The effect of strip thinning on tree transpiration in a Japanese cypress (Chamaecyparis obtusa Endl.) plantation
Introduction
Tree transpiration (Et) is a main part in the forest water balance and for modeling water, energy and carbon exchange in the forest ecosystem. In coniferous forests, Et may account for approximately 19.0–72.4% of evapotranspiration for different species and climates (Sun et al., 2014b). Et is influenced by environmental variables, including vapor pressure deficit, solar radiation, wind speed and temperature (Morikawa et al., 1986, Granier et al., 1996b, Oren et al., 1999, Clausnitzer et al., 2011), and by the availability of soil water within the rooting zone (Black et al., 1980, Breda et al., 1995, Simonin et al., 2007, Sun et al., 2014b). The thinning of forests results in more open stand canopies. Accordingly, the remained individual trees have apportioned a higher availability of site resources (e.g., soil water) due to thinning treatment (Black et al., 1980, Breda et al., 1995, Morikawa et al., 1986). However, thinning can affect various factors that influence the growth rate of trees, and it is difficult to determine the single most important factor affecting tree water use (Medhurst et al., 2002). Therefore, studies regarding changes in Et that are induced by thinning are necessary for predicting tree water use and for guiding integrated forest and water management.
Among several methods for quantifying Et at both temporal and spatial scales, the thermal dissipation sap flow technique (Granier, 1987) is the most useful, particularly in mountainous countries such as Japan because complex terrain and spatial heterogeneity does not restrain its applicability (e.g., Wilson et al., 2001). This technique can be effectively applied to estimate the tree water use on a continuous basis, which has made it feasible to examine the thinning effects on water uptake and Et at both tree and stand levels, at a high temporal resolution. When climatic data are properly collected simultaneously with sap flow data, this method can supply forceful insights into atmospheric-biological controls of tree water use (Whitehead, 1998).
Several studies have focused on the changes in Et by thinning in different species (e.g., Breda et al., 1995, Lagergren et al., 2008, Morikawa et al., 1986, Reid et al., 2006, Simonin et al., 2006, Simonin et al., 2007, Stogsdill et al., 1992). For example, Morikawa et al. (1986) reported that the stand Et decreased by 21.2% after 24% thinning in a Japanese cypress (Chamaecyparis obtusa Endl.) forest. In addition, the daily single tree Et was higher at a given range of solar radiation, except in the small tree class. Breda et al. (1995) reported that thinning caused the stand Et value to decrease in the thinned plot of an oak forest for the first year, whereas the stand Et approached the same level as that on the control plot after two years of thinning. Furthermore, the difference in the stand Et between the thinned and the control plot may not significantly decrease due to the drought periods. Simonin et al. (2007) found that the difference in the stand Et between the thinned and the control plot was much less when the soil water content was low in semi-arid Pinus ponderosa forests. Lagergren et al. (2008) reported that the stand Et in the thinned plot was rather higher than that in the control plot during the drought period in a mixed pine-spruce forest in Sweden.
In Japan, forests cover 67% of the total land area, and approximately 40% of the forested land area is composed of coniferous plantation forests (National Astronomical Observatory, 2009). Japanese cypress and Japanese cedar (Cryptomeria japonica D. Don) are the main two coniferous plantation species. However, these plantations received no management practices and have been abandoned since their planting, primarily after the Second World War, because of low wood value and high labor costs (Iwamoto, 2002). As a result, the lack of management leads to high stem density, dense canopy density, and sparse or no understory vegetation, particularly in Japanese cypress forests (Onda et al., 2010). Therefore, these cypress plantations need to be thinned about 40–60% to induce recovery of understory vegetation (Sun et al., 2014b). However, previous studies only examined changes in Et by light thinning (removing 24% of stems) (Morikawa et al., 1986) or during a short measuring period (two months before and after thinning, respectively) (Komatsu et al., 2013). Until now, little data have been available to document the changes in Et induced by heavy thinning during a long measurement period for Japanese coniferous plantations.
