Post-Drought Vegetation Dynamics on Arid Rangelands in Southern New Mexico
Results indicated that protection from rodents and rabbits increased the cover of annual grasses, whereas their presence increased the cover of annual forbs. However, protecting herbaceous plants from rodents, rabbits, or cattle did not give a major response
Broom snakeweed increased on sandy soils in wetter years.
Apparently, the absence of cattle increased honey mesquite cover, but the presence of cattle did not prevent honey mesquite increase
The factors affecting perennial grass yields were precipitation, soil water, soil characteristics, plant species, and plant cover. The variation in annual perennial grass production on all sites was sometimes dramatic
Precipitation is extremely variable, both temporally and spatially (Martin and Cable 1974, Paulsen and Ares 1962)
During the 1916?18 drought, black grama?s basal cover on the Jornada Experimental Range (JER) in southern New Mexico declined to 42% of predrought cover (Nelson 1934). Black grama was the major plant species on lighter-textured soils. It practically recovered its stand during two years of favorable precipitation (1919?20), then dropped very suddenly in the following two years of drought to the extremely low point of 11% of the original cover. Black grama cover remained at a low level for the remainder of the 1921?25 drought, then in-creased to its original size when summer rainfall was above average from 1926 to 1929.
black grama cover is mainly influenced by plant vigor at the start of the current growing season, as determined by rainfall from the previous summer. Changes in black grama basal area were closely correlated with precipitation during a 15-month period from July 1 of one year through September 30 of the next year
Herbel et al. (1972) found a direct relationship between drought damage to vegetation and soil depth during the 1951?56 drought. That drought was one of the most severe in the Southwest during the last 350 years, and, perhaps, was exceeded in severity only by the Great Drought of 1275?99 (McDonald 1956 and Schulman 1956)
The percentage composition of black grama increased during protracted dry periods, as other less drought-enduring species died.
When severe drought losses occurred over extensive areas, black grama recovery was slow, even with favorable moisture
greatest increase in basal [black grama] area following drought?even greater than that obtained by protection from grazing?was under conservative grazing (Nelson 1934, Paulsen and Ares 1962)
Canfield (1957) found black grama seedling survival was 39% on ungrazed sites and 35% on grazed sites.
Overgrazing at any time, but particu-larly during drought, reduces black grama cover (Nelson 1934).
Where drought reduces ground cover, there is an increase of bare soil and, consequently, wind erosion increases (Marshall 1973
tobosa and burrograss types are less susceptible to drought and recover relatively rapidly (Herbel et al.1972).
conducted on the Jornada Experimental Range (JER) in southern New Mexico (fig. 3) from 1957 to 1988
Lower Sonoran Life Zone Merriam (1898)
JER averages about 200 frost-free days. Temperatures are gen-erally moderate with an annual mean of 15°C. The diurnal variation is often 20°C. Maximum temperatures are highest in June, averag-ing 36°C; in January, the coldest month, the average maximum temperature is 13°C (Paulsen and Ares 1962)
Wind velocities average 2.4 to 3.2 km/hr for March through June ? average wind velocities range from 1.0 to 1.6 km/hr (Paulsen and Ares 1962)
The topography is gently undulating to nearly level uplands, inter-spersed with swales and lake beds formed during Pleistocene. The elevation at JER headquarters is 1317 m (Paulsen and Ares 1962,Dittberner 1971)
The soils have almost no humus and have little change in texture between surface soil and subsoil. All soil types have some calcium carbonates
Plant cover on the range differs primarily due to soil types.
Clay, loam, silt loam, sandy loam, clay loam, loamy sand,(moderate to deep all) sand (shallow),
Stocking rate is the number of animals grazing on rangeland per unit area. A heavy stocking rate may lead to a change in plant species and increased soil erosion that eventually could cause permanent deterioration to the system (Herbel and Pieper 1990)
The practice has been to use the amount of available forage in the fall to establish the stocking rates until the next summer (Herbel and Nelson 1969, Herbel 1973, Herbel and Gibbens 1981, Holechek and Herbel 1982
hectares per animal unit month (AUM)
average stocking rate was 9 ha/AUM for these seven units for this 20-year period.
