Habitat
Astragalus ripleyi occurs in the the Great Basin and Coniferous Woodland sections of the Southern Rocky Mountain province (Fenneman 1946, McNab and Avers 1994). In Colorado, the habitat is generally open ponderosa pine-Arizona fescue savanna (Pinus ponderosa-Festuca arizonica association), in open-canopy pinon-juniper (PJ) woodlands with an Arizona fescue (Festuca arizonica) understory, and on the edges of closed-canopy ponderosa pine and mixed conifer forest (Lightfoot 1995). In New Mexico, habitat seems a little more variable and occurrences are frequently in shrub-dominated habitat such as pine-oak (Quercus gambellii) communities, pinon-juniper sagebrush, sagebrush communities, and Chrysothamnus viscidflorus meadows.
Astragalus ripleyi is often observed growing under the canopy, or amongst the stems, of shrubs such as big sage (Artemisia tridentata), gambel oak (Quercus gambellii), rabbit brush (Chrysothamnus sp.), and juniper (Juniperus sp.). The association with shrubs may be due to a favorable microclimate or advantageous soil environment for germination and seedling establishment. In addition, or alternatively, growing in the midst of shrubs affords protection from large herbivores and may be a consequence of current or historical grazing pressures (Naumann 1990, Lightfoot 1995, R.A. Sivinski pers. comm. 2002). In one severely overgrazed, very weedy Chrysothamnus viscidiflorus meadow that had apparently been planted with Agropyron cristatum in the past, Astragalus ripleyi plants were confined to large shrubs. The entire meadow appeared to be similar habitat but the plants were only in a few clumps. This observation, made in 1998 (Colorado Natural Heritage Program element occurrence records accessed in 2002), suggests that either some habitat modification is tolerated or that habitat is not strictly confined to the limited types described in the past (Naumann 1990, Lightfoot 1995). It is unclear as to how many years had past since the grass seeding took place and one may conjecture that the root systems were present prior to habitat modification and that the surviving plants are relics.
A noteworthy observation is that Astragalus ripleyi invariably grows in areas where plant species diversity is high (Braun 1988). Tree species associated with Astragalus ripleyi habitat are Abies concolor, Juniperus monosperma, Juniperus osteosperma, Juniperus scopulorum, Pinus edulis, Pinus ponderosa, Populus tremuloides, and Pseudotsuga menziesii. Associated shrub species include: Artemisia frigida, Artemisia nova, Artemisia tridentata, Chrysothamnus greenei, Chrysothamnus naseosus, Cercocarpus montanus, Potentilla fructicosa, Symphoricarpus oreophilus, Rhus trilobata, and Quercus gambellii. Associated forb species include: Antennaria sp., Aster bigelovii, Astragalus drummondii, Astragalus hallii, Astragalus lonchocarpus, Calohotus sp. Castilleja sp., Gutierrezia sarothrae, Heterotheca villosa, Hymenoxys odorata, Picradenia richardsonii, Erigeron sp. Eriogonum sp., Eriogomun racemosum, Linium lewisii, Melilotus sp., Melilotus officinalis, Gilia sp. Oxytrophis lambertii, Penstemon griffinii, Penstemon secundiflorus, Taraxacum officinale, and Vicia americana. Associated grass and grass-like species include: Agropyron cristatum, Agropyron trachycaulum, Agropyron smithii, Blepharoneuron tricholepis, Bouteloua gracilis, Bromus tectorum, Carex sp, Danthonia intermedia, Elymus elymoides, Festuca arizonica, Festuca ovina, Festuca thurberi, Koeleria sp., Muhlenbergia sp., Muhlenbergia filiculmis, Muhlenbergia montana, Oryzopsis micrantha, Poa fendleriana, Poa pratensis, Stipa sp., Stipa comata, and Stipa hymenoides.
Plants occur at elevations between at 5,459 ft to 9,360 ft (1,664m to 2,835m). This is a considerable extension from the 7,000 to 8,250 feet range reported by Barneby (1964). However, where elevation was reported, the majority of occurrences are between 8,500 and 8,999 ft. Plants are found on level ground to slopes of approximately 30% with the most occurrences occurring on slopes of 10% or less. Although it has been found on slopes facing all aspects, northerly aspects were most common among the occurrences where aspect was reported. This, and a tendency to find them in east-west trending drainages, may imply a preference for more mesic sites in otherwise relatively dry environments.
Astragalus ripleyi occurs exclusively on volcanic derived soils associated with the San Juan volcanic field (Erhard 1994, Lightfoot 1995). In New Mexico, the soils that overlay volcanic rock are generally loamy mixed soils. One report in New Mexico (USFW ACSS 1993, New Mexico Natural Heritage Program element occurrence records accessed in 2001) indicates that plants grow in loamy clays overlying granitic bedrock but the town-range-section information places the occurrence on volcanically-derived soils. The occurrence is most likely on soils derived from plutonic rocks that are characteristically medium- to coarse-grained granitic textured (geological code: Xp, Anderson et al. 1997). Soils on this geological formation may well appear "granitic" rather than "volcanic" in the field. The precision of the occurrence data was such that it may, alternatively, be on the adjacent Los Pinos geological formation (geological code: Tlp) that comprises volcaniclastic conglomerate interbedded with basaltic flows (Anderson et al. 1997).
