Hydrologic, Edaphic, and Vegetative Responses to Microtopographic Reestablishment in a Restored Wetland

Microtopography is a characteristic feature of many natural wetlands that is commonly lacking in restored wetlands (RWs). Consequently, it has been suggested that microtopography must be reestablished in RWs to accelerate the development of wetland function. The objective of this research was to examine responses of hydrology, soils, and vegetation to microtopographic reestablishment at a 3‐year‐old RW site in North Carolina. Microtopography was reestablished by configuring hummocks (mounds) and hollows (depressions), on otherwise level terrain (flats) of intermediate elevation. For most of the 2003 growing season, mean water table depths were below the soil surface in the flats and 10 cm above the soil surface in the hollows. Analysis of variance revealed significant microtopography by time interactions for soil temperature (p < 0.05) and moisture (p < 0.001), indicating that differences between zones were not consistent throughout the growing season. Hummocks had significantly higher nitrate (p < 0.0001) and ammonium (p= 0.001) than flats and hollows for most of the growing season. Differences in microbial biomass carbon and denitrification enzyme activity across the microtopographic zones were not detected. Plant species richness was significantly different (p < 0.001) across the microtopographic zones, with hummocks < hollows < flats. Flats supported the greatest numbers of wetland species. Aboveground biomass differed significantly (p < 0.001) across the microtopographic zones and followed a different pattern than richness: hummocks < flats < hollows, owing to the growth of emergent wetland herbs in hollows.     

Morphological plasticity of Primula nutans to hummock-and-hollow microsites in an alpine wetland

Hummock-and-hollow microtopography is common in wetlands of the Qinghai–Tibetan Plateau. The physical environment of hummocks contrasts strongly with that of hollows. To address how Primula nutans Georgi, a herbaceous species broadly distributed on the plateau, can inhabit both hummocks and hollows, we investigated the plasticity of its morphology and biomass growth in relation to the environmental variables during the growing season. The results are as follows: (1) total daily photosynthetic photon flux density, maximum soil temperature, and diurnal soil temperature range were significantly larger on the hummocks than in the hollows; (2) individual ramets had smaller leaves, higher leaf mass per unit area, and shorter petiole and peduncle length on the hummocks, but leaf and root dry weights per ramet differed little between microsites; (3) P. nutans allocated most of its dry mass to peduncles and flowers and the proportion of reproductive mass was significantly higher for plants in the hollows than on the hummocks; (4) the coefficients of variation of most morphological and biometric parameters was higher on hummocks than in hollows. The results suggest that P. nutans favors hummocks but can tolerate hollows through morphological adjustments in that the mass investment to peduncles and flowers is much larger in plants in the hollows than hummocks.     

The dynamics of peat accumulation on bogs: mass balance of hummocks and hollows and its variation throughout a millennium

This study concerns the mass balance in hummocks and hollows on three ombrotrophic boreonemoral bogs in both a short (ca 10 yr) and long (1000 yr) time scale. Nitrogen, ¹⁴C. and ^?210Pb are used to establish detailed time scales and to estimate productivity and decay losses in tour different microtopographical units: hummocks with either Sphagnum or lichens and hollows with either Sphagnum lawns or bare hollows. The accumulation of N and ²¹⁰Pb was greater in hummocks than in hollows. The litter input was higher in Sphagnum hummocks (170-210 g m^?2yr^?1) than in lawns (110-145 g m ^?2- yr^?1) while its decay rate (0.011 -0.014 yr^?1) did not differ. The arotelm was deeper in Sphagnum hummocks than in lawns but because of less compaction in lawns, neither residence time (80 100 yr) nor decay losses (70-75%) differed. Productivity in lichen hummocks and bare hollows was insignificant and the mass balance negative. It is concluded that the higher productivity in Sphugnum hummocks maintains the microtopography on the mire surface. The mass balance in hummocks will determine not only the development in hollows but also the rise of the ground water mound, and the height increment of a bog. The addition of mass to the catotelm has generally been less in hollows than in hummocks. Since 800 BP the overall input to the catotelm has decreased from about 150 to < 50 g m ^?2 yr^?1 due to longer residence time increasing losses through decay in the acrotelm from < 20% to 70% and is the result of either climatic changes or autogenic processes in the bog ecosystem. Before recent centuries the whole bog surface must have been covered with Sphagnum mosses, forming an overall input of litter as large as in the recent Sphagnum hummocks and lawns. Due to the present lesser cover of peat forming mosses (20-50% of the surface), the recent overall input of peat-forming litter is only 50-65 g m^?2 yr^?1. The bogs no longer act as sinks for carbon since the input of carbon only just covers the losses as CH₄ and CO₂.     

