The Potential Role of Secondary Structure in Forest Renewal After Mountain Pine Beetle
by Philip J. Burton, Ph.D., R.P.Bio

The term “secondary structure” has been coined recently to refer to all trees in pine-dominated stands that will be left alive in the wake of the mountain pine beetle (MPB) outbreak that has been racing across BC and into Alberta. Although specific to pure stands or forest cover types dominated by a single tree species that finds itself susceptible to a particular pest or disease, the concept is simply an extension of the term “residual structure”, which refers to the remaining woody material (live or dead, upright or downed) after forest disturbances such as fire or logging. 

Secondary structure has two main components: (1) mature or canopy trees (Figure 1); and (2) understory trees, including seedlings, saplings and poles (Figure 2). Canopy trees that survive the insect outbreak are typically of non-host species (spruce, Douglas-fir, or broadleaf trees in the case of MPB), but also include the odd individual of the preferred host species that has exceptional genes, ideal microsite conditions, or unexplained luck. The abundance of mature trees likely to survive the MPB outbreak can be estimated from forest cover maps and air photos. Understory trees, on the other hand, are largely ignored in our forest inventories. This means that the abundance of seedlings, saplings and poles in most pine-dominated stands is unknown. Some of those understory trees consist of lodgepole pine individuals (especially on dry and open sites) too small to be attacked by MPB, but most of them consist of more shade tolerant fir and spruce species. Research is underway at the University of Northern BC and elsewhere to estimate understory tree densities from LiDAR and low-altitude aerial photography, and to use statistically-based spatial modelling to predict where we are likely to find full stocking of this unseen cohort of trees. 

Forest planners and managers in central BC are now paying attention to secondary structure as a key consideration in planning for the sustainability of timber and non-timber values. The last few years have seen several uplifts in allowable annual cut (AAC) levels in order to facilitate sanitation harvesting (to control beetle populations) or salvage harvesting (to capture economic value before the beetle-killed timber deteriorates). A consequence of the beetle outbreak combined with these uplifts will be a drop in the mid-term timber supply as the beetle and the mills consume, in just a few years, the pine we had been planning to harvest over the next several decades. For any given management unit or timber supply area severely affected by MPB, we are expecting to see a decrease in harvest rates and the availability of mature forest habitat starting 5 to 15 years from now, and lasting for up to 70 years. This gap in the mid-term timber supply represents the period of time after salvage logging is completed and until second-growth stands (primarily managed plantations) come on-stream. Most planners and managers recognize that the depth of this mid-term timber supply drought can be reduced by careful harvest scheduling. Harvesting priority is given to those stands with the greatest proportions or volumes of mature pine trees (dead, dying, or soon to die anyway). The key is to leave mixed stands and those dominated by non-pine species for logging only after the pure pine and pine-leading stands are harvested.

The benefits of protecting secondary structure dominated by understory trees and advance regeneration are less obvious. Many of these trees germinated at the same time as the overstory pine, however, the more shade-tolerant spruce and fir have been suppressed by the fast-growing pine for decades.

 Alternatively, some of these small trees may have established in the last few years as the forest has undergone succession - as canopy gaps have formed, and as seeds have drifted in from a few non-pine seed trees or adjacent stands. There are questions about how well these smaller trees will grow once they are freed from competition with the mature pine. Foresters in BC and elsewhere have often observed browned (sun-scalded) seedlings, toppled thickets of saplings and poles, or arrested growth when such advance regeneration is left after clearcut logging. But the fate of this secondary structure might be different after the MPB outbreak than after clearcutting, because the overstory is dying and opening up gradually, with mortality typically occurring over two or three years, and the loss of needles and fine branches taking another two to five years. As a result, the interior spruce, subalpine fir, and Douglas-fir found in the understory of these beetle-killed stands seem to have time to adjust to an environment that has less inter-tree competition, more light available, and often more soil moisture available. In more westerly portions of the current MPB outbreak, where the pine was killed eight to ten years ago, we are now seeing good leader growth (Figure 3) and survival on the part of that advance regeneration. Many lodgepole pine forests (e.g. on the Chilcotin Plateau, in the Flathead Valley, and in Colorado) have undergone severe MPB outbreaks in the past, but understory spruce and even small lodgepole pine trees have released and grown to maintain continuous forest cover.

