Responses of Sap Flow of Deciduous and Conifer Trees to Soil Drying in a Subalpine Forest
Co-occurring species may adopt different water-use strategies to adapt to limited soil water. In Jiuzhaigou Valley, a continuous decline in soil water after an initial recharge from the thawing of snow and frozen soil in early spring was observed, but its effects on the sap flow dynamics of co-occurring species are not well understood. To clarify the species-specific water-use strategy, variations in sap flow and environmental conditions were investigated for two co-occurring species (Betula
... species (Betula albosinensis Burk. and Pinus tabuliaeformis Carr.) in a mixed forest during a transition from the wet to dry period in 2014. Sap flow was measured using Granier-type thermal dissipation probes, and the soil-water content was measured using time-domain reflectometry probes for a successive period. Our study showed that B. albosinensis maintained relatively high transpiration until late into the season regardless of soil moisture, while the transpiration of P. tabuliformis showed a continuous decrease in response to seasonal soil drying. Sap flow for both species exhibited a marked hysteresis in response to meteorological factors and it was conditioned by the soil-water status, especially in the afternoon. We found that P. tabuliformis was sensitive to soil-water conditions, while for B. albosinensis, the sap flow was not very sensitive to changes in soil-water conditions. These results indicate that B. albosinensis could manage the water consumption conservatively under both dry and wet conditions. These results may have implications for evaluating the species-specific water-use strategy and carrying out proper reforestation practices. Forests 2018, 9, 32 2 of 15 has intended to increase the broadleaved species, especially Betula species in the subalpine forests to provide a more fascinating tourist attraction  . In addition, it is believed that the ratio change will lead to changes in evapotranspiration and therefore water balance [4, 5] . In recent years, with runoff decreasing and streams drying up, the water landscape of Jiuzhaigou Valley has ultimately been affected     . Therefore, for proper reforestation practices and landscape management, common species in JZG need to be investigated to understand their water-use strategies under different climatic and soil-water conditions and to determine whether they are suitable for developing a sustainable color forest landscape. The responses of sap flow to soil moisture and meteorological factors, such as solar radiation (R s ) and vapor pressure deficit (VPD), have been a topic of research in agricultural and forest management over the last two decades        . Previous studies have shown that despite a large variation in the threshold of transpiration responses to the VPD, when R s , VPD or the variable of transpiration (VT)-which is defined as an integrated index of R s and VPD-were below a certain threshold, transpiration increased rapidly. Then, when it exceeded the threshold value, transpiration maintained a relatively stable level. However, due to large differences in sensitivity to soil-water conditions, the transpiration response usually varies greatly among different species in a single wet-dry cycle  and is possibly conditioned by soil-water availability [11, 17] . Sap flow characteristics vary not only with meteorological factors and soil water conditions but also with species. It has been demonstrated that different species showed different responses to meteorological and edaphic parameters [10, 15, 16, 18] . Based on the different sap flow responses to meteorological factors under different soil-water conditions, tree species can be distinguished into types with contrasting drought sensitivity [11, 14, 19] . For example, by analyzing sap flow characteristics and climatic responses, Du et al.  found that the exotic Robinia pseudoacacia Linn. in a semiarid forest of China was drought-sensitive, while the naturally dominant Quercus liaotungensis Koidz. and the indigenous concomitant species Armeniaca sibirica Lam. were drought-insensitive. Similar studies in the greater Los Angeles area showed that the native California sycamore showed little response to shallow soil-water content, while the non-native Canary Island pine was sensitive to shallow soil-water conditions but tolerant of very low soil-water availability  . Therefore, the effects of soil-water availability on species distribution, forest structure and stand composition could be assessed by studying the species-specific sap flow characteristics in relation to meteorological factors and soil-water availability. This approach is particularly suitable in areas subjected to higher atmospheric demand precipitation and shallow soils  . In JZG, the mean annual value of precipitation and potential evapotranspiration for 1956-2014 was 762 mm and 806 mm, respectively (data are from the Songpan meteorological station (32 • 39 N, 103 • 34 E, 2852 m elevation) of the Chinese Meteorological Association). Additionally, the subalpine mixed forest is characterized by shallow soil depth, and our previous research showed that after a large amount of soil-water recharge in early spring, JZG experienced a general decline in soil moisture as the growing season progressed  . In this context, the approach is suitable for assessing species' water-use strategy by evaluating sap flow characteristics in response to meteorological factors and soil-water availability in JZG. In JZG, two dominant tree species in the secondary mixed forest are B. albosinensis and P. tabuliformis [1, 22] . Both of these species are widely distributed in the subalpine region and are pioneer, shade-intolerant species [3, 23] , but B. albosinensis has been regarded as a forest species that prefers a humid climate and that mostly occurs in hillside and canyon habitats . It has shallower roots and more fine roots near the surface than the conifer species and could take up sufficient water in a humid subalpine valley  . In contrast, P. tabuliformis prefers a dry and cold climate and is widely distributed in areas with precipitation ranging from 361-1038 mm  . However, the species-specific water-use strategy is still unknown in the experimental shed. We hence hypothesized that these two species have different water-use strategies to cope with the limitation of soil-water availability. To test the hypothesis, the sap flow for two dominant tree species and the relevant environmental data were measured in a secondary mixed forest of JZG. For the present study, we aimed to (1) investigate the sap flow and the water-use Forests 2018, 9, 32 3 of 15 characteristics of the typical deciduous and conifer species in cold-limited areas, (2) compare the difference in sap flow for each species to meteorological factors under different soil-water conditions, and (3) compare interspecies differences of sap flow responses. The results are expected not only to enrich the understanding of species-specific water-use strategy, but also to provide insight into species distribution, forest structure and stand composition. Materials and Methods Study Site The study site is located in Jiuzhaigou County of the Aba Tibetan and Qiang Autonomous Prefecture, Sichuan Province (32 • 53 -33 • 20 N, 103 • 46 -104 • 05 E). JZG lies in a transition zone of a northern subtropical area and a warm temperate area on the Tibet Plateau designated as a highland temperate monsoon climate. From 1951 to 2009, the mean annual precipitation was 762 mm, and the mean air temperature was 7.3 • C. The mean annual humidity is more than 70%. The mean annual potential evapotranspiration (806 mm) was slightly higher than the mean annual precipitation. JZG has an elevation range spanning from 2000 m to 4800 m and the vegetation types and vegetation compositions show distinct vertical zonation: coniferous and broadleaved mixed forest below 2800 m, coniferous forest between 2800 and 3800 m, and alpine bush and meadow above 3800 m. The experiment site for the current study was thus selected in a coniferous and broadleaved mixed forest (33 • 9.54 N, 103 • 52.86 E) with an elevation of 2478 m, which suffered from a forest harvest during 1964-1979. At the time of study, the forest was dominated by a secondary forest of pine (P. tabuliformis), oak (Quercus liaotungensis Koidz.), and sub-canopy species (B. albosinensis and Acer ginnala Maxim.) [1, 22] . B. albosinensis was generally dominant in the lower diameter classes, while P. tabuliformis was present in all diameter classes  . To make both species have similar access to light, all target trees were selected in sparse parts of the mixed forest. The general characteristics of B. albosinensis and P. tabuliformis in this study are presented in Table 1 .