Dennis F. Whigham

Restoration of the Mississippi Delta: Lessons from Hurricanes Katrina and Rita

Hurricanes Katrina and Rita showed the vulnerability of coastal communities and how human activities that caused deterioration of the Mississippi Deltaic Plain (MDP) exacerbated this vulnerability. The MDP formed by dynamic interactions between river and coast at various temporal and spatial scales, and human activity has reduced these interactions at all scales. Restoration efforts aim to re-establish this dynamic interaction, with emphasis on reconnecting the river to the deltaic plain. Science must guide MDP restoration, which will provide insights into delta restoration elsewhere and generally into coasts facing climate change in times of resource scarcity.

PLANT-MEDIATED CONTROLS ON NUTRIENT CYCLING IN TEMPERATE FENS AND BOGS

This paper reports on patterns in plant-mediated processes that determine the rate of nutrient cycling in temperate fens and bogs. We linked leaf-level nutrient dynamics with leaf-litter decomposition and explored how the observed patterns were reflected in nutrient cycling at the ecosystem level. Comparisons were made among growth forms (evergreen and deciduous shrubs and trees, graminoids and Sphagnum mosses) and between mire types (fens and bogs). A literature review showed that the predominant growth form was more important as a determinant of leaf-level nutrient-use efficiency (NUE) than mire type (fen vs. bog). Evergreens had the highest N and P use efficiency. The growth form differences in NUE were mainly determined by differences in N and P concentrations in mature leaves and not by differences in resorption efficiency from senescing leaves. Sphagnum leaves had lower N and P concentrations than the other growth forms, but because of a lack of data on nutrient resorption efficiency the NUE of thes...

Effects of forest fragmentation on breeding bird communities in Maryland, USA

Abstract Point surveys were used to estimate the abundance and diversity of forest birds in relation to the size, degree of isolation, floristics, physiognomy, and successional maturity of 270 upland forest patches in the coastal plain province of Maryland. Physiognomic and floristic characteristics of the tree, shrub, and herb layers of the forest were measured at each site. The local abundance of almost every bird species breeding in the interior of upland forests was found to be significantly influenced by forest area, isolation, structure, of floristics, or combinations of these factors. Highly migratory species tended to be most abundant in extensive stands of mature, floristically diverse forests that were only slightly isolated from sources of potential colonists. Densities of permanent residents and short-distance migrants tended to be less affected by these site characteristics, or showed responses opposite in sign to those of long-distance migrants. The impacts of forest fragmentation on bird populations are complex and species-specific. Many bird species respond strongly to factors other than, or in addition to, forest patch area and isolation. Dissection of the landscape into small highly isolated patches of forest adversely affects some bird species, but structural and floristic characteristics of the forest are more important than patch size and isolation for many species, given the existing distribution of forest patches in the coastal plain of Maryland.

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.

The impact of Hurricane Gilbert on trees, litterfall, and woody debris in a dry tropical forest in the northeastern Yucatan Peninsula

Hurricane Gilbert struck the northeastern portion of the Yucatan Peninsula in an area where we have been conducting studies of the vegetation and avifauna in a dry tropical forest since 1984. All trees in our study area were completely defoliated and most suffered heavy structural damage. Although few trees were killed outright, many died over the next 17 months, especially those that had been heavily damaged. Tree recovery was rapid as relative diameter growth for most species for the first year after the hurricane were greater than average diameter growth rates for three of the five prehurricane years. Biomass of litterfall (leaves and wood less than 10 cm in diameter) and nutrients generated by the hurricane exceeded the totals produced during any of the five previous years. The hurricane increased the mass and nutrients in coarse woody debris (wood greater than 10 cm in diameter) by approximately 50 percent. Mortality caused by fire was much greater than mortality caused by the hurricane.

Growth and senescence in plant communities exposed to elevated CO2 concentrations on an estuarine marsh

SummaryThree high marsh communities on the Chesapeake Bay were exposed to a doubling in ambient CO2 concentration for one growing season. Open-top chambers were used to raise CO2 concentrations ca. 340 ppm above ambient over monospecific communities of Scirpus olneyi (C3) and Spartina patens (C4), and a mixed community of S. olneyi, S. patens, and Distichlis spicata (C4). Plant growth and senescence were monitored by serial, nondestructive censuses. Elevated CO2 resulted in increased shoot densities and delayed sensecence in the C3 species. This resulted in an increase in primary productivity in S. olneyi growing in both the pure and mixed communities. There was no effect of CO2 on growth in the C4 species. These results demonstrate that elevated atmospheric CO2 can cause increased aboveground production in a mature, unmanaged ecosystem.

