Spectral analysis of hummock-hollow pattern in a weakly minerotrophic mire

Assessing runoff generation in riparian wetlands: monitoring groundwater-surface water dynamics at the micro-catchment scale

Riparian wetlands (RW) are important variable source areas for runoff generation. They are usually characterised by a combination of groundwater exfiltration-which maintains saturated conditions in low-lying organic-rich soils-and direct precipitation. Both processes interact to generate overland flow as a dominant runoff process. The small-scale details of groundwater-surface water (GW-SW) interactions are usually not well understood in RW. Here, we report the results of a study from an experimental catchment in the Scottish Highlands where spatio-temporal runoff processes in RW were investigated using isotopes, alkalinity and hydrometric measurements. We focused on perennial micro-catchments within the RW and ephemeral zero-order channels draining peatland hollows and hummocks to better understand the heterogeneity in GW-SW interactions. The 12-month study period was dominated by the wettest winter (December/January) period on record. Runoff generation in the RW was strongly controlled by the local groundwater response to direct rainfall, but also the exfiltration of groundwater from upslope. This groundwater drainage is focused in the hollows in ephemeral and perennial drainage channels, but in wet conditions, as exfiltration rates increase, can affect hummocks as well. The hollows provide the dominant areas for mixing groundwater, soil water and direct rainfall to deliver water to the stream network as hollows "fill and spill" to increase connectivity. They also provide wet areas for evaporation which is evident in enriched isotope signatures in summer. Although there is some degree of heterogeneity in the extent to which groundwater influences specific micro-catchments, particularly under low flows, the overall isotopic response is quite similar, especially when the catchment is wet and this responses can explain the isotope signatures observed in the stream. In the future, more longitudinal studies of micro-catchments are needed to better explain the heterogeneity observed.

Nitrogen deposition changes the distribution of key plant species in the meadow steppe in Hulunbeier, China

Improved understanding of how nutrient levels affect the distribution of plants can provide important insights into the potential impacts of increasing global nitrogen (N) deposition. We used point pattern analyses to examine the impact of nutrient addition on heterogeneity in the spatial distribution of the three main plant species of the meadow steppe community of Hulunbeier, Inner Mongolia: Leymus chinensis (Trin.) Tzvel (aka Aneurotepidimu chinense), a rhizamotous grass; Stipa baicalensis Rasher, a bunch grass; and Artemisia tanacetifolia Linn, a rhizamotous forb. The six treatments tested added nitrogen N in three different concentrations, N with phosphorus (P), P alone and a Control. Although the three plant species were randomly distributed at the start of the experiment in 2011, the spatial distribution of some species in some treatments had changed at the end of 3 years of nutrient addition. There was a significant increase in aggregation of L. chinensis at fine scales of analysis from application of N and P in tandem. However, S. baicalensis and A. tanacetifolia distributions remained random under all treatments. Positive associations of L. chinensis with S. baicalensis and with A. tanacetifolia were apparent at the lowest concentration of added N, 2.5 g N m–2 year–1, which represented an approximate doubling of global N deposition. These associations, which represent clustering among individuals of these species were also apparent where only P was applied. Negative associations, representing dispersion, were prevalent with higher N concentrations. The results indicate that increases in global N deposition up to about double current levels may have a positive influence on meadow steppe communities by increasing the niche overlap of different species. However, increases beyond that level may trigger substantial ecological change through increased competition for other, more limited, environmental resources, and disassociation between plants of the different dominant species. Our findings suggest that studies of the spatial patterning of plant communities can contribute to understanding the potential impacts of climate change.

Bryophyte community structure in a boreal poor fen II: interspecific competition among five mosses

A possible competitive hierarchy among five boreal bryophytes (Sphagnum angustifolium, Sphagnum fuscum, Sphagnum magellanicum, Ptilium crista-castrensis, and Pleurozium schreberi) was examined in a 2-year transplant experiment conducted in two habitats of a poor fen in northern Alberta, Canada, using a sixth moss (Aulacomnium palustre) as a phytometer. Change in relative growth rate in length and weight of A. palustre, measured as competitive effect, indicated the existence of a hierarchy in this peatland wherein the feather mosses are subordinates to the Sphagnum mosses. Relative competitive performance, which was calculated relative to the growth of the phytometer in monoculture, indicated that S. magellanicum exhibited the greatest competitive effect. Sphagnum magellanicum may be a strong competitor and partially inhibit S. fuscum from occupying its full fundamental niche. The competitive ability of S. fuscum and S. angustifolium in this peatland is low relative to that of the other study species. The feather mosses did not demonstrate competitive effect and may have facilitated growth of the phytometer.Key words: bryophyte, competition, feather moss, peatland, phytometer, Sphagnum.

Microtopography and vegetation in Atlantic white cedar swamps: the effects of natural disturbances

Disturbance may alter patterns of microtopography within wetlands, and may consequently alter the relationship between woody plants and microtopography. I measured microtopographic relief and the relationship of shrubs and tree seedling distributions to microrelief in undisturbed Atlantic white cedar (Chamaecyparis thyoides (L.) Mills.)) swamps in the New Jersey Pinelands, and in stands disturbed by blowdowns and fire. In undisturbed stands, hummock–hollow structures, at a scale of about 1 m, are poorly defined and often obscured by smaller scale and larger scale patterns of elevation. The swamp floor is, on average, 25 cm above the lowest hollows, but sites ≥ 100 cm high occur in undisturbed swamps and within blowdown gaps. Burned sites tend to lack microsites with high elevations, and have less well-developed pattern than undisturbed sites. In blowdowns, there is an increase in the frequency of high-elevation microsites, and more pronounced hummock–hollow patterning. All woody plant species occupy sites of intermediate elevation, are absent from the lowest 20% of the forest floor, and display little evidence of habitat differentiation among species along the microtopographic gradient. In disturbed sites, the species' ranges shift in response to disturbance-induced changes in the availability of microsites. Key words: microtopography, microsites, forested wetlands, swamps, spatial pattern, disturbance, Chamaecyparis thyoides, shrubs.