Recovery of Desert Plants in Various States of Vigor

by R. Dennis Child and C. Wayne Cook

ABSTRACT

Desert plants, when defoliated to the extent that vigor is even moderately reduced, require rather long periods of nonuse for complete restoration. Defoliation in the winter and again in the spring at only moderate intensities was considered deleterious to plant welfare. Late spring harvesting was significantly more harmful to plants than early spring harvesting. In the Intermountain area, livestock graze on desert ranges mainly during the winter, but in some instances cattle graze on these arid lands yearlong. Desert ranges in the basins of the Intermountain area are in a delicate balance and if incorrectly used deteriorate rapidly. Changes in plant vigor generally precede changes in the botanical composition and range deterioration. Relative plant vigor may also indicate the degree of range recovery from a lowered state of range condition. In general, vigor denotes health and vitality of the plant. For these reasons a study was conducted from 1959 to 1968 in desert ranges of western Utah to determine the recovery of desert range plants that were harvested during the first three years of the study at three different intensities during four different seasons. Plant vigor measurements were taken after seven years of rest to determine recovery from previous treatments.

METHODS AND PROCEDURES

The dominant forage species used in the study included three browse species: black sagebrush (Artemisia nova), big sagebrush (Artemisia tridentata), and shadescale (Artiplex confertifolia); two suffrutescent species: winterfat (Eurotia lanata), and Nuttall saltbush (Atriplex nuttallii); and two grass species: Indian ricegrass (Oryzopsis hymenoides), and squirreltail grass (Sitanion hystrix). Exclosures were built to protect the study plots from livestock and rabbit use.

During the period from 1959 to 1961, these plants were clipped at three intensities during four periods. The three clipping intensities were 30, 60, and 90 percent of the available herbage and the four periods were winter only, about January 1; winter and again in late spring, about January 1 and May 1; early spring only, about April 1; and late spring only, about May 1. In the browse and grass species, individual plants were treated as one sampling unit (Fig. 1). For the suffrutescent species the plants within a 9.6 square foot circular frame were used as a sampling unit. Ten units of each species were clipped at each intensity during each period and at each of the three locations. There were also ten control units at each location. During the summers of 1962 and 1968 vigor measurements on each sampling unit were made.

RESULTS

In order to determine the recovery during the seven-year rest period it was necessary to separate changes caused by previous treatments from changes caused by weather and other environmental effects displayed by control plants. Assuming that climate affected both the controls and treated plants alike, the difference shown in the controls can be used to adjust the treated plants (Mueggler, 1967).

As might be expected the amount of live crown cover was closely correlated with herbage yield for all seven species studied. Many investigators have suggested that either or both would be excellent indexes to vigor (Cook and Goebel, 1962; Cook et al., 1958; Hickey, 1961; Hyder et al. 1966; Mueggler, 1967; Voegel and Van Dyne, 1966).

Effect of Previous Seasonal Clippings

Crown cover alive from clipped plants compared with controls was the major criterion for measuring recovery from 1962 to 1968 (Table 1). Percent dead crown cover for the suffrutescent species in 1968 may have been somewhat underestimated because plants killed by previous treatments could not always be found in 1968 since the woody base occasionally disintegrated and disappeared from the plots (Table 2).

Plants judged to be in lowest vigor in 1962 were those harvested in the winter and again in late spring from 1959 to 1961. These plants had only about 42% as much live crown cover as the controls in 1962. The three browse species that were harvested twice a year produced only about 18% as much crown cover as controls in 1962 (Table 1). The two suffrutescent species and Indian ricegrass that were previously clipped twice a year had about two-thirds as much crown cover as the controls in 1962. Squirreltail grass had only about 45% as much crown cover as the control after being harvested twice a year for three years. In 1968 the browse species from these same treatments were producing about 28% as much crown cover as the controls, the suffrutescent species had completely recovered and grasses had from 55 to 76% recovery for squirreltail grass and Indian ricegrass, respectively (Table 1).

Clipping only in late spring about May 1 was the second most severe treatment from the standpoint of reduction in vigor of desert forage plants. The average of the seven species showed that plants previously harvested during late spring had only 51% as much crown cover as the controls in 1962. Plants, after seven years rest from late spring clipping produced only about 60% as much forage as the controls.

Past harvesting treatments during the winter only and during early spring only were less detrimental to the welfare of desert forage species than harvesting only in late spring or harvesting during the winter and again in the late spring. There were no significant differences (P<.05) in vigor measurements during 1968 between harvesting in the winter only compared to harvesting only in the early spring. The crown cover in 1968, after seven years of rest from previous clippings during the winter season and the early spring season, was approximately 81% of the control plants. Herbage yield in 1968, after seven years of rest for the plants previously clipped only in winter and only in early spring, was about 73% as much as control plants (Table 2).

Effect of Intensity of Clippings

The overall effect of past treatment at various intensities showed that normal plant cover, even after seven years rest, was less for each increased increment of harvesting (Table 1). The crown cover for previously clipped plants was 89, 77, and 52% of the controls in 1968 for light, moderate and heavy clipping intensity, respectively. The yield in 1968 from the previously clipped plants at 30, 60, and 90% was 81, 61, and 40% as much as controls, respectively.

The most severe treatment was harvesting during the winter and again in the late spring at 90% intensity. In 1968 after seven years rest, these plants were producing only about 23% as much forage as the controls and their crown cover was only about 44% of the controls. Previous harvests at 90% during late spring were the second most severe treatment for all species. These plants in 1968 were producing only 32% as much forage as the controls and they had only about 42% as much crown as the controls.