Nitrogen and carbon dynamics in C3 and C4 estuarine marsh plants grown under elevated CO2 in situ

SummaryCarbon dioxide concentrations were elevated in three estuarine communities for an entire growing season. Open top chambers were used to raise CO2 concentrations ca. 336 ppm above ambient in monospecific communities of Scirpus olneyi (C3) and Spartina patens (C4), and a mixed community of S. olneyi, S. patens and Distichlis spicata (C4). Nitrogen and carbon concentration (% wt) of aboveground tissue was followed throughout growth and senescence. Green shoot %N was reduced and %C was unchanged under elevated CO2 in S. olneyi. This resulted in a 20%–40% increase in tissue C/N ratio. There was no effect of CO2 on either C4 species. Maximum aboveground N (g/m2) was unchanged in S. olneyi, indicating that increased productivity under elevated CO2 was dependent on reallocation of stored N. There was no change in the N recovery efficiency of S. olneyi in pure stand and a decrease in the mixed community. Litter C/N ratio was not affected by elevated CO2 suggesting that decomposition and N mineralization rates will also remain unchanged. Continued growth responses to elevated CO2 could, however, be limited by the ability of S. olneyi to increase the total aboveground N pool.

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.

The Ecology of Freshwater Tidal Wetlands

ties of less than one percent, but insufficient flow to dampen upstream tidal movement. Odum et al. (1979) conservatively estimate that there are 500,0001,000,000 ha of freshwater tidal wetlands along the Atlantic and Gulf Coasts, of which 100,000-140,000 ha are in New Jersey. Almost all of the major East Coast cities from Trenton, NJ, to Richmond, VA, are near freshwater tidal wetlands. Consequently, these wetlands are greatly affected by human activities. Our discussion of the structure, function, and value of freshwater tidal wetlands is based on studies of three Delaware River wetlands: the Hamilton Marsh near Trenton, NJ, Woodbury Creek Marsh south of Camden, NJ, and Tinicum Marsh near Philadelphia, PA. In freshwater tidal wetlands the major system components-producers, consumers, detritus, sediment, and nutrients-are coupled by biological and physical processes that transfer materials and energy (Figure 1). Materials, such as organic matter, nutrients, heavy metals, and sediment, enter freshwater tidal wetlands from sources including the atmosphere, tides, point-source effluents, non-point-source runoff, groundwater, and consumer immigration. Outputs are via the atmosphere, tides, and consumer emigration. Along the urbanized upper Delaware River estuary, tidal waters provide the most important inputs, although point-source effluent and non-point-source runoff may locally contribute significant quantities of nutrients and heavy metals (Walton and Patrick 1973). Wetland function is ultimately controlled by climate, but hydrologic parameters such as duration and frequency of inundation, and the velocity and source of the water determine the physical and chemical properties of wetland substrates (Gosselink and Turner 1978). In turn, substrate characteristics dictate specific ecosystem responses, including primary production, species diversity, decomposition, and uptake and release of nutrients.

Elevated Atmospheric CO2 Effects on Belowground Processes in C3 and C4 Estuarine Marsh Communities

Belowgound carbon allocation is a major component of a plants carbon budget, yet relatively little is known about the response of roots to elevated atmospheric CO{sub 2}. We have exposed three brackish marsh communities dominated by perennial macrophytes to twice ambient CO{sub 2} concentrations for two full growing seasons using open top chambers. One community was dominated by the C{sub 3} sedge Scirpus olneyi, one was dominated by the C{sub 4} grass Spartina patens, and one was a mixture of S. olneyi, S. patens, and Distichlis spicata, a C{sub 4} grass. Root and rhizome growth were studied in the 2nd yr of exposure by measuring growth into peat cores previously excavated and refilled with sphagnum peat devoid of roots. Growth under elevated CO{sub 2} resulted in an 83% increase in root dry mass per core in the Scirpus community. Those roots were also significantly lower in percentage of nitrogen than roots from ambient-grown plants. There was no effect of elevated CO{sub 2} on root growth or nitrogen content in the Spartina community or in the C{sub 4} component of the Mixed community.