Michael S. Adams

Coexistence and the comparative light relations of the submersed macrophytes Myriophyllum spicatum L. and Vallisneria americana Michx.

SummaryThe Eurasian watermilfoil (Myriophyllum spicatum L.) has partially replaced wild celery (Vallisneria americana Michx.) as a community dominant in the littoral zones of lakes of Madison, Wisconsin. The two species have very different growth forms, with that of M. spicatum corresponding more closely to the optimal growth form simulated by the macrophyte production model WEED. The objective of this research was to investigate the mechanisms by which Vallisneria could compensate for its nonoptimal growth form and coexist with Myriophyllum.A quantification of midsummer growth form for the two species at a rooting depth of 80–90 cm showed that M. spicatum had 68% of its shoot biomass within 30 cm of the surface, whereas V. americana had 62% of its leaf biomass within 30 cm of the bottom. Vallisneria had a light extinction coefficient ranging from 0.013 to 0.019 m2·g-1, much higher than the value (ca. 0.006 m2·g-1) for M. spicatum. This indicates less effective penetration of light to lower leaves of V. americana. Half-saturation constants describing the light-dependence of carbon uptake in “shade” and “sun” tissues ranged from 60–197 microeinsteins·m-2·s-1 for V. americana, and 164–365 μeinsteins·m-2·s-1 for M. spicatum. The optimum temperature for photosynthesis was 33.6°C for M. spicatum and 32.6°C for V. americana, but Myriophyllum was nearly twice as effective at carbon uptake at 10°C. Integration of all of the above features with WEED showed that, for midsummer conditions, V. americana more than compensated for apparently disadvantageous morphological features by its greater physiological adaptability to low light regimes. Coupled with the temperature-dependence of photosynthesis, it appears that V. americana is favored by midsummer conditions, whereas M. spicatum is at an advantage at other times.

The macrophyte tissue nutrient pool of a hardwater eutrophic lake: Implications for macrophyte harvesting

Abstract Concentrations of 11 minerals in shoots of Myriophyllum spicatum L. were measured at 13 sites in a eutrophic hardwater lake at monthly intervals during the 1975 growing season (May–September). Tissues from sites near point sources of input to the lake were significantly richer in nitrogen and phosphorus than tissues from sites receiving diffuse inputs from vegetated shorelines. Macrophyte shoots contained unusually high amounts of iron and aluminum near a major storm sewer outfall. Phosphorus was the most nearly limiting mineral, although macrophyte growth in the lake did not appear to be nutrient limited. The optimum time for nutrient removal from the lake by thorough macrophyte harvesting was late August. Potential removal of nitrogen and phosphorus by harvesting at this time was approximately 16 and 37%, respectively, of the net annual inputs of these minerals to the lake. Harvesting can partially counteract the effects of allochthonous nutrient loading to the lake, but is not the functional equivalent of a reduction in loading and manipulates only a fraction of the lakes metabolically active nutrient pool.

Productivity, growth and photosynthesis of two small isoetid plants, Littorella uniflora and Isoetes macrospora

(1) The annual productivity and tissue nutrient contents were determined for populations of the isoetids Littorella uniflora var. americana and Isoetes macrospora in an infertile and a moderately fertile lake in northern Wisconsin, U.S.A. These species have crassulacean acid metabolism (CAM). (2) Annual leaf turnover, determined by biomass marking, ranged from 0-85 to 1 72 leaves per leaf for the study populations. Population annual production, including belowground biomass and allocation to vegetative spread, ranged from 63 to 332 g C m-2 yr-1. Leaf turnover rates were significantly greater for the Littorella populations than for the Isoetes populations and compared well with published values. (3) The Littorella population in the infertile lake had similar tissue nutrient concentrations and a significantly greater areal productivity than the population in the more fertile lake. The Isoetes population in the infertile lake had significantly greater tissue concentrations of N, P and K than the population in the more fertile lake. However, it received 40% less light (PAR) because it grew at greater depth. Leaf turnover was greater for the Isoetes population in the more fertile lake, but population productivities in the two lakes were similar. (4) These data along with data for similar plants suggest that the way in which these plants grow is well-adapted to low fertility sites with waters having a low inorganic carbon content.

