Aggregate height—A composite measure of stand density for tree seedling populations

doi:10.1016/j.foreco.2005.11.014

Forest Ecology and Management

Volume 223, Issues 1–3, 1 March 2006, Pages 336-341

https://doi.org/10.1016/j.foreco.2005.11.014 Get rights and content

Abstract

Aggregate height, a composite measure of seedling size and number per unit area, is proposed for describing stand density of seedling populations during early stand development. Aggregate height is analogous to other stand density measurements, such as basal area, that incorporate both size and number of trees into a single value. A large scale dataset (3452 subplots in 33 stands) from the central Appalachians and a long-term dataset (six stands from pre- to 20-years post-harvest) from the Missouri Ozarks were used to compare the utility of number of seedlings, mean height, aggregate height squared, and aggregate height for examining patterns of stand development. Comparisons of coefficients of determination (r²) indicate that aggregate height produces more deterministic models for describing the early development of tree seedling populations than do number of seedlings, mean height, and aggregate height squared. Stand developmental paths based on number of seedlings, basal area, and aggregate height reveal that the onset of the self-thinning stage coincides with the shifting of mean stand aggregate height, but not other measurements. The results indicate that aggregate height is an efficient measure of stand density to describe the relative prevalence of a species at a given stage of development and its ability to persist into future stages.

Introduction

The measurement of stand density has been one of the major research topics in forestry. Stand density is any numerical quantity obtainable by measurement of a stand and expressible on an area basis, such as number of trees, total basal area, total volume, stand density index, or other criteria per unit area (Ford-Robertson, 1971). Compared to a relative standard or normal condition, stand density measurements provide a means to assess the level of competition, area occupancy, and stand closure (Curtis, 1970). Basal area is often regarded as the standard unit for measuring stand density and has the sanction of long usage (Bickford et al., 1957). It is not only simple, objective, and easy to use; but it also efficiently describes overall stand conditions and the abundance of individual species. However, basal area is obviously not applicable to stands that are too young to take the standard measurement of stem DBH (diameter at the breast height (1.37 m)).

Increasing interest in hardwood regeneration, particularly in eastern mixed-oak stands in North America, has created a need to understand and predict stand density changes in young developing stands better. Early stand development is a seemingly chaotic period characterized by rapid tree growth and rapid changes in species composition. The environment, growth pattern, and size of each plant change more dramatically during this stage than during any other period (Oliver and Larson, 1996). Monitoring and understanding this critical period of stand development is difficult and challenging because young forest stands are often highly complex and exhibit seemingly unpredictable responses to a multitude of important factors. Hence, an appropriate measure of young stand density that can serve a similar role as basal area in older stands is desirable.

Although the standard measure of stem diameter is not applicable to very young stems, seedling height can be easily and accurately measured until the trees are large enough to take DBH measurements. Tree height has been used to measure stand density (Wilson, 1946). In this paper, we propose aggregate height as an alternative measure of stand density for young stands. Aggregate height is defined as the total height of all individuals of a species or species group per unit area. We argue that aggregate height is a useful approach for describing stand density in the early stage of stand development. We demonstrate that aggregate height more predictably describes stand density than do alternative measure of stand density. We also show that aggregate height has the useful property of increasing monotonically with stand development until approximately the point of maximum stand density, after which it decreases.

Section snippets

Data collection

Two datasets were used in this study. The first was collected in 33 mixed-oak stands in Pennsylvania. Depending on stand size, 15 to 30 permanent plots with 8.02 m radii (0.05 acre) were systematically installed in a square grid to represent the whole stand. Four permanent subplots with 1.13 m radii (0.001 acre) were established within each plot at a fixed distance from plot center. In total, 3452 subplots were established in the study area. On each subplot, tree seedlings (<5.1 cm in DBH) were

Results

Regression coefficients (b) and coefficients of determination (r²) for linear regression models comparing mean number of trees, mean height, mean aggregate height squared, and mean aggregate height between measurement periods are shown in Table 1. With one exception (red maple aggregate height squared 01 versus 00), aggregate height exhibited the strongest correlations between measurement periods based on coefficients of determination.

Changes in stand density were most variable between

Discussion and conclusions

Early stand development following major disturbances is a chaotic period characterized by rapid changes in tree number and size. Compared to other stages of development, this period is poorly understood partially because of the difficulty of measuring the relative potential of different species for future site occupancy. Our results indicate that aggregate height is a promising variable for measuring stand density in the early period of stand development. Aggregate height reveals the relative

Acknowledgements

This research was supported by the Pennsylvania Bureau of Forestry. We are grateful to the personnel of the Bureau of Forestry who provided the stands and field assistances. The authors also thank Daniel Dey of the U.S. Forest Service North Central Research Station for supplying the Missouri Ozark regeneration data.

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Aggregate height—A composite measure of stand density for tree seedling populations

Abstract

Introduction

Section snippets

Data collection

Results

Discussion and conclusions

Acknowledgements

For. Ecol. Manage.

Stocking, normality, and measurement of stand density

J. For.

Stand density measures: an interpretation

For. Sci.

Measuring and evaluating stocking and stand density in upland hardwood forests in the Central States

For. Sci.