December 2012 Number 1 - Utah Native Plant Society

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Utah Native Plant Society Figure 2. Frequency of Astragalus rusbyi by treatment over time at G.A. Pearson Natural Area near Flagstaff, AZ. treatments (G.A. Pearson Natural Area), Laughlin and others (2008) found A. rusbyi to be an indicator species of both thinned treatments and treatments that involved thinning plus burning (mean indicator value of 25.0; p
Calochortiana December 2012 Number 1 D.C. Laughlin (personal communication, 2009) collected trait data on 137 ponderosa pine understory species, including A. rusbyi, and found that, on average, it has a higher specific leaf area (24 mm 2 /mg) and higher nitrogen and phosphorous concentrations in its foliar tissue (4.4% and 0.18%, respectively) (Figure 4). Because of the high nitrogen content, it has a relatively high net photosynthetic rate. On average, it also has a lower leaf dry matter content (0.19 mg/mg) and foliar C:N mass ratio (10.7). Combined with its high photosynthetic rate and comparatively tall stature (with an average height of 31 cm), it is able to compete well with other understory species, but is not very tolerant of stresses such as deep shade. The combined trait data place it in the category of a competitive ruderal species (Hodgson et al. 1999). CONCLUSIONS As previously mentioned, little is known of the ecology of this locally abundant but narrowly endemic species, and much of its known range is slated to undergo various thinning and prescribed burning activities in the very near future. Many Astragalus species are long-lived, recruit slowly by seed, and maintain longlived seeds in the soil seed bank. Whether A. rusbyi utilizes a similar strategy is unknown but could be determined from additional research. We currently do not have a thorough understanding of the population dynamics of this species over time. Rigorous long-term demographic monitoring would be valuable in determining population baselines and is essential for understanding the ecology and conservation and habitat needs of this species. Such monitoring can also reveal patterns that might be caused by precipitation fluctuations. From the information available, it appears to have a large taproot, which should give some resistance to the impacts of drought and fire, but high-intensity fire or burning at peak growth times could be detrimental. It has shown positive to no effects from tree thinning and prescribed burning operations in ecological restoration research studies, but additional research that specifically targets this species would be useful before we can draw firm conclusions. Figure 3. Proportion of permanent monitoring plots through time containing Astragalus rusbyi at an ecological restoration study area near Flagstaff, AZ. Treatments were randomly assigned within each block and included (a) no thinning, no burning (control), (b) 1.5-3 tree replacement (high-intensity thinning), (c) 2-4 tree replacement (mediumintensity thinning), and (d) 3-6 tree replacement (low-intensity thinning). All treatment units were thinned in 1999 and subsequently treated with prescribed fire in spring 2000 (Block 3) and spring 2001 (Blocks 1 and 2). 161
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December 2012 Number 1 - Utah Native Plant Society

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