The selection pressures experienced by clonal plants in heterogeneous environments may be significantly affected by physiological integration among ramets via rhizome connections. We experimentally examined how connections affected the response to saline soil conditions in Ambrosia psilostachya clones from natural saline basins in eastern Nebraska. Paired stems connected by uniform lengths of rhizome were grown in partitioned pots in 3 watering regimes: (1) both stems watered with tapwater, (2) both stems watered with salt water (1% NaCl), and (3) one stem watered with salt water and one with tapwater. All plants survived and grew in salt water, yet dry weight gain of salt-salt plants was only 34% of that for plants in uniform tapwater. Salt plants connected to tapwater plants had 2-fold higher dry weight gain than salt-salt plants. Their tapwater neighbors had significantly smaller biomass than pairs with both stems growing in tapwater. Measurements of leaf stomatal conductance, transpiration rate and water potential, together with root-shoot allocation patterns, suggest that rhizomes transported both water and photosynthate from tapwater plants to their neighbors in saline soil. These results indicate that ramets in a locally inferior environment can be helped by their neighbors, but at some cost to the contributing ramet. We discuss the consequences of this phenomenon for the evolution of local adaptation in populations of rhizomatous plants.

Rhizomatous growth may permit the nonrandom placement of ramets into different environments, but whether clonal plants are able to use this means to exercise adaptive habitat choice is not known. Western ragweed (Ambrosia psilostachya) plants are shown to preferentially colonize nonsaline soil over saline soil patches, and clones with the strongest preference for nonsaline soil are those least able to grow when restricted to saline conditions. In clonal plant species, nonrandom associations of genotypes with specific environments may thus reflect habitat selection by plants as well as selective mortality imposed by different habitat patches.

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