SALT MARSH CANOPY ARCHITECTURE DIFFERS WITH THE NUMBER AND COMPOSITION OF SPECIES

In response to the debate about how ecosystem functions are affected by species richness vs. species composition, we explored canopy architecture (plant cover, height, and layering) in three contexts: a reference marsh, a field experiment, and a greenhouse experiment. Species-rich canopies (five or more species) in the reference marsh were taller and had more (13 vs. 7) layers than monotypes of Salicornia virginica, although both had 100% cover. Cover and layering were greater for three- and six-species plots than for one-species plots in both the field experiment (70 2 × 2 m plots, eight herbaceous halophytes) and greenhouse (same treatments). Cover (80–95%) and heights were similar among field experimental treatments after two growing seasons (18 mo after planting seedlings in April 1997). In years 3–4, heights (∼0.5 m) and cover (∼100%) were similar to those of the 1998 reference marsh data, but only 1–3 canopy layers had developed. Species effects became evident when assemblages were replicated in the greenhouse. The eight one-species treatments differed in cover, height, and layering, and complementary traits produced more layers in multispecies canopies than monotypes. Within the three- and six-species canopies, attributes were similar for most assemblages, but there was a slight height effect with S. virginica, as in the field. In general, the effects of individual species were best distinguished by heights, and species-richness effects by layering. Of relevance to the larger diversity debate is our finding that the attribute examined influenced perceptions of the importance of species number vs. species composition. To apply our results, managers would assess height and layering, not just vegetation cover. Salt marsh restorationists would establish species-rich assemblages in newly graded sites to accelerate the development of canopy height and layering. Monitoring efforts would continue until canopy architecture matches that of reference systems (>4 yr for Californian marshes planted on coarse, nutrient-poor soils). Finally, research on wildlife habitat quality (e.g., nesting potential for the endangered Belding's Savannah Sparrow [Passerculus sandwichensis beldingi]) would determine critical levels of canopy layering and height for nesting, feeding, and perching to defend territories.

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