Dense Monospecific Stands Research Articles

Physiological adaptations of the invasive cordgrass Spartina anglica to reducing sediments: rhizome metabolic gas fluxes and enhanced O 2 and H 2 S transport

Cordgrasses of the genus Spartina form dense monospecific stands worldwide, profoundly influencing the ecology of estuaries. One species, Spartina anglica, originated by allopolyploidy in the 1800s and has been particularly prolific as an invasive species worldwide. S. anglica tolerates low-lying estuarine mudflats that its progenitor species and other coastal halophytes cannot. However, very little is known of the physiology of S. anglica. In the present study, an automated flow-through respirometry system was used to quantify metabolic gas fluxes (O2, H2S, CO2, and NH3) of S. anglica rhizomes. Enhanced physiological mechanisms to transport O2 and H2S in both directions between the rhizosphere and the atmosphere were exhibited by S. anglica, but not by the native North American species S. alterniflora. These results suggest that tolerance of anoxia and H2S may assist S. anglica in colonizing extensively flooded environments. Enhanced sediment oxygenation by S. anglica may be potentially useful for phytoremediation of contaminated sediments, since microbial degradation of organic pollutants is often limited by O2 availability.

The biology of Canadian weeds. 118. Artemisia vulgaris L.

Artemisia vulgaris L. (mugwort) is an introduced rhizomatous perennial found primarily along roadsides, in waste areas, and in non-containerized nursery crops in eastern regions of Canada and the United States. Artemisia vulgaris is rapidly spreading throughout the Northern Hemisphere, and is currently found in nine Canadian provinces, as well as half of the states in the United States. Historically, A. vulgaris has been used as an herbal remedy and for flavouring beer, but recently has been identified as a primary pest of nurseries and urban landscapes, largely because of its ability to propagate easily from small rhizome fragments, and because of ineffective control strategies. The recent expansion of the nursery/landscape sector has accelerated the spread of A. vulgaris into turfgrass and landscape settings throughout the Northern Hemisphere, but especially westward towards the Pacific Coast. With few effective strategies for control, this aggressive weed has rapidly colonized new areas, often forming dense monospecific stands. Not surprisingly, species diversity of native flora in these habitats has declined following A. vulgaris colonization. The mechanisms of interference (e.g., allelopathy and competition) and current strategies for the control of A. vulgaris are discussed. Key words: Mugwort, ARTVU, Artemisia vulgaris, Asteraceae, Compositae, weed biology

Nekton use of a Phragmites australis marsh on the Hudson River, New York, USA

Recently, Phragmites australis (common reed) in North American wetlands has changed from a species with limited distribution to an invasive species producing dense monospecific stands. This expansion has forced out other wetland vegetation, potentially changing the functions of these ecosystems. Little research has focused on whether P. australis stands serve as valuable wetland habitat and foraging areas for terrestrial animals and resident marsh nekton. This study addressed the effect of different hydrologic and geomorphic conditions among P. australis stands on abundance of resident nekton in Piermont Marsh, located on the lower Hudson River Estuary. Three P. australis-dominated sites (erosional creekbank, depositional creekbank, interior) were sampled for nekton using bottomless lift nets during day and night spring high tides during the months of July, August, and September, 1998. Hydrology data were collected for ten continuous days during each sampling period and included mean flooding depth, duration, and frequency for the three sites. Species captured on the marsh surface included Fundulus heteroclitus (common mummichog), Palaemonetes pugio (dagger-blade grass shrimp), Callinectes sapidus (blue crab), and Uca minax (brackish water fiddler crab). Results indicate that nekton and macrobenthos are using all areas sampled, and measured densities are comparable to those measured in some non-P. australis dominant wetlands elsewhere on the Hudson River. Although no strong correlation was made between nekton densities and hydrology, definite patterns were observed in the data. The results indicated that additional variables, such as geomorphology, might interact with hydrology to influence nekton use of the marsh surface. Although exact mechanisms controlling spatial variation of nekton use within P. australis stands still need to be elucidated, P. australis does seem to serve as nekton habitat.

Limitations on photosynthesis of competing individuals in stands and the consequences for canopy structure.

