Podocarp Roots, Mycorrhizas, and Nodules

Podocarps are often found on low-nutrient soils and further exacerbate nutrient limitation through the production of recalcitrant litter. Understanding podocarp ecology therefore depends on understanding the root adaptations of podocarps to obtain nutrients. The roots of podocarps are heterorhizic (having two root forms) with abundant spherical determinate short roots, a unique form of largely unknown function, as well as indeterminate fine roots. There is no strong evidence for nitrogen fixation in the nodules. Although nodules are frequently and abundantly colonized by arbuscular mycorrhizal fungi, they are neither required for mycorrhizal infection nor dependent on mycorrhizal infection for their development. The fungal symbiont community associated with podocarps does not appear to be distinctive from that of other plants at a global scale, although it may differ from co-occurring plants within a site. Perhaps the most obvious, yet largely overlooked, hypothesis is that the form of the nodules is the function. The presence of nodules permits a doubling of root cortex volume, while simple geometry indicates that a spherical form involves the lowest possible cost in producing cell wall and membrane material per unit of root volume. Increased root volume permits greater arbuscular mycorrhizal interaction, and hence, nodules may be analogous to the determinate ectomycorrhizal roots of the Pinaceae as a structure to maximize fungal interactions at minimal root construction cost.