Morphology of the Structural Root System of Sitka Spruce 2. Computer Simulation of Rooting Patterns

In a previous paper (Henderson et al. 1983), it was suggestedthat the processes of root growth and development proceed witha geometric regularity and that, as a consequence, root distributionis further extended and spatially more even than if growth wereat random. We examine this hypothesis and the relative importanceof the component growth processes through computer simulationof a model for root distribution at a fixed time. Root segments were measured on 16 year trees and statisticaldistributions fitted to the occurrence of lengths, branchingfrequencies and growth directions. These distributions comprisethe model which assumes that a root system consists of a numberof first-order roots originating at the stem, a number of second-orderroots originating on first-order ones and so on. Each root includesa number of bends and lateral branching points and terminatesin either a fork or when diameter reaches 5 mm, smaller rootsnot being included. Parameter manipulation of the fitted distributions and furthersimulation showed that some regular growth mechanisms were necessaryfor the simulation of realistic rooting patterns. In particularit was important that direction changes at bends and of newroots at forks were typically small to ensure that the systemspread outwards, away from the stem. Lateral branches neededto subtend large angles to their parents in order to exploitseparate soil regions. Other necessary rules were that first-orderroots were almost regularly distributed around the stem andfor a tendency for azimuth changes at bends to be alternatelyclockwise then anticlockwise. Simulations were also used to examine the possibility of estimatingtotal root length from a study of only part of a root system.An example of excavating one quarter of the system is consideredand the results indicate that root systems may be so variablethat no reliable estimate can be obtained.