The canopy structure of a stand of vegetation is determined by the growth patterns of the individual plants within the stand and the competitive interactions among them. We analyzed the carbon gain of individuals in two dense monospecific stands of Xanthium canadense and evaluated the consequences for intra-specific competition and whole-stand canopy structure. The stands differed in productivity, and this was associated with differences in nitrogen availability. Canopy structure, aboveground mass, and nitrogen contents per unit leaf area (N area) were determined for individuals, and leaf photosynthesis was measured as a function of N area. These data were used to calculate the daily carbon gain of individuals. Within stands, photosynthesis per unit aboveground mass (P mass) of individual plants increased with plant height, despite the lower leaf area ratios of taller plants. The differences in P mass between the tallest most dominant and shortest most subordinate plants were greater in the high-nitrogen than in the low-nitrogen stand. This indicated that competition was asymmetric and that this asymmetry increased with nitrogen availability. In the high-nitrogen stand, taller plants had a higher P mass than shorter ones, because they captured more light per unit mass and because they had higher photosynthesis per unit of absorbed light. Conversely, in the low-nitrogen stand, the differences in P mass between plants of different heights resulted only from differences in their light capture per unit mass. Sensitivity analyses revealed that an increase in N area, keeping leaf area of plants constant, increased whole-plant carbon gain for the taller more dominant plants but reduced carbon gain in the shorter more subordinate ones, which implies that the N area values of shorter plants were greater than the optimal values for maximum photosynthesis. On the other hand, the carbon gain of all individual plants, keeping their total canopy N constant, was positively related to an increase in their individual leaf area. At the same time, however, increasing the leaf area for all plants simultaneously reduced the carbon gain of the whole stand. This result shows that the optimal leaf area index (LAI), which maximizes photosynthesis of a stand, is not evolutionarily stable because at this LAI, any individual can increase its carbon gain by increasing its leaf area.

Intraspecific competition and subterranean herbivory: individual and interactive effects on bush lupine

High mortality of plants growing in dense monospecific stands (i.e. self‐thinning) usually results from intense intraspecific competition. However, inconspicuous below‐ground insect herbivory might be a potent but overlooked source of mortality within dense stands of plants, particularly if crowding limits a plant's ability to compensate for herbivore damage. Here I ask how high conspecific density influences a plant's ability to cope with heavy below‐ground insect herbivory. I manipulated conspecific density and exposure to an abundant root‐borer, the ghost moth (Hepialus californicus), and examined the impacts on the fecundity, growth, and survival of bush lupine (Lupinus arboreus), a fast‐growing shrub that grows in dense monospecific stands in coastal grasslands. Both herbivory and intraspecific competition affected seed production, size, and mortality of bush lupine over the two years of the experiment. Plants consistently produced fewer seeds when growing at high versus low density and ghost moth herbivory also significantly reduced seed production. The negative effects of herbivory on plant fecundity were similar, regardless of plant density. In contrast, plant survival was affected by both competition, herbivory, and the interaction of these factors. In high density plots, plant survival was uniformly low (averaging 0.45–0.50); plants exposed to herbivores died from heavy herbivory, and plants protected from herbivores died due to intense intraspecific competition that compensated for losses due to herbivory. In low density plots, ghost moth herbivory similarly reduced lupine survival, from an average survival probability of 0.94 in plots protected from these herbivores to 0.55 in plots exposed to herbivory. Thus, results show that regardless of plant density, below‐ground herbivory can be a potent source of mortality.

Biomass Allocation and Light Partitioning Among Dominant and Subordinate Individuals inXanthium canadenseStands

Patterns of above-ground biomass allocation and light capture by plants growing in dense stands or in isolation were studied in relation to their height. A canopy model was developed to calculate light absorption by individual plants. This model was combined with data on canopy structure and patterns of biomass allocation for solitary plants and for plants of different heights in dense mono-specific stands of the dicotyledonous annualXanthium canadenseMill. There were four stands, and stand height increased with age and nutrient availability. The allometric relationship between height and mass differed considerably between plants in stands and those growing in isolation and also between plants of different heights within stands. The proportion of shoot mass in leaf laminae (LMR) decreased with increasing plant height, but solitary plants had a higher LMR than competing plants of the same height. Thus, in contrast to previous assumptions, LMR of competing plants is not strictly determined by biomechanical constraints but results from a plastic shift in biomass allocation in response to competition. Average leaf area per unit leaf mass (SLA) decreased with increasing photosynthetic photon flux density (PPFD) independent of nutrient availability. Consequently, taller, more dominant plants in stands had a lower leaf area ratio (LAR: LAR=LMR×SLA) than shorter, more subordinate plants. Dominant plants absorbed more light both per unit leaf area (Φarea) and per unit mass (Φmass) than subordinate plants. Their greater Φareamore than compensated for a lower LAR (Φmass=Φarea×LAR). We conclude that the greater Φmassof dominant plants is quantitative evidence that success in competing for light is disproportionately related to the size of the shoot (i.e. asymmetric competition). We know of no other study which has demonstrated this with quantitative estimates of light acquisition in relation to plant size. This result is contrasted with previous studies on multi-specific stands in which Φmassof dominant and subordinate species is similar, and we discuss why light competition in mono-specific stands is asymmetric whereas it can be size-symmetric in multi-specific stands, with similar or greater LAI.