It remains to be seen whether the release of natural regeneration, the normal successional sequence in the life cycle of old lodgepole pine stands, can be repeated under management. Research trials coordinated by FP Innovations–Feric Division have been initiated in the Prince George area, where harvesting operations designed to protect secondary structure are being monitored. Operating costs, harvesting productivity and impacts, and the survival and growth of regeneration are all being measured. Regardless of the efficacy of these “careful harvesting” trials, another concern is the fact that this understory regeneration is often clustered, with dense pockets of seedlings and saplings in some spots, and bare moss or dense shrubs elsewhere in the same pine stand. Over the course of natural stand development, some competitive dominance is usually expressed among the trees that start growing in dense thickets (though they may not exhibit optimal growth), and they eventually sort themselves out and grow into canopy trees. The spots not occupied by trees, however, mean that stocking (the full occupancy of growing space by desired tree species) is often incomplete or unreliable. If timber is harvested from such stands and the release of advance regeneration is counted on for stand renewal, fill planting may be needed to fill those holes, or modified stocking standards and greater regulatory latitude may be needed to allow such heterogeneity (which nevertheless has value to biodiversity). 

The greater value of secondary structure, whether found in the canopy or the understory, may be in facilitating the natural, unassisted recovery of beetle-disturbed forests. Even with the AAC uplifts and before the current downturn in lumber markets, it was estimated that about one-third of MPB-affected forest would not be harvested. Despite some non-renewable forest licenses being issued to industries interested in the production of wood pellets and biofuels, it is expected that large areas of dead pine will not be logged and managed under normal silvicultural practices. This is the especially true for unroaded watersheds, areas zoned for high non-timber values, stands with low volumes, and stands in which wood value has already deteriorated. If we are leaving some pine trees unlogged, wouldn’t it make sense to leave those surrounded by live trees, large or small, so that forest cover can continue without management expense? There are plans under the BC Forests for Tomorrow program to rehabilitate (underplant, or knock down and then plant) some dead stands that are not commercially desirable, often at great expense. This may be a sound decision where the advance regeneration is so small or sparse that a new plantation of vigorously growing seedlings will outpace the naturals in a few years. But where the saplings, poles or non-pine canopy trees already have volumes equivalent to a few decades of plantation growth, it would be more prudent to hold such stands in reserve, for they will achieve operability for future harvest sooner than a new plantation. As indicated by a retrospective analysis of the last spruce budworm outbreak in New Brunswick, strategic harvest scheduling and deferrals can significantly offset losses to the timber supply.

A final consideration in looking at secondary structure in the lodgepole pine forests of BC is the pervasive background of climate change. The MPB outbreak itself is partially due to milder winters and subsequent reductions in overwinter mortality on the part of beetle larvae. At the same time, there are rising concerns about the potential effects of warmer and moister summers on the incidence of fungal diseases (needle blight, stem rusts) in second-growth stands and plantations of lodgepole pine. In many ways, large parts of BC now occupied by mature lodgepole pine (perhaps an historical anomaly related to fire history) may soon no longer be optimally suited for growth by this tree species. Let’s be careful to not just plant all MPB-killed harvest blocks back to pure lodgepole pine, or we may set ourselves up for more widespread tree mortality in the near or distant future. Perhaps it is better to let nature present and filter an array of species to see how forest ecosystems maintain themselves over the next few decades. We don’t want to quash those natural experiments. The diversity of tree species and sizes that we call secondary structure are a big part of the raw material that will provide forest continuity into the future. 

Phil Burton is Manager of Northern Projects, Pacific Forestry Centre, Canadian Forest Service and Associate Professor (Adjunct), University of Northern British Columbia, in Prince George. Thanks to Dave Coates, Ken Hodges, Al Mitchell, Grant Nishio, and Roger Whitehead for the comments they provided. The recommendations presented here are solely the opinion of the author, and do not represent the policy of the Canadian Forest Service, Natural Resources Canada, or the Government of Canada.