Long-awaited Nutrient Budget Model serves as a reality check on yield assumptions
Originally published in Atlantic Forestry Review, July 2017: http://www.rurallife.ca/new-page
by Jamie Simpson
Soil scientist Kevin Keys and four colleagues recently published a paper about their study of forest soil productivity in Nova Scotia. Though it contains only a sampling of what their work can tell us about forestry and soil nutrients, it’s worth a close look. The paper, “A Simple Geospatial Nutrient Budget Model for Assessing Forest Harvest Sustainability across Nova Scotia, Canada,” (published in Open Journal of Forestry, 6, 420- 444), gives us a glimpse into the long-awaited Nutrient Budget Model (NBM), as it is known, which was created by a team of researchers including Keys, of the Nova Scotia Department of Natural Resources (NSDNR), and Dr. Paul Arp, of the University of New Brunswick.
The NBM measures the long-term sustainability of various forest harvesting scenarios on the various soil types and conditions throughout Nova Scotia. It accounts for nutrients added to the soil from the weathering of rocks and deposition from the air, and loss of nutrients due to leaching (which is exacerbated by acid precipitation) and forest harvesting.
Dr. Arp likens the model to a bank account. Nutrients, like dollars, can go up and down in the short term without causing major issues. The problem is that forest soils in much of Nova Scotia have a very small “bank account” of nutrients. It does not take a large loss for the account to dip below a critical level. Trees cannot live on a deficit, so if nutrients drop below this threshold, trees slow their growth and may be taken over by plants that can make do with poorer soils. When the model predicts this outcome as the result of a given harvest scenario, it displays its answer in red lettering: “unsustainable.”
Unfortunately, Keys and his colleagues do not reveal what their model might tell us about the predicted impacts of clearcutting and whole-tree harvesting across the entire province. However, they do tell us that in combination with the effects of acid precipitation, whole-tree harvesting and even conventional stem-only clearcutting can reduce growth potential in Nova Scotia’s forests. This result, the authors point out, contrasts with an assumption held by some forest managers that “increased yields predicted through intensive management are sustainable on any given site.”
The paper includes a case study comprising 25 spruce plantations, comparing their productivity as predicted by the forest managers, and as predicted by the NBM. According to the authors, as many as half of the plantations will not produce as much wood as the forest managers predict, which means wood supply forecasts, at least for those plantations, would be unsustainable. The authors suggest that forest managers can use the model to fine-tune harvesting, avoiding productivity declines by reducing harvest levels or increasing the time period between harvests.
Dr. David Patriquin, a retired Dalhousie biology professor, welcomes the study for the insight it provides on how forest harvesting can affect soil productivity. “This is perhaps the first formal acknowledgment from NSDNR that forest harvest can worsen the effects of acid rain,” Dr. Patriquin says. Moreover, he points to the study’s finding that “base saturation” (a measure of the soil’s ability to hold on to nutrients – an important aspect of soil fertility) is “much lower in much of Nova Scotia than previously believed… It’s 37 to 82 percent lower compared to earlier soil surveys.”
Beyond the direct impact on soil fertility, low base saturation can lead to damagingly high levels of aluminum, explains Dr. Patriquin. “Base saturation below 15 percent results in aluminum stress for many forest trees, and this new study shows that we have a base saturation of only five to 10 percent over much of Nova Scotia,” he says.
“Basically, this study helps show that our soils, forests, and aquatic systems are already highly stressed,” concludes Patriquin. “Clearcutting, especially in southwestern Nova Scotia, only increases the stress on these ecosystems.”
In a presentation to the Mersey Tobeatic Research Institute’s 2014 Science Conference, Keys noted that the NBM project was initiated by NSDNR in 2008 due to interest in harvesting forests for biomass energy. The department wanted a “decision support tool” to determine where biomass harvesting might be suitable.
NSDNR contracted Arp and his team at the University of New Brunswick to build the model, and they completed it in 2011. Since then, NSDNR has been clarifying, checking, and updating the model, but has not made the results public, beyond what appears in the Open Journal of Forestry. The article includes province-wide maps of site productivity for Red spruce and Sugar maple, based on NBM estimates, but these results are not considered accurate because they are based on decades-old soil surveys. The department is currently updating those data through a 5-year soil sampling program.
The province has made several announcements regarding biomass harvesting, starting in 2010, with a policy commitment to “prohibit the removal of whole trees from the forest site.” In the 2011 Natural Resources Strategy, titled “The Path We Share,” NSDNR committed to establishing “rules for whole-tree harvesting.”
In 2013, NSDNR released a discussion paper that recommended amending regulations in order to prohibit whole-tree harvesting in the context of forestry operations. The recommendation was based on a review of “provincial policies and current literature, preliminary nutrient budget model analyses, and compatibility with other Natural Resources Strategy initiatives.” That same year, the department released a 24-Month Progress Report on implementing the Natural Resources Strategy, stating – in apparent contradiction with reality – that the province had “prohibited whole-tree and full-tree harvesting on Crown or private lands.”
In its Five-year Progress Report, released in 2016, the department backpedaled on regulating whole-tree harvesting. This document states, “We have now developed tools that ensure that all harvest treatments are aligned with the nature-based requirements of Nova Scotia’s lands,” and it refers to a Nutrient Budget Model being “in development.”
Today there are no legal restrictions against whole-tree harvesting on private land. On Crown land, timber licensees are prohibited from removing “coarse or fine wood debris, tree crowns (tops), or stumps, . . . unless otherwise agreed to by both parties in writing.” In response to inquiries this June, Bruce Nunn, NSDNR media relations advisor, said there was no whole-tree or full-tree harvesting on Crown land as part of forestry operations in 2016.
NSDNR declined to make Keys available for a phone interview about the current status of the NBM, but Nunn conveyed the following statement from him: “Since the nutrient budget model can be applied at different spatial scales, each with its own set of limitations and assumptions, we are currently evaluating how best to integrate the model into forest management planning. We are considering two approaches: the stand and landscape level. Part of this evaluation involves assessing whether sustainable productivity estimates are best generated at an eco-site level to be used as a reference value, or generated at an individual stand level (or a combination of both).”