Methane Flux Influenced by Experimental Water Table Drawdown and Soil Warming in a Dry Boreal Continental Bog

To quantify the effects of water table drawdown and soil warming on CH₄ fluxes, we used a static chamber technique during the growing seasons (May–October) of 2011–2013 at hollow and hummock microforms at three sites of a continental bog near the town of Wandering River, Alberta, Canada: (1) Control, (2) Experimental drained, and (3) old Drained. To simulate climatic warming, we used open top chambers to passively warm half of the hollows and half of the hummocks at each of the water level treatment sites. Water table drawdown significantly reduced CH₄ flux by 50% in 3 years and 76% in 13 years of drainage. The hollows showed greater reduction of efflux as compared to hummocks. A persistent functional relationship of CH₄ flux with water level was found across all sites in all years. The relationship revealed that the contribution of change in vegetation type at hollows and hummocks to CH₄ production and emission was relatively less important than that of the water level. Hummocks and hollows responded to warming differently. At the control, experimental and drained sites, warming increased flux at hollows by 16, 21 and 26%, and reduced flux at hummocks by 4, 37, and 56%, respectively. The combined effect of lowered water table and warming on CH₄ emission was overall negative, although the interaction between the two contributing factors was not significant. Therefore, whereas climatic warming and subsequent lowering of water table are expected to reduce CH₄ efflux from dry ombrotrophic bogs of Alberta, different microforms at these bogs may respond differently with accelerated emissions at warmed, wetter (hollows) and reduced emissions at warmed, drier (hummocks) microforms. Overall, CH₄ efflux from Alberta’s dry continental bogs that are not underlain by permafrost might be affected only slightly by the direct effect of predicted climate warming, although initial water table position will be an important control on the overall response.     

An assessment of soil bacterial community structure and physicochemistry in two microtopographic locations of a palustrine forested wetland

We studied redoximorphic features, field indicators and bacterial communities of soils in hummocks and hollows of a palustrine forested wetland in Virginia. We hypothesized that presence of hydric soils, soil physicochemistry and soil bacterial community structure would differ between hummocks and hollows. We fingerprinted soils collected from different microtopographic locations using Length Heterogeneity Polymerase Chain Reaction (LH-PCR) to study their bacterial community structures. Two hummocks had silty/sandy loam soils with mean chroma values of > 4, showing no indication of ‘hydric soils’ (i.e., wetland soils). Two hollows, however, had clay loam soils with mean chroma values of 2 with gleying and redox concentrations observed, indicative of seasonally inundated wetlands. The soils of hollows also had higher organic matter content and soil moisture compared to the soils of hummocks (P < 0.05). Multidimensional scaling (MDS) and Analysis of similarity (ANOSIM) of the fingerprints revealed differences in soil microbial community structures between hummocks and hollows (Global R = 0.30, P < 0.01). The diversity measures of the fingerprints (Shannon’s H′) were also different by microtopography with higher diversity in hollows relative to hummocks (P < 0.05). LH-PCR proves to be a useful tool in examining bacterial community composition of wetland soils in this study. However, cloning and sequencing of specific community LH-PCR profiles of interest is necessary to fully characterize the community down to genus/species level. With species identities we should be able to not only better explain differences observed in the community profiles, but study their relations to hydrologic and/or physicochemical conditions of wetlands.     

