Abstract
Tawa (Beilschmiedia tawa (A. Cunn.) Kirk) remains a minor but significant hardwood timber in New Zealand, currently sourced from tawa-dominant forest on freehold and Maori land where selective harvesting under sustainable management plans is permitted. Sustainable management plans require reliable tree growth estimates, which are used to calculate annual volume increment and harvest levels. The aims of this study were to model the relationship between diameter growth rate and tree size using existing data, and to determine the influences of within-stand competition, local climate and soil parent material on growth rates. A dataset was compiled on the diameter growth rates of nearly 1800 trees from a wide range of published and unpublished sources for the middle North Island. Non-linear quantile regressions were used to model average growth and growth of the fastest 25% of stems, termed rapid growth. Across the middle North Island, average growth of a 400 mm DBH (diameter at breast height) tree was 1.8 mm year−1 and rapid growth was 2.8 mm year−1. Overall, the model accounted for 12% of the total variation in growth rate. The effect of size on diameter growth rate was modest but positive. Within-stand competition reduced tawa diameter growth rates, particularly when basal area exceeded 100 m2 ha−1. Also, climate (winter minimum temperature and annual vapour pressure deficit) had little influence on growth rates and explained 10% of variation in stem growth. The fastest growth rates were on soils derived from Tertiary mudstone in inland Taranaki. Most of the existing data on growth rates of tawa are for trees growing on volcanic substrates. This means that the markedly faster growth on some sedimentary rather than volcanic substrates suggests that growth rates and volume increment may be underestimated for many sites in the middle North Island beyond the Volcanic Plateau. A growth dataset more representative of the range of substrates occupied by tawa is needed for management plans elsewhere in the North Island.
Highlights
This study provides improved growth models for tawa that incorporate individual tree characteristics, withinstand competition and stand characteristics, local environment and geographic location, using new and existing data from the middle North Island
Growth was strongly influenced by basal area, i.e., within-stand competition, in the pooled National Vegetation Survey (NVS) plot data (Figure 1b)
Soil parent material may provide a simple guide to tawa diameter growth for sustainable forest management planning, with higher growth rates at the one site with Tertiary sedimentary soils
Summary
Sustained-yield management of indigenous forest was implemented in some State Forests from 1975 (New Zealand Forest Service, 1977) in response to growing public dissatisfaction with forest clearance and an increasing national awareness of the heritage values of indigenous forest It largely ended in 1984 with a government decision to halt harvesting in almost all indigenous forests in Crown tenure. The demand for high-quality indigenous timber persisted, which together with a growing recognition that timber supply and the maintenance of ecological integrity need not be mutually exclusive, led to the Forests Amendment Act 1993 This Act allows the continuing harvest of significant volumes of indigenous timber on freehold land under a ‘sustainable forest management plan’ approved by the Ministry of Primary Industries, setting annual or periodic timber harvest ‘at a rate matching the forest’s productivity and retaining its natural values and ability to continue to provide a full range of products and amenities in perpetuity’ (Ministry of Forestry, 1997). Some 116,000 ha of tawa-dominant forest is potentially available for sustainable management (Ministry of Primary Industries, unpubl. data)
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