SHRUB ROOTING CHARACTERISTICS AND WATER ACQUISITION ON XERIC SITES IN THE WESTERN GREAT BASIN Sara J. Manning David P. Groeneveld

ABSTRACT has data to support his hypothesis. In arid lands where plants are often widely spaced, light is rarely a limiting Competition for limited soil water and nutrients may resource while water, due to low precipitation, is. It can be hypothesized to give rise to root morphologies adapted thus be hypothesized that within arid environments the to survival on xeric sites. To test this hypothesis, root sys­ two most important resources that plants compete for are tems were excavated for a number of shrubs occurring on water and nutrients. Since water is typically responsible the alluvial fans in the Owens Valley, CA, including for mobilization and uptake of nutrients, the low water Haplopappus cooperi (Cooper goldenbush), Chrysotham­ availability on xeric sites we have examined may directly nus teretifolius (needleleafrabbitbrush), Tetradymia axil­ affect nutrient acquisition. Therefore, in this paper, we laris (longspine horsebrush), Artemisia tridentata have focused on water as a primary limiting resource. It (big sagebrush), Purshia glandulosa (desert bitterbrush), should be recognized that both water and nutrients are Hymenoclea salsola (white burrobrush), and Ephedra resources acquired by plant root systems, and it is the nevadensis (Nevada ephedra). Root system morphologies paucity of knowledge ofbelowground features for desert were species specific and predictable. Two examples of plants that severely limits differentiating between the divergent rooting strategies, H. cooperi, and C. teretifolius, effects of nutrients and water. representing shallow, highly branched versus deeper tap­ Data on root systems are difficult to obtain. Excava­ root systems, respectively, were chosen for more intensive tions are labor intensive and costly, and reporting of find­ ecophysiological investigation. Phenologic timing and ings in a meaningful manner has yet to be standardized. response to selective removal were consistent with a hy­ Many studies report root morphologies using sketches pothesis that deeper rooting provides buffering against drawn to scale (see, for example: Cannon 1911; Cody water deficit. Both species initiated growth contem­ 1986a; Spence 1937). Data on maximum rooting depth, porally, but in H. cooperi, flowering rapidly proceeded, length of roots per volume soil, changes in root density while C. teretifolius did not flower until fall. Water poten­ as depth and distance from the plant's main axis in­ tial~ ofH. cooperi were shown to be affected by neighbor­ creases, and degree of suberization of root tissue are just ing shrubs, but under similar densities C. teretifolius some useful pieces of information necessary to complete a water potentials showed no effect. picture of belowground phenomena. A correlation has been observed between root morphol­ Here we report our observations on root systems of a ogy and flowering time for each of the other co-occurring number of shrubs occurring on alluvial fans of the Owens species excavated. The authors, therefore, propose that the Valley, CA. Next, we report on a specific study of compe­ root systems of these species are fitted to a particular eco­ tition for water between two frequently co-occurring logical stratagem. On numerous Great Basin sites this shrubs with very different root systems. Finally, we sum­ suite of species can be found in associations of variable marize our findings by proposing a correlation between composition. Set rooting patterns that are unique to each root system morphology and shrub phenology and by species, such as those exhibited by H. cooperi and C. tereti­ speculating on the role of roots in community dynamics. folius, may permit these shrubs to avoid direct competition and to coexist under the limiting conditions imposed by their arid environment. SITE DESCRIPTION All excavations were carried out on east-facing alluvial INTRODUCTION fans, at the foot of the Sierra Nevada and on the west side of the Owens Valley, CA. The average elevation was Rooting characteristics are critical to our understanding 1,300m. of arid shrub communities. Tilman (1988) claims that the Precipitation is low on these sites; the average annual two most important resources for which plants compete precipitation, as recorded at the nearest weather station are light and nutrients, and for mesic environments he in Bishop, CA, is 142 mm per year, and three-quarters of this falls between October and April (NOAA 1988). It is uncommon for the water table on the alluvial fans to be high enough to contact the root zone, and by comparison Paper presented at the Symposium on Cheatgrass Invasion, Shrub Die­ Off, and Other Aspects of Shrub Biology and Management, Las Vegas, NV, to many locations that are affected by shallow ground­ April 5-7, 1989. water, we saw no evidence of a water table. Therefore, Sara J. Manning and David P. Groeneveld are with the lnyo County we believe these shrubs rely solely on water from Water Department, 301 W. Line Street, Bishop, CA 93514. precipitation.

