Successional Status

Pinon Woodland

The pinyon woodland is generally a climax vegetation type throughout its range, reaching climax about 300 years after disturbance, with an ongoing trend toward increased tree density and canopy cover and a decline in understory species over time. Woodlands may also expand into nearby grass and shrublands over time. The woodland type often occurs in a mosaic, with trees occupying the stonier soils where fires spread poorly and competition from shrubs and grass is minimal. Fire may have kept trees out of grasslands in the past, and it is debated whether the lack of fire is now allowing woodlands to invade true grasslands.

Causes of change

A variety of natural and anthropogenic processes can lead to changes in the spatial distribution of pinyon woodlands over time. Among these are 1) tree seedling establishment during favorable climatic periods, 2) tree mortality (especially seedlings and saplings) during periods of drought, 3) expansion of trees into adjacent grassland in response to overgrazing and/or fire suppression, and 4) removal of trees by humans, fire, or other disturbance episodes. Specific successional pathways after disturbance in singleleaf pinyon stands are dependent on a number of variables such as plant species present at the time of disturbance and their individual responses to disturbance, past management, type and size of disturbance, available seed sources in the soil or adjacent areas, and site and climatic conditions throughout the successional process.

After the woodlands

A general successional pattern in pinyon after overstory removal may be as follows: grasses and forbs dominate for the first 10 years; shrubs are well established within 20 years and dominate at 30 years; between 10 and 20 years, tree seedlings appear, their presence becoming important after about 50 years; site is again woodland with low understory cover after 70 to 80 years.

After a fire

A fire time study in the Great Basin followed this general pattern for grasses, forbs, and shrubs over time, although grass and shrub cover was maintained as late as 115 years following fire, and singleleaf pinyon had less than 10% cover 115 years after fire. After fire disturbance, several successional pathways from annual to shrub dominance are possible, including initial postfire dominance by shrubs.

Stages of woodlands

Successional stages in pinyon woodlands often have the same species present in different amounts and dominance on the landscape over time. Singleleaf pinyon may be present in early to mid-succession, but slow growth and establishment could prevent early dominance.

Variability in tree- and shrub-dominated communities complicates extrapolation of these results to sites of different growing conditions.

Establishment

Once trees establish, they continually increase their dominance of the site. Trees start competing with the understory when they are approximately double the size of the shrub nurse plant, and can exclude the understory within a 100-year period. The litter of singleleaf pinyon inhibits establishment of understory species through allelopathy. An understory may be maintained by periodic fire, reducing tree density.

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