Fig. 6 shows stocking on JER from 1945 to 1990. Note the precipitous decrease in 1954 because of drought
Perennial grass yields were obtained annually from 1958 to 1988 on about 17 transects in unit 2 [and many other units]
A set of four plots was constructed or marked at two locations in 1957. Each location had four, 38.1 x 45.7 m plots as follows: 1. a plot surrounded by 21 ga., 9.5 mm mesh hardware cloth, 121.9 cm wide, buried 15 cm in the soil, and flared at the top to exclude rodents, lagomorphs, and livestock (Ro); 2. a plot sur-rounded by 2.5 cm mesh poultry netting, 91.4 cm wide, buried 10 cm in the soil, to exclude lagomorphs and livestock (Ra); 3. a plot surrounded by four strands of 12 1/2 ga. barbed wire to exclude livestock (Li); and 4. a marked plot open to all influences ? [2 transects per year]
Perennial grasses: ear and sand muhlys, black grama, alkali sacaton, mesa dropseed, fluffgrass, and red and poverty threeawns
major perennial forbs were leatherweed, desert holly, rattlesnakeweed, Fendler?s bladderpod, and paperflower
major annual forbs encountered from 1957 to 1977 were Russian thistle and annual broomweed.
Table 3 [pdf 15] [rainfall to cover 57-77 on specific soil type, up to Table 14(maybe more).]
major shrubs encountered were Torrey?s and longleaf ephedra, tarbush, crucifixion thorn, and creosotebush.
number of transects taken annually ranged from 14 to 118
perennial grasses: bush, ear, and sand muhlys; black grama; alkali sacaton; mesa dropseed; poverty, red, and Wooton threeawns; vine mes-quite; fluffgrass; and Hall?s panicum. Sixweeks grama
perennial forbs encountered were leatherweed, desert holly, wooly sumpweed, Fendler?s bladderpod, trailing windmills, and scarlet globemallow
nnual forbs were Russian thistle, annual broomweed, desert marigold, Dakota vervain, purple scorpionhead, deerstongue, and gray goosefoot
Engelmann?s pricklypear, and soaptree yucca
major perennial forbs were dingy falsenightshade (1957), wrinkled globemallow
annual forbs (> 0.01% basal cover) were tansy mustard (1958), bitterweed (1958), and western fleabane (1973).
perennial forbs were leatherweed, Fendler?s bladderpod, paperflower, rattlesnakeweed, wrinkled globemallow, trailing windmills, western sensitivebriar, twinleaf senna, scarlet globemallow, and plains zinnia
major annual forbs on these soils were desert marigold, Russian thistle, Texas selenia, spike spiderling, wooly indianwheat, saucerleaf buckwheat, deerstongue, and gray goosefoot
major shrubs on Typic Calciorthids were honey mesquite, soaptree yucca, longleaf ephedra, creosotebush, three fans, and tarbush.
Apparently, protection from rodents (Ro) de-creased broom snakeweed basal cover
Precipitation and soil water determined yields
There was a large variation in all plants? cover and perennial grass production due to very uneven plant distribution on the landscape within each year, uneven precipitation distribution within a year and among years, and differences among observers.
Perennial grasses were the most prominent portion of the herbaceous plants on the sampled sites. They made up > 94% of the herbaceous plants? basal cover for the observations taken on soils D, E, F, and G; 88% on soil M; and 79?85% on soils O, P, R, S, and T. Perennial grass basal cover averaged 3.44% on soil G, 1.65% on soil D, 1.26% on soils E and F, 0.81% on soil M, and 0.52?0.57% on soils O, P, R, S, and T. (tables 3?12)
There was about a tenfold increase in perennial grass basal cover in the high years compared to the low years, and this change can occur in two years.
Generally, perennial grass basal cover was lower in the dry years of the mid-1960s and higher in the wet years of the late 1950s and the mid-1970s
Black grama cover was substantially reduced on deeper soils (such as O) by drought and did not recover
increase of honey mesquite was greater with severe drought damage
Factors affecting perennial grass yields were precipitation, soil water, soil characteristics, species, and plants? cover.
The variation in perennial grass production from year-to-year was sometimes dramatic and has definite management implications
Drought reduced ground cover and, on some sites, this led to increased wind erosion with reduced potential for production from the soil (UCAR 1989).