Healthy woodlots have dead and declining cavity trees
By Ken Elliot, Forestry Specialist, Ontario Ministry of Natural Resources

Woodlots with a natural mix of tree species, birds, mammals, insects and all other woodland life forms are more likely to be healthy, functioning ecological communities.  A woodlot with no cavity trees (trees with holes created by woodpeckers or through natural processes), is likely to lack certain cavity-using species, such as chickadees, nuthatches, northern flickers and flying squirrels. Forest management that sanitizes woodlots by removing every dead, declining or cavity tree will displace wildlife populations that depend on these habitat features.

In southwestern Ontario, researchers are studying the impacts of current harvesting practices and experimental silviculture on such cavity habitat. These studies cover a broad range of questions, from the natural regeneration and growth of various tree species, to the health and productivity of numerous woodland birds. Researchers have been fortunate to receive funding from a number of agencies. They have also benefited from the assistance of graduate students and numerous field staff in collecting data on how wildlife (dependent on tree cavities) respond to tree harvesting of various intensities. Research is being led by Ken Elliott, a forester, and Dawn Burke, an ecologist, from the Science and Information Section of the Ontario Ministry of Natural Resources (OMNR) in London, in cooperation with Erica Nol of Trent University.

As many as 50 species of wildlife use cavities in dead or declining trees to nest, roost, forage, den and escape from predators. And there are about 25 bird and mammal species that use cavities as an essential component of their habitat needs. These cavity-dependent species comprise a “nest web,” which is not unlike a food web. The primary cavity nesting species, usually woodpeckers (hairy, pileated, northern flickers, yellow-bellied sapsuckers, etc.), create cavities for their own nesting needs during an initial breeding season, but they do not return to these sites the next season. The nest holes then become available to a long list of secondary cavity nesters. These are species that cannot excavate their own holes but will gladly take up residence in cavities produced in previous years by other species. The secondary cavity nesters include great-crested flycatchers, house wrens and flying squirrels.

The southwestern Ontario studies found that the population of primary cavity nesters is significantly reduced by selection harvesting and heavy harvesting (diameter limits). However, the number of secondary cavity nesters did not appear to be affected by such harvesting. These findings became particularly clear when Elliott, Burke and Nol conducted experiments where woodlots were studied before and after harvesting.

In one study, three woodlots were harvested under provincial guidelines for single-tree selection. The researchers found that, even though the harvest met the provincial guideline for retaining six live cavity trees per ha, one-third of live cavity trees and 82 percent of dead cavity trees were felled in the harvesting operations. Many of these cavity trees and snags were removed to ensure worker safety during operations. However, the observed impact on primary cavity nesters remains a concern.

The continuous monitoring of active cavity nests and their reuse over many years has revealed how critically important the retention of good cavity trees can be in the life of both primary and secondary cavity-nesting wildlife. Research conducted by Kata Bavrlic, a masters graduate from Trent University, found that cavities were reused at least once, 60 per cent of the time, over a four-year period. Using a camera mounted on a telescoping pole, Bavrlic detected nest reuse in 236 of 392 cavities and also found that a number of the cavities were used in multiple years. Some cavities had both bird and mammal use (21 percent), while others were exclusively used by mammals (31 percent) or birds (8 percent).

When Bavrlic calculated the tree reuse rate for primary cavity nesters, she once again found that woodpeckers responsible for creating cavities returned to the same tree to make new cavities 18 per cent of the time (49 of 277 trees).  She observed that only 10 percent of the reuse trees were healthy, while 40 percent were declining and 50 percent were in snags (dead trees).

Bavrlic’s research also revealed that in addition to the well-recognized importance of pileated woodpeckers as a contributor to the cavity-nest web community, hairy woodpeckers were consistently one of the most significant creators of cavities in live trees, followed closely by northern flickers and yellow-bellied sapsuckers, which rely mostly on dead trees or dead limbs on live trees.

The cavity research clearly shows that trees retained with nest cavities are consistently reused in our managed woodlands and form an important component of the post-harvest habitat used by the cavity-nest web community. Therefore, it is critical that forest managers hire trained tree markers to identify good cavity trees for retention during harvest operations, so that diverse, healthy wildlife populations are maintained.