Tree Harvesting and its Effect on Soil Nutrients

We are currently experiencing a global resurgence in wood production, and as biomass derived from forest residues continues to become more popular as an alternative and renewable source of energy, questions regarding forest sustainability and land management inevitably become the focus of many discussions. And rightfully so. While a forest is so much more than its available timber, an important—and often overlooked—aspect of responsible forest stewardship concerns maintaining overall land quality.

As the science has repeatedly demonstrated, when managed properly, forest resources offer tremendous economic, social and environmental benefits. But in productive, heavily-forested areas, does the land itself suffer long-term due to repeated tree removals? Do the land’s soil nutrients demonstrate any measurable decline?

An extensive 2005 paper by the Wisconsin Department of Natural Resources (WDNR) titled, Effects of Logging Residue Removal on Forest Sites, notes that nutrient budgets on forest sites are cyclical based upon harvesting practices, and these nutrients exist in three primary forms:

• Inorganic: mineral components that exist in rocks and soil particles, which are the least available to plants and take a long time to become available through weathering.
• Organic: material originating from living matter, which will become available to plants as they are subjected to biological processes such as decay. Undecayed wood, forest residuals, etc. represent a storehouse of nutrients for future plant growth.
• Ionic: available nutrients that support current plant growth. Can be diminished or lost through leaching or run-off, which will have near-term impacts on site regeneration abilities.

Concerning the availability of these types of nutrients in merchantable wood from the forest, the paper also states that, “Of the living biomass in forest stands, nutrients contained in the boles or stems of trees will take the longest to become available due to the length of time it would take them to decompose. Nutrients in branches would take a relatively shorter time to become available while those in leaves, needles, and herbaceous materials would be available most readily due to rapid decomposition.” Most importantly, the paper says that, “It is also significant that leaves, needles, and herbaceous materials are produced annually and, consequently, provide more frequent inputs into the nutrient cycle.”

The fact that the most frequent sources of material that can quickly be incorporated into a plant’s nutrient cycle becomes available on an annual basis cannot be overstated when assessing the potential consequences of harvesting timber. A forest, therefore, has a constant supply of ionic, available nutrients working their way into the soil as the stand matures. The WDNR paper confirms that the largest proportion of essential elements is contained in tree foliage, “Especially fine residues (more so if incorporated into the soil) [that] provide soil protection, enhanced infiltration of moisture, reduced evaporation, and maintenance of vigorous soil microbial activity.”

So, how are these nutrient levels measured and tracked to assess overall forest health? A recent paper by the Northwest Advanced Renewables Alliance (NARA) seeks to answer this question by challenging some of the dated monitoring techniques used at Long-Term Soil Productivity Sites (LTSP), and redefining the scope of the measurement process moving forward. As NARA notes, “For these studies to be fully relevant, an accurate baseline measurement and understanding of soil nutrient levels in varied forest soils needs to be established. Traditionally, soil samples used to evaluate nutrient supplies and carbon pools were taken at the upper 15-20 cm (6-8 inches) of soil, but recent weathering measurements have detected soil nutrient levels to as much as 100 cm (~ three feet) below the surface.”

Measuring Soil Nutrient Levels

NARA notes that the “base mineral index” for testing, which was used for many years, relied on a toxic liquid to separate out minerals for evaluation. More recently, this liquid has been replaced with either sodium polytungstate or an agua regia analysis. Measuring the amount of nutrients in a soil sample is straightforward using these tools, but measuring the time from when a nutrient is released into the soil to when it becomes available for plant uptake has been difficult. “The paper releases new data that measures the release profiles of potassium and phosphorus over time in varied soils. The results show that nutrient release times and amounts vary depending on the parent materials (i.e. rocks) at varied soil depths. These data suggest that the amount of nutrients made available through deep root actions can be dependent on the soil mineralogy, which is at times, left out of soil quality assessments.”

Soil Organic Carbon

Globally, forests have a vital role in carbon sequestration. Soil organic carbon (SOC) is thought to be the largest deposit of terrestrial carbon, and tree roots play a part in storing it there. The amount of SOC in soils varies due to a number of factors, including soil type and overall site activity, but can often be accounted for at depths up to 300 cm and deeper. “In one study, soils sampled at 20 cm contained an average of 59 Mg C/ha, sampling at 50 cm showed 113 Mg C/ha, and sampling at 300 cm showed 202 113 Mg C/ha. The results suggest that shallow measurements for carbon can dramatically underestimate the level of available SOC to the ecosystem. And during a time when carbon stocks are being monitored due to climate change, a rethinking to how we measure SOC is suggested.”