238 This file was created by scanning the printed publication. Errors identified by the software have been corrected; however, some errors may remain. Temperatures are hot in the summer and cold in the ROOT SYSTEMS AND WATER winter, with mean high and low daily temperatures for ACQUISITION IN TWO FAN SPECIES July and January of 36.4 oc and 13.5 °C, and 11.6 °C and -5.9 °C, respectively (NOAA 1988). We hypothesize that fan shrubs have different niches, Soils on the sites have poorly developed profiles, are which can be delineated in part by root system morphol­ rocky, and have low nutrient contents (Manning and ogy. Though quantitative traits of roots, such as maxi­ Barbour 1988). mum depth and number oflaterals, will vary among members of the same species, the overall morphology ROOT EXCAVATIONS AND proves to be consistent within a species. The alluvial fan, thus, proves to be a habitat in which many species OBSERVATIONS of shrubs with similar aboveground characteristics have Methods belowground features enabling them to exploit environ­ mental resources differently. Pits were dug adjacent to 10 fan shrub species using a Community dynamics on the alluvial fan could also be backhoe. Once the pits were opened, water was sprayed influenced by belowground phenomena. Cody (1986b), for onto the roots to remove adhering soil. Root systems of example, presented data on spatial arrangement of many each shrub excavated were identified and photographed. perennials in the Mojave Desert. Among both plants of Sketches were then made from 35-mm slide transparen­ the same species and of different species, he found posi­ cies by camera lucida technique. tive associations to be more common than negative asso­ ciations and random assemblages to be least common. He concluded that the frequent positive associations he Results observed could develop because the plants involved had Root system sketches of the 10 shrubs excavated appear root systems that did not overlap and therefore were com­ in figures 1 through 10. All are drawn at the same scale. patible with each other. Competition for water between Ephedra nevadensis (Nevada ephedra) (fig. 1) has thick, two species with different root morphologies may be negli­ woody roots which do not grow much deeper than 0.5 m, gible; therefore, their roots would exploit different soil but which do spread laterally and produce clones. layers where water availability may not be the same. Roots of Grayia spinosa (spiny hopsage) (fig. 2) are Two of the species excavated in our study, H. cooperi shallow and diffuse, and there is no obvious taproot. This and C. teretifolius, were examined for evidence of both particular individual was growing through the center of a interspecific and intraspecific competition for water. Chrysothamnus teretifolius (needleleafrabbitbrush) These two relatively small composites commonly co-occur shrub. Grayia spinosa frequently utilizes other shrub on the alluvial fans of the Owens Valley. Similar timing species as nurse plants, particularly in areas of heavy ofleader growth in the spring, similarities in general grazing. aboveground morphology, and close systematic affinities Artemisia spinescens (bud sagebrush) also has a shal­ suggested the possibility of competition for resources, low, diffuse root system (fig. 3). such as water. However, differences in flowering time Coleogyne ramosissima (blackbrush) roots grow deeper and in root morphology implied a low to insignificant than those of the previous shrubs, but again the root sys­ degree of interference between species. We performed tem is diffuse (fig. 4). The individuals shown appear to be a shrub-removal experiment to determine the presence clones which could have arisen by mechanisms of stem of competition between these two species. splitting as described by Ginzburg (1963). The root system of Haplopappus cooperi (Cooper gold­ Methods enbush) (fig. 5) is similar to that of C. ramosissima: rela­ tively shallow and diffuse. A site dominated by Haplopappus and Chrysothamnus Hymenoclea salsola (white burrobrush) possesses a was chosen on an alluvial fan. Circular quadrats (r = 1 m) relatively short taproot with prominent laterals (fig. 6). were randomly located and data on frequency, density, Tetradymia axillaris (longspine horsebrush) (fig. 7) also and cover were recorded. A Poisson distribution for the has a taproot. Laterals emerging from the taproot show data was determin~d and a chi-square goodness of fit test strong downward growth. was run to assess association among members of the same Purshia glandulosa (desert bitterbrush) has a thick species. Association between the two species was deter­ taproot (fig. 8). Near-surface laterals were not evident. mined with a contingency table (Manning and Barbour The root system of our specimen of Artemisia tridentata 1988). (big sagebrush) (fig. 9) began as a tap, but spread laterally Twenty-four shrubs of each species were selected as the quite near the soil surface. The thick lateral roots then experimental or "target" shrubs for the removal experi­ turned downward some distance from the center of the ment. Criteria for selection included large enough size shrub. so as not to be harmed significantly by repeated sampling Chrysothamnus teretifolius (needleleafrabbitbrush) and the presence of at least three members of each species displays a thick taproot with prominent laterals (fig. 10). surrounding the target shrub within a 1.8-m radius. One of four treatments was then applied to each target shrub: I. All neighbors removed, II. Chrysothamnus neighbors removed,

