The Scientific Reserves of Auckland University. Ii. Quantitative Vegetation Studies
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TANE 29, 1983 THE SCIENTIFIC RESERVES OF AUCKLAND UNIVERSITY. II. QUANTITATIVE VEGETATION STUDIES by John Ogden Department of Botany, University of Auckland, Private Bag, Auckland SUMMARY Forest vegetation surveys, using both plots and point-centred-quarter (plotless) methods, were undertaken by students in the University reserves at Swanson, Huapai, Oratia and Leigh over the period 1979 to 1982. The two methods gave similar estimates of species composition, total basal area and total density, but both gave wide confidence intervals on the parameters being estimated. The "leading species", in terms of both density and basal area, are defined for each area. The differences between the rankings for these different vegetation parameters are commented upon A regression of age on height of kauri (Agathis australis) seedlings is presented. The relationship between age and trunk diameter is discussed. It is shown that in cores taken from kauri poles in ricker stands over the size range 10 to 40 cm DBH the relationship between age and diameter is very weak. When a wider size range is employed a highly statistically significant relationship is obtained. It is concluded that the highly skewed size class frequency distribution of kauri, characteristic of such stands, reflects a competitive hierarchy. The senile population structure of kanuka (Leptospermum ericoides) in ricker stands confirms their serai role in forest development. The basal area estimates are discussed in the context of other figures for New Zealand forests. It is concluded that although the values are high (average 70 m2 ha 1) they are nevertheless below those commonly found in mature kauri forest. INTRODUCTION The natural vegetation of the Waitakere Ranges is discussed by Esler (1983) in this volume. Before the advent of European settlement the ranges were covered with temperate forest dominated by kauri (Agathis australis) or mixtures of podocarp and broad-leaved hardwood trees. Much of this bush has been milled and/or burned, and is now regenerating through Leptospermum scrubland (described by Esler and Astridge 1974). Consequently much of the vegetation is serai, and, if undisturbed, will change in structure and species composition in the future. This is particularly true of the dense kauri "ricker" stands which characterise many ridges. These stands are mostly in the process of 163 replacing Leptospermum communities (usually L. ericoides), and are recruiting seedlings and saplings of hardwood species which may ultimately form the understorey of mature kauri communities. The early stages of this process have been documented by Mirams (1957). This paper describes some of the results of University field courses on quantitative descriptive forest ecology in the University reserves at Swanson, Huapai, Oratia and Leigh in the years 1979 to 1982. The aims of the field courses have been (in part): (1) to compare plot and plotless sampling techniques for estimating forest structure and composition; (2) to provide some quantitative descriptive parameters for the areas being studied; (3) to describe the population structures of the kauri ricker communities on the sites. The detailed locations of the sites, and general descriptive and historical information, are given in the first paper in this series (Thomas and Ogden 1983 - this volume). The aim here is to introduce the field methods employed, and to present some of the results obtained. These results comprise quantitative descriptions, both of the general nature of the forest community, and of some of the ricker stands occurring within the reserves. In view of the changing nature of the vegetation the data presented may become of increasing interest, although the variety of techniques employed limits their value in this respect. Working with a class of 20 or more students poses problems of standardisation, both of detailed technique and of species identification. However, a large amount of quantitative data can be collected quickly and this, to some degree, compensates for its variable quality. The problems can be reduced by careful briefing beforehand, and by standardised data collection sheets. Sometimes inconsistencies recognised during analysis can be allowed for, or corrected, but usually individual differences in measurement or recording accuracy are reflected in large standard errors on the parameters being estimated. Descriptive parameters in forest ecology The full description of forest vegetation requires data on the relative abundance and spatial pattern of the various tree species composing it, the quantity of plant material present (biomass) and its disposition through the different vertical strata. Understanding the dynamics of a forest requires also data on rates of productivity, energy flow and nutrient cycling, and some insight into past historical influences. The "abundance" of a