Great Basin Naturalist Volume 55 Number 2 Article 3 4-21-1995 Growth and reproduction in an alpine cushion plant: Astragalus kentrophyta var. implexus Wayne R. Owen University of California, Davis and White Mountain Research Station, University of California, Las Angeles Follow this and additional works at: https://scholarsarchive.byu.edu/gbn Recommended Citation Owen, Wayne R. (1995) "Growth and reproduction in an alpine cushion plant: Astragalus kentrophyta var. implexus," Great Basin Naturalist: Vol. 55 : No. 2 , Article 3. Available at: https://scholarsarchive.byu.edu/gbn/vol55/iss2/3 This Article is brought to you for free and open access by the Western North American Naturalist Publications at BYU ScholarsArchive. It has been accepted for inclusion in Great Basin Naturalist by an authorized editor of BYU ScholarsArchive. For more information, please contact [email protected], [email protected]. Great Basin Naturalist 55(2), © 1995, pp. 117-123 GROWTH AND REPRODUCTION IN AN ALPINE CUSHION PLANT: ASTRAGALUS KENTROPHYTA VAR. IMPLEXUS Wayne R. Owen1 ABSTRACf.-A two-year field experiment was conducted to investigate factors hypothesized to affect the reproduc­ tive potential ofAstragalus kentrophyta var. implexus and to test the importance oftrade-offs between growth and repro­ duction in this species. Levels of mineral nutrients, water, herbivory, and competition were manipulated. Seed output R."1d growth of individuals in treatment groups were compared against control plants. Neither water nor mineral nutri­ ents alone were shown to affect growth or reproduction. Herbivory was shown to be similarly unimportant in affecting growth and reproduction. Competition with other species influenced growth but not reproduction. No significant trade­ offs between growth and reproduction were detected within years. However, there did appear to be a trade-offbetween these major fitness components when compared between years. Key words: Astragalus, alpine, competition,!ecundUy, trade-off, White Mountains. The impact of resource availability on the limited. Experiments involving other organ­ reproductive output of plants is well estab­ isms from this habitat have shown that avail­ lished (Harper 1977, Schoener 1983, Fowler ability of resources influences the competitive 1986, Welden and Slausen 1986). Plants may ability and distribution of species (Wright and experience resource limitation as a result of Mooney 1965, Mooney 1966, Marchand 1973), competition (inter- or intraspecific) or poor though this is not generally true of all alpine hahitat quality. Resource limitations can also habitats (Korner 1989). Second, standing bio­ occur when a portion of a plant's photosyn­ mass and percent cover are substantially lower thetic organs are removed (e.g., by herbivory), on dolomitic soils than on adjacent sandstone­ damage which clearly interferes with the plan(s and granite-derived substrates, suggesting that ability to provision its offspring (Marquis plants on the dolomite barrens might be rela­ 1991). A number of authors (Cody 1966, tively resource limited (Mooney 1966, Owen MacArthur and Wilson 1967, Harper 1977, 1991). Third, A. kentrophyta plants routinely Grime 1979, Tilman 1982, Weiner 1988, 1990) abort the majority of flowers they produce have considered the ecological consequences each year (Owen 1991), a pattern that has been of resource limitation for individuals and pop­ attributed to resource limitations in a broad ulations and have described various strategies spectrum of species (Lovett Doust and Lovett that plants might be expected to pursue to optimize the allocation oflimited resources. Doust 1988). This study tests wbether tbe availability of An experiment was designed (1) to test resources limits the fecundity of Astragalus whether there are resource constraints on the kentrophyta Gray var. implexus (Canby) reproduction and growth of A. kentrophyta Barneby (hereafter, simply A. kentrophyta) and and (2) to assess the interactions between two to what extent trade-offs between growtb and major components of fitness (Le., growth and reproduction might influence patterns of reproduction) under different regimes of reproduction observed in this species. A. ken­ resource availability. To do this, a factorial trophyta is an alpine cushion plant indigenous field experiment was established in which sep­ to high elevations throughout the Intermoun­ arate groups ofplants would receive either (1) tain West ofNorth America (Barneby 1964). water or (2) nutrient supplements, (3) protec­ Many lines of evidence suggest that repro­ tion from herbivory, or (4) relief from the duction in A. kentrophyta might be resource potentially competitive influence ofneighbors. lUniversity ofCalifOrnia. Davis. and White Mountain Resem:ch Station, University ofCalifornia, Los Angeles. Present addres~: Boise National Forest, 1750 Front Sh'eet, Boise, 10 83702. 