© 2004 Heron Publishing—Victoria, Canada
Diurnal and seasonal variability in the radial distribution of sap flow: predicting total stem flow in Pinus taeda trees
Chelcy R. Ford (1, 2, 3), Carol E. Goranson (1, 2), Robert J. Mitchell (2), Rodney E. Will (1) and Robert O. Teskey (1)
1. Warnell School of Forest Resources, University of Georgia, Athens, GA 30602, USA / 2. Joseph W. Jones Ecological Research Center, Newton, GA 39870, USA / 3. Corresponding author (crford@uga.edu) / Received November 17, 2003; accepted January 26, 2004; published online July 1, 2004
Summary
We monitored the radial distribution of sap flux density (v; g H2O m–2 s–1) in the sapwood of six plantation-grown Pinus taeda L. trees during wet and dry soil periods. Mean basal diameter of the 32-year-old trees was 33.3 cm. For all trees, the radial
distribution of sap flow in the base of the stem (i.e., radial profile) was Gaussian in shape. Sap flow occurred maximally
in the outer 4 cm of sapwood, comprising 50–60% of total stem flow (F), and decreased toward the center, with the innermost 4 cm of sapwood (11–15 cm) comprising less than 10% of F. The percent of flow occurring in the outer 4 cm of sapwood was stable with time (average CV < 10%); however, the percentage
of flow occurring in the remaining sapwood was more variable over time (average CV > 40%). Diurnally, the radial profile changed
predictably with time and with total stem flow. Seasonally, the radial profile became less steep as the soil water content
(θ) declined from 0.38 to 0.21. Throughout the season, daytime sap flow also decreased as θ decreased; however, nighttime
sap flow (an estimate of stored water use) remained relatively constant. As a result, the percentage of stored water use increased
as θ declined. Time series analysis of 15-min values of F, θ, photosynthetically active radiation (PAR) and vapor pressure deficit (D) showed that F lagged behind D by 0–15 min and behind PAR by 15–30 min. Diurnally, the relationship between F and D was much stronger than the relationship between F and PAR, whereas no relationship was found between F and θ. An autoregressive moving average (ARIMA) model estimated that 97% of the variability in F could be predicted by D alone. Although total sap flow in all trees responded similarly to D, we show that the radial distribution of sap flow comprising total flow could change temporally, both on daily and seasonal
scales.
Keywords:
canopy conductance, capacitance, sap flow, temporal variability, time series analysis, transpiration.