As noted above, NARA’s research confirms the contention that measurements need to be taken at a number of different soil depths, including those deeper than three feet below the surface topsoil.

Tree roots provide water and nutrient uptake, and they anchor the plant firmly in the soil. Acidity due to root respiration and chemicals released by roots promote chemical weathering of mineral components deep in the soil. The fact that soil nutrients at deep levels change seasonally suggest that tree roots uptake nutrients in deep soil when growing and that mineral weathering, root systems and soil carbon levels are all linked.

Harvesting Methodology and Soil Nutrients

It is a fairly standard forestry practice to leave behind a portion of forest residues on harvesting sites, which can include leaving forest slash, or burning or chipping the residues and spreading them throughout the tract. This process returns the most nutrient-dense residues to the logging tract, where they will quickly decompose and cycle back into the soil to sustain new growth.

Whole-tree harvesting and chipping—which is one harvest method used in the production of biomass—is a practice that removes most of these residues from the logging site as well.  This methodology has also garnered recent attention due to the rise in the popularity of biomass. But is this practice more damaging to the land, or does it have an adverse effect on soil nutrients in the near- and long-terms?

The WDNR paper discusses a comprehensive 1982 study that measured the impact of clearcutting and different post-harvest residue treatments. The study focused on residual organic matter and post- harvest nutrient levels in lodgepole pine forests, and four treatments were analyzed: residues removed, residues chipped and spread, residues broadcast burned, and residues windrowed and burned. Woody material weights and the percentage of change from pre-harvest to post-harvest were: 

Approximately 400 lbs/acre of understory vegetation existed on all sites prior to harvest, and fifty lbs/acre remained post-harvest. The immediate impact on the nutrient content of surface organic material was dramatically different across treatment areas as illustrated below.

But after five years, nutrient levels in surface organic material approached those in the uncut forest. Interestingly, the treatment that came closest to restoring the nutrient levels seen in the uncut forest after five years was where the residue was removed. 

Conclusion

It is important to remember that there are also studies that arrive at, and present, different conclusions depending on location and geography. There are simply too many dynamic factors that affect a tract of land and the plant growth on that land to draw any sort of universal conclusion about timber harvesting and its effects on soil nutrients. That said, there are some general conclusions that both NARA and the WDNR draw from their respective research:

• In order to understand the amount of soil nutrients available within an ecosystem, samples should include soils at greater depths than are currently being measured. In addition, the type of minerals in soil can affect the amount of nutrients made available and should therefore be considered when evaluating a soil’s potential for plant productivity.
• It is important in assessing site impacts to distinguish between available nutrients and total nutrient capital. Harvesting of the bole and slash will remove a much larger proportion of available nutrients compared to total nutrient capital removals. Short-term impacts will depend on how quickly the available nutrients can be replaced through deposition, decomposition, and weathering. Long-term impacts will depend on how much and how often total nutrient capital changes.
• Research has been conducted in a wide variety of forest types growing on different soils. This research seems to suggest that the effects of slash removal on nutrient budgets are short-term in nature with sites recovering pre-harvest nutrient levels within approximately five years if the site has adequate reserves or will obtain adequate available nutrients through deposition. This is important data, especially in the US South where pine plantations with relatively short rotation periods are gaining in popularity; these tracts also typically receive additional nutrient applications to maximize growth.
• An important variable influencing the impact of slash removal on nutrient budgets is the amount of organic matter that is already incorporated in the mineral soil (not removed with the slash). The more of this matter that is in place, the less influence slash removal seems to have on nutrient budgets.
• Any impacts that slash removal has on nutrient budgets appear to occur in the top five centimeters of soil, and are influenced by the amount of mineral soil exposure.
• On already nutrient poor sites, the impact of slash removal will be more severe and long-term.
• Research suggests that the most important component of the slash that should remain on-site from a nutrient budget perspective is the fine material (needles, leaves, and small twigs). Winter-only harvesting in hardwoods or drying of the slash over summer in softwoods has been shown to conserve nutrients (since the leaves and needles remain on-site).