GREATER FUNDY ECOSYSTEM RESEARCH PROJECT

UNB Faculty of Forestry and Environmental Management


Forest Management Guidelines to Protect
Native Biodiversity in the Fundy Model Forest

Site Level Considerations

Snag and Cavity Tree Retention


Clearcutting and short rotational forest management are not compatible with the immediate needs of most species of wildlife which require tree cavities at some point during the year. Multiple-entry harvesting of plantations and thinned stands may extend the window of maturity for these sites. Selective removal of a portion of timber from a stand - or group of stands - is the type of timber harvest most compatible with implementing management prescriptions which protect, enhance or mitigate damage to the habitat required by most species of cavity-dependent wildlife. For most species of cavity-dependent wildlife, other components of "forest maturity" are required for successful occupancy, especially those which address food and cover.

Most species of wildlife dependent on tree cavities have different food, cover, and spatial requirements. A lone dead Maple tree in a clear-cut might be used for nesting by a Northern Flicker, Tree Swallow or American Kestrel, and is quickly labelled as a "wildlife tree." However, this does not tell the entire story. Such species select for open or partially open sites and, with intensive timber harvest, such species are seldom lacking for nesting or feeding habitat. They are not the species of most concern. Forest management guidelines requiring a token number of "snags" left per hectare of clear-cut are of little or no practical use to most species of cavity-dependent wildlife until the regenerating forest is at least greater than 20 years old. Guidelines for the benefit of most "cavity-dependent wildlife" will manage towards maintaining components of forest stand maturity, on a sustainable basis.


This large Yellow Birch has been left for wildlife values within a selection harvest site. (Photo: G. Forbes)

Forest stand maturity must be defined by ecological and not economic parameters. For example, a stand sufficiently "mature" to cut (e.g. a 60 year-old planted Black Spruce stand on a clear cut and scarified site) cannot be equated with an ecologically mature 150 year-old Red Spruce dominated stand which developed following fire and natural regeneration. The abundance of snags should reflect the post-disturbance environment for that forest type.



Information Sources

There are studies, mostly from outside New Brunswick, which have measured tree cavity selection and use by birds and mammals. Those studies of most relevance to the northeast have been summarised by Hunter (1990). There are also two Fundy Model Forest projects on cavity nesting species currently underway. Some preliminary results are also included. Cavity tree selection data from 1983 and 1984 are also available for northern New Brunswick (Parker, unpublished data).

General Principles:


  2. 1. At the Provincial level, the needs of tree cavity dependent wildlife (TCDW) can only be adequately accommodated through modifications to regional forest harvest and silviculture strategies.

  1. 2. TCDW require both nesting and feeding habitat. Large-scale clear-cutting, site preparations and conifer plantations are not conducive to providing such needed nesting and feeding habitats.

  1. 3. The selective removal of timber in a manner which maintains elements of stand ecological maturity within diversified horizontal and vertical profiles is the preferred manner of forest management for TCDW. In stand-replacing sites, clumps of standing trees need to be retained.

  1. 4. Research must be directed towards measuring specific life requirements of TCDW so that forest management strategies can be adaptive and incorporate new information and knowledge.

Specific Concepts

  1. Most cavity dependent mammals use existing tree cavities (they are termed secondary cavity users).

  2. Cavity dependent birds may be primary (excavate a new cavity each year) or secondary (use existing cavities) cavity nesters.

  3. In New Brunswick, most species of primary cavity nesting birds select live or partially dead deciduous trees for excavating nesting cavities, especially Trembling Aspen (in the south) and Beech (in the north).

  4. Bole diameter and tree height are important characteristics of cavity tree selection, and appear to vary with each bird species.

  5. In a mature mixed-forest, dead standing trees, or snags, commonly represent 5 - 10% of the trees.

  6. Most snags show some form of use by insectivorous birds. Most of this use is from searching for and feeding on insects.

  7. The influence of adjacent trees and shrubs (i.e. microhabitat) on the level of use of snags or cavity trees for feeding and nesting, is relatively unknown.

  8. All snags are not of equal value. A snag that is used heavily for feeding (or nesting) by woodpeckers in a closed mature forest will probably receive little or no use when left standing and exposed after a forest harvest.

  9. The science of snag ecology is still young. Much additional information on snags and their use is needed before useful predictive models can be developed.