Genetic diversity,sexual condition,and microhabitat preference determine mating patterns in Sphagnum (Sphagnaceae) peat‐mosses

In bryophytes, the possibility of intragametophytic selfing creates complex mating patterns that are not possible in seed plants, although relatively little is known about patterns of inbreeding in natural populations. In the peat‐moss genus Sphagnum, taxa are generally bisexual (gametophytes produce both sperm and egg) or unisexual (gametes produced by separate male and female plants). We sampled populations of 14 species, aiming to assess inbreeding variation and inbreeding depression in sporophytes, and to evaluate correlations between sexual expression, mating systems, and microhabitat preferences. We sampled maternal gametophytes and their attached sporophytes at 12–19 microsatellite loci. Bisexual species exhibited higher levels of inbreeding than unisexual species but did generally engage in some outcrossing. Inbreeding depression did not appear to be common in either unisexual or bisexual species. Genetic diversity was higher in populations of unisexual species compared to populations of bisexual species. We found a significant association between species microhabitat preference and population genetic diversity: species preferring hummocks (high above water table) had populations with lower diversity than species inhabiting hollows (at the water table). We also found a significant interaction between sexual condition, microhabitat preference, and inbreeding coefficients, suggesting a vital role for species ecology in determining mating patterns in Sphagnum populations. © 2015 The Linnean Society of London, Biological Journal of the Linnean Society, 2015, 115 , 96–113.     

Variation in wetland seed banks across a tidal freshwater landscape

Almost nothing is known about landscape-level variation in seed bank composition across complexes of hydrologically connected wetlands. We examined species composition of seed and spore banks in three habitats of tidal freshwater wetlands (marshes, swamp hollows, and swamp hummocks) along 48 km of tributaries throughout the tidal freshwater portions of the Nanticoke River watershed (Maryland and Delaware). Taxa and seedling density decreased with increasing distance upstream in the swamp hollows and hummocks, but increased or remained constant proceeding upstream in the marshes. Species rarefaction curves indicated equal taxa richness (28) between marshes and swamp hummocks at 175 individuals, with lower richness in swamp hollows (19). However, communities in swamp hollows were patchier and had an estimated total taxa richness of 52, similar to the marshes (50) and higher than swamp hummocks (41). Coefficients of variation for seedling emergence densities (136-180%) were greater than those of published seed bank studies conducted at smaller spatial scales in tidal freshwater marshes (36-117%). Our literature searches suggest that ours is the first study to document significant spatial trends in seed bank diversity and density across a wetland landscape.     

CH4 emission from a hollow-ridge complex in a raised bog: The role of CH4 production and oxidation

The aim of this study was to correlate magnitude andcontrols of CH₄ fluxes with the microtopographyand the vegetation in a hollow-ridge complex of araised bog. High CH₄ emission rates were measuredfrom hollows and mud-bottom hollows, while hummocksconsumed atmospheric CH₄ at a low rate. Thehighest emissions were measured from plots with Eriophorum vaginatum and Scheuchzeriapalustris. CH₄ emission ceased after Scheuchzeria had been clipped below the water table,indicating the importance of this aerenchymatic plantas a conduit for CH₄.Peat in the upper catotelm of hollows was younger andless decomposed than in hummocks. Potential CH₄production in vitro was higher and themethanogenic association was better adapted to highertemperatures in hollow than in hummock peat. Highertemperatures in hollows resulted in a strongerCH₄ source in hollows than in hummocks. Negativefluxes from hummocks indicated that even in wetlandsmethanotrophic bacteria exist that are able to oxidizeCH₄ at atmospheric mixing ratios, and thatoxidation controls CH₄ emission completely. TheCH₄ mixing ratio was low in the acrotelm, but itincreased within the catotelm. Comparing fluxesmeasured in static chambers with fluxes calculatedfrom the porewater CH₄ profiles it was deducedthat the zone of methane oxidation was located closeto the water table.In hollows, CH₄ production at in situtemperature was far higher than emission into theatmosphere, corresponding to an oxidation rate ofnearly 99%. The CH₄ flux between the catotelmand the acrotelm of hollows was also higher than theemission, indicating the importance of CH₄oxidation in the aerobic acrotelm, too. CH₄microprofiles showed that CH₄ oxidation inmud-bottom hollows was confined to the topmost 2 mm,and that in Sphagnum-covered hollows CH₄oxidation occurred at the lower edge of green Sphagnum-parts.     

