Browsing by Author "Sorrell, BK"

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    Mangrove forest and soil development on a rapidly accreting shore in New Zealand
    (Springer, 2010-04) Lovelock, CE; Sorrell, BK; Hancock, N; Hua, Q; Swales, A
    Mangrove forests are rapidly expanding their distribution in New Zealand, which is at the southern limit of their range. We investigated how these expanding mangrove forests develop through time. We assessed patterns in forest structure and function at the Firth of Thames, which is a rapidly accreting mangrove site in New Zealand where 1 km of mangrove of Avicennia marina has established seaward since the 1950s. Across the intertidal region, mangrove forest structure was highly variable. We used bomb-pulse radiocarbon dating to age the forest. Two major forest establishment events were identified; one in 1978–1981 and another in 1991–1995. These events coincided with sustained El Niño activity and are likely the result of reduced wind and wave energy at the site during these periods. We used the two forests of different ages to assess whether mangroves in New Zealand mature at similar rates as other mangroves and whether they conform to classic models of succession. The timing of forest maturation is similar in New Zealand as in more tropical locations with trees exhibiting features of mature forests as they age from about 10 to about 30 years. In older forest (~30 years old) trees become larger and stands more homogenous than in the younger forest (~10 years old). Carbon and nutrient concentrations in soils increased and soils become more aerobic in older forest compared to younger forest. Additionally, using fertilization experiments, we established that despite reduced growth rates in older forests, nitrogen remained limiting to growth in both older and young forests. However, in contrast to classic successional models leaf tissue nutrient concentrations and nutrient conservation (nutrient resorption from senescence leaf tissue) were similar in forests of differing ages and did not vary with fertilization. We conclude that mangrove forest expansion in New Zealand is influenced by climatic factors. Mangrove forests mature rapidly, even at the limits of their range and they satisfy many of the successional patterns predicted by Odum (1969) for the early stages of forest succession. © 2010, Springer.
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    Wetland development nutrient accumulation and runoff in New Zealand dairy pastures with very high annual rainfall
    (Aarhus University, 2012-06-17) Chagué-Goff, C; Sorrell, BK; Duncan, MJ; Cotton, S
    Agricultural activity in regions with high annual rainfall raises particular challenges for managing nutrient runoff. Here we report on a four-year study of patterns of nutrient accumulation and loss in dairy pastures from a catchment of the West Coast of the South Island of New Zealand, receiving up to 3500 mm annual rainfall. Due to poor drainage in the gley soils, farming practice involves contouring the pasture in broad corrugations, termed ‘humps and hollows’, to improve drainage; grazing is restricted to the humps, whilst wetland vegetation (predominantly a marsh of Juncus and Carex species) naturally colonizes the wetter drainage hollows. We aimed to quantify nutrient accumulation and runoff at this site, paying particular attention to the function of the wetland hollows in nutrient cycling. Subsurface flow was found to be negligible in the poorly draining soil, with water loss and nutrient yields restricted to surface flows. No fertiliser was applied the first year at one site. Specific nitrogen and phosphorus yields ranged from 11-56 kg N ha-1 yr-1 and 2.0-13.4 kg P ha-1 yr-1. The highest P yield (mostly in particulate form) was recorded even when no P fertiliser was applied. Wetland vegetation and soils showed prolonged accumulation of both N and P throughout their development, but failed to intercept peak events that occurred when cattle were allowed to graze the pastures following heavy rainfall events. Potential soil denitrification, measured from denitrification enzyme activity experiments, was very low, with median values < 50 ng N g-1 h-1 throughout the study, predominantly due to low nitrate concentrations. We conclude that (i) significant nutrient discharge is a feature of this system; (ii) the natural wetland formation in drainage hollows is an important nutrient sink that mitigates discharge; and (iii) much of the remaining discharge could be ameliorated by improved farming practices. © University, DCE – Danish Centre for Environment and Energy