Livestock grazing for weed management - Ecological Management of Agricultural Weeds - page 416

other planned management derive from differential species performance, the
availability of sites for the establishment of new individuals,and the availabil-
ity of propagules (Figure 9.1) (Pickett,Collins & Armesto,1987; Sheley,Svejcar
& Maxwell, 1996). These factors can be analyzed on time intervals of days,
months, or years, represented as T x and T x 1 in Figure 9.1.
Relative species performance is based on the mechanisms of resistance and
tolerance to grazing and trampling described earlier. These vary from one
plant species to another and drive changes in the proportions of different
plant species in a pasture or range. Relative performance may cause species
composition to remain relatively stable, cycle seasonally, or undergo large
cumulative changes. Variation in weather adds unpredictable variability to
species growth and reproduction. Year-to-year weather variations, even in
humid areas such as England and New Zealand, produce two- to six-fold fluc-
tuations in herbage production (Snaydon,1981).Relative species performance
also affects soil protection (Olson, 1999).
The availability of sites for new individuals depends on relative species per-
formance that leads to gaps in the sward and on weather conditions which
control soil temperature and moisture availability. Wiens (1984) proposed
that ecosystems are on a gradient from limited abiotic variability with large
effects of biotic interactions on community composition to high abiotic vari-
ability with small effects of biotic interactions on community composition.
The nature of site availability along the spectrum from temperate pastures
through semiarid rangelands illustrates this gradient. In a clover-perennial
ryegrass sward in humid New Zealand, Panetta & Wardle (1992) detected
more small gaps in summer (43% of points sampled) than in spring or autumn
(18% and 28% of points sampled). However, these temporary gaps frequently
were closed by vegetative growth of the surrounding clover and ryegrass.
Seedlings of weed and forage species, which vary in their ability to colonize
gaps of different sizes (Panetta & Wardle, 1992; Bullock et al ., 1995), suffer
severe interference from established plant species during establishment. In
contrast, in their study of the range weed Gutierrizia sarothrae in semiarid New
Mexico, McDaniel, Torell & Bain (1993) found new seedlings in only one year
of an 11-year study, primarily due to rainfall.The special establishment event
was characterized by a drought that weakened the existing vegetation, fol-
lowed by above-average fall and spring rainfall, which allowed both G. saroth-
rae germination and establishment. Similarly, during a 45-year period,
significant recruitment of Astrebla spp. occurred only in 1945 and 1987 (Roe,
1987).Arid climates have even more highly variable rainfall,and the establish-
ment of a new cohort for a given species may be a rare event (Westoby, Walker
& Noy-Meir, 1989).