Temperature and Microtopography Interact to Control Carbon Cycling in a High Arctic Fen

High arctic wetlands hold large stores of soil carbon (C). The fate of these C stores in a changing climate is uncertain, as rising air temperatures may differentially affect photosynthesis and ecosystem respiration (ER). In this study, open-top warming chambers were used to increase air and soil temperatures in contrasting microtopographic positions of a high arctic fen in NW Greenland. CO₂ exchange between the ecosystem and the atmosphere was measured on 28 dates over a 3-year period. Measurements of the normalized difference vegetation index, leaf and stem growth, leaf-level gas exchange, leaf nitrogen, leaf δ¹³C, and fine root production were made to investigate the mechanisms and consequences of observed changes in CO₂ exchange. Gross ecosystem photosynthesis (GEP) increased with chamber warming in hollows, which are characterized by standing water, and in hummocks, which extend above the water table. ER, however, increased only in hummocks, such that net ecosystem exchange (NEE) increased in hollows, but did not change in hummocks with chamber warming. Complementary measurements of plant growth revealed that increases in GEP corresponded with increases in C allocation to aboveground biomass in hummocks and belowground biomass in hollows. Our results and those of several recent studies clearly demonstrate that effects of climate change on the C balance of northern wetlands will depend upon microtopography which, in turn, may be sensitive to climate change.     

底栖生物影响下的潮滩微地貌演化数值模拟

潮滩是海岸带湿地的主要类型之一,其中分布的底栖生物对生态环境具有重要的调节作用。潮滩底栖微藻、泥沙与水动力之间存在相互作用,影响潮滩微地貌形态,明晰底栖生物对潮滩微地貌的演化机制至关重要。以黄河三角洲潮滩湿地为研究区,通过构建潮滩微地貌动力模型,探究底栖生物对微地貌格局演化的作用机制,分析底栖生物对微地貌系统稳定性的影响。结果表明(1)底栖微藻生长与泥沙扩散、水流再分配过程交互作用驱动下,潮滩上可形成底栖微藻覆盖的高丘与积水洼地交替分布的规则性微地貌斑图;(2)微地貌斑图的形成提高了潮滩生态系统初级生产力和泥沙淤积高度;(3)底栖微藻与泥沙、水流的交互作用使得潮滩微地貌系统对侵蚀扰动呈现非线性响应行为,系统存在临界点,且在一定侵蚀率范围内存在双稳态;(4)黄河口泥螺入侵使得微地貌系统抵抗侵蚀扰动能力减小,且系统稳定性随泥螺生物量的增加而降低。     

Microscale Spatial Distribution of Phragmites australis (Common Reed) Invasion into Spartina patens (Salt Hay)-dominated Communities in Brackish Tidal Marsh

Over the last century, Phragmites australis (common reed) has been expanding rapidly from the marsh–upland boundary into Spartina patens (salt hay)-dominated high marsh communities of the eastern US coast. Whereas direct and indirect human disturbances and changes in hydrology or salinity are likely to influence rates of spread at the landscape scale, the susceptibility of specific plant communities to invasion also influence rates of Phragmites expansion at the local scale. I measured microscale (0.25 m²) spatial patterns of culms (emerging buds and mature stems) in October 1993 at both expanding and stable boundaries of Phragmites populations within a S. patens-dominant matrix. In both expanding and stable plots, Phragmites culms were observed more frequently than expected on hummocks that were created by S. patens tussock-forming root structure. Culm density within a plot was correlated with the percent hummock cover within a plot. Further, Phragmites culms, particularly mature stems, were concentrated along the perimeter of the hummocks. Because the culms were not evenly distributed between hummocks and hollows, I suggest that invasion rates of Phragmites are limited in S. patens communities by microscale differences in hummock availability. The pattern of emergence suggests that expanding rhizomes of Phragmites encounter both competition with S. patens roots on the hummocks and physiological stressors (salinity, anoxia, sulfide concentrations) in the hollows.     

