Chapter 2: Stand structure and structural attributes

Chapter 2: Stand structure and structural attributes

Chapter 2: Stand structure and structural attributes

Chapter summary

This chapter reviews the literature concerning forest and woodland structure at the scale of an individual stand. Stand structure is defined in terms of structural attributes and stand structural complexity. Stand structural complexity is considered to be a combined measure of the number of different attributes present and the relative abundance of each of these attributes. The review indicates there is no definitive suite of structural attributes; different authors emphasise subsets of different attributes, and relatively few studies provide quantitative evidence linking attributes to the provision of faunal habitat or other measures of biodiversity, although a number of studies identify attributes that distinguish between successional stages. A summary of key structural attributes identified in the literature is presented under the following stand elements: foliage arrangement, canopy cover, tree diameter, tree height, tree spacing, tree species, stand biomass, understorey vegetation, and deadwood.

2.1 Defining forest and woodland structure

2.1.1 Ecosystem structure, function and composition

Ecosystems are frequently characterised in terms of their species and genetic composition (Hunter, 1999; Claridge et al.,1997; Magurran, 1988), even though this approach ignores ecological processes (e.g. natural disturbance, the decomposition of woody debris, the cycling of nutrients etc.), which are critical for the maintenance of biodiversity (Noss, 1990). An alternative approach, first suggested more than two decades ago, (e.g. Franklin et al., 1981), is to describe forest ecosystems by attributes relating to ecosystem structure and function in addition to those describing composition (Franklin et al., 2002; Noss, 1990; Franklin, 1988). In this approach: ? Structure refers to the spatial arrangement of the various components of the

ecosystem, such as the heights of different canopy levels and the spacing of trees; ? Function refers to how various ecological processes, such as the production of organic matter, are accomplished and to the rates at which they occur; ? Composition refers to the identity and variety of ecosystem components, as characterised by species richness and abundance.

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Chapter 2: Stand structure and structural attributes

2.1.2 Structural attributes

The structural, functional and compositional attributes of a stand are often interdependent, so that attributes from one group may also be surrogates for attributes from another group (Franklin et al., 2002; Ferris and Humphrey, 1999; Noss, 1990). For example a structural attribute such as dead wood can also be a good indicator of functional attributes such as decomposition and nutrient cycling processes (Franklin et al., 1981). Similarly, compositional attributes, such as species composition and abundance can be indicators of structural attributes such as canopy layering (Franklin et al., 2002), or of functional attributes such as flowering and bark shedding (Kavanagh, 1987). The division of attributes into three groupings is therefore by no means a clear categorisation. In order to define structure in an unambiguous manner, this paper will therefore pool structural, functional and compositional attributes into a single category simply called structural attributes.

2.1.3 Stand structure

Stand structure is commonly defined in terms of two components in the ecological literature - stand structural attributes and stand structural complexity. Stand structural attributes can include measures of: ? Abundance - e.g. density of large trees (Acker et al., 1998), volume of dead

wood (Wickstrom and Erickson, 2000); ? Relative abundance - e.g. foliage height diversity (MacArthur and MacArthur,

1961), dbh 2 diversity (Gove, 1996; Buongiorno et al., 1994), basal area of deciduous tree species (Spies and Franklin, 1991). ? Richness - e.g. overstorey species richness (Munks et al., 1996), eucalypt species richness (Bauer et al., 2000), shrub species richness (Seddon et al., 2001); ? Size variation - e.g. standard deviation of dbh (Spies and Franklin, 1991); ? Spatial variation - e.g. coefficient of variation of distance to nearest neighbour (Franklin et al., 1981); Attributes that quantify variation are particularly important because these can

2 dbh or diameter at breast height, is the diameter of a tree measured at a standard height above the ground on the uphill side of the tree. In Australia this standard height is 1.3m.

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Chapter 2: Stand structure and structural attributes

also describe habitat heterogeneity at the stand scale. For example the coefficient of variation of shrub cover would be a measure of the patchiness of understorey vegetation, a feature which is important for some macropod species (Lunney and Ashby, 1987).

Stand structural complexity is essentially a measure of the number of different structural attributes present and the relative abundance of each of these attributes. Structural complexity is used in preference to structural diversity because the latter term is considered ambiguous. This reflects the work of a number of authors in which a diversity measure, such as the Shannon-Weiner Index, has been used to quantify a single attribute, such as variation in stem diameter (e.g. Gove et al., 1995; Buongiorno et al., 1994). This quantity is then deemed a measure of structural diversity and to be indicative of biological diversity. In reality all that has been quantified is one of many possible attributes, and by most measures of diversity a system with one attribute or element has a diversity of zero (Magurran, 1988).

2.2 Stand structural attributes

2.2.1 Overview of attributes To be an efficient and effective biodiversity surrogate, any measure of structural complexity will need to be based on an appropriate suite of structural attributes. This suite should be sufficiently comprehensive to capture the variety of structural components that occur in forests and woodlands, reflect observed relationships with faunal diversity, and remain concise enough to function as a practical tool for land managers. In this section, I review the range of structural attributes used to characterise stand structure.

The studies that were reviewed and their associated attributes are summarised in Table 1. For clarity, attributes have been grouped in Table 1 under the stand element they aim to describe (e.g. foliage, tree diameter etc.). For studies where a large number of attributes were tested, only those attributes that proved significant in the modelling process have been included in Table 1.

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Chapter 2: Stand structure and structural attributes

Table 1: Attributes used to characterise stand structure. Attributes are grouped under the stand element they aim to describe. Superscript indicates an alternative measure used by a particular author.

