International audienc

The behaviour of the FracRoot model, which describes coarse-root architecture based on proximal root diameters using a recursive algorithm, was studied using field data on Populus deltoides Bartr. ex Marsh. in a short-rotation plantation. Roots were described as a branching, hierarchical network of connected links. Diameters of daughter links were estimated from the diameter of the mother link using scaling parameter p and allocation parameter q, which were based on sharing of the root cross-sectional area by daughter links. Parameters were estimated from complete root excavations. The length of each link and the vertical and horizontal branching angle distributions were recorded. Parameter p values were distributed log-normally, and q values followed a beta distribution both for the whole root system and within 5 mm link diameter classes. Including the variability of p and q in the model did not significantly improve root length estimates compared with the use of mean p and q values over all branching points. Including a coarse-root turnover factor based on field-observed evidence on root mortality improved the model fit to field data. Root length was more sensitive to parameter values and turnover factor than root mass. Field observations and the importance of the turnover factor to simulation accuracy suggest that coarse-root turnover should be considered in root research, at least under conditions of strong competition or other external stress.

The functional-structural tree growth model LIGNUM was developed as a general research tool that can be applied to several tree species. The growth simulation of short-rotation eastern cottonwood (Populus deltoides Bartr. ex Marsh.) inherits the basic LIGNUM modeling concepts including modular tree structure, L-system-based description of structural development, and carbon budget. New developments of LIGNUM model in this study were the incorporation of a biochemically-derived photosynthesis submodel; nested time steps for simulating physiological processes, structural development, and annual biomass production; incorporation of field-measured weather data for modeling the response of physiological processes to environmental variation; and application of a Monte-Carlo voxel space submodel for simulating the stochasticity of tree growth and improving computational efficiency. A specific parameter system was applied for modeling P. deltoides growth in the central Missouri, USA, environment. This adaptation of LIGNUM was applied on modeling growth of P. deltoides in a short-rotation agroforestry practice. The simulated height and biomass growth were close to field observations. Visualization of simulation results closely resembled the trees growing in an open site. The simulated response of tree growth to variations in photon flux input was reasonable. The LIGNUM model may be used as a complement to field studies on P. deltoides in short-rotation forestry and agroforestry.