Feedback control of the rate of peat formation.

The role of peatlands in the global carbon cycle is confounded by two inconsistencies. First, peatlands have been a large reservoir for carbon sequestered in the past, but may be either net sources or net sinks at present. Second, long-term rates of peat accumulation (and hence carbon sequestration) are surprisingly steady, despite great variability in the short-term rates of peat formation. Here, we present a feedback mechanism that can explain how fine-scale and short-term variability in peat-forming processes is constrained to give steady rates of peat accumulation over longer time-scales. The feedback mechanism depends on a humpbacked relationship between the rate of peat formation and the thickness of the aerobic surface layer (the acrotelm), such that individual microforms (hummocks, lawns, hollows and pools) expand or contract vertically in response to fluctuations in the position of the water table. Hummocks (but not hollows) 'evolve' to a steady state where changes in acrotelm thickness compensate for climate-mediated variations in surface wetness. With long-term growth of a topographically confined peat deposit, the steady state gradually shifts to a thicker acrotelm (i.e. taller hummocks) and lower rates of peat formation and carbon sequestration.     

Restoration of floating mat bog vegetation after eutrophication damages by improving water quality in a small pond

We studied vegetation changes in a small floating mat bog in Mizorogaike Pond (Kyoto, Japan), which had experienced a severe decrease in the number and area of hummocks caused by nutrient loading in the 1960s and 1970s, to examine whether reducing the extent of nutrient loading can restore degraded wetland vegetation. However, nutrient loading in the region has been minimized since the 1980s. We examined the distributions of hummocks and Sphagnum cuspidatum mats in 1980, 1988, and 2006, as well as nine major vascular plants that dominated the hollows on the floating mat in 1980 and 2006. The total area of normal hummocks formed by Sphagnum palustre increased from 5865.3 m² in 1980 to 5913.6 m² in 1988 and 8485.2 m² in 2006. The total area of the S. cuspidatum mats also changed, from 416.4 m² in 1980 to 322.3 m² in 1988 and 1012.5 m² in 2006. Examination of the spatial distribution patterns of major plants revealed that emergent plants decreased in the northern part of the mat, but increased in the southern part. Thus, the improved pond water quality was effective at restoring hummocks, although nutrient loading may have caused some irreversible changes in the wetland vegetation.     

Microtopographic specialization and flexibility in tropical peat swamp forest tree species

Tropical tree species distributions are determined by a wide range of biotic and abiotic factors, including topography and hydrology. Tropical peat swamp forests (TPSFs) are characterized in part by small‐scale variations in topography (‘hummocks’ and ‘hollows’) that create distinct microhabitats and thus may contribute to niche diversification among TPSF tree species. Using tree elevations calibrated to daily peat water levels collected using a data logger and a permutation test, we evaluated topographical microhabitat preferences for 21 tree species in a relatively undisturbed TPSF in Central Kalimantan, Indonesia, to determine whether these species show preferential association with hummocks or hollows and to quantify the prevalence of microhabitat specialization among them. Only one species, Tetractomia tetrandrum, emerged as a hollow specialist, with no hummock specialists among the species tested. The remaining 20 species, including Psydrax dicoccos, which had the lowest mean observed elevation, and Maasia hypoleuca, which had the highest mean observed elevation, showed no clear microtopographic preference. This suggests that many TPSF species may be resilient to the natural hydrologic variations that occur in relatively intact peat swamp forests. Such studies of microtopographic preferences of tree species in TPSF and other wetland forest ecosystems can inform selection of tree species for reforestation projects, and potentially also provide information on how future climate change may impact these habitats and their resident tree species.     

