Any water-related factors affecting stem diameter shrinkage are due to changes in the tree bole water potential gradient caused by tree transpiration [
30], and transpiration could cause changes in stem sap flow [
16,
31]. Interpreting growth and water dynamic signals from automatic dendrometer data is not always straightforward [
29,
32]. After dividing stem diameter changes into TWD and growth-induced irreversible stem expansion, TWD was found to be an effective index for analyzing stem diameter responses to tree bole moisture loss. As a stem diameter shrinkage indicator, TWD was a good choice for reflecting sap flow changes at a half-hour time resolution. It should be noted that the division mode adopted in this study was based on the zero-growth assumption (diametral growth has little or no effect on stem shrinkage). Zweifel et al. [
29] found TWD to be a very effective biological indicator for tree bole moisture, which is consistent with our results in this study. The hydration caused by relatively high stem sap flow can slow down the stem diameter contraction and increase water storage [
33]. TWD is produced by stem volume changes caused by the moisture status, and it is therefore advantageous in reflecting stem sap flow. This study proves that TWD can reflect a tree’s moisture status at a half-hour time resolution. Since stem diameter shrinkage indicators such as the maximum daily shrinkage, daily stem diameter increment, and daily stem diameter variation cannot reflect sap flow at a half-hour time resolution, TWD is considered a better choice for indicating sap flow when the time scale required is less than 1 day. Transpiration can cause variations in plant moisture indices, including plant moisture content [
34,
35], water potential [
36], and stem sap flow [
37,
38]. It would be good to acknowledge that coupled TWD and actual tree water content, for example, using time domain reflectometry probes or frequency domain reflectometry probes [
39], should be analyzed to confirm whether TWD is a good tree moisture indicator.
TWD has advantages for indicating stem sap flow at a daily time resolution compared to other indices. These secondary thickening belt tissues can store water inside trees and play a positive role in water transportation whenever necessary [
40]. Water loss and transportation via these tissues in the case of water deficit can be demonstrated by varying TWD and stem sap flow at a daily time resolution. To some extent, the stem diameter shrinkage indicator TWD can reduce or even eliminate the influence of division and enlargement of cambium living cells on stem diameter variations, which highlights the reversible stem shrinkage during daytime and moisture supply during the night. Compared to other indices, this represents an advantage for identifying sap flow changes caused by transpiration pull. The daily stem diameter increment places more emphasis on the seasonal growth of trees, and stem diameter variations due to moisture loss are considered to be noise. Daily stem diameter variation represents the swell of the stem diameter due to seasonal growth and moisture variation [
41]. Some studies [
42,
43,
44,
45] found that the maximum daily shrinkage worked well in diagnosing a plant’s moisture storage. They also showed that it shares a certain correlation with stem sap flow, but this was less significant when compared with TWD in terms of explaining stem sap flow variations. Maximum daily shrinkage is obtained by subtracting minimum and maximum stem diameter values, which indicates the shrinkage of the stem diameter due to moisture variation based on the irreversible property of trees—seasonal growth. Assuming that a tree’s seasonal growth takes place at a constant rate, the maximum daily shrinkage works well for indicating stem moisture loss. However, in reality, it cannot be ensured that the division and enlargement of cambium living cells take place at a uniform speed. Moreover, Zweifel et al. [
29] proved that some trees might stop their seasonal growth during a period of stem diameter shrinkage.
Although TWD was shown to effectively reflect the variation characteristics of sap flow on above 0 °C below 80% cloud cover days, above 0 °C large percentage cloud cover days, and low temperature below 80% cloud cover days, it could not demonstrate the variation in sap flow on low temperature large percentage cloud cover days (
Table 4). We concluded that this is a combined effect of temperature and cloudiness conditions, and accounted for this phenomenon through variations in relevant environmental factors. We found that sap flow responded significantly (
r = 0.55 and
p ≤ 0.05) to variations in vapor pressure deficit on low temperature large percentage cloud cover days. However, the correlation was not significant (
p > 0.05) under other conditions. The correlation between vapor pressure deficit and TWD did not reach significance (
p ≤ 0.05) under any conditions. The vapor pressure deficit, determined by air temperature and relative humidity, may be responsible for the variation in sap flow on low temperature large percentage cloud cover days. Eliades et al. [
26] found that during negative temperatures, reverse sap flow was occurring. This may be one of the explanations of the poor correlation of the TWD with sap flow. One of the main limitations of the thermal dissipation method [
25] used in this study is that it cannot measure reverse flows. There was a significant correlation between stem diameter shrinkage and sap flow changes [
46,
47], despite the sap flow density being different among trees from different species. Moisture movement inside the xylem is caused by water tension. Not only in theory, the practice also proves that sap flow variation is linearly related to stem diameter variation [
21]. This study found that such correlation can be more easily affected by other factors under low temperature, more cloud cover conditions. Effects of stem diameter shrinkage indicators on sap flow are reduced by other plant-specific physiological and biophysical processes [
29]. Stem diameter shrinkage can significantly respond to moisture variations on the soil-plant-atmosphere continuum (SPAC) [
1], and sap flow can only serve as a plant moisture variation index to indicate the response of tree growth to plant moisture in the SPAC. Plant growth responses to the combined action of the soil moisture [
48], the atmospheric moisture [
49], and other environmental factors [
50,
51] require further studies.