Vegetation diversity, growth, quality and decomposition in managed grasslands

The relationship between vegetation growth rate, quality, litter decomposition and diversity was investigated in four grazed grasslands differing in topography, management and fertility in the southern North Island of New Zealand. Vegetation samples were clipped at two sampling times (summer and autumn) from 0.5 m2 areas excluded from grazing. Crude protein (CP), in vitro dry matter digestibility (IVDMD), neutral detergent fiber (NDF), average daily growth (ADG) rate, species composition and the relative extent of tissue degradation by soil microorganisms were determined for the vegetation samples. It was hypothesized that vegetation diversity would be negatively related with measures of vegetation growth, quality and decomposition. Comparisons made across all environmental and management conditions agreed with the hypotheses. Higher vegetation diversity was associated with lower ADG rate, lower CP concentrations, lower IVDMD and higher NDF concentrations. Comparisons within management regimes (MR), however, were less consistent and possible reasons for this are examined. Litter decomposition extent was closely related to the tissue quality characteristics of the vegetation, especially NDF concentration, but not species diversity. The observed local-scale relationships between the vegetation diversity, growth, quality and decomposition characteristics were contended to be largely the result of the influence of environmental resource richness on community structure of the managed grasslands studied. There is a need to promote a functionally diverse plant community over the entire managed area rather than promoting high localized species diversity within grasslands.