Soil-dependent canopy-die back and plant mortality in two Mojave Desert shrubs

Authors: Hamerlynck, Erik; MCAULIFFE, J. - DESERT BOTANICAL GARDEN

Submitted to: Journal of Arid Environments
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/6/2008
Publication Date: 6/18/2008
Citation: Hamerlynck, E.P., Mcauliffe, J.R. 2008. Soil-dependent canopy-die back and plant mortality in two Mojave Desert shrubs. Journal of Arid Environments. 72:1793– 1802. . Journal of Arid Environments.

Interpretive Summary: The desert regions of the Southwestern US have recently experienced unprecedented levels of drought, which has resulted in widespread, though patchy, mortality of adult plants. This study showed how soil characteristics that affect seasonal soil hydrology affect the degree of branch and whole-plant mortality in the two dominant shrubs of the Mojave Desert, the drought-deciduous shrub white bursage (Ambrosia dumosa) and the evergreen creosotebush (Larrea tridentata). Overall levels of branch mortality (“canopy die-back”) and whole-plant mortality were greater in white bursage than in creosotebush. Dead plants of both species tended to be smaller than surviving plants, especially in channel and hillslope locations, which indicate that juvenile plants were particularly vulnerable to drought in locations that receive additional water from runoff under normal climate conditions. Canopy-die back was very pronounced in larger plants growing in young, weakly-developed soils, while smaller plants growing in older, well-developed soils showed much less branch and plant mortality. We attributed these differences to the distribution of large rocks through the soil profile, which were more common in older soils. Sub-surface rocks might create large numbers of favorable soil microsites, resulting in more water available throughout the soil, which would buffer against drought effects and also reduce the intensity of between- and within-species competition for water.

Technical Abstract: Recent drought conditions have led to unprecedented levels of plant mortality across the arid Southwestern US. An unaddressed feature of the effects of this drought is how variation in soil characteristics and soil hydrological behavior has affected desert plant canopy die-back and mortality. Here, we present a multi-year study in the Mojave Desert that assessed canopy die-back and whole-plant mortality in the drought-deciduous shrub white bursage (Ambrosia dumosa) and the evergreen creosotebush (Larrea tridentata) growing in different soil hydrological environments. The soils varied in age-dependent surface and sub-surface horizon development, and topographic (hillslope vs. channel) positions. Canopy-die back and whole-plant mortality was more widespread A. dumosa than in L. tridentata, and dead A. dumosa and L. tridentata plants tended to be smaller than surviving plants, especially in channel and hillslope locations. This suggests that juveniles were particularly vulnerable in locations where plants depended heavily on augmentation of incident precipitation by runoff from adjacent areas. Canopy-die back was more extensive in young, weakly-developed soils that favored extensive plant growth preceding the drought, while plants growing in older, well-developed soils showed markedly lower levels of branch and plant mortality, especially in A. dumosa. We attributed these differences in plant response in part to variation in the distribution of large rocks within soil profiles, which might affect soil hydrological heterogeneity and influence the intensity of plant competition for water.