
  32. Johnson, E.A. and J.L. Kovner, Effect on streamflow of cutting a forest understory. Forest Science, 1956. 2(2): p. 82-91.
  33. Baker, M.B., Effects of ponderosa pine treatments on water yield in Arizona. Water Resources Research, 1986. 22(1): p. 67-73.
  34. Bren, L., P. Lane, and G. Hepworth, Longer-term water use of native eucalyptus forest after logging and regeneration: The Coranderrk experiment. Journal of Hydrology, 2010. 384(1): p. 52-64.
  35. Stokes, R.A., Streamflow and groundwater responses to logging in Wellbucket catchment, south Western Australia / R.A. Stokes, F.E. Batini. Report (Water Authority of Western Australia) ; no. WH3., ed. F.E. Batini and B. Water Authority of Western Australia. Hydrology. 1985, Leederville, W.A: Water Authority of Western Australia.
  36. Rahman, A., et al., Effects of forest thinning on direct runoff and peak runoff properties in a small mountainous watershed in Kochi Prefecture, Japan. Pakistan Journal of Biological Sciences, 2005. 8: p. 259-266.
  37. Stoneman, G.L., Hydrological response to thinning a small jarrah (Eucalyptus marginata) forest catchment. Journal of Hydrology, 1993. 150(2): p. 393-407.

12 thoughts on “How does thinning affect key hydrological processes?

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  6. rees 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 [5]. However, so

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