Karen L. McKee

Oxygen Deficiency in Spartina alterniflora Roots: Metabolic Adaptation to Anoxia

The aerenchyma (air-space) tissue in the wetland macrophyte Spartina alterniflora conveys sufficient oxygen to roots for predominately aerobic respiration in moderately, but not highly, reduced substrates. Continuously flooded plants survive by respiring anaerobically, although growth is decreased. Two metabolic adaptations to flooding are displayed in this species, depending on the degree of soil reduction.

Nitrogen vs. phosphorus limitation across an ecotonal gradient in a mangrove forest

Mangrove forests are characterized by distinctive tree-height gradientsthat reflect complex spatial, within-stand differences in environmentalfactors,including nutrient dynamics, salinity, and tidal inundation, across narrowgradients. To determine patterns of nutrient limitation and the effects ofnutrient availability on plant growth and within-stand nutrient dynamics, weused a factorial experiment with three nutrient treatment levels (control, N,P)and three zones along a tree-height gradient (fringe, transition, dwarf) onoffshore islands in Belize. Transects were laid out perpendicular to theshoreline across a mangrove forest from a fringe stand along the seaward edge,through a stand of intermediate height, into a dwarf stand in the interior ofthe island. At three sites, three trees were fertilized per zone for 2yr. Although there was spatial variability in response, growth byR. mangle was generally nitrogen (N) -limited in thefringe zone;phosphorus (P) -limited in the dwarf zone; and, N- and/or P-limited in thetransition zone. Phosphorus-resorption efficiency decreased in all three zones,and N-resorption efficiency increased in the dwarf zone in response to Penrichment. The addition of N had no effect on either P or N resorptionefficiencies. Belowground decomposition was increased by P enrichment in allzones, whereas N enrichment had no effect. This study demonstrated thatessential nutrients are not uniformly distributed within mangrove ecosystems;that soil fertility can switch from conditions of N to P limitation acrossnarrow ecotonal gradients; and, that not all ecological processes respondsimilarly to, or are limited by, the same nutrient.

Seedling recruitment patterns in a Belizean mangrove forest: effects of establishment ability and physico-chemical factors

A field study was conducted to evaluate the relative importance of factors affecting seedling establishment and survival on a mangrove-dominated island in Belize. An examination of spatial patterns of seedling relative densities in relation to reproductive adults and physico-chemical conditions provided correlative information on factors affecting mangrove regeneration patterns. Distance from reproductive adults explained 89–94% of the variation in relative density of Rhizophora mangle seedlings, whereas availability of resources (light and NH4) explained 73–80% of variation in Avicennia germinans seedling relative density. Just after dispersal (December), 89% of the variation in Laguncularia racemosa seedling relative density was attributable to distance from reproductive adults, but 7 months later (July) 74% of the variation was explained by intensity of flooding- and salinity-related stresses. Survivorship (after 2.5 years) of propagules and seedlings of R. mangle and A. germinans transplanted to zones of contrasting physico-chemical conditions demonstrated that: (1) mortality was highest during the establishment phase and major causes were failure to strand before viability was lost, consumption by predators and desiccation; and (2) after establishment, differences in sensitivity to physicochemical stress factors such as flooding (A. germinans) and initial orientation of the seedling axis (R. mangle) exerted a further influence on seedling survival. The results indicate that seedling recruitment in these neotropical forests is strongly influenced by dispersal patterns, differential establishment abilities and effects of physico-chemical factors that vary with elevation and distance from the shoreline.

The relationship of smooth cordgrass (Spartina alterniflora) to tidal datums: A review

An analysis of data relatingSpartina alterniflora Loisel. to tidal elevations along the Atlantic and Gulf coasts demonstrated that although this species is primarily confined to the intertidal zone, its elevational limits. of occurrence do not correspond to a consistent elevation relative to a tidal datum in all marsh locations. The variation in the vertical distribution of this species reported among marsh studies was attributed primarily to differences in mean tide range (MTR). A positive correlation between MTR and elevational growth range (r=0.91) demonstrated that theSpartina alterniflora zone expands with increasing tidal amplitude. Differences in MTR among marsh locations accounted for 70 and 68% of the statistical variation in the upper and lower limits, respectively, ofS. alterniflora growth. Among marshes of similar tidal amplitudes, the upper limit of occurrence ofS. alterniflora in northern marshes was significantly lower than that in marshes at lower latitudes. These results, in combination with regional differences in plant species distribution across the upper intertidal zone, suggested that some of the variation in the upper limit was due to latitudinal differences in growth conditions and/or differences in interspecific competition. Local and regional differences in other factors such as salinity, nutrients, or physical disturbance may have also contributed to the variation in the limits of growth relative to a tidal plane within and among marshes.

