Effect of site on honey mesquite transpiration

Honey mesquite (Prosopis glandulosa) is a problem brush species in much of the Southwest and is purported to use large amounts of water. In the Southwestern United States water is a limiting resource, both in terms of availability for plant growth and consumption by ever-increasing human populations. This study was conducted to determine the influence of site on the transpiration rate of honey mesquite.

In 1985 two sites, a sandy loam site of the Cosh series (Udic Rhodustalf) sandy loam-R, and a loamy bottomland site of the Weswood series (Fluventic Ustochrept), were selected for the study. Transpiration rates, soil moisture, vapor pressure deficits (VPD), photosynthetically active radiation (PAR), leaf conductance, xylem pressure potentials (XPP), soil temperatures, and wind were used to compare sites. In 1986, a second Cosh sandy loam site, the sandy loam-H site, and a Mangum clay site (loamy substratum variant, Vertic Ustochrept), the clayey bottomland site were added.

Two points of eight trees on each site were sampled for transpiration rate and leaf conductance every other hour from 0.5 hours post sunrise (HPS) until dusk. Vapor pressure deficits and wind data were collected for each one-hour sampling period. Soil moistures and soil temperatures were taken on each sampling date, and XPP were sampled predawn.

In 1985 transpiration rates peaked in the spring and early summer, declined rapidly, and then more slowly as the summer progressed. In 1986, due to the drier dormant season conditions, the early season transpiration peak was not as great as in 1985, but a second transpiration peak occurred on the sandy loam-R and sandy loam-H sites in late summer after rainfall occurred at the sites. Early in the growing seasons low soil temperatures inhibited transpiration. As the summer progressed low soil moisture was the primary factor influencing transpiration rates on the sandy loam-R, loamy bottomland, and sandy loam-H sites. The sandy loam-R site, due to its better surface soil structure and more surface vegetation, had greater water infiltration and higher soil water contents than the loamy bottomland site. Trees on the sandy loam-R site had higher average transpiration rates than trees on the loamy bottomland site in both years, 6.2 µg cm^-2 s^-1 to 4.9 µg cm^-2 s^-1 in 1985 and 5.5 µg cm^-2 s^-1 to 4.5 µg cm^-2 s^-1. In 1986, photosynthetically active radiation, soil temperature, and VPD also had a significant effect on mesquite transpiration. Seasonal transpiration curves on the clayey bottomland site peaked during the hot midsummer period when the water table receded from under the site, soil temperatures increased and PAR peaked. In 1986 the clayey bottomland site transpired at a rate (5.8 µg cm^- 2 s^ -1) similar to that of the sandy loam-R site, but greater than that of the sandy loam-H site (5.2 µg cm^-2s^-1) or the loamy bottomland site.

Transpiration rates in honey mesquite do not increase dramatically as the sun impinges on a plant, 1 HPS was generally the lowest point in the daily transpiration cycle. Under very favorable or very harsh conditions transpiration rates were relatively high 1 HPS. The extent of the daily transpiration peak is regulated by a complex interaction of environmental variables, but in this study soil moisture at the time of, and leading up to, data collection appeared to primarily influence transpiration.

Transpiration rates generally peaked 5 HPS regardless of environmental conditions. Transpiration rates declined to a level similar to that of 1 HPS, at 13 HPS regardless of the height of the daily transpiration curve (DTC).

The transpiration rates of honey mesquite on the sandy loam-R site were consistently higher than those of the loamy bottomland site. The exception to this was under extremely harsh conditions in July of 1986, when the loamy bottomland site was better able to maintain moderate transpiration rates. When the sandy loam-R, loamy bottomland, and sandy loam-H sites were under the greatest drought stress, and exhibited low DTC, the clayey bottomland site exhibited its greatest DTC,