Vegetative Growth and Mortality of Drummond's Goldenweed (Isocoma drummondii [T. & G.] Greene: Compositae) as Influenced by Soil Moisture and Site Characteristics

Abstract

Substantial emergence of Drummond's goldenweed (Isocoma Drummondii (T. & G.) Greene; Compositae) seedlings on the Texas Coastal Prairie occurred in 1 of the 2 yr study. Immediately following seed dispersal in January 1976, average density of new seedlings (2667/m2) on a grey sandy loam was nine times that of a tight sandy loam and almost 30 times that on a honey mesquite (Prosopis glandulosa Torr. var. glandulosa)-dominated loamy sand site. No new seedlings survived the spring season under natural conditions on any of the three sites, although seedling densities remained high in a fenced exclosure, especially where artificially seeded. Cumulative annual mortality of second-year seedlings was 100% on the loamy sand, 47% on the grey sandy loam, and 25% on the tight sandy loam site. Initial establishment of two populations of Drummond's goldenweed appear to have coincided with mechanical removal or serious disturbance of the mixed-brush overstory. Maximum rate of stem elongation (1.77 mm/day) of mature Drummond's goldenweed growing in full sunlight occurred during late summer and was associated with high rainfall. Stem growth of juvenile Drummond's goldenweed on a deep, calcareous site with high extractable phosphorus varied closely with soil water availability (r =0.85). On a heavier-textured, poorly-drained soil with a shallow argillic horizon where salts and sodium accumulated, stem growth was suppressed regardless of soil water availability. Average stem elongation rates of juvenile and mature Drummond's goldenweed across all sites and sampling dates were 0.49 and 0.77 mm/day, respectively. The perennial Drummond's goldenweed is a heavily-branched, suffrutescent subshrub. The rounded canopy may reach 1 m in height, 1 Approved by the Director, Texas Agricultural Experiment Station as TA 13887. Senior author was formerly a graduate research assistant, Texas Agricultural Experiment Station, College Station.