In 1968 there were no significant differences (P<.05) in vigor between the controls and those previously harvested at 30% during any season except those harvested twice a year during the winter and again in late spring. For all practical purposes and for sustained optimum yield the plants harvested at 60% or less during winter and early spring were not harmed (Cook, 1971). He pointed out that harvesting any plant at 90% during any season and at 60% twice a year (winter and spring) and during late spring was detrimental to sustained yield.
Thus any past treatment that lowers vigor, even moderately, has a rather lasting effect upon vigor of desert plants.

Recovery of Individual Species

Shadscale appeared to make better recovery during the seven years rest following defoliation than did black sagebrush or big sagebrush (Table 1). Black sagebrush made somewhat less recovery compared to the controls than big sagebrush but this was not consistent among treatments.

For the most part the two suffrutescent species (winterfat and Nuttall saltbush) made recovery of crown cover comparable to the controls for all treatments (Table 1) but yields, in some cases, were still significantly lower than the controls (Table 2). The differences between extent of recovery for winterfat and Nuttall saltbush over the seven years of rest were not substantially different. For some vigor measurements Nuttall saltbush appeared to have made better recovery but for others winterfat recovery was somewhat better.

The two grass species, like the three browse species, appeared to have suffered some long lasting damage from defoliation treatments over the three year period from 1959 to 1961. Squirreltail grass had significantly lower percent crown cover and herbage yield (P<.05) than Indian ricegrass when compared to the controls in 1968. Both grass species harvested at 90% were significantly lower in crown cover and yield (P<.05) than the controls during all seasons. Indian ricegrass appeared to tolerate 30% use during all seasons and 60% use during winter and possibly during early spring.

Indian ricegrass plants made vigor recovery comparable to the controls when previously harvested at 60% at all seasons. Squirreltail grass vigor in 1968 was comparable to the controls only for 30% use during the winter or during early spring.

Black sagebrush, big sagebrush, and shadscale had the lowest recovery in vigor after seven years rest compared to the controls; whereas, Nuttall saltbush, winterfat, and Indian ricegrass had the highest. The slow rate of vigor recovery of black sagebrush might have been the result of the rather low level of vigor following defoliation in 1962 and the rather harsh sites supporting this species compared to other species. In 1962, one year after cessation of clipping treatments, black sagebrush and big sagebrush had the lowest vigor measurements of all species and shadsclae and squirrletail grass were only slightly better in this respect. Nuttall saltbush, winterfat, and Indian ricegrass were less affected by defoliation treatments from 1959 to 1961 than the other species as shown by vigor measurements in 1962 (Table 1). Indian ricegrass was somewhat less resistant to treatments than the two suffrutescent species, however.

By 1965 Nuttall saltbush, winterfat, and Indian ricegrass had made full recovery from all treatments except when clipped at 90% in late spring and when clipped heavily in winter and again in late spring (Fears, 1966). The most rapid recovery therefore was made by these three species but they also had a higher state of vigor when clipping was discontinued in 1962. Nuttall saltbush made the most extensive recovery since it had 98% of normal crown cover in 1968. Big sagebrush and shadscale recovered form 37 and 43% of normal crown cover respectively in 1962 to 56 and 60% normal crown cover in 1968 (Table 1).

The data within a species suggest that the lower the vigor the longer the recovery period. It takes progressively longer for increased vigor through increased crown cover to occur when plants are in extremely low vigor compared to recovery of plants that have had their crown cover only slightly reduced. This is not necessarily true among species however, since big sagebrush had only 37% of normal crown cover in 1962 and gained at the rate of 2.7%/year during the seven years of rest. Black sagebrush had 40% of normal crown cover in 1962 but gained only 1.1%/year in crown cover over the 7-year period of protection. These were made at a rather constant rate over the 7-year rest period. Indian ricegrass gained crown cover at the rate of 3.1%/year but most of this recovery was made from 1962 to 1965. Squirreltail grass gained at the rate of only 1.6%/year during seven years of rest.

SUMMARY

From 1962 to 1968 a study was conducted on the desert ranges of western Utah to determine the recovery of desert range plants that had previously been subjected to three years of harvesting at three different intensities during four different seasons.

The three clipping intensities used were 30, 60, and 90%. The four periods were: (1) winter only, about January 1; (2) winter and again in late spring, about January 1 and May 1; (3) early spring only, about April 1; and (4) late spring only, about May 1. Vigor was measured during the summers of 1962 and 1968.

Rate of recovery within a species was proportional to the state of vigor, the lower the vigor the less rapid the recovery. Plants that were in lowered vigor had a smaller area of live crown cover, shorter and fewer seedstalks, less current year's growth, a larger percentage of dead crown cover and produced less herbage than did plants in higher vigor.

It was found that desert plants clipped heavily during any season or harvested even moderately during late spring or twice a year, in winter and again in late spring, still differed significantly in vigor measurements from untreated plants even after seven years of protection.

Shadscale made faster and more complete recovery than either of the other two browse species, black sagebrush and big sagebrush. Winterfat and Nuttall saltbush made complete crown cover restoration for all clipped plants during a seven year rest period. Indian ricegrass had restored crown cover comparable to controls for all previous defoliation treatments except when clipped at 90% during late spring and twice a year during winter and again in late spring. Squirreltail grass made only slight recovery and this was made at a very slow rate.