Soil is a complex growth media comprised of many chemical, physical and biological processes which integrate to provide trees with physical growth support, water and nutrients. It is effectively a non-renewable resource that requires intensive ameliorative inputs if it has been allowed to degrade. Any activity or practice that threatens the ability of the soil to support and sustain good stand production must be avoided or minimised. Concerns exist over the impacts of certain forestry management practices on the quality and functioning of our plantation soils, and consequently on long-term sustainability of our plantations.
by Louis Titshall

Current recommendations are to avoid using heavy machinery on wet soils, which can lead to compaction and rutting.

A burning question – is it sustainable to burn harvest slash?

Why should we worry?
Generally, forestry management practices are aimed at supporting efficient harvesting and removal of timber and subsequent crop re-establishment. It is during this period that the greatest amount of soil disturbance occurs. Depending on the type and level of disturbance and the soil properties, there may be an immediate and short-term impact on the success of the new crop. In some cases, the impact may be small but cumulative over time, with longer-term consequences. In addition to the confounding effects of climate, silviculture and genetics, slow declines in soil quality may thus not be immediately evident and only manifest in future rotations as a loss in stand productivity. Understanding the mechanisms, site thresholds, and magnitude and persistence of any impacts, as well as relating this to timber production, is key to providing management guidelines that support sustainable multiple-rotation production.

The areas of concern
Management practices with the greatest impacts on the soil are those operations that affect the soil physical environment (typically ground-based mechanised operations) and the removal, destruction or redistribution of harvest slash. Soil properties most likely to be impacted by these operations include soil organic carbon, nutrient capital, availability and fluxes, soil pore size and distribution, soil water fluxes and strength, where soil organic carbon is considered a key driver. Although soil organic carbon forms only a small fraction (0.5-12%) of most forest soils, it is crucial for the maintenance of soil productivity, as it has beneficial effects on soil water holding capacity, structure, strength and aggregate stability, nutrient storage and supply and organisms, as well as resisting many degrading impacts, including erosion and compaction. The loss of topsoil through erosion has been recognised as the largest risk to soil quality and sustainable agricultural production worldwide. Much of the local and international research has focused on these aspects.

Soil’s physical impacts
The size and distribution of pores in the soil (or porosity) is a key attribute that affects soil aeration, it’s water holding capacity and drainage, and indirectly, nutrient availability, soil biology and strength. Of primary concern are large reductions in porosity and increases in soil strength associated with compaction and kneading of the soil during ground-based mechanised operations, especially under wet conditions. This, however, is a two-edged sword. Research has found that in the case of coarse textured, sandy soils (such as in parts of Zululand), a decrease in the number of macro-pores is beneficial in improving the water holding capacity of these soils, by reducing excessive water drainage. However, in fine or more clayey textured soils, compaction can result in an excessively high proportion of fine pores. This lowers the water holding capacity and aeration of the soil and may reduce water infiltration, leading to higher surface run-off. Regardless of the soil type though, compaction almost always increases soil strength, and when this is too high, root growth is reduced and tree growth is negatively affected. A further concern is that much of the forestry in South Africa occurs on soils that do not have strong shrink-swell properties that allow for self-amelioration of any compaction, so the effects may persist across rotations.

The ICFR has produced guidelines for a range of soil types and site management scenarios that aim to minimise any negative impacts. Recommendations include avoiding using machinery on wet soils and, where possible, retaining as much organic matter and slash on site, as this can reduce the direct impact on the soil, which is important for ameliorating any damage. This has been confirmed in later field experiments, where in almost all instances, where harvest slash was retained on the site, the compaction impact of mechanised operations on the soil was reduced. Current research at the ICFR is focused on establishing the relationships between stand productivity and physical impacts caused by mechanised operations. Research to date indicates that responses between sites and even species are variable, but that there is a risk for a loss of production under severe compaction.

Site nutrition
The majority of processes contributing to site nutrient and organic carbon losses are affected by harvest slash management, which may include residue removal, reduction (on-site burning) or retention on a site. Slash management is perhaps the single largest operation that has the potential to alter both the short and long-term nutritional sustainability of a site. Harvest slash is considered beneficial through its contribution to soil organic carbon, nutrients, and as a protective barrier against erosive and mechanised impacts on the soil. However, slash retention may result in temporary nutrient immobilisation (especially in the cooler areas where there can be a build-up), and reduce the ease of re-establishment, while also increasing fire fuel loads

