Effects of defoliation on net photosynthesis and regrowth of western wheatgrass.

Net photosynthesis (PN) and regrowth of 60-day old Agropyron smithii Rydb. plants were examined over a lo-day period following defoliation to simulate grazing. Plants grown hydroponically in full strength Hoagland’s solution were moderately defoliated (l/2 tillers clipped at 5 cm), heavily defoliated (3/4 tillers clipped at 5 cm), or left as unclipped controls. Thirty minutes after clipping, PN rates of the youngest fully expanded leaf of a remaining undamaged tiller had declined by 6%7oj, in both groups of defoliated plants. Rates of PN were subsequently monitored on the same leaves at 2-day intervals. By Day 2, PN (per unit of leaf area) of both defoliated groups had increased to rates 5-10% higher than those preceding treatment, while PN of control plants had decreased about 6%. From Day 2 through Day 10, PN rates of control plants averaged 90% of their preclipping PN rates, while PN rates of moderately and heavily defoliated plants averaged 106% and 114% of their preclipping rates, respectively. Defoliation had no significant effect on tiller production over this lo-day period. While total new biomass production of controls was almost twice that of either of the defoliated groups, the proportion of the new growth allocated to shoots, crowns and roots did not differ among the three groups. In most terrestrial ecosystems, herbivores remove less than 10% of the annual aboveground net primary production (Chew 1974), but they may consume 50% or more of the annual aboveground production of grasslands (Chew 1974, McNaughton 1976). To achieve better understanding of grassland ecosystem function, it is essential to understand how these herbivores cause departures from the linear transfers of energy depicted in most models (Chew 1974). For example, in addition to reducing leaf biomass, grazing may potentially alter energy flow in the system by changing photosynthetic or respiratory rates, growth rates, and carbon allocation patterns. Following partial defoliation, photosynthetic rates in remaining undamaged leaves frequently increase or decline less rapidly with age than do those in equivalent leaves on nondefoliated plants (Gifford and Marshall 1973, Hodgkinson et al. 1972, Hodgkinson 1974, Detling et al. 1979b, 1980). By contrast, Ryle and Powell (1975) reported slightly reduced photosynthetic rates in remaining leaves of partially defoliated uniculm barley (Hordeum Kindred Culm 97) plants. Net photosynthesis was also reduced in remaining Authors are graduate research assistant and associated director, Natural Resource Ecology Laboratory, Colorado State vniversitv. Fort Collins 80523. The research was supported by National Science Foundation Grant DEB 76 81821. Authors thank Donna Winn and Vicky Chamberlain for their technical assistance, and Marilyn Campion for performing the statistical analyses. Manuscript received October 30, 1979. 66 tissue of individually damaged leaves (Hall and Ferree 1975, 1976; Detling et al. 1979a). Carbon allocation and growth patterns also change following defoliation. Ryle and Powell (1975) found that the proportion of photosynthetically fixed 1% translocated from leaves to roots was markedly reduced, while the proportion used in regrowth of new leaf tissue increased, following defoliation of uniculm barley. Similarly, leaf blade growth accounted for over half of the total new growth of defoliated blue grama (Bouteloua gracilis) plants, but only one-third of the total growth of control plants, over the initial 10 days after clipping (Detling et al. 1979b). Relatively few studies have been conducted to evaluate the photosynthetic responses of North American prairie grasses to defoliation (Detling et al. 1979a, b, 1980). The purpose of this study was to examine the effects of simulated grazing on rates of net photosynthesis (PN) and regrowth of western wheatgrass (Agropyron smithii Rydb.), an important component of the North American shortgrass and mixed-grass prairies. It is a Cs species (Williams and Markley 1973) and is a preferred food of several species of grasshoppers (Hewitt 1978), of ungu! ates such as bison and domestic cattle (Peden et al. 1974, Peden 1976), and of small mammals such as the black-tailed prairie dog (Summers and Linder 1978). Methods and Procedures Western wheatgrass seedlings selected for uniformity were cultured hydroponically in full strength Hoagland’s solution in a growth chamber with 14-hr photoperiods and 20/ 15OC day/night temperatures. quantum irradiance (PAR) at midcanopy height was 450 PE l ml s-l. Experimental plants were maintained under these conditions for the duration of the experiments. Estimates of PN were made by measuring C& exchange of a single leaf blade with Beckman 3 15 infrared gas analyzer, utilizing an open system and methods described by Williams and Kemp (1978). The cuvette was cooled by circulating water from a controlled-temperature water bath. Air in the cuvette was mixed by a magnetic stirring bar to maintain low boundary layer resistance. Air temperature in the cuvette was maintained at 20” C f 0.5”) measured with a shaded YSI thermister. Quantum irradiance within the cuvette was 1200 PE l me2 l s-l. Experimental plants were 60 days old and each had four tillers with from one to four fully expanded leaves. The tiller used for PN measurements was not clipped and it had two fully expaned leaves and one emerging leaf blade. All COZ exchange measurements were made on the youngest fully expanded leaf. Plants were clipped 5 cm above the crown, at two intensities. Six plants were moderately defoliated by clipping one-half of all tillers over 5 cm JOURNAL OF RANGE MANAGEMENT 34(l), January 1961 tall. Six others were heavily defoliated by removing three-quarters of any tillers over 5 cm tall. Thus, PN rates were determined on 25 r Control similarly aged leaves of comparably developed undamaged tillers remaining after defoliation. Likewise, COZ exchange was measured on similar leaves of seven untreated plants which served as controls. On the day of treatment (Day-O), PN was measured immediately before and 30 minutes after clipping. Rates of PN were subsequently determined on the same leaves of defoliated and control plant every other day over the next 10 days. All data were normalized by expressing PNasa percentage of the preclipping PN rate of that leaf. Regrowth of western wheatgrass was analyzed in another experiment-utilizing 108 plants. Plants were treated as before, and dry weights determined for shoots, crowns, and roots of 9 plants of each treatment on Days 0, 2, 6, and 10. Total tiller number and number of tillers with expanded leaves were also recorded on those days. Results and Discussion Immediately prior to treatment on Day 0, PN rates $t 1 SE) were 24.1 f 1.9, 24.8 f 2.2, and 22.9 Z! I 0.8 mg CO2 dm* h-r for the controls, moderately defoliated, and heavily defoliated plants, respectively. These rates are similar to those found by Detling et al. (1979a), for soil-grown western wheatgrass plants transplanted from the field. Over the ten days of the experiment (Fig. l), PN of the controls decreased 10% to 2 1.6 mg CO, * dme2 h-' . This decline also corresponds closely to results of Detling et al. (1979a) and is typical in aging leaves (Jewiss and Woledge 1967).