ALTERED ECOHYDROLOGIC RESPONSE DRIVES NATIVE SHRUB LOSS UNDER CONDITIONS OF ELEVATED NITROGEN DEPOSITION

Authors: WOOD, YVONNE - UC RIVERSIDE, CA; MEIXNER, THOMAS - UC RIVERSIDE, CA; Shouse, Peter; ALLEN, EDITH - UC RIVERSIDE, CA

Submitted to: Journal of Environmental Quality
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/25/2005
Publication Date: 1/3/2006
Citation: Wood, Y.A., Meixner, T., Shouse, P.J., Allen, E.B. 2006. Altered ecohydrologic response drives native shrub loss under conditions of elevated nitrogen deposition. Journal of Environmental Quality. Vol 35:76-92

Interpretive Summary: Native plants exert strong local control over the subsurface hydrology of arid and semi-arid landscapes and the loss of a dominant vegetation type can drastically alter soil water and nutrient dynamics. Many regions of Coastal Sage Scrub (CSS), one of the dominant native shrub communities of southern California, are rapidly declining across low elevation hillslopes, and being replaced by exotic annual grasses and forbs resulting from a suite of environmental pressures brought about from a burgeoning human population. Our results suggest a major shift in the hydrologic response of degraded shrub ecosystems as mature plants, and their root conduits, decrease in number. As a result, the top foot of the soil becomes the predominant zone for water movement, with increasing amounts of anthropogenic (photochemical smog) nitrogen being concentrated in the topsoil. Such a shift in the hydrologic response will increase the rate of Coastal Sage Shrub ecosystem loss as shrub seedling establishment rates decline and infiltration of rainwater is reduced. This will limit the growth of mature shrubs and favor early season exotic grasses due to increased water and nutrient-N concentrations in the surface soils.

Technical Abstract: Many regions of southern California’s Coastal Sage Scrub (CSS) are rapidly declining as exotic annual plants replace native shrubs. During this conversion, the subsurface hydrology of the semi-arid hillslopes that support CSS may be altered. This could chronically suppress the ability of native shrubland to re-vegetate the landscape since ecosystem processes of nutrient availability and of seedling establishment rely on spatial patterns of available soil water. In this work, soil water and nutrient-N regimes were compared over a two-year period between a southern California site where CSS has declined (approximately 5% shrub cover) with high additions of anthropogenic N, and one where CSS remains dominant (over 50% shrub cover) with predominantly background atmospheric additions of N. These two sites have similar climate, bedrock lithology, soils, and topography, and had the same vegetation type (Riversidian CCS) thirty years ago. We found that the depth and rate of rainwater percolation into wildland hillslope soils in response to early season storm events has been greatly reduced after loss of CSS shrubs and vegetation type conversion to invasive grassland. With decreased rainwater redistribution to soil depths of 100 to 150 cm, the predominant zone of soil water has become the upper 25-cm. This shift exacerbates vegetation type conversion by (1) concentrating smog-produced nitrogenous (N) chemicals in the uppermost soil, where they become readily available, along with high soil water, to shallow-rooted exotic grasses early in the growing season and (2) depriving adult and juvenile shrubs of deeper regolith water