Salt Marsh Canopy Architecture Differs With the Number and Composition of Species

Abstract

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.

Diversity–function relationships changed in a long-term restoration experiment

“…It has a central tidal channel and two adjacent tidal plains, all excavated from disturbed upland in 1996 to connect a natural tidal channel to a tidal lagoon. The south tidal plain was planted to test BEF theory (Keer and Zedler 2002) as follows. In April of 1997, we established 87 2 3 2 m experimental plots with assemblages of 0, 1 (solos), 3 (trios), or 6 (sextets) native salt marsh plant species drawn from the eight native species that are common to the marsh plain of Tijuana Estuary and were known to cooccur at small spatial scales (often in 0.25-m 2 monitoring quadrats).…”

“…All seedlings were grown from Tijuana Estuary seed for five months in greenhouse flats before planting (Keer and Zedler 2002), and each 2 3 2 m plot was planted with exactly 90 seedlings to avoid selection effects arising from differential establishment (Huston 1997). Because our assemblages were randomly chosen, the total number or seedlings planted varied slightly from species to species (low ¼ 755, high ¼ 885; Fig.…”

“…During the first three growing seasons (1997)(1998)(1999)(2000), with weeding only through October 1998, species richness increased eight response variables (shoot biomass, root biomass, soil nitrogen accumulation, plant cover, mean canopy height, maximum canopy height, canopy layering, and resistance to invasion [Keer and Zedler 2002, Lindig-Cisneros and Zedler 2002, Callaway et al 2003). A parallel greenhouse experiment (Sullivan et al 2007) showed similar positive BEF relationships, and, based on eightfold replication of the 32 assemblages and eight monotypes planted in our field site, we attributed the positive BEF relationships to selection effects.…”

Accelerating the Restoration of Vegetation in a Southern California Salt Marsh

“…New techniques for reestablishing plants are needed for sites that impose severe stress on seedlings. Earlier experiments showed that species-rich plantings in salt marsh restorations can promote recruitment (Lindig-Cisneros and Zedler 2002), increase canopy complexity (Keer and Zedler 2002) and enhance rates of biomass and nitrogen accumulation (Callaway et al 2003); however, to promote diverse vegetation on severe sites, it might be necessary to manipulate the topography (Boeken and Shachak 1994;Whisenant 2002), plant in clusters (MacMahon 1997) and/or amend the soil to ameliorate stress on newly establishing vegetation (Handa and Jefferies 2000;Callaway 2001).…”

Seed Supply and Revegetation Dynamics at Restored Tidal Marshes, Napa River, California

“…Such species will likely require active introduction by restoration practitioners to achieve reference site levels of native species diversity. Achieving greater native plant diversity in restored marshes will conserve native biodiversity and increase wetland ecosystem services, such as habitat complexity and nitrogen accumulation (Zedler et al 2001;Keer & Zedler 2002;Callaway et al 2003;), making it an increasingly endorsed restoration evaluation metric (Zedler et al 2001;Peterson & Lipcius 2003).…”