Water uptake and movement in the clonal plants, Lycopodium annotinum L. and Diphasiastrum complanatum (L.) Holub

summary Lycopodium L, sensu stricto and Diphasiastrum complanatum (L.) Holub demonstrate a ‘guerilla’ growth strategy whereby the growth of the stolon apex is subsidized by resources from root systems several years old. In the dry sub‐arctic/sub‐alpine birch forests of Swedish Lapland where the soil cover is very heterogeneous and the annual precipitation is low, the subsidized water balance of the stolon apex is critical to the survival of the plant. Some aspects of water relations of L. annotinum L. and D. complanatum (L.) Holub were studied in the field. Water potentials were more negative towards the stolon apex, and water movement, traced using acid fuchsin dye, showed that the dye moved 14 times further from an isolated root system in a distal direction towards the stolon apex than in the proximal direction. A single root system initiated 8 years previously, and 65 cm from the stolon apex, was shown to be capable of supporting the growth of this apex. The excision of 3 and 4 year old root systems resulted in the greatest effect on depression of the water potential of the stolon apex, suggesting that these provided most of the water used by the stolon apex. The preferential allocation of water to the stolon apex was very effective, and the functional morphology of the plants seems to be related to harsh, patchy environments. However, when crossing large patches of inhospitable terrain, or under periods of drought, those stolon apices dependent on old, distant root systems for their water supply can experience lethal water stress. As the health of the stolon apex effects its degree of correlative inhibition of sub‐dominant stolons, the architecture and foraging ability of the clone is affected.