Abstract
BackgroundTree allometric equations are critical tools for determining tree volume, biomass and carbon stocks. However, there is a lack of species-specific biomass equations for juvenile trees of many of New Zealand’s indigenous species. The aim of this study was to provide allometric equations for total above- and below-ground biomass and total root biomass and length for eight common evergreen conifer and broadleaved species.MethodsIn a plot-based field trial, growth metrics of conifers Prumnopitys taxifolia (matai), Agathis australis (kauri), Prumnopitys ferruginea (miro), Podocarpus totara (totara), Dacrycarpus dacrydioides (kahikatea) and Dacrydium cupressinum (rimu) and broadleaved species Alectryon excelsus (titoki) and Vitex lucens (puriri) were measured annually. These species were selected based on their potential role as a long-term solution for mitigating erosion in areas of marginal land proposed for new afforestation/reforestation and as an important carbon (C) sink.ResultsRoot collar diameter (RCD) provided the best fit for tree height, total above-ground biomass (AGB) and total below-ground biomass (BGB), and all regressions were highly significant (P = 0.001). Most species showed significant increases in annual growth and, by year 5, the BGB ranged between 21 and 42% of total biomass and decreased with increasing plant age. Of the conifers, Podocarpus totara had the greatest mean maximum root spread (2.2 m) exceeded only by the broadleaved Vitex lucens (2.5 m). For all species, and in each year of the trial, 100% of the BGB remained confined to within 0.5 m of the ground surface. With the exception of Vitex lucens and Podocarpus totara, > 90% of the total root length remained within a 0.5-m radius of the root bole. The species-specific mean tree biomass of 5-year-old plants ranged from 0.32 to 4.28 kg plant−1. A mixed-species forest established at 1000 stems per hectare (spha), consisting of 200 of each of the best performed of the trialled species, would amass ~ 2.3 t ha−1 of biomass and a forest carbon stock of 3.8 t CO2 ha−1 within 5 years.ConclusionsInter-species differences in the allocation of BGB and AGB appeared to be age dependent. The root-growth metrics of these common indigenous forest species, as candidates for erosion control, have improved our understanding of their potential usefulness for stabilising marginal land. Whole-plant biomass of juvenile trees will greatly improve the accuracy of current estimates of forest carbon stocks for proposed new areas of indigenous afforestation/reforestation.
Highlights
Tree allometric equations are critical tools for determining tree volume, biomass and carbon stocks
Larger still were Podocarpus totara and Dacrycarpus dacrydioides, with a similar Root collar diameter (RCD) as each other (~ 48–50 mm), while Vitex lucens was the best performed of the trialled species with an RCD of 77.6 mm, almost twice that of the best performed species and more than three times that of the slowest growing Prumnopitys ferruginea (Additional file 2)
As suggested by Wagner and Ter-Mikaelian (1999), RCD may serve as a better predictor of both the above- and below-ground biomass as it is measured at the intersection of the base of the stem with the root bole
Summary
Tree allometric equations are critical tools for determining tree volume, biomass and carbon stocks. New Zealand was almost completely vegetated with indigenous evergreen podocarp (mainly conifer) and broadleaved forest in the lowlands (~ 200 m), transitioning to Nothofagus forest at elevations above 600 m to the tree line at 1050 m and alpine-subalpine shrubland and grasslands on the highest parts of the axial ranges (McGlone 1988; Wilmshurst 1997). Exotic forests established on former grasslands since 1990 are likely to provide the major carbon offsets, but it has been recognised that an important additional carbon sink could be created through afforestation/reforestation of steep, erosion-prone pastoral hill country that environmentally is marginal for long-term agriculture by using indigenous shrubs and trees (Tate et al 2000; Tate et al 2003, Trotter et al 2005). A range of additional objectives in sustainable environmental management will be achievable These include improving indigenous biodiversity, erosion mitigation and soil conservation and consequent improvements in water quality. As suggested by Hasselmann (1997), active net carbon accumulation over such time frames is consistent with the prolonged effort likely required to effect significant reductions in atmospheric carbon dioxide (CO2) levels
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