SPATIAL DYNAMICS OF A HIMANTHALIA ELONGATA (FUCALES, PHAEOPHYTA) POPULATION

ABSTRACT In dense monospecific stands of plants intraspecific competition usually results in self‐thinning, the concurrent increase in biomass and decrease in density over time. Self‐thinning may also result in a change in the spatial pattern of individuals, but so far the spatial dynamics of marine plants has not been investigated. The brown alga Himanthalia elongata (L.) S. F. Gray forms dense monospecific stands on many northern temperate rocky shores, and various attributes (including its simple form) facilitated the study of the spatial dynamics of this species. The spatial pattern of settling zygotes was examined in the laboratory. In the absence of water movement, substratum heterogeneity, and a point source, zygotes usually settled in clumps rather than randomly. Within the clumps zygotes appeared to be regularly distributed at a scale similar to the size of the zygotes themselves. Furthermore, the clumps themselves seemed to be regularly distributed. On the shore, well‐established stands of “button‐stage” Himanthalia populations were examined during a period of extensive growth and self‐thinning. Individual plants were initially highly regular in spatial pattern but became less so over time. The pattern of plants dying during self‐thinning was also highly regular and probably reflected existing spatial regularity. However, using a hypothesis of mortality as a random event, I found that smaller plants had a less than average survival potential, while larger plants had a greater than average chance. A consideration of the spatial pattern of plants alive at the end of the study revealed regularity at a scale of 2–7 mm but a random spatial pattern at larger scales, which might indicate a small sphere of influence of competing individuals. The best predictor of mean nearest neighbor distances in the populations was mean plant diameter.

Host Specificity of Three Potential Biological Weed Control Agents Attacking Flowers and Seeds of Lythrum salicaria (Purple Loosestrife)

Abstract Lythrum salicaria is a Eurasian herbaceous perennial that has become a serious invader of wetlands in the United States and Canada. Dense monospecific stands replace a diverse native flora resulting in the degradation of these wetland habitats. There are presently no satisfactory means of control. Biological control offers the most promising method of resolving this problem. A root-mining weevil and two leaf-feeding chrysomelids from Europe were released in North America in 1992. The host specificity of three additional flower- and seed-feeding species was investigated. The two weevils, Nanophyes marmoratus and N. brevis, have a wide geographic and ecological range. Both develop exclusively on Lythrum salicaria within its native European range and were found to be highly host specific during screening tests. Minor adult feeding was observed in no-choice tests on a few other species within the Lythraceae. Successful larval development was restricted to purple loosestrife. The only known field host of the third species, the gall midge Bayeriola salicariae, is purple loosestrife. Oviposition and successful larval development of B. salicariae in cages and the open field occurred on potted test plants of another three Lythrum species. The introduction of N. marmoratus and N. brevis into North America is expected to further reduce seed output and lessen the competitive ability of purple loosestrife. Their introduction was approved in 1994.

Population Biology and Rates of Invasion of Three Introduced Impatiens Species in the British Isles

The rate and extent of spread of three introduced species of Impatiens (I. glandulifera Royle, I. parviflora DC and I. capensis Meerburgh) in Britain was studied. The dates for the first records in each vice-county were ob- tained, and used to estimate rates of spread in England since their introduction. I. glandulifera was estimated as having a rate of spread of up to 38 km yrl, compared to a rate of up to 24 km yrl for L parviflora. The use of vice- county data as opposed to 10 km squares is discussed. Comparative growth experiments were performed both in garden plots and in the field, in an attempt to identify char- acters which enabled I. glandulifera to be so successful. Frost tolerance and seed output seem to be important factors; there are no doubt others not identified. Occasional long distance dispersal events, probably aided by man, are considered to be responsible for the rapid spread of I. glandulifera. Impatiens glandulifera is a pest plant because it forms dense monospecific stands in damp woodland, swamping all other plants. It is able to do so because of early germina- tion and rapid growth to a height higher than any other British annual. Local spread is limited to 2 m yrl from its own ballistic dispersal and to a small multiple of that from

