The ecology of lotebush (Ziziphus obtusifolia (T. & G.) Gray var. obtusifolia) in North Texas

Abstract

Lotebush, a spinescent, highly-branched shrub, is codominant with honey mesquite on almost 4.8 million ha of rangeland in the Rolling Plains of Texas. Three facets of lotebush ecology were investigated during 1979 and 1980 on the W.T. Waggoner Estate near Vernon, Texas: life history, diurnal and seasonal water relations and influence on the composition and production of associated herbaceous vegetation. Selected aspects of lotebush phenophasing (leaf and fruit development, twig elongation and flowering) were observed through the growing season. Leaf bud swell was initiated in late March and leaves were fully expanded by early July. Emergence of new twigs began in late April and elongation was essentially completed by mid July. However, additional twig growth occurred in late August of 1979, apparently in response to late summer precipitation. Flowering commenced in mid June and was complete by late July. Fruit-set did not occur immediately following flowering, but instead the ovary overwintered on the dormat plant with fruit development occurring the next spring. Xylem pressure potential was measured diurnally throughout the growing season in 10 individual plants. The values were usually highest at dawn and lowest at midday. Diurnal fluctuation in water potential was less than -1.3 MPa. Basal vegetation cover and standing crop were estimated along transects radiating from the lotebush canopy in each of the cardinal directions. Basal cover and standing biomass were generally lowest near the shrubs; however, buffalo grass was less abundant in the brush-free zone. Texas winter-grass basal cover and standing crop were greatest in the shrub-free areas except at the north and east driplines where environmental conditions were apparently ameliorated by the lotebushes. Subordinate herbaceous species were most abundant in those locations where buffalo grass and Texas winter-grass influences were the least. Lotebush is not seriously competing with the herbaceous vegetation. Therefore, the impact of lotebush on rangeland in north Texas is concentrated directly beneath the shrub canopy where vegetation is sparse and unavailable to large herbivores.