In fact, in many cases where biomass-density relationships have been documented, it is not a single cohort that has been followed over time, but a series of crowded populations at different densities (and possibly different ages) that have been compared. In such cases, it is more correct to speak of a species boundary line - a line beyond which combinations of density and mean weight appear not to be possible for that species (Weller, 1990). Indeed, since what is possible for a species will vary with the environment in which it is living, the species boundary line will itself subsume a whole series of population boundary lines, each of which defines the limits of a particular population of that species in a particular environment (Sackville Hamilton et al., 1995).
Thus, a self-thinning population should approach and then track its population boundary line, which, as a trajectory, we would call its dynamic thinning line - but this need not also be its species boundary line. For example, the light regime, soil fertility, spatial arrangement of seedlings, and no doubt other factors may all alter the boundary line (and hence the dynamic thinning line) for a particular population (Weller, 1990; Sackville Hamilton et al., 1995). Soil fertility, for example, has been found in different studies to alter the slope of the thinning line, the intercept, neither, or both (Morris, 2002).
The influence of light is also worth considering in more detail, since it thinning slopes of -1 highlights a key feature of thinning and boundary lines. A slope of roughly -3/2 means that mean plant weight is increasing faster than density is decreasing, and hence that total biomass is increasing (a slope of -1/2 on a total biomass-density graph). But eventually this must stop: total biomass cannot increase indefinitely. Instead, the thinning line might be expected to change to a slope of -1: that is, loss through mortality is exactly balanced by the growth of survivors, such that the total biomass remains constant (a horizontal line on a total biomass-density graph). This can be seen when populations of Lolium perenne (Figure 5.31b) were grown at low light intensities. A boundary (and thinning line) with a slope of—1 was apparent dynamic thinning and boundary lines need not be the same at much lower densities than it would otherwise be. Clearly, the light regime can alter the population boundary line. This also emphasizes, however, that boundary lines with negative slopes steeper than —1 (whether or not they are exactly -3/2) imply limits to the allowable combinations of plant densities and mean weights that set in before the maximum biomass from an area of land has been reached. Possible reasons are discussed below.
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