A method for studying vegetation dynamics when there are no obvious individuals: Virtual-population analysis applied to the tundra shrub Betula nana L.

A method is presented to study dynamics of plants that cannot be separated into individuals such as many grassland, salt marsh and tundra species. A virtual population is created by using a permanent transect line through the vegetation and individuals are defined as the branch segments distal to the intercept with the transect line. Addition and loss of individuals together with growth or shrinkage form the basis for constructing a size-structured transition matrix. A discrete-event simulation demonstrates that: 1) a virtual population of individuals grows at the same rate as the parent population; and, 2) size-structured transition matrices for a virtual population and parent vegetation have similar dominant and subdominant eigenvalues so a virtual population can be used to describe the dynamics of a parent vegetation.Dwarf birch, Betula nana L., was studied in the northern foothills of the Brooks Range, Alaska, by using photography to record branch intercepts along permanent transect lines. The distal branch segments constitute a virtual population of the parent vegetation. Transects were photographed in 1985 and again in 1986 and changes of branch segments were used to construct two transition matrices for shrubs with and without elevated fertilizer treatment. Analysis of the virtual populations suggests that although Betula nana may show increased branch growth with increased fertilizer, in the long run this shrub may decline in the tundra in response to such treatment.