Leaf water relations in Juniperus ashei in Central Texas woodlands

Background/Question/Methods        

Predicted changes in precipitation and temperatures of the south central portion of the U.S. may increase water stress in woodlands which developed in this region following the last Ice Age.  Future drought tolerance by woody species is dependent on their ability to withstand water potentials that emerge due to low soil water and high evaporative demand. In this study, we examine mid-day leaf water potential, the relationships to sap flux, and potential impacts of leaf morphological characteristics in Juniperus ashei, a dominant evergreen tree found in Central Texas. Leaf water potentials for samples taken in the mid-morning and early afternoon at four different heights and three locations around the canopy for five mature junipers were measured using the WP4-T Dew Point Potentiameter. Sampling was conducted from Fall 2006 through Summer 2008 in order to capture as much of the range in leaf water conditions as possible. Other environmental variables measured include specific leaf water content, air temperature, relative humidity, soil water, and irradiance in the form of photosynthetically active radiation.

Results/Conclusions

Results show that a broad range of leaf water potentials exist ranging from -0.35 MPa to -7.10 MPa.  Specific leaf water content values, the ratio of the mass of water in the fresh sample to the oven dry sample mass, also show a broad range of values (62 to 128 percent). Sap flux data measured at the same time indicate that J. ashei transpires at very low leaf water potentials, with a non-linear decline in flux values with decreasing leaf water potentials. The results of the systematic sampling indicate that leaf water potential was strongly correlated with leaf water content, as well as with vapor pressure deficit values calculated from temperature and relative humidity values measured at the time of sampling. Leaf water potential also correlated strongly with irradiance values.  There were no significant differences observed in the leaf water potential - leaf water content relationship by canopy height leading to speculation about internal leaf morphological differences.  Analysis of electron microscopy cross-sections of leaves collected synoptically at varying heights also indicate no difference with canopy height.  These results concur with the findings of other water use studies of J. ashei and indicate that the species is likely to be a highly competitive species under future water limited conditions.