Depending on the harvesting system being used, variable amounts of biomass (and consequently nutrients) are removed from a site. Perhaps the lowest impact is where only utilisable stemwood is removed and all remaining biomass (bark, branches, twigs, leaves) is left on the site. Nonetheless, nutrients are still being exported from the site and the subsequent slash management is important to reduce these losses. Slash burning results in a direct loss of organic matter and some nutrients (notably volatilisable nitrogen) depending on fire intensity and residue quality. Burning generally increases short-term nutrient availability of elements such as phosphorus and calcium, and has an alkalising effect on the soil. However, there is evidence to suggest that long-term nutrient capital and balances can be negatively impacted on sites where the losses exceed inputs. Where slash is retained, it generally results in a slow release of nutrients back to the soil, though short-term nutrient availability is often lower than under burning or complete biomass removal. Furthermore, alternative management practices, such as windrowing, broadcasting or mulching, result in differences in the distribution of soil organic carbon and nutrients across a site. Broadcasting and mulching of residues generally produce a more even distribution, whereas windrowing or brush-piling concentrates the nutrients to a specific area. Total slash removal, such as with whole tree harvesting or biomass collection for biofuel, has been shown to cause very large losses of organic matter and nutrients from sites, typically far in excess of any natural nutrient inputs, and this can lead to a loss of stand productivity over time.

Studies that have considered how slash management practices affect tree growth have shown that nutrient-poor soils generally have reduced growth with increasing levels of slash removal, primarily being attributed to higher nutrient loss. Growth responses to residue management on more fertile sites have varied, although significant decreases in soil nutrient levels and fluxes have often been reported. Locally, there are two key process studies that have substantially enhanced our understanding of the impacts of various slash management treatments on site nutrition and stand production. The first was part of the CIFOR network of sustainability trials established on a clay soil in the KwaZulu-Natal Midlands (the ‘Karkloof’ study), while a subsequent study was established on nutrient-poor sandy soil in Zululand (the ‘Dukuduku’ study). Both studies investigated slash management treatments which included total biomass removal, slash retention and slash burning. It was found that biomass removal generally resulted in the greatest loss of nutrients and also resulted in the greatest loss of productivity. Burning increased nutrient availability in the short-term with a concomitant boost in early stand growth, but there was no late-rotation advantage in this (Type 1 growth responses). Slash retention was best for maintaining the nutrient capital of a site, though not necessarily for stand growth, perhaps due to the unfavourable planting conditions created by slash retention. The nutrient budget of the Karkloof study showed that even with total biomass removal, the site was relatively well buffered against nutrient loss, while the Dukuduku site was at risk of nutrient decline. In both studies, the long-term multiple rotation impacts have not yet been investigated.

Erosion
Soil erosion has been recognised as a serious threat to soil productivity through the loss of nutrient- rich topsoil. Activities associated with harvesting and site preparation are most commonly linked to increased incidence of run-off and erosion due to increased site disturbance and removal of soil protective coverings. If these activities coincide with high intensity rainfall events, there is typically a sharp increase in runoff and erosion.

Locally, there have been some studies that have investigated run-off and erosion due to site management. This research has shown that soil loss was greatest from slash burnt treatments and lowest where slash was retained, and that the magnitude of soil loss was also greater on the steeper slopes and under higher intensity rainfall. Furthermore, it was found that soil loss decreased sharply after vegetative cover had been established, even in burnt treatments.

Some subsequent studies also confirmed that bare and disturbed soil surfaces were at greatest risk of soil loss due to erosion and that this increased dramatically with an increase in slope gradient. Soil losses from sites with windrowed or broadcast slash were considered negligible, highlighting the importance of a protective soil cover. None of these studies have reported on the long-term impacts over multiple rotations, nor has the actual magnitude of nutrient loss been adequately quantified. Unlike some of the other management impacts on site quality, the loss of topsoil is largely irreversible and requires intensive inputs to rehabilitate the site to a productive condition. The value of maintaining protective soil cover, as well as minimising the fallow inter-rotation period after harvesting, cannot be over-emphasised.

How are we doing?
Evidence to date has not definitively demonstrated that our current forestry site management practices have resulted in marked losses in stand productivity. This may be attributed to generally resilient soil types, the long crop growth periods (seven to 25 years), improved genetics and silviculture that mask any insidious changes, and generally conservative site management practices.

Currently, whole tree harvesting and stump extraction is not common practice, nor is biomass extraction for biofuel, thus the negative impacts associated with these practices is not yet evident. Off-site debarking, delimbing and slash burning are, however, occurring more commonly, especially under increasingly mechanised operations, which will affect the return of organic matter and nutrients back to a site. In addition, as more mechanised operations take place, it is likely that there will be a greater impact on the soil’s physical environment with greater levels of compaction and soil surface disturbance.

Despite the lack of conclusive evidence of soil quality decline and impacts on production, a number of studies have shown that some soils with less conservative management, these sites may be at risk of slow decline. Understanding these conditions is key to providing the guidelines to managing operations to minimise the impact. There is also a distinct need for long-term monitoring to allow us to detect any impacts that may not appear immediately, but that are having long-term consequences.

Ameliorating compaction is costly and time consuming and can be avoided through careful management.

Soil is an essential resource for plantation forestry and should be managed in a sustainable manner.

Retaining residues is considered advantageous for site nutrition and soil protection, but reduces re-establishment efficiency.

*Dr Louis Titshall can be contacted on Tel: 033 386 2314 • louis.titshall@icfr.ukzn.ac.za

**Published in April 2014