USE OF THEISSEN AREAS IN MODELS OF NUTRIENT-UPTAKE IN FORESTED ECOSYSTEMS

Nutrient uptake models have generally incorporated simplifying assumptions, one of which is that roots are regularly arrayed in space. This allowed an evaluation of an average root to be extrapolated to a whole plant by simply multiplying the single root result by the total root length. In low-fertility soils, particularly those with very low buffer powers, this regular root assumption causes nutrient uptake to be overestimated. Nutrient uptake models would be more realistic if Theissen areas were used as the area of influence of roots. A computer program for computing Theissen maps for a hydrological application has been adapted, tested, and applied to nutrient uptake by roots in a forested ecosystem. Nutrient uptake using Theissen areas is compared with uptake based on the assumption of regularly arrayed roots and with worst-cast situations of uptake influenced by nearest-neighbor roots. For slash pine (Pinus elliottii Engelm. var. elliottii) growing on a Spodosol of the lower Coastal Plain of the USA, root patterns of the A horizon have been found to be random, not regular. Computer simulations of nutrient uptake showed that the average root procedure was appropriate for K under the conditions investigated, but suggested that P uptake would be overestimated unless the spatial pattern of roots was included. Ratios of Theissen radii to 1/2 nearest neighbor distances show that root areas of influence tend to be noncircular or that roots are eccentrically positioned, either of which are violations of assumptions used in nutrient uptake models.