Live Oak (Querucus virginiana var fusiformis)-Ashe Juniper (Juniperus ashei) Interactions in a Central Texas Savanna
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Woody plants have increased in abundance and distribution in the grasslands and savannas of North America since the 1800's. This encroachment has been attributed to land use practices such heavy grazing by domestic livestock and fire suppression, climate change, enrichment of atmospheric CO2, and the introduction of exotic species. On the Edwards Plateau of central Texas, Ashe juniper (Juniperus ashei) has increased in abundance since European settlement in the mid-1800's and is a predominant member of the woody assemblages that often occur beneath live oak (Quercus virginiana var. fusiformis) trees, which possibly function as nurse plants. To evaluate the effect of Ashe junipers and other shrubs on the overstory live oak, an understory removal experiment was conducted on live oak-shrub clusters in a central Texas savanna parkland. Predawn water potentials of oaks ranged from - 0.04 to -1.01 MPa over a year, but did not differ (p= 0.9) between controls (intact understory) and understory removal treatments (n = 10). By comparison, midday water potentials, which ranged from -1.02 to -2.82 MPa, averaged 0.12 MPa lower (p = 0.05) for live oaks without an understory relative to controls. Mean stomatal conductances varied from 58 to 414 mmol-m-2*s-i, and were significantly greater during both morning (p = 0.02) and afternoon (p = 0.03) periods in live oaks with understory removed, as compared to control live oaks; however, treatment differences were generally less than 22%. Over 6 sampling dates, net photosynthesis ranged from 9.9 to 23.9 pmol CCfe-m^-S'1 (n = 10) and was significantly greater (p = 0.04) in removal oaks as compared to control oaks. No significant treatment differences were found for intrinsic water use efficiency or relative chlorophyll content (p = 0.68; p = 0.10, respectively). In addition, there were no significant differences between treatment groups in stem length (p = 0.27), total shoot biomass (p = 0.66), or leaf litter production (p = 0.93). Thus, while the removal of the Ashe juniper-dominated understory had a positive effect on live oak gas exchange, the effects were small, were not closely coupled to changes in plant water potential, and did not translate into increased growth or production. These findings imply that the intensity of understory competition on the overstory live oaks is rather weak, at least at this site, and may, therefore, be insufficient to competitively exclude the live oak nurse tree that initially facilitated these understory shrubs. Studies were also conducted to test for continued overstory facilitation of mature Ashe juniper shrubs. Predawn and midday shoot water potentials of Ashe junipers exhibited a wide range throughout the year (between -6.69 and - 0.38 MPa), especially when compared to the range measured in the live oaks. During both the predawn and midday sampling periods, there were no significant differences (p = 0.66; p = 0.73, respectively) between water potential measurements of Ashe junipers under live oak canopies and those not associated with any trees. These data suggest that, at this site, live oaks were not facilitating mature Ashe juniper shrubs.
CitationRamirez, P. A. (2002). Live oak (Querucus virginiana var fusiformis)-ashe juniper (Juniperus ashei) interactions in a central Texas savanna (Unpublished thesis). Southwest Texas State University, San Marcos, Texas.