DOI: 10.5061/dryad.51c59zwj8
title: Species-specific water-use characteristics of trees in old-growth and secondary tropical forests of Thailand

description: Tree water use is a critical component of the forest water cycle and is
 influenced by global climate changes, such as shifts in precipitation
 patterns. These changes may disproportionately affect forest runoff
 depending on how sensitive tree water use is to environmental conditions.
 Therefore, understanding the water-use strategies of different tree
 species is essential for predicting how forests will respond to
 environmental changes. This study investigated how daily sap flux density
 (Js), representing water flow per a unit of sapwood area, varies with
 environmental factors in eight common tree species in successional
 tropical forests in Thailand. Using thermal dissipation probes, we
 measured Js in an old-growth (OF) and a young forest (YF). When averaging
 across all species, trees in OF were highly sensitive to rising vapor
 pressure deficit (VPD) under low soil moisture, responding to high
 atmospheric demand, while those in YF maintained their water use rate
 regardless of changing VPD. Additionally, species-specific response
 patterns were observed across different soil moisture conditions at both
 sites. In OF, Syzygium syzygoides and Cinnamomum subavenium exhibited
 conservative water use under low soil moisture, potentially preventing
 them from negative effects from droughts. In YF, Js of Adinandra
 integerrima saturated earlier than other species in dry soils, possibly
 indicating greater drought tolerance compared to others. These findings
 provide valuable insights into species-specific water-use patterns across
 different successional forests which may benefit forest management,
 particularly the selective planting of suitable species in reforested
 areas.
description: The tree species were selected based on their relative basal area
 abundance and number of trees within each plot, using inventory data from
 the sites (W. Chanthorn, pers. comm, 2018). This was determined by
 calculating each species' basal area as a proportion of the total
 basal area across all species in each site. We focused on common tree
 species<em>, </em>i.e., those in the top 20 tree species with
 high relative basal area ranking and more than 100 trees. However, our
 choice of species selection was limited to the requirement that the
 sampled trees for sap flux measurement had to be within 25 m radius from
 the data logger. This requirement resulted in 21 trees of eight evergreen
 species in total, with one species existing in both sites
 (<em>Syzygium antisepticum</em>). Sap flux density (g
 cm<sup>-2</sup> s<sup>-1</sup>) was measured using
 self-constructed thermal dissipation probes (Granier, 1985). Details about
 sap flux density measurement and its estimation are described in
 Ampornpitak et al. (2023). Our monitoring period
 covered January 2021 to April 2022 in OF, and September 2020 to June 2023
 in YF. The data covered at least one year in both sites, representing a
 wide range of environmental conditions. Wet canopy conditions may inhibit
 tree water use when the leaf surface is covered with water
 droplets (Aparecido et al., 2016); hence, we selected the days under
 rain-free conditions to perform the analysis. Unfortunately, rainfall data
 in OF were unavailable from 20 February 2021 to 6 October 2021 due to
 technical issues with the rain gauge. Nevertheless, we found a strong
 correlation between the rainfall data from OF and YF (<em>R
 </em>= 0.60, <em>p</em> &lt; 0.0001), allowing us to
 assume similar rainfall patterns. Therefore, we referred to the rainfall
 from YF to filter out rainy days for further analysis in OF during the
 period of missing data. To evaluate how sap flux
 density varies with environmental factors in common tree species in
 successional tropical forests, we performed a boundary line analysis
 (Ewers et al., 2001; Schäfer et al., 2000) to obtain the response of daily
 sum sap flux density (J<sub>s</sub>; g cm<sup>-2
 </sup>day<sup>-1</sup>) to environmental factors on the
 daily scale under non limiting conditions. This technique allows us to
 obtain responses with a single factor without the effects from other
 confounding factors. For each site, the species average
 J<sub>s</sub> was used to assess the response of
 J<sub>s</sub> to environmental factors with the boundary line
 analysis. Generally, VPD, REW, and PAR are the environmental variables
 that affect J<sub>s </sub>(Phillips & Oren 2001;
 Wright et al., 2023; Zhao et al., 2021). Based on the data from the
 monitoring period, we found a high correlation between VPD and PAR in both
 sites (<em>R</em> = 0.71, <em>p</em> ≤ 0.001 in OF
 and <em>R</em> = 0.73, <em>p</em> ≤ 0.001 in YF).
 Therefore, we mainly focused on VPD and REW as the environmental driving
 factors in this study. Before performing the analysis, we filtered the
 data by selecting those under no rainfall conditions. Next, we partitioned
 soil moisture data into two classes based on REW distribution, including
 low soil moisture (REW &lt; 0.1 for YF and &lt; 0.2 for OF) and
 high soil moisture (REW &gt; 0.1 for YF and &gt; 0.2 for OF).
 These filtering and partitioning resulted in two subsets of data in each
 forest plot and each common species. To perform the boundary line
 analysis, we then (1) partitioned J<sub>s </sub>data of each
 REW class into at least five VPD intervals, (2) calculated J<sub>s
 </sub>mean and its standard deviation of each VPD interval, (3)
 removed outliers using Z score (95% confidence interval), (4) chose the
 data above mean plus one standard deviation, and (5) averaged the chosen
 data for each VPD interval. The data set from step (5), which represents
 the maximum of mean J<sub>s</sub>, were then analyzed using
 regression analysis with VPD in different soil moisture conditions (i.e.,
 REW classes). Data management (e.g., partitioning, filtering) and data
 analysis were performed in R software (version 1.3.1073), and regression
 analyses were done in SigmaPlot version 15.0 (Systat Software, Inc., San
 Jose, CA USA).
description: # Species-specific water-use characteristics of trees in old-growth and
 secondary tropical forests of Thailand
 [https://doi.org/10.5061/dryad.51c59zwj8](https://doi.org/10.5061/dryad.51c59zwj8) ## Description of the data and file structure This study investigates the variation of tree water use with environmental factors in two successional forests in Khao Yai National Park, Thailand. Sap flux density (Js), expressed as the rates of water flow per unit sapwood area, was used in the analyses. The Js data were monitored from January 2021 to April 2022 in the old-growth forest, and September 2020 to June 2023 in the young forest. We used daily sum values in all analyses presented in the corresponding paper. The descriptions of labels in the data set are as follows. **Site:** ·       OF = the old-growth forest (&gt;200 years old) ·       YF = the young forest (&lt;10 years old) **REW (Relative extractable water) class:** ·       low = low soil moisture (REW &lt; 0.2 for OF and &lt; 0.1 for YF) ·       high = high soil moisture (REW &gt; 0.2 for OF and &gt; 0.1 for YF) **VPD** = vapor pressure deficit in kPa **Js**= daily sum sap flux density in g cm-2 day-1 **Dataset 1** contains processed Js data from the boundary line analysis, averaging across all species within each forest, in relation to VPD for each REW class. **Dataset 2** contains processed Js data from the boundary line analysis, averaging across all trees for each species within each forest, in relation to VPD for each REW class.