tree species can be described by three parameters: (1) Density: Number of stems equal to or greater than some chosen minimum diameter per unit area. (2) Basal area: Sum of the cross-sectional areas of the trunks equal to or greater than the chosen minimum (at a specified height), again 164 expressed per unit area. (3) Frequency: The proportion of plots (of a specified size) in which the species occurs. All three parameters can be transformed into relative values by expressing them for each species, as a percentage of the sum for all species. Such relative values can be summed to give "importance values". Because density and basal area are largely independent, measure different attributes of species, and (unlike frequency) can be applied to the vegetation as a whole, we have concentrated on these two parameters. Further discussion can be found in most text books of quantitative ecology, (e. g. Greig-Smith 1964, Mueller-Dombois and Ellenberg 1974) and in Ogden and Powell (1979). METHODS Plot sampling Most of the vegetation description work was done at Swanson and Huapai in February 1980, and at Leigh in 1981. A single large (subdivided) plot was enumerated at Oratia in October 1982. Plot sampling employed 7 x 7 m plots arranged in a stratified random manner over a 1 ha area at both Swanson (24 plots) and Huapai (27 plots). At Leigh 28 10 x 10 m plots were arranged randomly along three transects running at different heights along the contours, and covering the whole of the bush patch. Plot sizes were determined by prior sampling, and represented a compromise between the "minimal area" required to ensure inclusion of most species and the practical difficulties of enumerating large plots. A minimum stem size of 10 cm "diameter at breast height" (DBH) was set. Tree ferns were recorded. Plotless sampling In most cases point-centred-quarter (pcq) sampling was performed on a square grid of 25 points located at 20 m intervals along five parallel transects 20 m apart. Thus the total sample covered approximately 1 ha. Each point was marked temporarily with a stake, and the space around it conceptually divided into four quadrants using the cardinal compass points. In each quadrant the nearest tree or tree-fern ( > 10 cm DBH) was chosen and identified. Its diameter and distance from the point was measured. Thus the final data sample usually comprised 100 trees identified and measured (DBH) and 100 point-tree distances. The latter were used to estimate tree density as follows: mean point-tree distance = m (in metres); mean area (MA) per tree = m2; density (stems per ha) = 10 000/5i2. 165 Note that the individual point-tree distance measures can be used to calculate a standard error and confidence limits for m, and hence for the density estimate. Thus an estimate of the reliability of the density estimate can be obtained. When 100 trees are included in the sample the relative density of each species is simply the number of times it is recorded. Basal area is obtained by calculating the basal area of each tree using If r2 (r = radius at breast height) and obtaining the mean. The mean tree basal area is then multiplied by the mean density per ha to obtain an estimate of basal area per na. There is some evidence that basal areas obtained by this method overestimate the true value. This appears to arise primarily from the non-random distribution of the smaller size classes, which tend to be aggregated in the "gaps" between the larger trees (Mark and Esler 1970). Size frequency distributions in ricker stands Detailed studies of the kauri ricker stands were made at all sites, enumerating stems ^2. 5 cm DBH in single rectangular plots divided into contiguous sub-plots. The total area sampled at each site, and the dates, are given in Table 4. As the original aim of this sampling was to obtain DBH frequency distributions for kauri and kanuka these species only were recorded at Swanson and Huapai in 1979 and 1980. Subsequently (in 1981) a repeat sample was taken in the Huapai ricker, and further samples at Leigh and Oratia. In all these latter cases all species with stems >2. 5 cm DBH were recorded. Tree ages Ages of kauri trees were obtained from increment cores. The cores were mounted and sanded to a fine polish so that individual cells and annual ring boundaries could be clearly seen using a low-powered binocular microscope. Twenty cores were obtained from the Swanson ricker, ten from Matheson's bush, and 34 from Huapai (including cores from some older trees on the ridge at Huapai). At Oratia two trees only were cored. Age was estimated from the number of rings, with a correction for the missing portion where the core failed to transect the pith. From each site (and some others) a few seedling kauris were cut at ground level. Sections were taken from these seedlings at various heights and the number of rings counted. From these data an average height growth rate was calculated and an addition made for the number of years required for growth to coring height.