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Open AccessArticle

Sparse Pinus Tabuliformis Stands Have Higher Canopy Transpiration Than Dense Stands Three Decades After Thinning

1
College of Soil and Water Conservation, Beijing Forestry University, Key Laboratory of Soil and Water Conservation, the State Forestry and Grassland Administration, Beijing 100083, China
2
Badaling Forest Farm, Beijing 102112, China
3
Gongqing Forest Farm, Beijing 101300, China
4
Department of Geography, University of North Carolina at Chapel Hill, Chapel Hill, NC 27599, USA
*
Author to whom correspondence should be addressed.
Forests 2020, 11(1), 70; https://doi.org/10.3390/f11010070
Received: 12 December 2019 / Revised: 1 January 2020 / Accepted: 2 January 2020 / Published: 6 January 2020
(This article belongs to the Section Forest Ecophysiology and Biology)
Hydrological effects of forest thinning have been studied at small watershed scales using the paired watershed approach since the 1920s. However, how forest transpiration, a critical component of evapotranspiration, changes decades after thinning is not well understood despite its importance for modifying drought resilience of forest ecosystem under climate change. In a semi-arid mountainous area of northern China, we measured growing season sap flow of Pinus tabuliformis, a widely planted afforestation species, in 44-year-old monoculture plantation stands with low (983 stems ha−1), medium (1688 stems ha1), and high (2160 stems ha1) density. Three decades after thinning, diameters at breast height (DBH) were larger in sparse stands than in dense stands. While its relation with sapwood area was density independent, the accompanying high sapwood area at the tree level for sparse stands resulted the highest stand sapwood area in the medium density stand (33.26 m2 ha−1), rather than in the high density stand (29.84 m2 ha−1). Similar to short-term studies, sparse stands demonstrated higher sensitivity to climatic fluctuations and drought depressions than dense ones. Nevertheless, stand density had no effect on the isohydric strategy of Pinus tabuliformis. Contrary to the positive relation between stand density and stand canopy transpiration soon after thinning, sparse stands exhibited higher growing season canopy transpiration than dense stands three decades later. In the dry year 2014, these density differences were relatively most pronounced, with July-September transpiration totals of 56.7 mm, 31.1 mm, and 22.1 mm in the low, medium, and high density stands, respectively. Our findings highlighted that stand density was not an appropriate indicator of thinned forest transpiration over long time scales. Interactions between soil droughts and thinning on forest transpiration need to be further clarified, especially in longer periods of time. View Full-Text
Keywords: thinning; stand density; sap flow; transpiration; DBH; sapwood area; soil water content; climatic response; physiological control thinning; stand density; sap flow; transpiration; DBH; sapwood area; soil water content; climatic response; physiological control
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Chen, Z.; Zhang, Z.; Chen, L.; Cai, Y.; Zhang, H.; Lou, J.; Xu, Z.; Xu, H.; Song, C. Sparse Pinus Tabuliformis Stands Have Higher Canopy Transpiration Than Dense Stands Three Decades After Thinning. Forests 2020, 11, 70.

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