Salt Tolerances and The Distribution of Fugitive Salt Marsh Plants

Fugitive species often live under harsh physical conditions, yet their ability to cope with environmental extremes has received little attention. We studied the association of fugitive plants and stressful physical conditions using salt marsh plants that live in hypersaline bare patches. In New England salt marshes, primary space is dominated by dense monospecific stands of perennial turfs, but disturbances frequently leave bare space that rapidly becomes hypersaline. Elevated salinities of bare patches result form direct exposure of soil to solar radiation and evaporative water loss. A number of fugitive plants that are rare in undisturbed salt marsh vegetation are commonly associated with bare patches before being displaced during secondary succession. Greenhouse studies show that plants that dominate undisturbed vegetation are stunted by high substrate salinities, whereas the photosynthesis and growth of patch—dependent fugitives are relatively independent of typical salinity variation. Moreover, in the field, watering more than doubled the survivorship and growth of seedlings in patches and more strongly affected dominants than patch—dependent fugitives. Our results show that physical conditions in salt marsh bare patches can limit plant colonization, and suggest that high salt tolerances may permit fugitive plants to utilize bare patches as refugia from competitors. Since fugitive plants are commonly restricted to physically stressful habitats, the ability to withstand harsh physical conditions is likely often an important aspect of their biology.

Sex Ratio and Neighbor Effects in Monospecific Stands of Juniperus Virginiana

Sex ratio, sexual dimorphism, and neighbor effects were investigated in 20 monospecific stands involving 3500 trees of Juniperus virginiana, a dioecious gymnosperm, near its northern limit in eastern Ontario. We tested the hypothesis that the intensity of competition for a given tree will depend on the sex of its nearest neighbor because of differential costs of reproduction in male and female plants. Population sex ratio was significantly male biased in only one stand but was male biased when data from all stands were combined. Stand sex ratio was not related to age structure, stand density, or local competition intensity (based on separation distance/size regression). Male trees were significantly taller and had greater diameters in only three stands. In the combined data from all stands, however, there was a greater representation of male trees among the larger diameter trees and among the taller trees. Male trees were also significantly taller in the pooled data set. These results suggest that female trees pay a slightly but significantly greater cost for reproduction in terms of reduced vegetative growth. However, the effects of close neighbor proximity did not depend on a neighbor's sex, and male and female trees were randomly distributed relative to each other. Hence, males and females appear to compete with similar intensities against each other and are not spatially segregated into separate "niches." The overall greater height for males may be interpreted as a consequence of sexual selection, as greater height in male trees would confer greater success as a pollen donor, especially in dense monospecific stands characteristic of this species.

The biology of Canadian weeds. 99. Matricaria perforata Mérat (Asteraceae)

Scentless chamomile, Matricaria perforata Mérat, (=M. maritima var. agrestis (Knaf) Wilmott), is an introduced weed having an annual to short-lived perennial life cycle. Two cytotype races are found in Canada with a distinctive geographic distribution. The tetraploid (2n = 36) is abundant in agricultural habitats of the Prairie region, whereas the diploid (2n = 18) is found mainly in ruderal habitats of the Atlantic region. Infestations are found in many annual and perennial crops, pastures, wasteland, lawns, gardens, roadsides, fence lines, and ditches. The plant reproduces entirely by seed, producing up to 1.8 million seeds m−2 in dense monospecific stands. A density of 25 annual plants m−2 can result in a 55% reduction in spring wheat yield under cool wet conditions. Shallow preseeding tillage in late autumn or early spring is an effective method of cultural control. Several herbicides are recommended for control of the weed in crop and noncrop areas. Scentless chamomile is considered a suitable candidate for biological control. Key words: Weed biology, scentless chamomile, Matricaria perforata, Matricaria inodora, Matricaria maritima var. agrestis, Tripleurosperum inodorum

Growth and Competition Between Seedlings of Protea repens (L.) L. and the Alien Invasive, Acacia saligna (Labill.) Wendl. in Relation to Nutrient Availability

Growth and Competition Between Seedlings of Protea repens (L.) L. and the Alien Invasive, Acacia saligna (Labill.) Wendl. in Relation to Nutrient Availability

Invasion of Microstegium vimineum (Poaceae), An Exotic, Annual, Shade-Tolerant, C 4 Grass, into a North Carolina Floodplain

