The interaction of density and resource levels in monospecific stands of plants: a review of hypotheses and evidence

The series of asymptotic yield-density curves traversed by even-aged monospecific stands of plants at the same resource level is described by the function f(w_max y _max), where w_max= spaced plant weight and y_max= maximum yield possible at high densities. If the resource level at which the stands are grown is reduced, two sets of outcomes are possible. The stands at the decreased resource level may show essentially the same crowding effects as the control stands, but at a slower rate (altered-speed hypothesis). This hypothesis implies a common family of yield-density curves and a common thinning line for stands at different resource level may occur not only at a different rate at a harvest in the stands of higher growth. Alternatively, crowding effects at a reduced resource level may not only occur at a different rate to the controls but take a different course, leading to more intense crowding effects at a harvest being observed in the stands of lower growth (altered-form hypothesis). Thus a different family of yield-density curves and different thinning lines would apply at each resource level. For yield-density curves, the altered-speed hypothesis applied if f(w_max, y_max) is the same at each resource level. If this function differs with resource level, the altered-form hypothesis applies at a harvest provided the ratio y_max/w_max is less at the low resource level. Cases intermediate between these two hypotheses may occur. For thinning lines differences in the position of the non-thinning/self-thinning boundary and in f(w_max, y_max) can affect the intercept and slope of thinning lines. The theoretical magnitude of these differences are discussed. Examples of both hypotheses are reviewed. For the altered form cases differences in non-thinning crowding effects between resource levels are capable of causing significant differences in thinning lines.