Plant Assemblage Composition Explains and Predicts How Biodiversity Affects Salt Marsh Functioning

Knowing that diverse plantings enhanced biomass and nitrogen (N) accumulation in a restored California salt marsh, we asked if the “biodiversity effect” was due to species selection or complementarity. In a two-year greenhouse experiment, we found positive biodiversity effects on total, root, and shoot biomass, total and root N crop, and on biomass and N allocation; negative effects on root and shoot N concentration; and no effect on shoot N crop. Overyielding among trios and sextets was supported by significant deviations in observed yield from that expected relative to solo yields (DT). However, both trios and sextets strongly underyielded relative to the highest yielding solo in the assemblage (Dmax) in all attributes, and to the dominant species in the assemblage (Ddom) in most attributes. When we decomposed biodiversity effects on shoot characteristics, selection effects primarily drove over- and underyielding. The only complementarity effect was underyielding of sextet shoot biomass. These analyses were possible because we replicated assemblages and evaluated 11 response variables. One species (Salicornia virginica) dominated functioning when present; when absent, another dominated (e.g., Frankenia salina). Effects varied with the response tested, however. For both shoot biomass and N crop, S. virginica was the dominant overyielding species (based on Dij and comparisons of trios ± target species). For shoot N concentration, however, the dominant was Triglochin concinna, a species that had low biomass but was capable of reducing assemblage performance, presumably by concentrating N. Evidence for strong species selection effects led us to predict that three species would eventually dominate our parallel field experiment that tested the same assemblages. Exactly that happened in nine years, but (we predict) without losing function, because the site retained the three highest-performing species. Biodiversity loss was nonrandom in the field, and because trios with two top performers sustained critical functions in the greenhouse, we predict that many functions will not decline, even if the salt marsh becomes dominated by a single species, e.g., S. virginica. Unmeasured functions (e.g., resilience) might not persist, however. Knowing how assemblages perform made biodiversity–ecosystem function theory both explanatory and predictive

Species-Rich Plantings Increase Biomass and Nitrogen Accumulation in a Wetland Restoration Experiment

Our test of the hypothesis that biomass and nitrogen would increase with more species-rich plantings simultaneously vegetated a salt marsh restoration site and demonstrated that on average, randomly chosen, 6-species plantings accumulated more biomass and nitrogen than the mean for 0- and 1-species assemblages, with the mean for 3-species assemblages being intermediate. In addition, we found that individual species (from the pool of eight native halophytes) differed in their functional capacity, with Salicornia virginica (Sv) and Jaumea carnosa contributing the greatest biomass when planted alone, while Triglochin concinna had the highest tissue N concentrations. When planted alone, Sv accumulated comparable amounts of biomass and nitrogen as in the multispecies plots, indicating that individual species can have a large effect on particular functions. Soil TKN in the surface 0–5 cm was greater in 6-species plots than unplanted plots in 1999, while both 3- and 6-species plots were greater than unplanted plots in 2000; however, there were no differences at 5–20 cm depth and no species-specific effects. Root and shoot biomass both increased with species richness, with total biomass of 6-species plots averaging 995.6 ± 120.5 g/m2 in 2000, compared to the mean for 1-species plots (572.1 ± 90.3 g/m2) and unplanted plots (164.5 ± 24.7 g/m2). Still, at the age of three years, root biomass was only about one-third that of the species-rich reference site, and shoot biomass was one-half to one-fifth the maxima reported for reference salt marshes. Species-specific effects were found for Sv, which had high biomass of both roots and shoots in the multispecies plots (55% of aboveground biomass in 3-species plots and 41% in 6-species plots) and the highest pool of N (52% of the N pool in 3-species plots and 42% in 6-species plots), even though only one-eighth of the initial plantings were Sv. However, when plots with this species were excluded from the analysis, the species-richness effect persisted. Thus, ecosystem function, as measured by biomass and N accumulation, increased with species richness regardless of dominance by the highly productive Sv. We conclude that manipulating the richness and composition of plantings offers ecosystem restorationists an effective tool for accelerating the rate of functional development

Diversity-Function Relationships Changed in a Long-Term Restoration Experiment

The central tenet of biodiversity-ecosystem function (BEF) theory, that species richness increases function, could motivate restoration practitioners to incorporate a greater number of species into their projects. But it is not yet clear how well BEF theory predicts outcomes of restoration, because it has been developed through tests involving short-run and tightly controlled (e.g., weeded) experiments. Thus, we resampled our 1997 BEF experiment in a restored salt marsh to test for long-term effects of species richness (plantings with 1, 3, and 6 species per 2 x 2 m plot), with multiple ecosystem functions as response variables. Over 11 years, 1- and 6-species assemblages converged on intermediate richness (mean = 3.9 species/ 0.25-m2 plot), and composition changed nonrandomly throughout the site. While three species became rare, the two most productive species became co-dominant. The two dominants controlled and increased shoot biomass, which appeared to decrease species richness. Diversity-function relationships became less positive over 11 years and differed significantly with (a) the species-richness metric (planted vs. measured), and (b) the indicator of function (shoot biomass, height, and canopy layering). The loss of positive relationships between species richness and function in our restored site began soon after we stopped weeding and continued with increasing dominance by productive species. Where species-rich plantings are unlikely to ensure long-term restoration of functions, as in our salt marsh, we recommend dual efforts to establish (1) dominant species that provide high levels of target functions, and (2) subordinate species, which might provide additional functions under current or future conditions

