John E. Titus

Carbon Balance for Two Sphagnum Mosses: Water Balance Resolves a Physiological Paradox

Sphagnum fallax is generally found closer to the water table than S. nemoreum along the hummock-hollow gradient of an Adirondack bog. However, at low water contents S. fallax un- expectedly fixes carbon at higher rates than S. nemoreum, as we have previously reported. We here compare water and carbon balances for these species to resolve this puzzling contrast. S. nemoreum maintains a more favorable water balance by virtue of its greater water-holding capacity, lower rates of water loss from moist plants, and apparently more effective capillary conduction of water. The consequence is a significantly higher tissue water content in the field compared to adjacent patches of S. fallax for a wide range of dates and heights above the water table. We developed a simulation model, PEATMOSS, to evaluate the relative importance of contrasting physiological relationships and water balances of the two species. The model integrates physiological and field data to estimate 24-h carbon balance for these mosses in adjacent patches at two heights above the water table under different conditions. PEATMOSS estimated a greater carbon gain for S. fallax at the low site, where it is a clear dominant over S. nemoreum, and a markedly more favorable carbon balance for S. nemoreum at the hummock site, where S. nemoreum is common but S. fallax is infrequent. Morphological features of S. nemoreum which effect its more favorable water balance apparently confer an advantage over S. fallax when water availability is limited.

Contrasting Water Relations of Photosynthesis for Two Sphagnum Mosses

Sphagnum fallax and S. nemoreum differ in their vertical distributions above the water table on hummocks in Bloomingdale Bog (Adirondack Mountains, New York), and both exhibit decreasing water content with increasing height above the water table. We tested the prediction that S. netnoreum, a species better developed on hummock tops, would photosynthesize at greater rates when relatively dry than would S. fallax, which is more abundant in hollows and on hummock bases. Net photosynthesis declined sharply at tissue water contents (fresh-/dry-mass ratio) below 7.2 for S. fi/lax and 8.9 for S. nemoreum collected in spring. These values fell to 5.1 and 7.4, respectively, for plants collected in late summer. Thus both species exhibit adaptive seasonal acclimation, as reflected by the maintenance of relatively high net photosynthetic rates to lower water contents after a dry summer. Neither species showed significant response differences when collected from different heights above the water table. In direct contradiction to our hypothesis, Sphagnuin falax fixed carbon at greater rates than S. nemoreuln at low water contents for both spring and summer collections. This physiological contrast appears inconsistent with observed field distribution patterns for these species.

DIFFERENTIAL NITROGEN AND PHOSPHORUS RETENTION BY FIVE WETLAND PLANT SPECIES

Riparian wetlands have a demonstrated ability to filter and control nitrogen (N) and phosphorus (P) movement into streams and other bodies of water; few studies, however, have examined the roles that individual plant species serve in sequestering N and P pollutants. We evaluated the potential for growth and consequent N and P accumulation by five species of wetland perennials. We planted blocks consisting of 900-cm2 plots of each species at 11 sites within a riparian wetland that receives large inputs of agricultural runoff. Plant shoots and roots were collected at the time of peak standing crop to determine net accumulation of biomass, N, and P for one growing season. A portion of the plant shoots was placed in decomposition litterbags in the field to determine biomass, N, and P losses for 60, 120, and 150 days. Of the five species, but reed (Sparganium americanum) had the greatest aboveground accumulation of N and P but had the lowest belowground accumulation values. In contrast, woolgrass (Scirpus cyperinus) had the lowest aboveground values for N and P accumulation but had the highest belowground value for P. Soft rush (Juncus effusus) and reed canary grass (Phalaris arundinacea) showed high values for both aboveground and belowground N and P accumulation, while blue joint grass (Calamagrostis canadensis) showed low values for aboveground N and P. The five species also showed wide variations in the retention of N and P in decomposing shoots.Juncus effusus had the highest percentages of N and P remaining in litter after five months (87% N and 69% P), whileP. arundinacea retained only 28% N and 18% P.Sparganium americanum had high retention rate for N in litter (74% N) but showed low P retention values (35%).Scirpus cyperinus andC. canadensis also showed high retention rates of litter N but lower values for P retention. Our study suggests that species show differential accumulation and release of N and P and may influence the overall potential of a wetland to retain agricultural nutrients.

