A new mechanistic theory of self-thinning: Adaptive behaviour of plants explains the shape and slope of self-thinning trajectories

The scaling exponent of the biomass-density relationship of even-aged plant populations – often described as the slope of the self-thinning line - and its presumed universality has been a subject of debate for a long time. Comprehensive observational studies, mainly in the last century, yielded even shifting slopes, for which, until now, the theoretical basis was not fully clarified.With a new mechanistic individual-based plant growth model, the BETTINA_ibm that considered allometric adaptation to resource supply, we identified two regimes of the self-thinning process:(i) The Geometrical thinning, which is driven by the ground area occupied by individual plants. For this, the slope is controlled by the allometric relations of the plant and thus roughly fitting the −3/2 power law. Age dependent processes impacting the allometry (e.g., secondary girth growth) result in a deviation from the original geometrical assumptions, and this may alter the slope accordingly. The intercept depends on species-specific allometric relations, site characteristics and the competition mode. (ii) The Maximum maintainable biomass per ground area, for which, if reached, the slope is −1. The intercept depends on resource supply (light and below-ground resources), as derived by the logarithm of the maximum total volume per area.The actual self-thinning line follows the minimum of both lines, and it is capped by the maximum individual plant size. Depending on the intercepts of (i) and (ii), the slope of the self-thinning line may be controlled by (i) geometrical thinning, (ii) resource limitation, or a switch between both.These two regimes and the shift from one to the other comply with experimental observations from the literature. Overall, morphological plasticity explains the variability of the slope of the self-thinning line when geometrical thinning is dominating.