Читать книгу Ecology - Michael Begon - Страница 145

If we now turn to the third type of study, listed in Section 5.9, we can note that although, again, combinations of density and mean weight for a particular species are being used to generate a relationship between them, 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). In such cases, it is more correct to speak not of a thinning line but 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).

dynamic thinning and boundary lines need not be the same

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. 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).

thinning slopes of −1

The influence of light, in particular, is worth considering in more detail, since it 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. But eventually this must stop: total biomass cannot increase indefinitely. Instead, the thinning line might be expected to change to a slope of −1, which would mean that loss through mortality is being exactly balanced by the growth of survivors, such that the total biomass remains constant. This can be seen when the populations of Lolium perenne discussed previously were grown at low light intensities (Figure 5.36b). A thinning line with a slope of −1 was apparent at much lower densities than it would otherwise be. This emphasises 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 in the next section.