239 Scale: 3.5 em = 1 m

Figure 1-Ephedra nevadensis. Figure 2-Grayia spinosa.

Figure 3-Artemisia spinescens. Figure 4-Co/eogyne ramosissima.

Figure 5-Hap/opappus cooperi. Figure 6-Hymenoc/ea sa/sola.

240 Scale: 3.5 em = 1 m

Figure 7-Tetradymia axillaris. Figure 8-Purshia glandulosa.

/

Figure 9-Artemisia tridentata. Figure 1~Chrysothamnus teretifo/ius.

241 III. Haplopappus neighbors removed, and 0 IV. Control: no neighbors removed. -I Shrubs were removed in early spring, 1986. Predawn .... -2 ~ HACO I water potential of all target shrubs was then measured ~• HACO II t: -3 throughout the growing seasons of 1986, 1987, and 1988. ~ HACO II! -~ -4 HACO! V Leader growth, leaf senescence, and extent of flowering ~ were also monitored in these years by recording length, • ... CWE I .., -s CI-4TE II Ia number ofleaves, and number of flowers on 30 randomly ~ CHTE Ill -6 ... located branches of each target shrub. - CHTE ! V -7 Results and Discussion -I Haplopappus proved to have a random distribution on the site, while Chrysothamnus exhibited a clumped distri­ ....• -2 bution. The two species were randomly associated with ~ -3 each other (Manning and Barbour 1988). A review of the ~ -4 -~ literature shows that it is difficult to draw conclusions on • the interactions between species from aboveground vege­ ~ -5 Ia tation sampling alone. Cody (1986b) suggested that a ~ -6 clumped distribution indicated compatibility among the - plants, but he did not present physiological data on plant -7 interactions. Fonteyn and Mahall (1981) found no meas­ urable competition for water among their clumped Ambro­ -I sia (ragweed) shrubs; however, Ehleringer (1984) and Robberecht and others (1983) found significant interaction .... -2 for water among the clumped species in their studies. Age • .....~ -3 of the plants involved most likely accounted for the mixed ~ -4 results of these studies: young plants could be actively L •Ia -5 interacting with neighbors, while older plants may have Ia already established dominance on a site at the expense of ~ -6 some neighbors. - -7 From early spring to late summer, predawn water po­ tentials of both shrub species became progressively lower -8 ISO 180 210 240 270 300 (fig.11). Haplopappus water potentials fell much lower 90 120 Day or year than Chrysothamnus water potentials. Haplopappus water potential also was affected by presence of neighbors, Figure 11-Results of predawn xylem potential though neighbor effects appeared to become less pro­ (XPP, in MPa) measurements for all target shrubs nounced with time. In 1986, Haplopappus control shrubs of the removal experiments, 1986-88. Ordinate is had significantly lower water potentials than Haplo­ day of year. Open symbols correspond to Hap/a­ pappus shrubs around which all neighbors were removed, pappus cooperi (HACO) and closed symbols while water potentials of Haplopappus shrubs in the par­ represent Chrysothamnus teretifolius (CHTE). tial-removal treatments (II and III) remained intermedi­ Roman numerals signify the removal treatment ate between the other two treatments. Chrysothamnus applied: I = all neighbors removed, II = CHTE neighbors removed, Ill = HACO neighbors re­ shrubs showed no effect of neighbors on either its own or moved, IV = control. the other species in the years examined. Leader growth and degree of senescence paralleled the predawn water potential results (see table 1). At the end Removal experiment results clarify the ecological role of the 1986 season, the leaf senescence rate for control of the root systems of each shrub species. Haplopappus Haplopappus shrubs was higher than that of any treat­ has a shallow root system capable of taking up water and ment, and by 1987, branches on Haplopappus control nutrients from the upper layers of soil. Since most of the shrubs had the least average leader growth of any of the soil water is replenished in the winter months, Haplopap­ experimental shrubs. pus adds new growth