Water‐Repellent Soils, Fire, and Annual Plant Cover in a Desert Scrub Community of Southeastern California

Surveys relating soil wettability and the establishment of annual plants were conducted on soil hummocks under burned and unburned shrubs and trees in a desert scrub community 4 years following a fire. Hummocks under burned and unburned individuals of Larrea divaricata, Prosopis juliflora, and Cercidium floridum were nearly devoid of established annual vegetation, whereas the surrounding soil was densely populated with several species of annual plants, predominately Sphaeralcea emoryi. Hydrophobic (water—repellent) layers of soil were found at various depths in the barren hummocks. In general, the layers were further below the soil surface under burned shrubs or trees, and the degree of water repellency was greater. The soil surrounding the hummocks did not contain hydrophobic layers. The mosaic of annuals in the desert scrub vegetation is probably the result of a reduction of available soil moisture due to the formation of water—repellent soils under the shrubs. The effect was more pronounced following fire.

A PRODUCTION MODEL FOR MYRIOPHYLLUM SPICATUM L.

The mathematical model WEED was derived to study the productivity of the submerged macrophyte Myriophyllum spicatum in the littoral of Lake Wingra, Madison, Wisconsin. The model coordinates four biomass compartments with selected physiological processes evaluated for a stratified meter—squared water coloumn. Light and temperature are the primary environmental forcing functions. The model considers growth form (depth distribution of biomass), total biomass/meter—squared, the depth profile of photosynthesis and macrophyte contributions to dissovled and particulate organic matter pools in the water column. The model output was tested against field data for seasonal patterns of standing crop, depth distribution of biomass and depth profile of photosynthesis for a rooting depth of 150 cm. WEED has permitted the simulation of macrophyte growth response to changes in environmental conditions and has outlined areas of needed research. See full-text article at JSTOR

The contribution of crassulacean acid metabolism to the annual productivity of two aquatic vascular plants

SummaryNet annual productivity and annual carbon budgets were determined for populations of Littorella uniflora var. americana and Isoetes macrospora in a mesotrophic and oligotrophic lake in northern Wisconsin, to assess the contribution of Crassulacean Acid Metabolism (CAM) to annual productivity of the species in their natural environment. Nocturnal carbon accumulation (CAM), daytime uptake of external CO2 via the C3 mechanism, and refixation of endogenously generated CO2 from daytime respiration were the sources of carbon income. CAM activity as diurnal acid rhythms reached maxima of 89 to 182 μeq·g-1 leaf fresh weight for the various populations.Maximum rates of daytime 14C uptake ranged from 0.56 to 1.46 mg C·g-1 leaf dry wt.·h-1 for the study populations. Refixation of daytime respired CO2 averaged 37% for the four populations. Carbon loss was due largely to “dark” respiration, during the day and night. Nocturnal carbon accumulation, daytime CO2 uptake and 24-h dark respiration were of similar magnitude, indicating dark respiration was equivalent to ∼50% of gross photosynthesis.Net annual production was measured for each population by following leaf turnover. Turnover rates for the Littorella populations were 1.56 and 1.72·yr-1, and for the Isoetes populations, 0.85 and 1.00·yr-1. Measured net annual productivity and calculated net annual productivity (based on carbon exchange) agreed within an average of 12% for the four populations. While CAM activity was greater for the more productive population of each species, the results suggest that the contribution of CAM to annual productivity is greater for the less productive population of each species. CAM contributed 45 to 55% of the annual carbon gain for the study populations.

