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.
This paper presents results from 34 years of the Glendhu Experimental Catchment Study, established in 1979 by the former New Zealand Forest Service in upland east Otago in New Zealand's South Island to determine the hydrological consequences of converting indigenous tussock grassland to plantation forestry. A traditional paired catchment approach was adopted; after a 2.5‐year pretreatment period, one catchment (GH2, 310 ha) was planted over two thirds of its area in Pinus radiata, and an adjacent catchment (GH1, 216 ha) was left in tussock as a control. The average annual reduction in water yield from the planted catchment between canopy closure in 1991 and 2013, compared with that in tussock, was 273 mm (33%). Annual water yields from the planted catchment continued to decline relative to the tussock catchment until 2010. Since then, the difference in annual water yields between the two catchments has narrowed. Ripping before planting caused some redistribution of the total streamflow from stormflow to baseflow. Following canopy closure, afforestation has reduced the low flow (Q95) by an average of 26% compared with the tussock catchment. Average peak flows for small events (2–5 L/s/ha) were reduced by 78%, but only by 37% for larger, less frequent storms (>15 L/s/ha), suggesting that peak flows during high magnitude storms are less dependent on the prevailing land cover.
This study tested the effect of oat catch crops on mineral nitrogen (N) leaching losses from cool season fodder beet grazing. Undisturbed soil monolith lysimeters were collected from two grassland sites with soils featuring contrasting texture and water holding capacity (WHC) characteristics. After simulated fodder beet grazing in late autumn or winter, synthetic dairy cow urine was applied. Nitrogen leaching losses were measured from lysimeters sown with oats after urine application and compared with those under fallow conditions until spring. Oat dry matter (DM) production and N uptake measurements were obtained. Sowing oats reduced total mineral N leaching losses by up to 59%. Reductions in mineral N leaching were inconsistently affected by soil type but were strongly influenced by urine application timing. Nitrogen uptake by oats (52–143 kg N ha−1) drove reductions in N leaching losses compared with fallow soil. Oats yielded 4–17 t DM ha−1, and both yield and N uptake were strongly affected by urine application timing (winter > autumn) and soil type (high WHC > low WHC). Sowing oats after fodder beet grazing instead of leaving the ground fallow can reduce the environmental impacts of these systems, while simultaneously increasing annual feed supply. Catch crop gains can be maximised by avoiding or delaying autumn grazing of fodder beet, particularly on low WHC soils.
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