Response of a freshwater marsh plant community to increased salinity and increased water level

Abstract Field and greenhouse experiments in which salinity and elevation were manipulated demonstrated that the response of freshwater marshes to saltwater intrusion may be variable and dependent upon a number of factors including: species composition; level, duration, and abruptness of exposure to saline water; flooding depth; a source of propagules of more salt-tolerant species. In the field, saltwater intrusion was simulated by transplanting swards of a freshwater marsh to a higher salinity area. The three dominant species, Panicum hemitomon Schultes, Leersia oryzoides (L.) Swartz, and Sagittaria lancifolia L., succumbed to the sudden increase in salinity to 15%. However, the denuded plots were rapidly invaded by more salt-tolerant species that were present in the higher salinity marsh. Although growth was reduced, Panicum hemitomon and Leersia oryzoides were relatively tolerant of salinities up to 9.4% for at least 1 month in the greenhouse. Sagittaria lancifolia was less tolerant of salinity increases and showed symptoms of tissue damage at 4.8%. A decrease in elevation, which resulted in an increase in mean water level and a more reduced soil environment, caused a significant decrease in live above-ground biomass and stem density in the experimental field plots.

EFFECTS OF NUTRIENT ENRICHMENT ON WITHIN-STAND CYCLING IN A MANGROVE FOREST

Within-stand nutrient cycling is dependent on many factors, including pri- mary productivity, nutrient-use efficiency, nutrient resorption, sclerophylly, decomposition, nutritional quality of plant tissue, and allocation to defense. The efficiency of these plant- mediated processes depends on nutrient availability in the environment and inherent func- tional properties of plants. However, little is known about how nutrient availability will affect these processes in forested wetlands in the tropics. In a factorial experiment we fertilized 48 dwarfed Rhizophora mangle (red mangrove) trees along tidal-elevation and water-depth gradients at Twin Cays, a range of intertidal, peat-based offshore mangrove islands in Belize, Central America. Initial results indicated that phosphorus (P) deficiency is a major factor limiting primary productivity. Phosphorus-fertilized trees had a significant decrease in P-use efficiency and P-resorption efficiency, but a significant increase in nitrogen (N)-use efficiency and N-resorption efficiency in their leaves compared with controls and N-fertilized trees. Sclerophylly decreased dramatically in P-fertilized trees, while the nu- tritional quality of the plant tissue increased. Phosphorus fertilization did not affect P leaching from green leaves. We found no fertilizer effect on the decomposition rates of leaf tissue, possibly due to higher phenolic concentrations in the P-fertilized trees compared with controls and N-fertilized trees. However, belowground decomposition of cotton strips increased in the substrate associated with P-fertilized trees. Environmental conditions re- lated to position along a tidal gradient may be as important as nutrients in controlling belowground decomposition.

Mangrove Species Distribution and Propagule Predation in Belize: An Exception to the Dominance-Predation Hypothesis1

Predation on mangrove propagules was studied to assess its impact on the intertidal distribution of Rhizophora mangle, Avicennia germinans, and Laguncularia racemosa. Field experiments conducted on a mangrove island range (Twin Cays) in Belize showed that rates of predation (percent of propagules rendered nonviable in 9 days) just after peak dispersal (December) were highest for A. germinans (60 ? 8%) and significantly lower for R. mangle (18 ? 5%) and L. racemosa (28 ? 9%), which were not significantly different. Rates varied seasonally, however, with highest losses of propagules at the beginning of dispersal (July). Major predators were the mangrove crabs, Goniopsis cruentata and Ucides cordatus. The high predation rate on A. germinans propagules was attributed to a relatively small size (facilitating burial in crab burrows), a higher nutritive content, and lower amounts of defensive chemicals such as tannins. Interspecific differences in predation rates, however, could not explain the relative dominance and spatial distribution of two of the three species that comprise this intertidal community. Comparison of predation rates in forest locations dominated by either R. mangle or A. germinans demonstrated that consumption of R. mangle propagules was highest in areas where it dominated the canopy and lowest where A. germinans was dominant. Predation also could not account for the infrequent occurrence of L. racemosa trees at Twin Cays, since rates of propagule consumption were not different from that of R. mangle and significantly less than that of A. germinans. High densities of L. racemosa seedlings further suggested that predators were not preventing this species establishment here. Only the predation pattern for A. germinans was consistent with intertidal dominance of trees. The results indicate that differential predation plays a less important role in the zonation of these island mangrove forests than has been reported for other geographic regions.

