The effects of spatial scale of environmental heterogeneity on the growth of a clonal plant: an experimental study with Glechoma hederacea

1) Habitat heterogeneity is manifested as patches differing in quality at a variety of spatial scales, durations or contrasts, but little is known about its effects on the capacity of plants to forage for resources and to grow. This paper investigates the effects of the spatial scale of heterogeneity upon growth of the clonal herb Glechoma hederacea. 2) Clones were grown in eight experimental environments, each containing the same total amount of two types of soil distributed in separate patches. Contrast between patch types was the same in all treatments. The number and size of patches differed between treatments, from two 25-cm × 50-cm patches to 64 6.25-cm × 6.25-cm patches. In six of the treatments roots could grow freely between patches. Partitions prevented root growth between patches in the remaining treatments. 3) Although all treatments provided the same quantity of nutrients, clone biomass was dependent on the scale of heterogeneity. Biomass was highest in the 25-cm × 25-cm patch-size treatment and declined significantly at smaller patch sizes. It varied by a factor of four when only the treatments allowing root growth between patches were compared, and by a factor of seven when the treatments preventing root growth between patches were included. 4) Clones displayed a scale-dependent capacity to locate roots selectively in nutrient-rich patches. Although the proportion of biomass allocated by clones to above-ground structures and roots did not differ significantly between treatments, a significantly greater proportion of the root biomass of clones was located in rich than in poor patches in the larger patch-size treatments, promoting more efficient foraging for nutrients in these treatments. As patch size decreased, the proportion of clone root biomass located in the two patch types became more equal. 5) Root: shoot ratio within clones responded to patch scale and quality. In the larger patch-size treatments, in which clones foraged more efficiently, parts of clones located in rich patches had a higher root:shoot ratio than parts of clones located in poor patches, thus enhancing nutrient acquisition from rich patches. However, G. hederacea behaved like plants with a single rooting point in the smaller patch-size treatments, in that root:shoot ratio increased when nutrients were scarce. 6) Thus, the foraging response of G. hederacea was coarse-grained in environments where patches were large. In comparison, G. hederacea apparently responded to environments with small-scale patchiness as if they were homogeneously poor. It could not adjust its morphology rapidly enough to respond to these less predictable environments where changes in patch quality were more frequent.