Ecosystem engineering by seagrasses interacts with grazing to shape an intertidal landscape

Self-facilitation through ecosystem engineering (i.e., organism modification of the abiotic environment) and consumer-resource interactions are both major determinants of spatial patchiness in ecosystems. However, interactive effects of these two mechanisms on spatial complexity have not been extensively studied. We investigated the mechanisms underlying a spatial mosaic of low-tide exposed hummocks and waterlogged hollows on an intertidal mudflat in the Wadden Sea dominated by the seagrass Zostera noltii. A combination of field measurements, an experiment and a spatially explicit model indicated that the mosaic resulted from localized sediment accretion by seagrass followed by selective waterfowl grazing. Hollows were bare in winter, but were rapidly colonized by seagrass during the growth season. Colonized hollows were heavily grazed by brent geese and widgeon in autumn, converting these patches to a bare state again and disrupting sediment accretion by seagrass. In contrast, hummocks were covered by seagrass throughout the year and were rarely grazed, most likely because the waterfowl were not able to employ their preferred but water requiring feeding strategy ('dabbling') here. Our study exemplifies that interactions between ecosystem engineering by a foundation species (seagrass) and consumption (waterfowl grazing) can increase spatial complexity at the landscape level.     

DIFFERENTIAL AVOIDANCE OF CORAL SNAKE BANDED PATTERNS BY FREE-RANGING AVIAN PREDATORS IN COSTA RICA

Empirical studies of mimicry have rarely been conducted under natural conditions. Field investigations of some lepidopteran systems have provided a bridge between experiments examining artificial situations and the mimicry process in nature, but these systems do not include all types of mimicry. The presence of dangerous or deadly models is thought to alter the usual rules for mimicry complexes. In particular, a deadly model is expected to protect a wide variety of mimics. Avoidance of different types of mimics should vary according to how closely they resemble the model. Coral snake mimicry complexes in the neotropics may provide natural systems in which these ideas can be examined, but there is no direct evidence that the patterns of venomous coral snakes or potential mimics are avoided in the wild. Plasticine replicas of snakes were used to assess the frequency of avian predation attempts as a function of color pattern. Avian predators left identifiable marks on the replicas, the position of which indicated that replicas were perceived as potentially dangerous prey items by birds. The number of attacks on unmarked brown replicas was greater than that on tricolor coral snake banded replicas. This result was true whether replicas were placed on natural or plain white backgrounds, suggesting that coral snake banded patterns function aposematically. In a separate experiment, replicas representing all six patterns of proposed coral mimics at the study site were attacked less often than unmarked brown replicas. Within these six banded patterns, some were attacked significantly more often than others. This study provides direct field evidence that coral snake banded patterns are avoided by free-ranging avian predators and supports theoretical predictions about mimicry systems involving deadly models.     

Linking habitat modification to catastrophic shifts and vegetation patterns in bogs

Paleoecological studies indicate that peatland ecosystems may exhibit bistability. This would mean that these systems are resilient to gradual changes in climate, until environmental thresholds are passed. Then, ecosystem stability is lost and rapid shifts in surface and vegetation structure at landscape scale occur. Another remarkable feature is the commonly observed self-organized spatial vegetation patterning, such as string-flark and maze patterns. Bistability and spatial self-organization may be mechanistically linked, the crucial mechanism being scale-dependent (locally positive and longer-range negative) feedback between vegetation and the peatland environment. Focusing on bogs, a previous model study shows that nutrient accumulation by vascular plants can induce such scale-dependent feedback driving pattern formation. However, stability of bog microforms such as hummocks and hollows has been attributed to different local interactions between Sphagnum, vascular plants, and the bog environment. Here we analyze both local and longer-range interactions in bogs to investigate the possible contribution of these different interactions to vegetation patterning and stability. This is done by a literature review, and subsequently these findings are incorporated in the original model. When Sphagnum and encompassing local interactions are included in this model, the boundaries between vegetation types become sharper and also the parameter region of bistability drastically increases. These results imply that vegetation patterning and stability of bogs could be synergistically governed by local and longer-range interactions. Studying the relative effect of these interactions is therefore suggested to be an important component of future predictions on the response of peatland ecosystems to climatic changes.     