and flowers in the spring. By the Evidence for competition is consistent with root mor­ end of the summer, upper soil layers are often quite dry phology of these two species. A review of figure 5 shows (data presented in Manning and Barbour 1988) and water Haplopappus to have a shallow, diffuse root system. The potentials of plants surviving in this soil tend to reflect shrubs excavated had roots growing no deeper than 1.2 m. the soil water potential and are extremely low. Water in There is no obvious taproot, and laterals begin proliferat­ upper soil layers may become limiting before growth and ing into the soil at 10-cm depth. Chrysothamnus, as seen flowering are complete, and thus presence of neighbors in figure 10, has a thick taproot which, in the shrubs exca­ around Haplopappus reduces an individual's ability to vated, was still 14 mm in diameter at 1.8-m depth. Most grow and speeds annual senescence. of the laterals branch from the tap at approximately Chrysothamnus root morphology enables it to exploit 50-cm depth, and laterals continue to emanate from the water, not only in the region of Haplopappus roots, but taproot at lower depths. 242 Table 1-Hap/opappus cooperi (HACO) and Chrysothamnus teretifo­ SUMMARY lius (CHTE) growth and senescence responses to removal treatments. Values shown are averages of the six shrubs The Owens Valley is a transition area. Moving north­ per treatment plus or minus standard deviation. Letters in ward through the Owens Valley, vegetation changes from common (a,b,c) denote no significant difference among that more characteristic of the Mojave Desert, a warm treatments for that species desert, to vegetation more commonly associated with the Great Basin, a cool desert. Precipitation in both deserts 1987growth 19861eafsenescence Treat· falls mainly in the winter months; and, in the Great ment HACO CHTE HACO CHTE Basin, a majority of this precipitation is snow. The xeric ------Millimeters ------Percent ------fan shrubs we have excavated in the Owens Valley are of both Mojave and Great Basin origin. 5.62±3.00a 6.74±7.26a 38.83±8.46a 35.78±10.39a In the Owens Valley, winter temperatures are cold, II 2.01±0.97b 4.01±3.02a 46.00±16.59a 39.08±6.21 a and growth does not usually occur before late February. Ill 2.08±0.58b 1.60±1.53a 51.52±16.08ab 30.90±14.19a About the time that growth begins, the period of maximal likelihood for precipitation is completed, and shrubs must IV 1.50±0.67b 2.06±2.33a 67.66±13. 75bc 34.15±16.88a then rely on water already absorbed by the soil during the winter to complete their annual growth and reproduction. The alluvial fan soils are sandy and contain numerous cobbles and rocks; thus they have a low field capacity. also in deeper soil layers. Therefore, Chrysothamnus can Precipitation falling on these coarse soils during the initiate growth at approximately the same time as Haplo­ period of lowest evapotranspiration readily percolates pappus, grow, maintain this spring growth into late sum­ to increase soil water storage. mer, and then flower in early fall. If soil water is not Since xeric shrubs depend on winter-precipitation­ limiting at the depths to which Chrysothamnus roots derived soil water storage to carry out their growth andre­ grow, or if the density of roots at these depths is low, then production cycles, those with shallow root systems must removal experiments would not show competition for complete their cycles when both the near-surface soil water water between Chrysothamnus shrubs. Chrysothamnus is available and temperatures are conducive to growth. In may take water away from Haplopappus since there is the Owens Valley, these coincide during the spring. some overlap of the root systems, but Chrysothamnus still All the alluvial fan shrubs, with the exception of, has access to other water while Haplopappus does not. perhaps, P. glandulosa, have a portion of their roots in the uppermost 0.5 m of the soil. Nitrogen in desert soils

