Abstract
The responsiveness of four types of stem diameter shrinkage indicators to sap flow changes was compared under four air temperature and cloudiness conditions: above 0 °C below 80% cloud cover days; above 0 °C large percentage cloud cover days; low temperature below 80% cloud cover days; and low temperature large percentage cloud cover days. In this study, we investigated the effects of indicative functions of relatively easy-to-access stem diameter shrinkage on variation characteristics of sap flow. High-resolution-based stem diameter shrinkage is related to changes in tree moisture content. Stem diameter shrinkage indicators are adopted to confirm sap flow changes resulting from transpiration pull, which may enhance the power of stem diameter shrinkage as an index for tree bole moisture loss. After measuring stem diameter variations, the following stem diameter shrinkage indicators were calculated: maximum daily shrinkage, daily stem diameter increment, daily stem diameter variation, and tree water deficit-induced stem shrinkage (TWD). Sap flow was measured synchronously, and stem diameter shrinkage indicators were analyzed to confirm their responses to sap flow. TWD was positively correlated (r ≥ 0.317) with daily variations in sap flow and reached extremely significant levels (p ≤ 0.001) under all conditions. TWD and maximum daily shrinkage were able to better reflect the correlation between changes in stem diameter and sap flow on a daily scale, except large percentage cloud cover days with low temperatures. Changes in stem diameter had no correlation with sap flow during low temperature and large percentage cloud cover days. Among all stem diameter shrinkage indicators, TWD showed the highest correlation (r ≥ 0.601 and p ≤ 0.001) with sap flow under all conditions, except during large percentage cloud cover days with low temperatures. The stem diameter shrinkage indicators did not reflect sap flow changes during large percentage cloud cover days with low temperatures. The indicator that best reflected moisture loss of trees was TWD.
Highlights
Stem diameter shrinkage based on high time resolution is closely related to changes in tree moisture levels [1,2]
Propose the following hypotheses: (1) stem diameter shrinkage indicators can help explain some characteristics of sap flow during above 0 ◦ C below 80% cloud cover days; and (2) since the indicator tree water deficit-induced stem shrinkage (TWD) eliminates the influence of seasonal growth on stem diameter variations and retains the reversible shrinkage caused by water loss, TWD can better explain sap flow when compared to other stem diameter shrinkage indicators under all air temperature and cloudiness conditions
We propose two hypotheses: (1) stem diameter shrinkage indicators can help explain some characteristics of sap flow during above 0 ◦ C below 80% cloud cover days and (2) TWD
Summary
Stem diameter shrinkage based on high time resolution is closely related to changes in tree moisture levels [1,2]. High-resolution measurements of diameter shrinkage are considered very effective tools for evaluating growth changes in tree diameter in the context of tree bole moisture [3,4,5]. This approach has been widely used for agricultural irrigation guidance [6], crop growth monitoring [7,8], and forestry research [9,10]. Many studies [11,12] have shown that stem diameter shrinkage is mainly determined by sap flow changes.
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