Density-independent mortality, density compensation, gap formation, and self-thinning in plant populations

Abstract

The timing and relative contributions of mortality agents in plant populations depend on the ways in which density-dependent (DD) mortality compensates for density-independent (DI) mortality rate. This relationship, in turn, determines the degree of canopy closure, or its complement, gap area. A model is presented that integrates the effects of DD and DI mortality agents, and it permits exploration of the "density compensation" (DC) effect, or the degree to which mortality caused by DI factors is alleviated by reductions in DD mortality, and changing gap area through time. DI mortality factors can be additive, and they affect the entire population early and late in life. If DI motality is low, crowding results in a DD mortality rate that is rather constant when plants are growing rapidly, and DD mortality descreases as plants approach maximum size. DD mortality rate compensates for DI mortality rate in populations of plants that are young and at high density: Although increased DI mortality contributes to increasing gap area as plants mature, such an increase is not necessary for gap area to increase. Reduced density compensation with age (declining plant growth rate) results in increased gap area even if per-capita mortality rates descrease. Complete canopy coverage shifts form a stable to a neutrally stable equilibrium with advancing stand age. Net ecosystem production (NEP) is maximized earliest during stand development at somewhat less than complete canopy coverage. This maximum occurs later in stands having lower canopy caverage, because of low DD mortality rate, and in stands having higher canopy coverage, because of high density. Low canopy coverage early in stand development will be associated with delayed maximum NEP if that low canopy coverage is associated with low density and with early maximum NEP if low canopy coverage results instead from high DI mortality rate. The self-thinning rule arises as a special case of a more general relationship that demonstrates the effects of canopy and DI mortality on the percent change in density that attends a percent change in standing crop. Because of high DC early in stand development, the tendency of this slope coefficient to become more negative with declining canopy coverage prevails over the tendency of DI mortality to make this coefficient less negative. With declining DC late in stand development the situation is reserved. © 1992.