Ecology
Astragalus ripleyi behaves as a long-lived individual that primarily allocates resources into survival of the individual rather than into reproduction (Burt 1997, 1998, 1999). It does not compensate for herbivore activity by producing more stems or leaves to replace those which are lost but tends to produce fewer reproductive organs and, presumably, builds up root stock reserves for subsequent years growth. The habitat of Astragalus ripleyi, namely in open savannahs and shrublands, open canopy ponderosa pine forest, and along the edges of closed canopy forests and woodlands, suggests that it is a mid-successional species. It rarely occurs in recently disturbed sites such as road cuts but is frequently found in areas that have disturbance, such as fire, recorded within a decade. It likely benefits from an intermediate disturbance regime and in pre-settlement times Astragalus ripleyi may have occupied habitats that were periodically opened up by fire (Naumann 1990, Lightfoot 1995).
Burt (1999) reported a correlation between precipitation and number of stems observed at each site. The observations indicated an increase occurred between 1996 and 1997 and a net increase between 1996 and 1998. However, there was intervening decrease in population size between 1997 and 1998. This variation emphasizes the importance of multiple year monitoring. In addition, although precipitation is often ascribed to explain variation in population size, as measured by above-ground stems, the precipitation in 1996 and 1998 did not appear substantially different. It may be that the heavy snowfall in April was particularly important in stimulating stem growth in 1998. This observation suggests that it is not only total precipitation but also when precipitation occurs, and perhaps the form in which the moisture occurs, that is significant. Another factor to consider in interpreting the trend is that precipitation records are for the general region and local precipitation levels may differ.
There are no data on longevity of seed or seed bank dynamics. The degree of reported seed predation by insects is variable but may be a cause of significant seed loss in some years (Coles 1996, Burt 1997). Evidence of seed predation can be found in some herbarium specimens, for example collected by O'Kane (1996) at the University of Colorado Herbarium (COLO) that had "pin-prick" holes, most likely present prior to collection, in the legumes. There is no information on seedling ecology and the rates of recruitment and mortality are unknown. Few age and size class data are available but plants with large, robust and flowering stems one year will appear small and immature the next (Burt 1999). This observation makes casual comments on seedling presence difficult to evaluate.
No studies have been undertaken to determine the genetic structure of either range-wide or local populations. Studies undertaken on the Rio Grande National Forest collected some valuable demographic data over a period of three years (Burt 1997, 1998, 1999). Unfortunately only limited conclusions can be drawn as the data was not fully analyzed, but some deductions and inferences can be made. From observations made during this three-year study, it is clear that individuals do not follow a linear progression from seedling to non-reproductive individual to reproductive individual but may be reproductive one year, small vegetative plants the next, or may remain dormant for at least one year without any above ground stem (Burt 1999). From observations made on the Rio Grande and Carson National Forests (G. Long pers. comm. 2002, J. Burt pers. comm. 2002) it is likely that patches of Astraglus ripleyi roots lie essentially dormant under canopy cover and sprout when suitable conditions recur. For example, within a "couple of weeks" of a burn on the Carson Forest stems of Astragalus ripleyi were observed in areas where no plants were documented in the past (G. Long pers. comm. 2002). Astragalus ripleyi stems were likely from pre-existing root stocks because precipitation appeared to be inadequate for seed germination and no other annual species were observed in the burn areas. Although roots appear to undergo dormancy for extended periods, direct evidence is only available to support a dormancy period of two years. Plants tagged in 1996 did not reappear in 1997 but came up in 1998 despite there being a high amount of moisture in 1997 (Burt 1998).
Reproduction
Astragalus species are generally insect pollinated (Geer and Tepidino 1993) and Astragalus ripleyi appears to be no exception. Bees and ants have been observed on flowers and Burt (1997) reported that bumblebees (Bombus ternaries) were the most common arthropod visitor. As part of the same study Burt (1997) bagged flowering stems to exclude pollinators and these produced no fruits whereas the unbagged stems did. This may indicate some degree of self-incompatibility exists or that flowers up the same stem did not mature at same time. Although this observation provides indirect support for incompatibility in Astragalus ripleyi, because no hand pollinations were performed, the environment of the pollination bag may have caused sterility and cannot be completely discounted.
Astragalus ripleyi flowers from June into July. The earliest date flowers have been reported is June 5. Within a population, a high percentage, frequently on the order of 80 to 90 percent, of individuals produce flowers and pods (Romero 1992, CNHP and NMNHP element occurrence data). Fruits tend to be persistent on the plant through at least October (Lightfoot 1995). The patchy nature of its spatial distribution suggests seed dispersal may often be limited and localized around the parent plant. Seed dispersal has been speculated by ants, mice and other seed storers, tumbling dispersal of dried plants, and wind or water transport although little evidence has been documented for any particular mechanism (Braun 1988, Naumann 1990, Lightfoot 1995, J. Anderson pers. comm. 2002). Rodents cache fruits in small piles near plants (Burt 1999).
There is no evidence of hybridization between Astragalus ripleyi with other Astragalus species. The sympatric Astragalus lonchocarpus is closely related but no intermediates have been observed (Lightfoot 1995).