title: Internal Water Potential of an Engelmann Spruce Stand in Relation to Soil and Atmospheric Factors
Creators: name: Meyn, Richard L.
Publisher: name: Utah State University
Year: 1973

description: The primary objective of this study was to determine the degree of correlation between internal water potential of Engelmann spruce trees and selected environmental factors. The secondary objective of the study was to determine the effect of summer precipitation, both in meadow and forested sites, on the soil drying cycle.
Four study plots (each roughly 154 square meters) under spruce cover and two study plots (100 square meters) in a small meadow were established. At each spruce plot, a thermocouple psychrometer was installed at one meter above the ground on the north side of three selected, mature Engelmann spruce trees. Thermocouple psychrometers were installed at 5, 15, and 30 cm depths in the soil at the meadow plots and at 15, 30, 45, and 90 cm depths at the spruce plots. A tensiometer and two soil temperature thermocouples were installed near the thermocouple psychrometers at each of the plots but at 15, 30, 45, and 90 cm soil depths. Throughfall gages were placed at each plot to obtain an estimate of rainfall which was not intercepted by the canopy but which fell to the forest floor.
Gross precipitation was measured by a 20.3 cm diameter recording gage located at the meadow. Global radiation and wind were recorded at the meadow area also. Air temperature and relative humidity were recorded in wooden shelters. One shelter was located at the meadow area and one was located in the vicinity of the four spruce plots.
Except for wind, weather parameters were recorded continuously. Wind movement was totalized on a digital counter and recorded on data sheets when needed. Psychrometric, pressure chamber, and soil data were obtained according to two schedules. Diurnal measurements of trunk water potential in six trees and independent variables were made during seven days in the summer months of 1971. Seasonal measurements of trunk water potential in twelve trees and independent variables were done at periodic intervals during the summer averaging every three to four days. Data to evaluate the effect of summer precipitation on the soil drying cycle was obtained periodically the summer of 1970 and concurrently (for the most part) with tree water potential measurements during 1971.
Significant findings of this study included the following: as shown by analyses of variation of the data, within-season variations of tree water potential (by psychrometer and pressure chamber measurements) were statistically significant. In other words, fluctuations in water potential with time were large enough that they could not have been due to chance alone.
On an hourly basis for seven cycles of diurnal measurements, daily peaks in wind movement corresponded roughly with trunk water potential minima, peak radiation occurred about two hours before water potential minima, and maximum vapor pressure deficits corresponded closely to water potential minima. Diurnal fluctuations in water potential of small understory branches, as determined by the pressure chamber, did not correlate well with weather factors.
Seasonally, trunk water potential appeared to be highly correlated with fluctuations in vapor pressure deficit and less correlated with global radiation and wind. Soil factors such as matric potential and temperature were not correlated with trunk water potential. Correlations between water potential determined by the pressure chamber and environmental factors were not consistent. Pressure chamber values of water potential, however, did correlate roughly with trunk water potential during the latter half of the summer of 1971.
By multiple regression analysis, a predictive equation was devised to predict trunk water potential on a daily and on a seasonal basis. With diurnal input data, radiation, vapor pressure deficit, and a transformation of vapor pressure deficit were significant variables. The model explained 81 percent of the diurnal variation in trunk water potential. Evaluation of the model with seasonal data input showed only one variable, vapor pressure deficit, highly significant. With seasonal data, 72 percent of the variation in trunk water potential was explained.
Analysis of 18 rainstorms which occurred during two summers of study showed that roughly 0.25 cm of rain must fall before interception storage of a spruce canopy is satisfied and measurable amounts of rain can fall to the forest floor. Variability among sample catches of a given storm was great, apparently reflecting the irregular nature of the overmature spruce canopy. On the average, 96 percent of rainfall from storms between 0.0 0 and 0.13 cm, 70 percent between 0.13 and 1.3 cm, and 38 percent between 1.3 and 2.5 cm was intercepted. The influence of rain on soil matric potential was restricted to the first 45 cm of soil at the spruce plots with no substantial increase of matric potential before the onset of fall rains. The effect of rain on matric potential of soil in the meadow was more pronounced. The matric potential of the 5, 15, and 30 cm depths fluctuated greatly. A temporary increase in matric potential of these depths following summer storms was noted while more marked increases in matric potential were measured after heavier fall rains.
The major conclusions made as a result of this study are (1) relative vapor pressure in the trunks of Engelmann spruce changes markedly from hour-to-hour and from day-to-day during the summer months, (2) trunk water potential as measured by thermocouple psychrometers is functionally related to atmospheric factors of radiation, wind, and vapor pressure deficit, (3) fluctuations in trunk water potential with weather factors imply a causal relation with transpiration, (4) precipitation during the summer months modifies soil matric potential-but only in the shallow profiles, (5) matric potential fluctuations in the meadow areas are extreme (from saturation to low as - 40 bars) and would impose a serious threat to the water economy of young Engelmann spruce seedlings established in such meadows, and (6) Peltier type thermocouple psychrometers are useful instruments for investigation of the soil-plant-atmosphere continuum in field situations.