Salt marsh restoration surprise: A subordinate species accumulates and shares nitrogen while outcompeting salt marsh dominants

Selectively planting native species could guide ecosystem development toward wetland restoration targets, once we understand how influential species function, alone and in combination. Knowing that Triglochin concinna (arrow grass, Juncaceae) accumulates N in its perennial roots, we asked how it would influence N dynamics on an excavated salt marsh plain at Tijuana Estuary, in southern California. We hypothesized that it would (a) accumulate N in roots and shoots, (b) reduce biomass of other marsh plain plants or, alternatively, (c) share N with neighbors as its litter decomposed and released N. We used 15N stable isotope enrichment to quantify N transfer between Triglochin and the marsh plain’s seven-species halophyte assemblage in field and greenhouse experiments. We also examined the effect of Triglochin on individual marsh plain species’ biomass and N accumulation. Triglochin had low shoot biomass (0.96 ± 0.5 g m−2 in field plots and 17.64 ± 2.2 g m−2 in greenhouse pots), high root:shoot ratios (4.3 in the field and 2.0 in the greenhouse), and high tissue N content (1.9 ± 0.2% in the field and 1.7 ± 0.1% in the greenhouse). Two productive perennials, Sarcocornia pacifica (pickleweed) and Frankenia salina (alkali heath), outgrew Triglochin; yet these biomass dominants produced 44%–45% less shoot biomass in greenhouse pots with Triglochin than without. However, we did not find this reduction in the field where roots were unconfined. In the greenhouse, δ15N values were higher for species grown with 15N-enriched Triglochin, indicating that this species made N available to its neighbors. The δ15N values for plants grown in the field exceeded background levels, also indicating that the marsh plain assemblage took up N released by Triglochin. We conclude that Triglochin can influence the restoration of salt marsh vegetation by accumulating N and releasing its tissue N to neighbors as leaves and roots decompose, while simultaneously reducing the biomass of neighbors. The seasonally deciduous Triglochin is low in shoot biomass, yet competitively superior in N uptake. Because this often-ignored species has limited tidal dispersal, we suggest restoration plantings, including tests of its ability to facilitate diversity where S. pacifica, the marsh plain dominant, might otherwise form monocultures

Ecological Restoration: Guidance from Theory

A review of the science and practice of ecosystem restoration led me to identify key ecological theories and concepts that are relevant to planning, implementing, and sustaining restoration efforts. From experience with actual restoration projects, I provide guidance for improving the restoration process. Despite an abundance of theory and guidance, restoration goals are not always achieved, and pathways toward targets are not highly predictable. This is understandable, since each restoration project has many constraints and unique challenges. To improve restoration progress, I advise that sites be designed as experiments to allow learning while doing. At least the larger projects can be restored in phases, each designed as experimental treatments to test alternative restoration approaches. Subsequent phases can then adopt one or more of the treatments that best achieved goals in earlier phases while applying new tests of other restoration measures. Both science and restoration can progress simultaneously. This phased, experimental approach (called “adaptive restoration”) is an effective tool for improving restoration when monitoring, assessment, interpretation and research are integrated into the process.</p

Ecological Restoration: Guidance from Theory

A review of the science and practice of ecosystem restoration led me to identify key ecological theories and concepts that are relevant to planning, implementing, and sustaining restoration efforts. From experience with actual restoration projects, I provide guidance for improving the restoration process. Despite an abundance of theory and guidance, restoration goals are not always achieved, and pathways toward targets are not highly predictable. This is understandable, since each restoration project has many constraints and unique challenges. To improve restoration progress, I advise that sites be designed as experiments to allow learning while doing. At least the larger projects can be restored in phases, each designed as experimental treatments to test alternative restoration approaches. Subsequent phases can then adopt one or more of the treatments that best achieved goals in earlier phases while applying new tests of other restoration measures. Both science and restoration can progress simultaneously. This phased, experimental approach (called “adaptive restoration”) is an effective tool for improving restoration when monitoring, assessment, interpretation and research are integrated into the process

Practical Handbook for Wetland Identification and Delineation by J. G. Lyon

Volume: 41Start Page: 241End Page: 24