Toward predicting reproductive success in submersed freshwater angiosperms

Reproduction is central to changes in distribution and abundance patterns of submersed freshwater macrophytes. In this paper we seek to identify potential ‘bottlenecks’ limiting sexual or vegetative reproduction or its predictability for these plants. To this end we consider the regulation of seed production, seed fate, vegetative propagule production, and vegetative propagule fate in turn, both in general terms and for the perennial Vallisneria americana Michx. in particular Plant biomass appears to be an important determinant of the incidence of flowering for greenhouse-grown Vallisneria americana: 88% of 425 plants either failed to flower if below a threshold dry weight of 0.75 g, or flowered if above it. The number of flowers (in females) or inflorescences (in males), however, was only moderately well correlated with Vallisneria biomass, and the flowering ‘threshold rule’ weakened considerably in the field. Closer examination of the dependence of seed output on plant biomass and pollination intensity, and of the importance of seed or fruit abortion is needed for submersed species. Our ignorance of seed fate is exemplified by the paucity of quantitative studies on dispersal distances and seed deposition patterns, on seed banks, and on features of seed microenvironments pertinent to germination, although the sensitivity of seed dormancy and germination to a variety of environmental conditions has been tested in the laboratory for many species. Further, losses of seeds and seedlings as a result of unfavorable environments, sediment disturbance, flotation, pathogens, and herbivores are generally not well characterized. Overall, the principal limiting step for sexual reproduction may in some cases be lack of floral induction, and in others, ineffective pollination, but dispersal away from the local population, failure of seeds to germinate, and the challenges of seedling establishment cannot be ruled out. Plant size is well correlated with vegetative propagule production for both greenhouse and field Vallisneria populations. As with seed germination, the germination of vegetative propagules has been relatively well studied in the laboratory, but important regulators of germination and establishment in the field warrant further study. We suggest that more investigations at two interfaces of physiological ecology and demography may be particularly fruitful. Regarding propagule production, experimental focus on allocation patterns which ‘translate’ accumulated resources into seeds or vegetative propagules should prove rewarding. Regarding propagule fate, post-dispersal mortality of seeds and vegetative propagules may be rooted in phytiological tolerance limits, but could also be attributable to pathogen or herbivore attack.

Neighbor Influences and Seasonal Growth Patterns for Vallisneria americana in a Mesotrophic Lake

SummaryBiomass of the submersed macrophyte Vallisneria americana rose steadily from early June to 50 g dry wt·m−2 in ealry August. Leaves dominated the biomass of this perennial from mid-June through August, after which plants senesced to leave only winter buds as perennating organs. Only 24% of plants sampled flowered during the 1978 growing season, yielding a population mean of less than 0.6% of dry weight allocated to sexual reproduction, with a maximum value of 7.8% for one plant. In contrast, allocation to winter buds accounted for 11% of total dry weight at the close of the growing season.Total dry weight, leaf length, stolon length, rosette number, and winter bud production of Vallisneria americana at the close of the growing season had significantly higher mean values for individual plants in a monospecific stand of Vallisneria than at sites dominated by Potamogeton amplifolius or Chara vulgaris. The basis of site differences in growth by V. americana is unknown, although we demonstrate significantly higher interstitial soluble reactive phosphorus concentration and organic content of sediment at a Vallisneria site.Removal of neighbors of some Vallisneria plants at the Vallisneria, Potamogeton, and Chara sites resulted in longer leaves, but fewer rosettes for plants with neighbors than plants without neighbors. It appears that this changing growth pattern permitted Vallisneria plants to compensate for the presence of neighbors, and thereby attain similar biomass and sustain similar winter bud production in the presence and absence of neighbors.

Comparative desiccation tolerance of two Sphagnum mosses

SummarySphagnum fallax (Klinggr.) Klinggr., a moss growing in hollows close to the water table, is more desiccation tolerant than S. nemoreum Scop., a hummock former distributed high above the hollows. Sphagnum fallax recovered to a greater proportion of its predesiccation photosynthetic rate after one and five days of tissue dryness. Further, a greater percentage of S. fallax plants survived five and ten day periods at low tissue water contents. Longer desiccated periods and lower water contents during these periods decreased both photosynthetic recovery and survival.Water contents measured in Bloomingdale Bog (Adirondack Mountains, NY, USA) showed that S. fallax probably dries more frequently and for longer periods than S. nemoreum. These results support previous findings that the greater ability of S. nemoreum to remain moist in the field is the most important character in its success as a hummock former. Greater tolerance of desiccation helps S. fallax to compensate for its greater tendency to become dry, and is a key physiological feature enabling it to dominate hollows.