It is clear that integrated forest-wildlife management strategies must consider what types of trees cavity-nesting species of birds need and select. It is also clear from the available data that most primary cavity nesting birds utilise mature Aspen in southern New Brunswick and Beech in the north. Since most snags are alive or only partially dead, leaving dead snags as potential nesting sites for most species of primary cavity nesters is pointless. Such trees may, however, be used by primary cavity nesters for feeding. Mean bole dbh for such trees should be 30 to 50 cm. The optimum distribution of potential nest trees is unknown, but 10 - 12 /ha, would be a reasonable number. Potential nest trees are of little use if left solitary in clearcuts. In fact, with very few exceptions, lone trees left scattered in clearcuts are of little benefit to cavity nesting species of birds. Exceptions might be Northern Flicker, Eastern Bluebird, Tree Swallow and American Kestrel. Most are secondary cavity nesters.

Management of nest trees for primary cavity nesters must be done in concert with selection harvesting techniques. Selection harvesting leaves elements of a mature forest intact, along with potential nest trees, thus providing required cover and feeding sites for most primary cavity nesters.

There are great differences between trees used for nesting and trees used for feeding. Most nest trees used by primary cavity users are living or partially dead hardwoods, usually Aspen species and Beech. Most feeding occurs on partially dead or fully dead hardwood and softwood trees. Although the snag component of the forest is important to cavity nesters, the presence of snags alone with the living component of the forest having been removed, would be misdirected. It is estimated that a Hairy Woodpecker, for example, requires 160 snags per 40 ha of habitat, a Pileated Woodpecker 14 snags per 40 ha (most are for feeding, a few may be used for roosting). However, the territory of a Pileated Woodpecker may be 12 times that of a Hairy Woodpecker. Thus, the Pileated has access to a greater area and thus a greater variety of snags.

Best Practices Recommendations for Snags and Cavity Trees

  1. 1. Forests can be best managed for cavity nesting species of birds by selection harvesting techniques. If, for example, 40% of commercial timber is removed during the first cut, the best management for cavity nesters is to leave a minimum of 12-15 snags (defined as standing dead trees, preferably 20 cm or greater dbh) per hectare for feeding plus 10-12 live, or partially dead, mature Aspen or Beech. In the absence of Aspen or Birch, Maple and Yellow Birch with a minimum dbh of 25 cm may be substituted for nesting (White Birch is of limited value as a nesting tree in this region). Subsequent interventions should strive to maintain those numbers and ratio. Single snags or live trees in clearcuts less than 4 hectares in size may be useful as feeding and nest trees for certain species of cavity nesters. As live Aspen, Beech, Maple or Birch (Yellow preferred over White; Grey of little value) soon die, become partially dead, or otherwise become suitable as feeding and nest trees, those species are preferred over conifer species. Trees should be mature, with a minimum dbh of 25 cm. Clumps of trees are preferable over single trees. The minimum number to be left should be no less than 12-15 single trees per hectare; more is always better than less. Solitary trees left in clearcuts greater than 4 hectares are of limited value, but, when possible, the recommendations for smaller clearcuts should apply. In larger clearcuts, managers should pay special attention to leaving scattered clumps of live trees, both deciduous and coniferous mixed; large clumps are always better than small clumps. Although larger dbh's are best, a diversity of diameters is possible in clumps. Harvest and silviculture operations should be site specific.

  2. 2. In an area of forest clear-cut operations, managers should have an inventory of potential nest trees (live Aspen and Beech 25 cm dbh) and snags (dead trees 20 cm dbh) as well as the amount of commercial timber. In cuts less than 4 hectares, a minimum 10-12 potential nest trees and 12-15 snags should be left per hectare. Clumps of trees are better than single trees. In clear-cut operations greater than 4 hectares, the same numbers apply, except managers should pay special attention to leaving clumps of trees rather than single trees. Potential nest trees in company of other trees (snags or merely non-commercial) is preferred. Dead and down trees are very useful to certain species of insectivorous birds, as well as a wide range of other living organisms. In selective timber harvest operations, managers should strive to maintain that element of remaining forest structure. Where dead and down trees do not present a hazard or otherwise interfere with selective timber removal, they should be left as an important component of the forest ecosystem. No numerical recommendations are practical, other than leave whenever possible.


Go to Guidelines for:
Patch Size (Forest Stand Size)
Connectivity
Stand Age - Provisions for Mature Forest Stands
Incorporating Maturity into Forest Harvest Schedules
Guidelines for Mature/Overmature Coniferous Forest
Plantations
Roads
Stand Conversion
Protected Areas
Water Course Buffers


Habitat Considerations for Specific Species
Special Status Tree Species
Coarse Woody Debris

Return to Table of Contents

 


 


The UNB Forestry Home Page

 

Information provided by:
Dr. Graham Forbes
Faculty of Forestry and Environmental Management at UNB
Last Update: Dec 17, 1997
This document: http://www.unb.ca/web/forestry/centers/cwru/snag.htm