117 118 GREAT BASIN NATURALIST [Volume 55 STUDY AREA as "Water," (2) Another 50 plants received sup­ plemental nutrients. These plants were given The study was conducted on the alpine approximately 17 g of a balanced general-pur­ dolomite barrens of Sheep Mountain Pass pose fertilizer (Scott's All-Purpose Builder, above the Patriarch Grove bristlecone pine 12:10:12 N:P:K), providing each plant with 2,0 g forest, in the White Mountains of Mono N (in the form of ammoniacal nitrogen, ureas, County, CA, Elevations at the site range from and water soluble nitrogen), 1.7 g P (from 3535 m (11,600 ft) to 3660 m (12,000 ft), and phosphoric acid, P20 S), and 2,0 g K (from sol­ topographic relief of the site is minimaL In the uble potash, K20), These quantities are equiv­ White Mountains A kentrophyto occurs only alent to application rates of 13,8, 11.7, and on dolomitic soils (Lloyd and Mitchell 1973, 13.8 kg ha-1, respectively, A balanced fertilizer Hall 1991), was chosen because experiments by Chambers Weather data were obtained from the White et al, (1987) and Shaver and Chapin (1980) have Mountain Research Station, Mt. Barcroft shown that plants in cold environments re­ Laboratory, located 6 km north of the study spond most vigorously to resource augmenta­ site at an elevation of3800 m, Soils on the dolo­ tion with fertilizer containing a balance of mite barrens have a high cation exchange essential nutrients. The dry fertilizer was scat­ capacity and are depauperate in nitrogen, tered in an approximately 2-cm-wide ring phosphorus, and potassium (Mooney et al. around the perimeter of each test plant. 1962, Wright and Mooney 1965, Brayton and Summer seasonal precipitation in 1989 was Mooney 1966, Mooney 1966, Marchand 1973, apparently sufficient to solubilize the fertilizer 1974), The moisture-holding capacity of and deliver it to the soil profile, as the granules dolomite-derived soils is equivalent to that of had completely disappeared from the surface adjacent granitic soils (Mooney et al. 1962, in approximately one month. This treatment Wright and Mooney 1965, Marchand 1973), group will be referred to as "Fertilized," (3) A Vegetation of the White Mountains is general­ third treatment was designed to protect plants ly xerophytic; this trend is especially prevalent from herbivory and predation on flowers and on the dolomite barrens (Lloyd and Mitchell young fruits, Two locally common insects ha­ 1973), bitually consume the reproductive parts of A, kentrophyta, The more common of these in­ MATERIALS AND METHODS sects, a darkling beetle (Tenebrionidae: Coleop­ tera), consumes flowers. Larvae ofa locally com­ In June 1989, 195 healthy A kentrophyta mon Lycinid butterfly species (Lycaenidae: plants were selected randomly from within an Lepidoptera) occasionally consume immature area ofapproximately 0,2 ha, Decadent (senes­ A, kentrophyta fruits, "Tangle-foot" brand cent) plants were disqualified from inclusion sticky-trap was applied in a circle around each in this experiment. The specific location of the of 25 plants to exclude potential herbivores, site was chosen for its apparent homogeneity Tanglefoot barriers were repaired as needed. with respect to soil physical characteristics, This treatment group will be called "No vegetation, and topographic profile, Plants Predation," (4) The fourth treatment sought to were randomly allocated to five treatment relieve a group of 20 A kentrophyta plants regimes: (I) 50 plants were provided with from neighborhood competition, A 0,25-m­ three separate 1-L applications of water dur­ radius circle around a central target A. kentro­ ing the 1989 growing season, Plants were phyta plant was cleared of all other plants by watered during the driest part of the summer cutting them off at ground level. This method (4 July, 2 August, and 19 August) to maximize minimized ground surface disturbance. the beneficial impact of the treatment. Water Clearings were 0.2 m2 in area. The average was applied slowly (to maximize infiltration) in number of neighbors (ramets) removed was 63 a radius of 12,5 cm around each plant. This (mostly tillers of Poa rupicola), covering an treatment supplied 6, I cm ofmoisture to each average of 15% of the ground surface, plant, Expected precipitation for the three­ Excavations ofA. kentrophyta plants show that month growing season is 8,7 cm (Pace et al. its roots grow straight downward into the soil 1968), The 1989 summer precipitation was I.I with minimal lateral root spread (Owen 1991), cm, This treatment group will be referred to Roots of the target plants were therefore 1995J ASTRAGALUS GROWTH AND REPRODUCTION 119 thought to