Stand element Foliage Canopy cover

Tree diameter

Tree height Tree spacing

Attribute

Foliage Height Diversity

Number of strata

Foliage density within different strata Canopy cover Gap size classes Average gap size and the proportion of canopy in gaps Proportion of tree crowns with broken and dead tops Tree dbh

Standard deviation of dbh (coefficient of variationA) Tree size diversity Horizontal variation in dbh

Diameter distribution

Number of large trees

Height of overstorey Standard deviation of tree height Horizontal variation in height Height class richness Clark Evans Index, Cox IndexB, percentage of trees in clustersC

Number of trees per ha

Source

Sullivan et al., 2001; Tanabe et al., 2001; Berger and Puettmann, 2000; Gove, 1996; MacArthur and MacArthur, 1961; Tanabe et al., 2001; Van Den Meersschaut et al., 1998; Maltamo et al., 1997; Uuttera et al, 1997; Koop et al., 1994; MacArthur and MacArthur, 1961;

Bebi et al., 2001; Ferris-Kaan et al., 1996;

Parkes et al., 2003; Watson et al., 2001; Van Den Meersschaut et al., 1998; Newsome and Catling, 1979

Tyrell and Crow, 1994;

Tanabe et al., 2001; Ziegler, 2000; Tyrell and Crow, 1994;

Spies and Franklin, 1991;

Tanabe et al., 2001; Ziegler, 2000; Ferreira et al., 1999; Uuttera et al., 1997; Acker et al., 1998; Spies and Franklin, 1991; Zenner, 2000; Acker et al., 1998; Van Den Meersschaut et al., 1998; Spies and Franklin, 1991; Franklin et al., 1981A; Wikstrom and Erickson, 2000; Gove, 1996; Buongiorno et al., 1994;

Zenner, 2000;

Bachofen and Zingg, 2001; Uuttera et al., 2000; Ferreira and Prance, 1999; Maltamo et al., 1997; Kappelle et al., 1996; Tyrell and Crow, 1994; Koop et al., 1994; Ziegler, 2000; Acker et al., 1998; Van Den Meersschaut et al., 1998; Tyrell and Crow, 1994; Koop et al., 1994; Spies and Franklin, 1991;

Dewalt et al., 2003 ; Bebi et al., 2001 ; Means et al., 1999 ; Spies, 1998; Kappelle et al., 1996; Koop et al., 1994;

Zenner, 2000;

Svennson and Jeglum, 2001; Zenner, 2000;

Sullivan et al., 2001 ;

Bachofen and Zingg, 2001; Svensson and Jeglum, 2001B; Bebi et al., 2001C; Zenner, 2000; Pretzsch, 1997;

Bachofen and Zingg, 2001; Uuttera et al., 2000; Ferreira and Prance, 1999; Acker et al., 1998; Kappelle et al., 1996; Spies and Franklin, 1991;

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Chapter 2: Stand structure and structural attributes

(Table 1 continued)

Stand element Stand biomass Tree species

Understorey vegetation

Deadwood

Attribute

Source

Stand basal area

Stand volume

Species diversity and / or richness Relative abundance of key species Herbaceous cover and/or its variation

Shrub cover

Shrub height Total cover of understorey Understorey richness Understorey stems (SaplingD) per ha

Number, volume or basal areaE of stags (by decay classesF, by diameter classG)

Volume of coarse woody debris Log volume by decay or diameterH classes Log lengthI or cover Coefficient of variation of log density

Litter biomass or coverJ

Denslow and Guzman, 2000; Ziegler, 2000; Means et al., 1999; Ferreira and Prance, 1999; Kappelle et al., 1996; Koop et al., 1994;

Uuttera et al., 2000; Means et al., 1999; Ferreira and Prance, 1999; Spies, 1998;

Sullivan et al., 2001; Uuttera et al., 2000; Van Den Meersschaut et al., 1998; Maltamo et al., 1997; Uuttera et al., 1997;

Berger and Puettmann, 2000; Ziegler, 2000; Wikstrom and Erickson, 2000; Spies and Franklin, 1991;

Parkes et al., 2003; Watson et al., 2001; Van Den Meersschaut et al., 1998; Spies and Franklin, 1991; Newsome and Catling, 1979;

Parkes et al., 2003; Watson et al., 2001; Berger and Puettmann, 2000; Spies and Franklin, 1991; Newsome and Catling, 1979;

Berger and Puettmann, 2000;

Spies and Franklin, 1991;

Sullivan et al., 2001; Van Den Meersschaut et al., 1998;

Dewalt et al., 2003; Van Den Meersschaut et al., 1998D; Spies and Franklin, 1991D;

Dewalt et al., 2003; Bachofen and Zingg, 2001; Svennson and Jeglum, 2001G; Sullivan et al., 2001; Van Den Meersschaut et al., 1998E; Tyrell and Crow, 1994E; Spies and Franklin, 1991F; Franklin et al,. 1981F;

Sullivan et al., 2001; Svennson and Jeglum, 2001; Ziegler et al., 2000; Wikstrom and Erickson, 2000; Tyrell and Crow, 1994;

Dewalt et al., 2003; Van Den Meersschaut et al., 1998H; Tyrell and Crow, 1994; Spies and Franklin, 1991; Franklin et al., 1981; Parkes et al., 2003I; Watson et al., 2001; Newsome and Catling, 1979;

Spies and Franklin, 1991;

Denslow and Guzman, 2000; Parkes et al., 2003; Watson et al., 2001; Newsome and Catling, 1979;

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