Strip thinning, which is a heavy and cost-effective thinning method, has been extensively carried out in these poorly managed plantation stands in Japan primarily because this method does not need to select trees that are involved in conventional selective thinning operations and thus requires less time and skill (Taniguchi, 1999), whereas it cannot effectively improve the stand structure. Furthermore, strip thinning results in different changes in the canopy density and in the structure of the forest compared with other forestry practices (e.g., selective thinning and partial cutting). The different forest structures can lead to resultant changes in environmental variables (Oguntunde and Oguntuase, 2007, Wilson et al., 2000), in the availability of soil water (Aboal et al., 2000, Molina and del Campo, 2012, Stogsdill et al., 1989), and in boundary layer conductance (Teklehaimanot et al., 1991). For example, Teklehaimanot et al. (1991) reported that the greater ventilation (i.e., wind speed) with an increase in tree spacing resulted in greater boundary layer conductance per tree in Picea sitchensis (Bong.) Carr forest stands. Thus, Et responses to different management strategies would be different. However, studies on the strip thinning effect on Et are limited and are necessary for achieving an optimized water and forest management.
This study aimed to examine the effect of strip thinning on Et at individual tree and stand levels for a Japanese cypress plantation. The study period was divided into the pre-thinning period (November 2010–October 2011) and the post-thinning period (November 2011–October 2012). Sap flow densities were measured using thermal dissipation (Granier-type) sensors.
Section snippets
Site description
Our study was carried out in a 156-m2 plot (12 m × 13 m) at a mean elevation of 198 m on a mountain slope (31°) with southwest exposure. The study plot is in the headwater catchment K2 in Mt. Karasawa, Tochigi Prefecture (36°22′ N, 139°36′ E), central Japan (Fig. 1a). The forest in catchment K2 consists of even-aged 32-year-old Japanese cypress stands. However, these plantations have been abandoned since their planting (Sun et al., 2014a). The drainage area is 13.3 ha. The understory vegetation is
Environmental conditions and sapwood area estimates in pre- and post-thinning
The time series of meteorological factors in the pre- and post-thinning periods are shown in Fig. 3. The meteorological variables (e.g., RH, T, VPD, Rs and PET) show clear seasonal trends and reached higher values during the growing season (May–October) in both periods. The day-to-day variations in Rs generally corresponded to Pg and were low during the regular rainy season from mid-June to mid-July in Japan. The day-to-day variations in the VPD generally corresponded to those changes in T and
Effect of thinning on As_stand and JS
As_stand in the present study decreased by 46.4% after 50% thinning (Table 1). The sapwood area at xylem bands of 0–20 and 20–40 mm also showed similar trends and declined by 47.5% and by 44.0%, respectively (Table 1). The decline of As_stand corresponded to the ratio to thinning. A linear relation between DBH and As_tree was found for Japanese cypress plantations (Fig. 4). Several studies estimated the As_tree from a power function-based regression (Kumagai et al., 2007, Vertessy et al., 1995,
Conclusions
This study elucidated the variations in Et and Gc, in addition to tree-to-tree and radial Fd, by 50% strip thinning in a Japanese cypress forest. Our results showed that the Fd at the outer xylem (0–20 mm) increased remarkably, whereas the Fd at the inner xylem (20–40 mm) had no significant change in three tree classes (large, medium and small) after thinning. This result implies that thinning only enhanced the capacity of conducting water at the outer xylem. Correspondingly, the JS_A values were
Acknowledgments
We acknowledge Drs. Yoshinori Shinohara, Kenji Tsuruta, and Takami Saito for their support of this research. Special thanks are also given to Dr. Makiko Tateishi (Kyushu University, Japan) and Dr. Teramage Tesfaye (Tsukuba University, Japan) for providing critical comments. This study was supported by the Core Research for Evolutional Science and Technology CREST project entitled “Development of innovative technologies for increasing in watershed runoff and improving river environment by the
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