Gas exchange and water use by saltcedar (Tamarix gallica) in the southern Great Plains



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Texas Tech University


Seasonal and diurnal patterns of photosynthesis were studied for two years at 3 locations within a dense saltcedar (Tamarix gallica) stand (interior, periphery, and exterior) growing on a sandy loam soil around a saline pond in Terry County, Texas. Trees within the interior of the stand had lower assimilation rates during the sampling period than the trees at the exterior and on the periphery of the stand. The differences were significant in the morning because the trees within the interior were shaded until noon. The differences were not significant in the afternoon when the radiation was received equally at different locations within the stand. Drought significantly reduced the assimilation rates in saltcedar mainly by reducing the mesophyll conductance. Photosynthetic rates peaked early in the morning at three and a half hours post-sunrise (0930 hours) in the trees at the exterior and on the periphery of the stand, and declined progressively as the leaf temperatures and the vapor pressure deficit of the air (VPD) increased. However, the maximum photosynthetic rate in the trees within the interior of the stand was not reached until noon when the plants received full radiation.

Optimum leaf temperature for photosynthesis for late spring/early summer and early fall were 30°-24o C, respectively. Assimilation rates were negatively correlated with leaf temperatures and VPDs (R^2 = 0.88). Diurnal changes in photosynthetic rates were not related to the twig water potential or soil water content in the 60-cm profile during the growing season. The extremely dry top soil did not have an adverse effect on the assimilation rates, indicating that saltcedar is mainly absorbing water from sub-saturated soil.

Photosynthesis in saltcedar was primarily limited by nonstomatal factors rather than stomatal factors. A high correlation (r = 0.95) was found between assimilation rate and mesophyll conductance. In the dry and hot periods of the summer, the effect of partial stomatal closure on photosynthesis was mainly due to increased leaf temperature rather than decreased intercellular CO2 concentration.

Seasonal trends of photosynthetic rates of saltcedar were high at the end of the spring and early summer, and low in the fall. These differences were not due to low availability of soil water or increase in environmental drought, but primarily to leaf senescence.

Transpiration of saltcedar was greatest during 1991 (the wet year); and even in 1990 (the dry year), saltcedar transpired more than the associated facultative phreatophytes receiving ample water. Trees at different locations within the stand transpired similarly throughout the sampling period. Transpiration rates were reduced by 50% through stomatal closure in dry and hot environmental conditions. During the dry periods of the growing season, transpiration rates peaked in the morning before the maximum VPD was reached, but in wet periods transpiration reached the maximum concomitantiy with the maximum VPD in the aftemoon. Adequate stomatal conductance were maintained during the extremely hot and dry weather conditions which enabled this saltcedar to continue to assimilate when other species such as mesquite had a negative carbon balance.

Transpiration rates were greatest at the end of the growing season mainly because of low leaf temperamre, low VPD, and high stomatal conductance brought about by the insensitivity of the stomata to water potential changes with leaf age. Early in the season transpiration was restricted by low stomatal conductance induced by a dry environment. Twig water potential was relatively constant during the course of the growing season except after periods of high rainfall in June 1991 when it substantially increased Saltcedar had the lowest water use efficiency (WUE) than other phreatophytes sampled because high rates of transpiration were maintained even in extremely dry conditions due to a relatively high stomatal conductance. The WUE was significantly greater at the exterior and on the periphery of the stand than within the interior. The WUE was high early in the season because of high assimilation rates and low at the end of the growing season when assimilation rates became low relative to the high transpiration rates.

Depth to the water table did not affect photosynthesis or water use by small saltcedar trees growing at locations with different depths to the water table. The water table within 0.30 m of the soil surface did not have a negative effect on photosynthesis and stomatal conductance of saltcedar, indicating that this species is capable of adequately assimilating carbon even under high water table (0.3 m). Small trees had significantly higher assimilation and transpiration rates than the large trees throughout the growing season mainly because of both high stomatal conductance and mesophyll conductance.