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

Abstract

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 ha−1), and high (2160 stems ha−1) 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.

Heat and drought impact on carbon exchange in an age-sequence of temperate pine forests

“…At the 80-year site, Skubel et al ( 2015 ) found a slight increase in transpiration in remaining trees, despite 2012 being a warm and dry year. Past studies have reported that thinning results in greater availability of soil water to remaining trees, leading to higher transpiration and hence photosynthesis (Bréda et al 1995 ; Reid et al 2006 ; Chen et al 2020a ). This may have been the case in our 80-year-old stand.…”

“…Plantation forests in Eastern North America are managed with the vision to enhance their growth, resilience to stresses, and to maximize their carbon sequestration potential (Meehl et al 2007 ; Sippel et al 2016 ). As heat and drought events are likely to be more frequent in the area, due to the predicted warmer climate in the future (IPCC 2014 ; 2018 ; Niinemets 2010 ; Xu et al 2020 ; Chen et al 2020a ; Fernández-Martínez et al 2020 ), their combined effects especially over multiple years, may have serious implications for net carbon sequestration in temperate conifer forests of Eastern North America. The impact of concurrent and consecutive extreme events should be considered while developing forest management practices for climate resiliency and sustainability with the aim of climate change mitigation by way of greenhouse gas reduction.…”

Biophysical regulations of transpiration and water use strategy in a mature Chinese pine (Pinus tabulaeformis) forest in a semiarid urban environment

“…However, the slope between m and the reference canopy conductance was significantly lower than 0.6 regardless of soil moisture conditions (Figure 9b). Chen, Zhang, Chen, et al (2020) found that the slope is close to 0.6 in a natural environment, and P. tabulaeformis can be classified as an isohydric tree species (Table 2). Hence, tree water use behaviours need to be confirmed by the dynamics of leaf water potential and stomatal conductance (Klein, 2014; Siddiq et al, 2017).…”

Optimal Harvesting Decision Paths When Timber and Water Have an Economic Value in Uneven Forests

“…In reality, other stand variables, such as rooting depth and canopy resistance, may have a strong effect on local water balance [33]. However, simulating these variables and their interactions with local water balance is a complex task [61,62]. For example, Chen and coauthors [62] report on a case where thinning led to increased transpiration through complex biophysical interactions.…”

“…However, simulating these variables and their interactions with local water balance is a complex task [61,62]. For example, Chen and coauthors [62] report on a case where thinning led to increased transpiration through complex biophysical interactions. In this study, as the model only simulated moderate thinning, we do not expect that such effects would have a large impact on the results.…”