Hardwood management must reverse encroachment by low-value species
by Gaetan Pelletier
Hardwood-dominated forests continue to be an important part of Eastern Canada’s economy, providing saw logs and pulpwood, as well as non-timber forest products. They also play an important role in terms of wildlife habitat and biodiversity, as well as carbon sequestration and climate-change mitigation. Managing hardwoods requires well-planned treatments that not only tend and improve existing trees but also regenerate desirable species. The successful recruitment of new cohorts of key tree species is vital to sustain our resource.
It is generally accepted that, for hardwoods, timber value is driven by the proportion of high-grade products, such as veneer logs and prime or select saw logs. The value differential between these products and hardwood pulpwood or biomass can be almost ten-fold – more than five times the differential between high-grade and low-grade softwood products.
Just how well are our hardwood-dominated forests doing in the Atlantic region? Well, from a timber standpoint, not that great. Species composition, stocking, tree quality, vigour, health, and growth rates are some of the factors we consider critical to the production of timber. Careful examination of the attributes of hardwood-dominated stands reveals some challenges for land owners and managers. However, the good news is that through silviculture, these problems can be controlled, mitigated, and even eliminated.
Currently, one of the biggest issues facing our hardwood forests is the undeniable change in species composition over time. It is happening at the expense of our prime tolerant hardwood species – Sugar maple – which happens to be one of the most demanding in terms of site, environment, and climate conditions. Sugar maple can be severely impacted by drought, and by thawing-freezing cycles during the winter; it is highly sensitive with respect to moisture, temperature, and nutrients (Figure 1).
A review of 20 years of inventory data and re-measurements of permanent sample plots in the Maritime provinces shows that, on average, the proportion of mature and commercial-size Sugar maple in our working forests has declined by more than 20 percent (Figure 2). This alarming trend seems to be mostly caused by relatively unsuccessful recruitment of new cohorts to promote seedlings and saplings into larger tree sizes.
So, what species are taking over the niche once occupied by Sugar maple? The answer to this question is equally disconcerting; in addition to a gradual increase in the proportion of Red maple (a species with limited potential to produce high-value timber products) in the mature strata, the gap in the regeneration layer is being filled by non-commercial species and American beech. While most non-commercial species will be curtailed as the canopy closes in, beech will continue occupying the gaps in the regenerating layers created as other species die (Figure 3).
In northeastern North America, Beech bark disease has become one of the key drivers of hardwood stand dynamics. Not only has it changed the structure of stands, it has also modified the growth and yield patterns of hardwood-dominated stands. Due to the complexity of the resulting stand conditions, it is often difficult to make silviculture decisions to improve them. Beech that is affected by the disease complex is of very poor quality, and produces very little high-grade material.
Results from mainstream research initiatives are unanimous: when beech is present in a stand, small-gap harvesting such as single-tree selection (or even no harvesting at all) will certainly lead to beech gradually replacing the overstory. This has important consequences for timber management objectives, as well as for species diversity. However, our results provide further evidence that this impact could be averted by the application of intensive harvesting regimes with high removals, to limit beech dominance over the more commercially valuable hardwood species. Given the close association between Sugar maple and American beech, management practices must be modified to prevent beech from dominating the canopy, especially on drier sites.
It is undeniable that our climate is changing, bringing higher temperatures, increased drought, more frequent episodes of high winds, and highly variable precipitation. Unfortunately, these effects will exacerbate the decline of Sugar maple. Indeed, beech and Red maple may very well be the species best adapted to these changes. To better understand this situation, an important study has been undertaken by scientists at UNB, the Northern Hardwoods Research Institute (NHRI), and FORUS Research. The goal is to understand the mechanisms that will trigger changes in species distribution, to model the extent of the new ranges, and to recommend adaptation methods through innovative silviculture.
Where beech is not present, or present at very low proportions (less than 20 percent), choosing the appropriate silviculture regimes will promote desirable species while reducing the proportion of less valuable ones. This is achieved by controlling conditions such as light availability, providing shelter to seedlings that are susceptible to sudden exposure, creating germination microsites for new seedlings to establish, and tending young trees of the preferred species. The NHRI silviculture prescription system (SPS) is one of several diagnostic tools that can be used to determine the most suitable treatments. Better silviculture will lead to better productivity and a higher ratio of more valuable trees. We think that it is also compatible with other values and objectives.
ACCEPTABLE GROWING STOCK
Another important aspect of the sustainable production of high-value timber in hardwoods is the notion of quality. Foresters and silviculturists often use the terms “acceptable growing stock” (AGS) or “unacceptable growing stock” (UGS) to describe trees in term of species preference, health status, shape, form, and size. AGS comprises trees of species that have potential for high-grade products, with low risk of losing vigour and value, and of a certain geometry that will produce quality logs now or in the future.
Because of past forestry practices, poor markets for low-grade products, and forest health issues, in our region today it is a rare occurrence to find stands with very high proportions of acceptable growing stock (more than two thirds AGS). When the right silviculture treatments and regimes are used, AGS trees with potential to continue increasing in value are left until subsequent re-entries – while UGS trees containing high-grade products, but at risk of deteriorating, are targeted for harvesting. Poor-quality trees that compete with future crop trees are also removed to achieve the desired density or basal area.
A tree classification system has been created to provide forest managers with objective ways to assess trees and to make silvicultural decisions. The system uses dichotomous keys to assign form (eight classes) and risk of losing vigour and value (four classes). Early adopters of the system in New Brunswick and Maine have found it is also of great value in making more accurate predictions with respect to growth and yield, product distribution, and internal decay (Figure 4).
Minimizing competition against crop trees is the first step towards healthy stands, but land managers must also pay attention to forest health issues such as pests and pathogens, climate-related damage, and browsing pressure from herbivores. The latter issue is not negligible; in some areas, browsing by moose and deer is so extensive that the only recourse is to regulate the distribution of treatments at the landscape level, in order to reduce overall habitat suitability.
INCREASING GROWTH RATES
The last element to consider is the productivity of the stand itself. Annual growth rates (periodic annual increments) in the range of 3-4 m3/ha of gross merchantable volume, or 0.3-0.5 m2/ha in basal area, can be sustained in our region. High stand productivity is maintained by removing unhealthy and older trees, and reducing competition on crop trees by releasing their crowns on at least two or three sides. It is a delicate exercise in balance, as releasing too much of the crown will, in most species, increase the likelihood of large branches developing.
In the end, sustaining our resource comes down to maintaining and growing an inventory of quality trees of the desired species, and practicing adaptive management. Properly planned and implemented silviculture can greatly reduce the negative impacts and gaps described here. The managed northern hardwood forests should continue to be the focus of partial harvesting in the region. A thorough understanding of the relationships among species, site, environment, and climate – informing a sound silviculture framework – will allow forest managers and landowners to balance timber production objectives with habitat and biodiversity values. This can be challenging, but our experience has shown that forest management regimes to promote species such as Sugar maple and Yellow birch are not incompatible with other objectives.
(Gaetan Pelletier is executive director of the Northern Hardwoods Research Institute Inc., based in Edmundston, N.B. This article is inspired by the work of current and past NHRI employees – in particular, Sharad Kumar Baral, Gabriel Danyagri, Pamela Hurley-Poitras, Stéphanie LeBel-Landry, and Monique Girouard.)