Effect of Fire on Big Bluestem Production

Microclimatic conditions associated with fire-induced changes in a tall-grass prairie environment were investigated, and estimates of the net photosynthetic response of Andropogon gerardi Vitm. to these changes were made by laboratory determination of the temperature dependence of net photosynthesis of A. gerardi plants collected from burned and unburned sites. Plants from both sites had temperature optima for net photosynthesis between 25 C and 30 C, and similar maximum rates. Increased production of A. gerardi in the field following burning was attributed primarily to the more favorable environmental conditions for net photosynthesis on the burned site from the time of leaf emergence through late June. Warmer soil temperatures and more light on the burned site appear to be important in initiating early season growth. Total carbon gain per unit area was greater on the burned site than on the unburned, primarily because more photosynthetic leaf and shoot area was available on the burned site earlier in the growing season. Beginning in late June and continuing for the rest of the growing season, measured environmental parameters were similar on both sites. INTRODUCTION An increase in dry matter production and flowering when tallgrass prairie is burned has been amply documented (cf., Brown, 1967; Curtis and Partch, 1948; Daubenmire, 1968; Ehrenreich and Aikman, 1963; Hadley and Kieckhefer, 1963; Kucera and Dahlman, 1968). Several factors including release from allelopathic effects and decreased competition from cool season grasses have been suggested (Robocker and Miller, 1955; Curtis and Partch, 1948; Ehrenreich, 1959; Old, 1969) as reasons for increased production after fire, but litter removal has often been cited as the primary factor (Curtis and Partch, 1950; Old, 1969). On unbumed tall-grass prairie the light-colored litter layer is often at least 10 cm deep, and quite dense. Its removal affects many microclimatic parameters and it is not clear which of these changes are responsible for increasing production. The most obvious ways in which litter removal affects the microclimate are by allowing increased light penetration and warmer temperatures near and below the soil surfaces (Hurlbert, 1969). These factors have been cited by Ehrenreich and Aikman (1963), Ehrenreich (1959) and Weaver and Rowland (1952) to account for earlier growth on burned compared to over 114 gm2 (1000 lbs acre-1) unburned areas. An additional effect of litter on microclimate was proposed by Brown (1967), who observed less net radiation in litter areas, sug1 Present address: Department of Vegetable Crops, Cornell University, Ithaca, New York 14850. 2 Department of Biology, Central State University, _Edmond, Oklahoma 73034,

The importance of belowground mineral element stores in cattails (Typha latifolia L.)

Abstract We measured the amount of N, P, K, Ca, Mg, Fe, B, Mn, Na, Sr, Cu and Zn in above- and belowground parts of cattails ( Typha latifolia L.) every 2 weeks during the growing season (April–October) in plants growing in a marsh on the shore of Lake Mendota, Wisconsin. Elements differed considerably in their distribution between above- and belowground parts and the amount of apparent exchange between parts. The ratio of the amount of an element in aboveground plant parts to that belowground (A:B) was between 1:1 and 2:1 for most elements, as compared with the 2.2:1 ratio of biomass. The maximum amounts of Fe and Zn belowground exceeded their aboveground maxima, while K, Ca and Mn had A:B ratios greater than 2:1. N, P and K in belowground plant parts decreased considerably during the spring, and belowground decreases were large enough to be potentially important sources of these elements for shoot growth. Belowground stores of Ca, Mg, Mn, Na and Sr decreased little in the spring and do not function as reserves.

Phytoplankton as a factor in the decline of the submersed macrophyte Myriophyllum spicatum L. in Lake Wingra, Wisconsin, U.S.A.

A dramatic decline in biomass and areal coverage of the submersed macrophyte Myriophyllum spicatum in Lake Wingra, Wisconsin, USA during the mid-1970s is documented using aerial photography, vegetation surveys, and quadrat biomass sampling. Over the same period, light penetration as measured by Secchi disc transparency and extinction coefficient decreased substantially. During this period, extinction coefficient was closely correlated with chlorophyll a levels implicating phytoplankton as the major source of decreased light penetration. A growth model for M. spicatum predicts a substantial decline in macrophyte biomass when extinction coefficient is increased to the levels reported since 1977. Available data do not show whether the phytoplankton increase preceeded or followed the macrophyte decline. Nonetheless it is clear that phytoplankton growth can account for a substantial portion of the decline and that macrophyte recovery will be difficult given the phytoplankton-induced decrease in water clarity.

The distribution of submerged aquatic macrophyte biomass in a eutrophic stream, Badfish Creek: the effect of environment

The distribution of biomass of the macrophyte community in Badfish Creek was examined in three sections (A–C) totaling ten kilometers. Biomass samples were taken in a stratified-random manner, with sediment characteristics, depth, current velocity and incident light measured at each site to correlate individual biomass samples with environmental factors. Total community biomass decreased in the downstream section (C), with the biomass ofElodea canadensis decreasing abruptly below section A. The only environmental factors which were correlated with the decrease in macrophyte community biomass, especially that ofPotamogeton pectinatus, in section C was the increase in substrate heterogeneity and sand substrates which lacked surface gravel. The change in substrate was related to channelization. Considering the stream as a whole, the biomass of the dominant species,Potamogeton pectinatus, was correlated with incident light. Other species present wereCeratophyllum demersum andCladophora glomerata. Analysis of riparian vegetation type indicates that tree cover significantly reduced macrophyte biomass by incident light reduction.