Soil and vegetation responses to hydrological manipulation in a partially drained polje fen in New Zealand

Abstract

Anthropogenic drainage causes loss of natural character in herbaceous wetlands due to increased soil oxygen penetration. We related vegetation gradients in a New Zealand polje fen to long-term effects of drains by using hydrological, edaphic and vegetation data, and a before-aftercontrol-impact (BACI) design to test responses to experimental drain closure. Soil profiles and continuous water level records revealed a site subject to frequent disturbance by intense but brief floods, followed by long drying periods during which areas close to drains experienced lower water tables and more variable water levels. Classification of vegetation data identified 12 groups along a moisture gradient, from dry areas dominated by pastoral alien species, to wet communities dominated by native wetland sedges. Lower total species diversity and native representation in pastoral communities were related to the high proportion of alien competitor and competitor-disturbance species, compared with the stress tolerator-dominated flora of other groups. Species-environment relationships revealed highly significant correlations with soil water content and aeration as measured by redox potential (E H ) and steel rod oxidation depth, as well as soil nutrient content and bulk density. Comparison of root anatomy confirmed greater development of floodtolerant traits in native species than in pastoral aliens, and vegetation N:P ratios indicated that most communities were probably nitrogen-limited. Flooding rapidly re-established wetland hydrology in dried sites in the impact area, and lowered E H and soil oxidation depth, but had no effect on N and P availability. Presence and cover of pastoral alien species decreased in these areas. This study supports the use of hydrological manipulation as a tool for reducing soil oxidation and thus the impact of alien plant species at restoration sites with minimal intervention, but suggests the need for detailed information on species flooding tolerances and hydrological preferences to underpin this approach.

Temporal variation of invertebrate communities in perennial wetlands

“…Fast, bedmoving flows that disturb benthic invertebrates are unlikely. For instance, Sorrell et al (2007) found that water depth in the central main drain (N2) of Bullock Creek increased by up to seven times during a rainfall period, whereas velocity only doubled, from 0.2 to 0.4 m/s. This was still not fast enough to initiate movement of the gravel-bed substrate.…”

“…and podocarp conifers (Podocarpaceae). The wetland is a mosaic of vegetation types, separated by a drainage network established during the first half of the twentieth century to convert the wetland into pasture (Sorrell et al 2007). Three site groupings have been identified within the BCW (Suren et al in press), based on differences in water chemistry, presence of aquatic vegetation, and substrate nature.…”

Phosphorus and nitrogen allocation in Allium ursinum on an alluvial floodplain (Eastern France). Is there an effect of flooding history?

“…This may explain why P was less accumulated in bulbs and leaves during growth in unflooded forests (Table 4). Nitrogen seems to be the limiting factor in the flooded sites as has been shown by Sorrell et al (2007) and Antheunisse et al(2006), probably due to the nitrogen removal by denitrification which commonly occurs in saturated soils (Rutherford and Nguyen 2004). Indeed, we have shown that bulb and leaf N:P ratios are highly correlated to total nitrogen in soils and not to soil phosphorus (Table 5).…”

“…While most studies of nutrient bioavailability matched up plant nutrient content with upland soil fertility (Aerts and Decaluwe 1994;Boerner 1984;Vitousek 1982), the importance of flooding in nutrient availability was widely highlighted (Ogden et al 2007) but still remained controversial as some recent studies showed that flooding re-establishment had no effect on N and P bioavailability (Ogden et al 2007;Sorrell et al 2007). …”

Long-Term Impacts of Forest Ditching on Non-Aquatic Biodiversity: Conservation Perspectives for a Novel Ecosystem

“…Additionally, those species that have special traits for tolerating flooding may be replaced by generalists, less flood-tolerant alien species, and native taxa typical of drier forests [2], [31], [32]. The new conditions may support some rare species absent from wetlands; for example, terrestrial orchids seem to benefit from the shade and nutrient release in northern decayed-peat forests [33].…”