Twenty-Fourth Annual Meeting and Symposium of the Desert Tortoise Council, March 5-8, 1999
Abstracts

Restoration efforts have focused on returning structure by planting an appropriate species composition on a site. The assumption is that ecosystem functioning will subsequently be returned to that resembling the undisturbed habitats. Increasingly, however, anthropogenic activity changes not only community composition but also broad-scale ecosystem functioning through direct soil disturbance coupled with indirect human activities. Two perturbations, exotic species invasions and nitrogen deposition pose a serious problem for restoring native communities because they alter ecosystem functioning. The resulting pattern in the western Mojave is a shift in species from a shrub-dominated ecosystem to one dominated by exotic annual grasses and forbs. We postulate that this shift reflects a "third axis" of the teeter-totter model between grassland and shrubland communities.

Schlesinger and colleagues proposed that cattle grazing destroyed the grasses thereby resulting in nutrient loss. Under low nutrients, shrubs invaded forming "islands of fertility" and inhibited native perennial grasses from re-establishing. Currently, the composition of the west Mojave is predominantly shrubs that are widely dispersed. Many native interspace annuals are N fixers or N scavengers. Today's perturbations are different than those a century ago. Nitrogen deposition from automobiles and agriculture into the Mojave shrublands is increasing the available surface nutrients, particularly N. In addition, the increasing incidence of roads provides corridors for the invasion of exotic annuals that are largely nitrophilous.

Based on observations we know that these shifts change the saprobic and mycorrhizal composition. We hypothesize that manipulation of these below ground community components could facilitate restoration by manipulating ecosystem functioning. This will require new approaches. Some useful restoration approaches may include manipulating the carbon to nitrogen ratio with recalcitrant mulches and spatial manipulations of water and mycorrhizal fungal inoculum. Since most desert plants are adapted to low soil nitrogen, additions of recalcitrant mulches can be used to immobilize soil nitrogen, making it unavailable for weedy species while shifting competitive advantages toward natives. Competition between shrubs and exotics can be further minimized, by planting seedlings with deep (10 cm) inoculum and by using irrigation techniques such as deep pipe watering. Based on our survey data, native annual forbs, should be more successfully restored if few or no mycorrhizae are present at the soil surface. Ultimately, restoration techniques, like these, that take advantage of ecosystem functioning may be the only "smart bombs" we have in our restoration arsenal that will overcome disturbance under altered environmental conditions.