The effects of aeolian nitrogen deposition gradients on sexual reproduction and competitive relationships of yucca (Hesperoyucca whipplei)

Background/Question/Methods

Air pollution from industrial and transportation sources affects both reproductive effort and competitiveness of plants, through both direct toxicity and dry deposition of nitrate, potentially favoring non-native species. Hesperoyucca whipplei, a species native to Southern Californian chaparral, can reproduce vegetatively, but relies obligately on the yucca moth for pollination. In the Los Angeles area, populations of yucca are fragmented, located mostly in alluvial washes surrounded by urban matrix, and may be particularly sensitive to inadvertently fertilized invasive competitors. I recorded GPS locations of 106 reproductive yuccas, representing seven populations along a predicted gradient in aeolian nitrogen in the San Gabriel foothills. For each plant, I recorded rosette height, clonality, surrounding grass cover, and soil nitrogen concentration. Leaf samples for use in genetic analysis were also taken.

Results/Conclusions

I found, contrary to predictions, no significant geographical gradient in soil nitrate. I found significant negative relationships between rosette height and elevation and between nitrate and grass cover, and positive relationships between clonality and both latitude and longitude. While the negative relationship between rosette height and elevation is intuitive, the other results were unexpected. My sample size may have been too small to detect a significant gradient in soil nitrogen, given high within-site variability. Grass cover may be lower in sites with high nitrate due to associated airborne pollutants, which counteract any benefits of fertilization. The significant distribution of clonal individuals might be attributed to variation at the subspecies level, or a plastic shift in reproductive strategy, perhaps related to pollinator abundance.