UNUSUALLY LOW-CARBON ISOTOPE RATIOS IN PLANTS FROM HANGING GARDENS INSOUTHERN UTAH

Leaf carbon isotope ratios (delta(13)C) and photosynthetic gas exchang e were measured on plants growing in hanging garden communities in sou thern Utah, USA. Hanging gardens are unusual, mesic cliff communities occurring where water seeps from the sandstone bedrock in an otherwise extremely arid region; there is very limited overlap in species distr ibutions inside and outside these gardens. Solar exposure in hanging g ardens varied with orientation and one of the gardens (Ribbon Garden) was shaded throughout the day. The leaf delta(13)C values of plants in hanging gardens were significantly more negative than for plants from either nearby ephemeral wash or riparian communities. In Ribbon Garde n, the observed delta(13)C values were as low as -34.8 parts per thous and, placing them among the most negative values reported for any terr estrial plant species growing in a natural environment. Hanging garden plants were exposed to normal atmospheric CO2 with an average delta(1 3)C value of -7.9 parts per thousand and so the low leaf delta(13)C va lues could not be attributed to exposure to a CO2 source with low C-13 content. There was a seasonal change toward more negative leaf delta( 13)C values at the end of the growing season. The observed leaf delta( 13)C values were consistent with photosynthetic gas exchange measureme nts that indicated unusually high leaf intercellular CO2 concentration s associated with the relatively low light levels in hanging gardens. Thus, extremely negative leaf delta(13)C values would be expected if s ignificant amounts of the seasonal carbon gain occur at light levels l ow enough to be near the light compensation point. Maximum observed ph otosynthetic rates varied with light levels at each of the gardens, wi th maximum rates averaging 20.3, 14.6, and 3.1 mu mol m(-2) s(-1) at D ouble Garden, Lost Garden, and Ribbon Garden, respectively. Leaf nitro gen contents averaged 18.5 mg g(-1) in species from the more shaded ha nging gardens (Lost and Ribbon). When expressed on a leaf area basis, nitrogen contents averaged 117 mmol N m(-2) at Lost Garden and 65 mmol N m(-2) at Ribbon Garden (shadiest of the two gardens). Leaf nitrogen isotope ratios averaged -2.3 parts per thousand (range of -0.7 to -6. 1 parts per thousand), suggesting that most of the nitrogen was derive d from a biological fixation sourer which is most likely the Nostoc gr owing on the sandstone walls at the seep. These values contrast with l eaf nitrogen isotope ratios of 5-9 parts per thousand which have been previously reported for arid zone plants in nearby ecosystems.