Composition and spatial heterogeneity of submersed vegetation in a softwater lake in Wisconsin

Big Muskellunge Lake, a softwater lake (pH 7.5–8.0, alkalinity 0.36 meq/L) in northern Wisconsin, harbors a diverse (25 species) submersed macrophyte vegetation. The present submersed flora combines species generally thought distinct in their ecological affinities, and is very similar to that reported in 1935. The only differences are the apparent loss of three previously infrequent Potamogeton species, and the addition to the flora of two infrequent Potamogeton species and the now abundant Elodea canadensis. Dramatic differences in composition and pattern of the vegetation occurred between sites of contrasting exposure and sediment type. Substantial compositional variation, at scales ranging from tens of centimeters to tens of meters, occurred along transects of contiguous quadrats at uniform depth and exposure. Analyses of species sequences along line transects detected significant segregation of species that is most plausibly explained by biotic factors such as colonization, clonal growth, and competition. Heterogeneity at small scales of a few meters or less is common in Big Muskellunge Lake. Such pattern has been neglected in macrophyte ecology, in spite of its significance for neighbor relationships which may critically influence growth and reproduction of aquatic plants.

Submersed macrophyte growth at low pH

The submersed macrophyte Vallisneria americana was grown for seven weeks in a greenhouse to test for differences in the ability of three different sediments to support growth stimulation in response to CO2 enrichment at low pH. Plants accumulated 21- to 24-fold greater biomass at 10 × ambient CO2 concentrations than at ambient CO2 on all sediments. At both CO2 levels, plants grown on sediment from an acidified lake accumulated ca. 81%, and those grown on oligotrophic lake sediment ca. 47% as much biomass as plants grown on alkaline lake sediment. Despite striking CO2 and sediment effects on biomass accumulation, there was no significant interaction (using log-transformed data) between CO2 and sediment effects, indicating that all sediments allowed similar proportionate growth responses to CO2 enrichment. Plants grown on the less fertile sediments showed greater relative allocation to horizontal versus vertical growth by producing more rosette-bearing stolons in relation to plant height (leaf length) than plants grown on relatively fertile, alkaline lake sediment. Tissue analysis suggested that sediment effects on Vallisneria growth could be attributed neither to mineral putrient (nitrogen and phosphorus) limitation nor to aluminum toxicity in these low pH treatments. In any case, CO2 availability can be an important regulator of submersed macrophyte growth at low pH on a variety of sediment types, including those from oligotrophic and acidic lakes.

Submersed Macrophyte Vegetation and Distribution Within Lakes: Line Transect Sampling

ABSTRACT Line transects often provide a practical basis for quantitative, nondestructive sampling of aquatic vegetation; here four types of line transect arrangements are briefly compared: 1) transects perpendicular to the shoreline have been widely used, but may oversample nearshore areas where shorelines are convex; 2) transects parallel to each other at regular intervals may provide the best basis for mapping an area; but 3) parallel transects at (stratified) random intervals may reasonably represent different areas within the site and avoid possible biases of regular sampling. The tendency of all three methods to oversample some depth ranges and to combine data from different community types may be obviated by using 4) transects along depth contours near a given shoreline of nearly uniform exposure. Line transects may in some cases be sampled with a line intercept method, which is probably best suited for floating-leaved species, but most often a belt transect consisting of contiguous quadrats is more...

Heterophylly in the yellow waterlily, Nuphar variegata (Nymphaeaceae): effects of [CO2], natural sediment type, and water depth.

We transplanted Nuphar variegata with submersed leaves only into natural lake sediments in pH-, [CO(2)]-, depth-, and temperature-controlled greenhouse tanks to test the hypotheses that more fertile sediment, lower free [CO(2)], and shallower depth would all stimulate the development of floating leaves. Sediment higher in porewater [NH(4)(+)] favored floating leaf development. Low CO(2)-grown plants initiated floating leaf development significantly earlier than high CO(2)-grown plants, which produced significantly more submersed leaves and fewer floating leaves. Mean floating leaf biomass was significantly greater than mean submersed leaf biomass but was not influenced by CO(2) enrichment, whereas mean submersed leaf biomass increased 88% at high [CO(2)]. At the shallower depth (35 cm), floating leaves required 50% less biomass investment per leaf than at 70 cm, and a significantly greater proportion of plants had floating leaves (70 vs. 23-43% at 35 vs. 70 cm, respectively) for the last three of the eight leaf censuses. Sediment type, water depth, and especially free [CO(2)] all can influence leaf morphogenesis in Nuphar variegata, and the development of more and larger submersed leaves with CO(2) enrichment favors the exploitation of high [CO(2)] when it is present in the water column.