A model on optimal root–shoot allocation and water transport in clonal plants

Experimental studies have shown that interconnected ramets of clonal plants may exchange resources and adjust root–shoot allocation patterns when exposed to spatially heterogeneous habitats, thereby enhancing the efficiency of resource extraction from patchy environments. If ramets of a clonal plant are placed in conditions of high light and low water availability, and connected ramets are exposed to a low light and high water supply, they can reciprocally translocate water and assimilates. In addition, ramets in both patch types can adjust their root–shoot allocation in a way to increase the uptake of a locally abundant resource. Resource exchange and morphological specialization in the uptake of above- versus below-ground resources (analogous to a ‘spatial division of labour’ in economic systems) can considerably enhance the performance of clonal plants. A mathematical model is constructed to formalize the primary processes held responsible for the beneficial effects of a spatial division of labour in clonal plants. The model simulates water flow through a two-ramet system as function of environmental conditions, plant-related characteristics and finds the optimal root–shoot allocation for the two ramets. Optimality is defined as the total rate of photosynthesis of the two ramets. Simulations are run in spatially homogeneous and heterogeneous environments. The model predictions suggest that interconnected ramets exposed to complementary patch types can maximise their performance by a functional specialization (i.e. plastic changes in root–shoot allocation) in the uptake of a locally abundant resource and by internally redistributing captured resources. This is in qualitative agreement with the results of experimental studies. The model outcome is largely determined by plant tissue conductivities for water transport, i.e. by root conductivity for water uptake, leaf conductivity for water loss and by internode conductivity for water sharing between ramets. The results of this study suggest a crucial role for the conductivity of stolon internodes in determining the degree of specialization and co-operation in clonal plants.