A long‐term study of Pinus banksiana population dynamics

Abstract

Abstract. The long‐term population dynamics of a pure, naturally‐established stand of Pinus banksiana (jack pine) in southeastern Manitoba, Canada is described. The study was initiated at stand age 15, when all 468 trees in a plot were mapped and their trunk diameter at breast height (DBH) measured. The plot was remeasured eight times — every five years (six years in one case) — and mortality and DBH changes recorded. Total mortality over the 41‐yr study period was ca. 84 %. Mortality was initially very low, increased once the stand entered the self‐thinning stage from ages 25–46, and declined at ages 46–56. Mortality was restricted to the smallest size classes throughout. The stand reached the self‐thinning line at ca. age 30. The self‐thinning slope was significantly less steep than the theoretically expected value of −0.5.The distribution of DBH values was initially symmetric, showed increasing positive skewness during the period of highest mortality, and became symmetric again at later stages. Size (DBH) inequality was highest just prior to the onset of density‐dependent mortality, and subsequently declined. Tree DBH values were positively autocorrelated both initially and at later stages of stand development, but were spatially independent during the period of highest density‐dependent mortality. The stand initially had a strongly clumped pattern at all spatial scales. Patterns of mortality were non‐random during stand development, however, resulting in increased spatial regularity over time. Mortality was initially restricted to high density patches of the stand, but occurred throughout the plot once the self‐thinning line was reached. Mortality during the self‐thinning stage deviated from random expectation at local spatial scales (1–2m radius), suggesting that individuals were competing with their immediate neighbours. It is argued that an integrated approach, incorporating both population size and spatial structures, is essential in improving our understanding of long‐term plant population dynamics.