Thresholds Database > Meadows: dominance of sagebrush vs. native vegetation in riparian meadows subjected to overgrazing

Background

Riparian vegetation communities along drainage channels in the Great Basin are largely determined by ground water level. At high water table, the dry meadow type, characterised by grasses, sedges and herbs, occurs, while at low water table, the vegetation is dominated by a mixture of the sagebrush Artemisia and the grass Leymus sp. The rather discreet boundaries between both types in undisturbed habitats suggests a threshold along water table depth, at which the transition occurs. When perturbed by overgrazing or other anthropogenic disturbances, sites previously occupied by either one of those vegetation types became pure Artemisia stands.

The example is based on a study aimed at quantifying the water table threshold at which transition occurs and the extent to which the pure Artemesia stands represent an alternative stable state to the unperturbed vegetation. The study also investigated restoration potential related to water table depth. Burning sagebrush and seeding of grasses and herbs was applied at sites of different water level.

Alternate Regimes

at low water table:
1) Artemesia/Leymus vegetation type
2) pure Artemesia stands

at high water table:
1) dry meadow type
2) pure Artemesia stands

Mechanism

The threshold of water table at which the dry meadow type is replaced by the Artemesia/Leymus type lays at ca 250 cm water table depth. At higher water table, Artemesia can not establish due to water-saturated soils at low depth.

The burning-and-seeding program sucessfully removed Artemesia and led to re-establishment of the unperturbed vegetation. The restoration was more successful at high water tables. The mechanism is not directly investigated, but the authors suggest competitive release of herbaceous vegetation by burning Artemesia plants may induce changes in soil properties such as soil temperature, water holding capacity, infiltration and nutrient content, among vegetation types and thus differential plant growth.

The temporal scale of the experiment was too short to assess the alternative stability of the contrasting states at different water table heights.

Management Decisions in Each Regime

At low water tables, restoration of the original vegetation type requires the more extensive burning-and-seeding program, while at high water table, just burning was sufficient for restoration.