Table 2-Root system morphology and flowering time of the shrubs excavated on the Owens Valley alluvial fans

Species Family Root system Flowering time

Ephedra nevadensis Gnetaceae shallow March-April Grayia spinosa Chenopodiaceae shallow, diffuse March-June Artemisia spinescens Asteraceae shallow, diffuse April-May Co/eogyne Rosaceae rei. shallow, April-June ramosissima diffuse Haplopappus· Asteraceae rei. shallow, March-June coo peri diffuse Hymenoc/ea sa/sola Asteraceae taproot March-June Tetradymia axillaris Asteraceae taproot April-May Purshia glandulosa Rosaceae taproot April-June Artemisia tridentata Asteraceae taproot August-October Chrysothamnus teretifolius Asteraceae taproot September-November

243 has been shown to decrease exponentially with depth with other deep-rooted species, and thus may be found in (West and Klemmedson 1978), and Groeneveld (these positive associations. This clumping could occur because proceedings) has found root density to follow a similar of nonlimiting soil water at depth and heterogeneous distribution. We believe that the near-surface roots are terrain, upon which seedlings are variably successful. essential for nutrient uptake. Thus, co-occurring plants Shallow-rooted species, with short time periods in which compete for the relatively nutrient-rich, near-surface to carry out their physiological activities, would tend to soil water. This resource, therefore, tends to be depleted compete for water and may exhibit negative distributions comparatively rapidly with the effect that shrubs with on a site since such competition could act to eliminate shallow roots are usually forced into dormancy by early individuals with access to fewest resources. summer. By contrast, deeply rooted shrubs growing in A study of community dynamics cannot focus on a the same habitat may have a longer period over which to single factor; root morphology alone cannot explain all carry out the same physiological activities since they have the complex phenomena occurring in a shrubland. Other access to water stored at depth. factors responsible for community composition include, Our study of Haplopappus and Chrysothamnus pro­ and are not limited to, sera} stage, seed dispersal proc­ vided a good example of adaptations provided by different esses, seed germination requirements, and herbivory. root system morphologies. Both species relied on the Nevertheless, we believe that root systems must be exam­ near-surface water for leader and leaf growth. Presence ined for a thorough study of community dynamics in of neighbors near Haplopappus influenced its xylem wa­ aridland communities. ter potential and its growth, while near neighbors around Chrysothamnus appeared not to cause measurable re­ REFERENCES sponses in target Chrysothamnus shrubs. Haplopappus leaves senesced by late summer, while Chrysothamnus Canon, W. A. 1911. The root habits of desert plants. Car­ leaves remained viable until the end of summer when the negie Institution ofWashington Publ. 131. Washington, shrub flowered. The influence ofbelowground factors on DC: Carnegie Institution of Washington. 96 p. such phenologic characteristics demonstrates how two Cody, Martin L. 1986a. Structural niches in plant commu­ shrubs of close taxonomic affinity and similar above­ nities. In: Diamond, J.; Case, T. J., eds. Community ground morphology have adapted differently to the same ecology. San Francisco: Harper and Row: 381-405. environment. Cody, M. L. 1986b. Spacing patterns in Mojave Desert For the shrubs in our study, there appears to be a quali­ plant communities: near-neighbor analyses. Journal of tative correlation between type of root system and time of Arid Environments. 