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We examined whether limber pine and skunkbush sumac individuals co-existing on the Pawnee Buttes of northeastern Colorado have the ability to access and utilize deep soil water resources. The δ18O signature of source water to the plants (deep soil water and precipitation) and plant cell water were measured in June 2000. The δ18O signatures of the two woody plant species were not significantly different from each other. However, the average δ18O signature of skunkbush sumac (−9.4‰) differed from the δ18O signature of deep soil water (−13.0‰), while the average δ18O signature of limber pine (−11.3‰) did not. Variability in the δ18O signature across plots and between individuals within a plot was relatively high for both species. These results suggest limber pine and some individuals of skunkbush sumac are able to access and utilize deep soil water resources at this site. This ability may confer an ecological advantage to these plants given the semi-arid climate of the site.

Soil seed bank dynamics, greenhouse seed germination and field seedling survival were compared in the weakly-clonal shrub Rhus aromatica Ait. and the strongly clonal shrub R. glabra L. (Anacardiaceae). Although both species had persistent soil seed banks, their sizes and dynamics differed greatly. The high seed density (5668 ± 1267 m−2 to 8739 ± 3303 m−2) in R. glabra was depleted by ca. 5% annually and replenished with 1102 ± 316 to 1828 ± 486 seeds m−2, whereas the low seed density (9 ± 5 to 138 ± 68 m−2) in R. aromatica was depleted by ca. 50% annually and replenished with 37 ± 7 to 69 ± 18 seeds m−2. Seeds of both species germinated within established populations. However, recruitment from seedlings was successful in R. aromatica but not in R. glabra, which depended exclusively on root suckering for aboveground population increase. This difference in local recruitment modes between the two species was correlated with differences in soil seed bank dynamics and other life history traits, such a...