Vegetation Dynamics of a Live Oak-Juniper Savanna: An Isotopic Assessment

Date

2001-08

Authors

Jessup, Kirk E.

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Abstract

Vegetation of the Edwards Plateau of central Texas prior to European settlement (i.e., 150-200 years ago) is not well characterized. Anecdotal accounts suggest grasslands and plateau live oak savannas (Quercus virginiana var. fusiformis) may have covered more of the region than they do at present, while the invasive woody Ashe juniper (Juniperus ashei) was restricted to fire-protected refugia such as steep cliffs and riparian areas. Stable carbon isotope (δ13C) values from vegetation and soil organic matter were measured to test for shifts in the relative contribution of C3 and C4 plants in grasslands, as well as to investigate the stability of live oak cluster and Ashe juniper woodland patches during the last several hundred years at an upland savanna parkland in the eastern Edwards Plateau. In addition, soil organic carbon, soil nitrogen, and stable nitrogen isotopes (δ15N) were measured to investigate differences between the woody patches and grasslands in nutrient storage and cycling. The δ13C profiles beneath grassland sites changed significantly with depth and averaged -20.8 ± 1.3‰ (in surface litter) to -13.2 ± 0.1‰ (in soil at 20-30 cm depth), indicating that a significant decline in the relative importance of C4 species has occurred in these grasslands over the last several hundred years. In addition, grassland δ13C profiles were distinct at all depths from profiles beneath discrete oak clusters, which averaged -26.7 ± 0.2 and -16.7 ± 0.3‰, for surface litter and soil between 20-30 cm, respectively. With depth, soil δ13C values from sites within an Ashe juniper woodland converged with those of an adjacent savanna, except for values from the most interior site which was situated near a steep, north-facing cliff. Soil organic carbon and total nitrogen values were greatest at live oak clusters, intermediate within the juniper woodland, and lowest in grasslands. The results suggest: (1) though the grassland vegetation currently contains many C3 species known to increase under grazing pressure, the relative importance of C4 species was much greater in these grasslands several hundred years ago, and likely declined due to grazing, (2) discrete live oaks have been present in these savannas > 300 years, (3) the contribution from C4 and/or CAM species was greater at live oak sites in the past, possibly reflecting a more grassy understory which has since been replaced by woody shrubs. ( 4) Ashe juniper occurred in sites associated with steep cliffs and/or drainages prior to European settlement, but the density and range of this invasive species has increased in recent times, and (5) encroachment of woody plants on Freeman Ranch serves to increase soil carbon and nitrogen sequestration.

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Keywords

Vegetation dynamics, Live oak, Junipers, Savanna ecology, Hays County

Citation

Jessup, K. E. (2001). <i>Vegetation dynamics of a live oak-juniper savanna: An isotopic assessment</i> (Unpublished thesis). Southwest Texas State University, San Marcos, Texas.

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