Consequences of patchy distribution of light for the growth of the clonal herb Glechoma hederacea

The hypothesis that the clonal plant Glechoma hederacea can forage for light in environments with spatially patchy light supply was tested experimentally using artificial habitats in the glasshouse. There were five light treatments. Two treatments were homogeneous, with uniformly low or high light. Three treatments were patchy, each consisting of one high light patch and one low light patch. In the patchy treatments the position of the light patches differed with respect to the position at which the clone commenced growth. Several morphological responses were predicted if foraging for light through morphological plasticity took place: (1) there should be a greater concentration of ramets and biomass, a greater frequency of branching, greater leaf area, a lower proportional allocation of biomass to stolons and a higher proportional allocation of biomass to leaves in high light than in low light patches; (2) there should be increased petiole lengths and a higher proportional allocation of biomass to stolons and petioles in low light patches than in high light patches; (3) different primary stolons should assume different morphologies when exposed, simultaneously, to different light regimes. Structures directly involved in light acquisition in G. hederacea showed the greatest responses to changes in light conditions. In the patchy light treatments the frequency of branching, the leaf area of primary ramets and the proportional allocation to leaf biomass were significantly greater in high light patches and petiole length and proportional allocation to petiole biomass were significantly greater in low light patches. The greater leaf area, coupled with greater branching, allowed clones to harvest light energy more efficiently in hight light conditions and the increase in petiole length allowed clones to escape from poor light conditions. However, these foraging responses were shown only when clones grew from low light to high light patches or when different primary stolons were exposed simultaneously to either high or low light. Foraging responses were suppressed when clones grew from high light to low light patches. These results imply that responses to patchy light conditions are not clone-wide, but localised within individual stolons, and that individual stolons can adjust morphology rapidly only when high resource patches are encountered after low resource patches. The converse - adjustment to a low resource patch following a period in a high resource patch