Microstegium vimineum, an Asian annual C4 grass that is very shadetolerant, has invaded floodplains, streambanks and adjacent mesic slopes in the North Carolina Piedmont during the past 30 years. A 3-year study of its invasive characteristics revealed that M. vimineum is slow to invade undisturbed vegetation, but rapidly fills disturbed, mesic, shaded areas, such as streamsides where floods scour existing vegetation or sewer-line rights-of-way which are mown once a year. Its seeds remain viable for at least 3 years in the soil seedbank and rapidly germinate to produce a new cohort if a disturbance removes an existing cohort. On fertile floodplain sites, soil fertilization in March had no effect on seed production in October. When M. vimineum seeds were sown into existing vegetation, seed production was negatively correlated with soil potassium, calcium, silt and pH, probably because the more fertile sites also supported a denser ground vegetation layer. These qualities, in addition to its cleistogamous or apomictic reproduction, help explain how M. vimineum has spread throughout the eastern U.S. since its introduction approximately 70 years ago. INTRODUCTION Microstegium vimineum (Trin.) A. Camus is an annual grass species from Asia which recently invaded floodplains, streamsides and adjacent mesic slopes in the North Carolina Piedmont. It appears to be advancing into the existing ground cover of floodplains, which consists mainly of Japanese honeysuckle Lonicera japonica Thunberg. These two species often occur in dense monospecific stands which are separated by a strikingly narrow zone of mixing. This pattern of an introduced annual grass invading established stands of Japanese honeysuckle, a perennial woody vine which is known for its aggressiveness on fertile sites of the Carolinas (Bruner and Shearin, 1964), led me to ask the following questions: (1) Is M. vimineum invading undisturbed stands of L. japonica? (2) Does M. vimineum create a seedbank? (3) Can M. vimineum invade undisturbed stands of L. japonica if its seeds are sown in the stands in large numbers? (4) What environmental variables are related to success of M. vimineum sown in stands of L. japonica? (5) Does M. vimineum respond to an increase in essential soil elements with greater reproductive success? (6) Does M. vimineum alter the soil so as to inhibit growth of L. japonica? (7) What is the role of disturbance in the invasion of M. vimineum? Microstegium vimineum is in the tribe Andropogonae, subfamily Panicoideae. It was first collected in the United States in 1919 near Knoxville, Tennessee (Fairbrothers and Gray, 1972). By 1933 it had been collected in the mountains of western North Carolina (Blomquist, 1948; Fairbrothers and Gray, 1972) and by 1964 in 35 counties in North Carolina, primarily in the Piedmont (Radford et al., 1968). By 1972 the species had spread to at least 14 eastern states, from Florida to New Jersey and W to Ohio and Mississippi (Fairbrothers and Gray, 1972), and by 1978 to Arkansas (Smith, 1978). In Asia the species occurs in Japan, Korea, China, Ryukyus, Formosa, Malaysia, India and the Caucasus (Ohwi, 1984). Microstegium vimineum is unusual in that it is a C4 plant which, unlike typical C4 plants, is adapted to low light conditions (Brown, 1977, Winter et al., 1982). Under netting which transmitted 18% full sunlight, dry matter production was not significantly reduced from production in full sunlight; at 5% full sunlight, growth was 17% of that at full sunlight (Winter et al., 1982). Apparently, the C4 pathway itself has no inherent disadvantages, compared to the C3 pathway, under conditions of low light (Pearcy, 1983; Pearcy and Troughton, 1975).

The nutritive value of three species of water weeds

In a recent paper (2), the possibility of utilizing common fresh-water pest plants as feedstuffs was considered. Based on leaf protein extraction data, chemical analyses of dehydrated samples and standing crop estimations, a number of species appeared to be potential food plants. Further research on the biological value of aquatic plants and the economic feasibility of their utilization was suggested. Three of the most serious aquatic pest plants (6), water hyacinth (Eichhornia crassipes), water lettuce (Pistia stratiotes) and Hydrilla sp. were not included in the previous research (2). These plants occur in almost all tropical and subtropical nations (6). All three species produce dense monospecific stands which often cover hundreds of hectares. Therefore, methods for their utilization would develop an important resource in many areas. Several studies (6) have indicated that E. crassipe's can be used as a forage. However, there is a paucity of information on the nutritive value of E. crassipes and almost no data are available for P. stratiotes and Hydrilla sp. The present research is an evaluation of the chemical composition of these species.