11: 199-217. flowering (see table 2). Neither of the late-season flower­ Ehleringer, J. R. 1984. Intraspecific competitive effects on ing shrubs have shallow, diffuse root systems; they have a water relations, growth, and reproduction in Encelia taproot and prominent laterals emerging at some depth in farinosa. Oecologia. 63: 153-158. the soil. Among the spring-flowering shrubs, most have Fonteyn, P. J.; Mahall, B. E. 1981. An experimental analy­ shallow root systems (E. nevadensis, G. spinosa, A sis of structure in a desert plant community. Journal of spinescens, C. ramosissima, Hymenoclea salsola, and Ecology. 69: 883-896. Haplopappus cooperi). Ginzburg, C. 1963. Some anatomical features of splitting A deep root system does not preclude a shrub from of desert shrubs. Phytomorphology. 13: 92-97. flowering in the spring. As an example, T. axillaris has a Manning, S. J.; Barbour, M.G. 1988. Root systems, spa­ relatively deep taproot, but flowers during April and May. tial patterns, and competition for soil moisture between In the Owens Valley, this shrub is usually found low on two desert subshrubs. American Journal of Botany. the fans where soils tend to be less rocky and quite sandy. 75(6): 885-893. Though we do not have soil water data for sites occupied NOAA. 1988. Local climatological data; annual summary by this species, it is possible that water is not held very with comparative data: Bishop, CA. long in the soil even at the depths to which these root Robberecht, Ronald; Mahall, Bruce E.; Nobel, ParkS. systems penetrate. Another exception to the trend of deep 1983. Experimental removal of intraspecific rooting/late season flowering is P. glandulosa, which flow­ competitors-effects on water relations and productivity ers in spring, but has the deepest trending taproot system of a desert bunchgrass, Hilaria rigida. Oecologia. 60: that we observed. Witl}in the Owens Valley this species 21-24. has a more montane distribution. When it is found on the Spence, L. E. 1937. Root studies of important range plants alluvial fans, it tends to grow at higher elevations or in of the Boise River watershed. Journal of Forestry. 35: washes, suggesting that it has a higher water require­ 747-754. ment than some of the other fan shrubs. Furthermore, Tilman, D. 1988. Dynamics and structure of plant commu­ the paucity of near-surface roots in P. glandulosa and its nities. Princeton, NJ: Princeton University Press. association with a nitrogen-fixing actinomycete (Torrey 360p. 1978) may serve to reduce its dependency on nutrients in Torrey, J. G. 1978. Nitrogen fixation by actinomycete­ upper soil layers. Including P. glandulosa in a compari­ nodulated angiosperms. Bioscience. 28: 586-592. son of xerophytic fan shrubs may be imprecise, since its West, N. E.; Klemmedson, J. 0. 1978. Structural distribu­ rooting ecology is quite different. tion of nitrogen in desert ecosystems. In: West, N. E.; Root systems conceivably influence shrub distribution Skujins, J. 0., eds. Nitrogen in desert ecosystems. as well as community dynamics on a site. As Cody US/IBP Synthesis Ser. 9. Stroudsburg, PA: Dowden, (1986b) hypothesized, deep-rooted shrubs may be more Hutchison and Ross: 1-16. compatible with members of their own species as well as

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