Fate of oxygen losses from Typha domingensis (Typhaceae) and Cladium jamaicense (Cyperaceae) and consequences for root metabolism

The objective of this work was to determine whether radial oxygen loss (ROL) from roots of Typha domingensis and Cladium jamaicense creates an internal oxygen deficiency or, conversely, indicates adequate internal aeration and leakage of excess oxygen to the rhizosphere. Methylene blue in agar was used to visualize the pattern of ROL from roots, and oxidation of a titanium-citrate solution was used to quantify rates of oxygen leakage. Typhas roots had a higher porosity than Cladiums and responded to flooding treatment by increasing cortical air space, particularly near the root tips. A greater oxygen release, which occurred along the subapical root axis, and an increase in rhizosphere redox potential (E(h)) over time were associated with the well-developed aerenchyma system in Typha. Typha roots, regardless of oxygen release pattern, showed low or undetectable alcohol dehydrogenase (ADH) activity or ethanol concentrations, indicating that ROL did not cause internal deficiencies. Cladium roots also released oxygen, but this loss primarily occurred at the root tips and was accompanied by increased root ADH activity and ethanol concentrations. These results support the hypothesis that oxygen release by Cladium is accompanied by internal deficiencies of oxygen sufficient to stimulate alcoholic fermentation and helps explain Cladiums lesser flood tolerance in comparison with Typha.

Relative growth ofSpartina patens (Ait.) Muhl. andScirpus olneyi gray occurring in a mixed stand as affected by salinity and flooding depth

Mixed stands ofSpartina patens andScirpus olneyi occur in brackish marshes along the Gulf Coast of Louisiana.Scirpus olneyi is considered to be an important wildlife food, and marshes are often managed to favor its dominance overS. patens. Two environmental factors that affect growth of the two species are salinity and water regime. The objectives of this study were to determine the effects of salinity and water depth, under controlled greenhouse conditions, on relative dominance of the two species, chemical properties of soil interstitial water, and nutrient concentrations in the plant tissue. Treatments imposed in a factorial design were salinities of 0, 5, 10, 15 and 20 ppt and water depths of −10, +10, and +30 cm relative to the soil surface. Results indicated that salinity treatments above 10 ppt reduced growth of both species, butS. olneyi was more drastically affected thanS. patens. Increased flooding depth reduced growth ofS. patens but had little effect onS. olneyi. Concentrations of inorganic ions (Na+, K+, Mg+2, Ca+2, Cl−1) in plant tissue were greater inS. olneyi thanS. patens, indicating that ion accumulation may be the principal salt tolerance mechanism ofS. olneyi. Extrapolated to field conditions, these results indicate that increasing salinity favors productivity ofS. patens relative toS. olneyi, while increased depth of flooding favorsS. olneyi.

Recovery of freshwater marsh vegetation after a saltwater intrusion event

Greenhouse mesocosms of freshwater marsh vegetation were exposed to a simulated saltwater intrusion event followed by a recovery period during which water levels and interstitial water salinity were adjusted over a range of conditions. Virtually all above-ground vegetation, including the three dominant species, Sagittaria lancifolia L., Leersia oryzoides (L.) Swartz, and Panicum hemitomon Schultes, was killed by the initial saltwater intrusion event. P. hemitomon did not recover, but S. lancifolia and L. oryzoides, as well as many of the other species initially present, exhibited some ability to recover depending on post-saltwater intrusion conditions. Increasingly harsh recovery conditions (for freshwater marsh vegetation), including more reduced soil conditions, higher interstitial salinities, and higher interstitial sulfide concentrations were associated with decreased live above-ground biomass and species richness. The effect of elevated salinity on vegetative recovery became more pronounced under flooded conditions. This experiment illustrates that the response of a freshwater marsh community to the long-term disturbance effect of a transient saltwater intrusion event will be strongly influenced by post-intrusion salinity and water levels.