Competitive dominance mediates the effects of topography on plant richness in a mountain grassland

Small-scale landforms influence plant species richness, but their mechanisms and effects in semi-natural dry grasslands have been poorly investigated. In this study we compared vascular plant richness, species composition, plant traits, soil properties and biomass nutrient content of convex (hillocks) and concave (hollows) karst landforms in a mountain pasture of the Central Apennines (Italy), at a small spatial scale (1 m² plots). We found hillocks had significantly higher species richness than hollows. On hillocks, smaller Specific Leaf Area and Lateral Width, together with greater allocation of resources to Below-Ground Organs, indicated lower water availability, whereas hollows had deeper (thus moister), more acidic and more fertile soils, with aboveground plant biomass displaying higher nutrient levels. Partial correlation and regression tree models suggested that fine-scale richness patterns were not directly determined by abiotic properties, but were rather the result of competition levels associated with the cover of Agrostis capillaris (=A. tenuis) – a calcifuge and drought-sensitive grass able to achieve dominance only in hollows. The higher functional convergence exhibited by hollows suggests that A. capillaris is a strong competitor both above- and below-ground, mediating the effects of topography by imposing a biotic filter. On hillocks, competition is released by lower levels of available soil water in summer and higher soil pH, resulting in higher species richness and a more functionally divergent assemblage.     

Effects of microtopography on mycorrhizal infection in Atlantic white cedar (Chamaecyparis thyoides (L.) Mills.)

 The presence and intensity of mycorrhizal infection in wetland trees has received relatively little attention. We report here a study of mycorrhizal infection in Atlantic white cedar (Chamaecyparis thyoides), a member of the Cupressaceae, which forms monospecific stands in swamps throughout the Atlantic Coastal Plain of North America. The trees grow on the tops of elevated hummocks, but the fine roots extend along the sides of the hummocks to the flooded hollows. Roots from all microtopographic positions on the hummocks are colonized by vesicular-arbuscular mycorrhizae (VAM). In addition to arbuscules and vesicles, occasional hyphal coils are observed within the cortex cells. However, frequencies of occurrence of vesicles and arbuscles are significantly higher on the tops and sides than at the bottoms of the hummocks. These differences correspond to higher concentrations of acetylglucosamine in the roots at these positions. Frequencies of all mycorrhizal structures (arbuscles, vesicles and hyphae) in roots at the base of the hummocks are very low. These results suggest that mycorrhizal colonization in wetland trees is greater in aerobic microsites, a finding in accord with results from studies of both herbaceous wetland plants and other wetland trees. Accepted: 5 September 1998     

C<Subscript>4</Subscript> grasses in boreal fens: their occurrence in relation to microsite characteristics

C₄ plants are rare in cool climates, an ecological pattern attributable to their poor photosynthetic performance at low temperatures relative to C₃ species. However, some C₄ species are able to persist at high latitudes and high elevations, possibly due to the characteristics of the particular microsites they inhabit in these otherwise unfavourable environments. One such species is Muhlenbergia glomerata, which occurs above 60°N in Canada and is found in the atypical C₄ habitat of boreal fens. In this study, we evaluate how microsite features affect the success of M. glomerata in boreal fens. We surveyed 19 populations across northern Ontario during the summers of 1999 and 2000. The ground coverage by woody vegetation was the most important parameter affecting the presence or absence of M. glomerata. Woody plants covered over 50% of the ground area in plots where M. glomerata is absent, but less than 20% where it is present. The minimum light intensity threshold for the presence of the C₄ species was about 32% of full-sunlight at plant height. Surprisingly, in boreal fens M. glomerata was largely restricted to the wetter moss hollows, rather than occurring on the dry hummocks where its greater water use efficiency might have been advantageous. Woody species dominated the hummocks, but were uncommon in the hollows. In these cool northern climates M. glomerata apparently persists because sufficient periods of temperatures favourable to C₄ photosynthesis occur, but this persistence likely requires some factor that suppresses the woody vegetation.