3.2 Stand structure

In British Columbia, typical forest management objectives were designed to optimize the production of wood products from the working forest. Consequently, the goal of stand level activities was to harvest the sites and quickly restock them with commercial species, and to apply stand-tending practices that would ensure full site occupancy and maximum growth production. In some environments where uneven-aged stands originally occurred, (note that these are not all that common) stands have been simplified through forest management. In other environments where large areas of one age and species occurred naturally, forest management has increased forest-level and stand level diversity.   

Forest management has impacted the stand structural diversity in some or all of the following ways:

• Reduced size, quality, and distribution of coarse woody debris
• Reduced the natural distribution of standing dead trees/wildlife trees
• Reduced canopy layering and gaps
• Reduced diversity of understorey vegetation
• Reduced variability in tree sizes, and ages
• All of these contribute, in varying degrees, to the risk of reducing stand level biodiversity in BC. We do not yet know the severity of the risk, but many suggest that there will be a decline in native species diversity, and an increase in the number of endangered species.

A diminished level of diversity could also reduce opportunities for economic diversification.

Components of Stand Structure 

Stand structures refers to the vertical and horizontal make-up or appearance of the stand. Old forests tend to have high structural diversity, with some or all of the following characteristics:

• Multiple canopy layers of trees
• Wide variety of tree sizes (heights and diameters)
• Some very large trees (relative to the rest of the trees), scattered throughout the stand
• Mixed species of conifers and deciduous patches
• Significant amounts of course woody debris of all sizes and stages of decay
• Scattered windthrown trees
• Brush pockets associated with canopy gaps
• Varying amounts of decadence, (i.e., broken tops, split trunk, multiple tops, large limbs)
• Significant amounts of lichens growing above ground level on very large trees

These features create a range of habitats for a wild variety of plants and animals. Many management practices aimed at promoting biodiversity actually focus on promoting or protecting some of these features. The structural diversity of stands reflects the natural disturbance patterns across a landscape, and the natural successional processes that have occurred since the disturbance. (Figure 7, right)

Before reading on, what do you already know about them?

The horizontal structure is a mix of successional stages that creates a habitat mosaic of older forest, younger forest, openings or gaps in the canopy (containing early seral vegetation), and edges (See Figure 7, right).

Name the locations of several good examples of horizontal stand structures

Horizontal structure describes the patchiness in forest stands, as affected by factors such as:

• Variations in soil depth, moisture, and productivity
• Presence of natural opening (e.g., rock outcrops, bogs, rock talus, disease pockets, or minor windthrows)
• Variations in microtopography, such as hummock/depression complexes

Habitat patchiness provides:

• A mix of foraging, nesting, and resting habitat
• Increased diversity of habitats that can support more species over time

Patchy habitats (with canopy openings or gaps) are created by disturbance factors such as windthrow, disease (e.g., root rot centres), insects, small-scale fires, and human activities.

These influence the horizontal variability within a stand.

For example, there is often a trend in tree species mix and performance as you progress further upslope, away from a wet depression or bog. There also is an associated understorey change from wet to drier microsites.

Vertical structure describes the top to bottom structure of a forest stand. A diverse stand would have multiple layers from tall to smaller trees, shrubs, forbs, grasses, mosses and coarse woody debris. (See Figure 7, right).

Name the locations of several good examples of vertical stand structures

The deep, multi-layered canopies are more likely to develop in low-density stands that allow light penetration through to the forest floor.

The multi-layered canopies:

• Provide structural habitat for various birds that forage, nest and roost at intermediate heights
• Intercept and retain snowfall, resulting in lesser accumulations of snow on the forest floor
• Maintain moderate stand temperatures by reducing convective and radiative heat loss. These climatically buffered areas are often selected by ungulates as winter range
• Provide an array of branches that are a substrate for invertebrates and arboreal lichen growth. These are an important food source for deer and caribou on their winter ranges

A forest stand with diverse stand structure provides a greater diversity of habitats than do sites with very little structural diversity. For example, the presence of understorey brush provides protective and thermal cover for small mammals and nesting birds. These brush areas also play an important role in forest productivity, through litter fall and in their association with nitrogen-fixing bacteria. 

Multi-layered canopies are able to provide a number of habitats for different bird species. The upper canopy is frequented by raptors, for perching and nesting. As well, researchers are discovering unique populations of insects, lichens, and epiphytic plants in the upper canopies of old forests. 

Insect feeding birds, such as the pygmy nuthatch and brown creeper are found foraging for insects from the bark in the mid and lower layers of a stand. Juncoes and winter wrens are more frequently found feeding within the ground layer of a stand.

The structural diversity of older forest often holds more aesthetic and recreational value, than do younger more uniform managed stands.

Vertical diversity influences the interior environment of a forest. An old forest, for example, is noted for its relatively stable microclimate (moderated wind, high humidity). These conditions are often critical for the survival of some forest-dwelling plants and animals such as the Northern Spotted Owl.

Are there other reasons? If so, list them.

With these possible impacts to biodiversity there are a number of ways the structural diversity of forests can be maintained or enhanced to balance resource use with biodiversity stability and conservation.

Add other applications that should be considered. Name specific habitat management practices for items 2 & 3

The practices to use should consider the following applications:

1. Maintain structural diversity elements in our forests to provide suitable habitat conditions to meet most needs of the native species in the forest 
2. Apply specific habitat management practices to protect endangered species 
3. Apply practices that emulate natural disturbances patterns to maintain ecosystem functions 
4. Maintain structural diversity with partial cutting and reserve areas that are appropriate for locally occurring species. 
5. Apply forest management practices that reflect local natural disturbance patterns. 
6. Use a diversity of forest management regimes. 
7. Use partial cutting to maintain a variety of stand structure, conditions and dynamics. 

Review of learning outcomes

1. Are you able to describe the first stand level component-stand structure?
2. Are you able to identify the applications for forest management for stand structures? 
3. Can you describe the role of the stand level components in forest biodiversity? 

Recall

Using a tree diagram, identify the stand structure applications for forest management (without referring back). Write each application on a branch of the tree (color each branch a different color). Add as many branches as you need. Add the other the applications that you have thought of. Add details using either single words or phrases about each application on individual smaller branches. The details only have to trigger your thoughts. 

When you are finished, refer back to the material and add (in another color of pen) the ones you omitted.

Transfer of learning

List 3 – 5 ways that that you can apply what you have learned about stand structures to your present/future job.

Reflection

• What is there about stand structures that you still need to know?
• How can you locate this information?