The effects of drought on host plant canopy condition and survival of the endangered Astragalus jaegerianus (Fabaceae)

Background/Question/Methods Astragalus jaegerianus Munz, (the Lane Mountain milkvetch) is a federally endangered species that exists in small fragmented populations restricted to granite outcrops in the central Mojave Desert. A scrambling, weak-stemmed, herbaceous perennial, A. jaegerianus is dependent on the canopy of common desert shrubs for structural support, but host shrubs may also provide protection from herbivores, as well as a modified microclimate conducive to the growth and recruitment of A. jaegerianus. For the past eleven years, we have conducted yearly A. jaegerianus surveys on nine 1 ha permanent plots. During this period, this already rare plant has undergone alarming population contractions. Since 1999, our monitored populations have declined to 12% of their previous size with no recruitment observed in 11 years. A number of A. jaegerianus subpopulations have dropped to critically low levels, and are in danger of local extinction. The decline of A. jaegerianus has occurred simultaneously with recent severe drought in the Mojave Desert. These drought conditions began in 1999 and are predicted to continue for decades, or may continue indefinitely under warmer temperature conditions projected by global climate change-type drought. This study investigates the indirect effect of drought on A. jaegerianus through the deterioration of its host shrubs.

Results/Conclusions Our results suggest that drought conditions may have direct and indirect affects on A. jaegerianus by killing or degrading its host shrub. Within A. jaegerianus sites, host shrubs have decreased in shrub volume and cover since the onset of drought. Mortality of these long-lived shrubs has been high, and the recruitment of new shrubs has been too low to maintain their populations at previous levels. Our results confirm that canopy condition affects the microclimate beneath and within A. jaegerianus host shrubs: shrubs with open canopies have light levels five times higher than shrubs with closed canopies, and soil surface temperature beneath shrubs with open canopies can be 20°C higher than shrubs with closed canopies. While most shrubs do not have completely open canopies, among host shrubs originally surveyed in 1999 and 2003, the average host shrub had only 45 percent of its canopy intact in 2009. Furthermore, our results show a significant increase in survival of A. jaegerianus among host plants with more intact canopies. These results support our study hypothesis that drought-related changes to host plant canopies affect A. jaegerianus survival and represent an indirect negative effect of long-term drought on A. jaegerianus populations.