Vegetation Organization and Dynamics of Lichen Woodland Communities in the Northwest Territories, Canada

The variances of species abundances from 141 upland stands are partitioned into habitat and fire frequency. Principal components analysis is then performed on each of these partitions. The habitat ordination has a topographic—canopy coverage gradient and a nutrient gradient. The fire frequency ordination has one gradient which orders species according to their temporal response after fire, from shorter lived, faster growing, competitively poor species to longer lived, slower growing, competitively effective species. The fire frequency ordination interacts with the habitat ordination by changing a site's canopy cover for approximately 10 yr after fire. The nutrient gradient is only slightly affected by the fire frequency. The recovery of vegetation after fire is explained by using the information on the adaptation of species as shown in the two ordinations and from existing life—history information. Most species found in older stands are present in the first years after fire. Recovery by vascular plants, mosses and lichens is by vegetative reproduction and invasion by propagules. Buried viable seeds play little role in recovery. Lichen abundance is best explained by different habitat requirements rather than successional sequences or caribou grazing. Feather mosses are most abundant in specific sites which develop closed canopies and have greater soil nutrients. The habitat and fire frequency ordinations represent two environmental complexes for which species are adapted, and consequently these are also the two predominant levels of vegetation dynamics. The fire frequency ordination represents shorter term dynamics which cause changes primarily in abundance but not species composition. The habitat ordination represents longer term dynamics which cause major changes in species composition.