Stem Dynamics of the Shrub Alnus Incana SSP. Rugosa: Transition Matrix Models

E-ISSN： 1939-9170|68|5|1234-1242

ISSN： 0012-9658

Source： Ecology, Vol.68, Iss.5, 1987-10, pp. : 1234-1242

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Abstract

We constructed transition matrices from observed rates of sprout production and stem growth in two New York state populations of a clonal shrub, Alnus incana ssp. rugosa. The largest real eigenvalue of the transition matrix, l, summarized current population rates of growth. An old—field alder thicket showed slow population decline over all 3 yr of study. The other population, in an undisturbed natural swamp, showed decline in each year but overall increase when data from all 3 yr were combined. Sensitivity matrices were calculated from the stable size distributions and reproductive value vectors for each 3 yr matrix. Sensitivity of population growth rates to specific transitions was illustrated by changing values of the matrix elements and calculating resultant 1 values. Population growth rates were least affected by changes in survivorship of small stems, and most affected by altering sprout production rates and large stem survivorship. Annual variations in demographic rates, and comparisons between actual stem size class distributions and the stable distributions predicted by the matrix models, confirmed the inadequacy of assuming constant transition probabilities. In such a case, transition matrix models are best viewed not as projections of long—term population behavior, but as indicators of current demographic trends.