Q+A - Forestry and debris flows

Roughly 7% of New Zealand’s total landmass is covered by exotic plantation forests.

The forestry industry contributes an annual income of around $5 billion to our economy, makes up 3% of GDP, and provides jobs for close to 20,000 people.

Forestry is undeniably important to New Zealand – socially, economically, and even environmentally. As our government pushes towards an ambitious ‘carbon zero by 2050’ goal, the role of planted forests in removing CO2 from the atmosphere through photosynthesis (a process termed ‘carbon sequestration’) will become vital.

 Source: USDA, Northern Research Station.

Source: USDA, Northern Research Station.

However, forestry also has its downsides, and one of them, the increasing occurrence of debris flows, needs to be addressed.

In the following Q+A we’ll take you through the ins and outs of debris flows, how they occur, what their effects are and how they might be managed more effectively in the future.

What are debris flows?

Debris flows are described by the NZ Forest Owners Association as “very rapid to extremely rapid surging flows of saturated debris in a steep channel.”

They tend to contain a 60-80% concentration of sedimentary material (matter that settles to the bottom of a water body, e.g. sand, silt, pebbles, clay) and may or may not also hold woody material from forestry.

Debris flows usually move in surges, growing gradually in mass through entrainment (surface sediment picked up by a fluid flow and incorporated into a larger body).

Debris avalanches or shallow landslides can transform into debris flows once they become confined to a channel.

How do they occur?

The primary cause of a debris flow, as described by the NZFOA, is the “generation of sediment by mass movement into a channel.”

This generally occurs when a land area characterised by high, steep slopes and plentiful sedimentary ground matter is hit by heavy rainfall, causing large volumes of debris to erode vertically into a channel. Once the flow begins, it is generally only halted by a change in gradient or a reduction in flow depth, e.g. - on a fan or where a steep stream exits onto a flood plain.

Planted forests which sit in the post-harvest, pre-canopy closure phase (which usually lasts 5-6 years) typically contain high volumes of sediment, putting them in what is known as the ‘window of vulnerability’ for debris flows.

During this time, the removal of the forest canopy leaves the soil exposed to rainfall, making it increasingly vulnerable to surface erosion and water infiltration. Additionally, as the trees have been felled, their remaining roots begin to rot, reducing their soil reinforcement capacity.

When a period of high rainfall hits an area of forest in the ‘window of vulnerability’, debris flows tend to occur.

What are the impacts of debris flows?

Debris flows from forestry can have devastating effects on nearby towns and infrastructure, a fact highlighted by two recent examples in New Zealand.

In February of this year, in Marahau, a period of extremely high rainfall (brought on by Cyclone Gita) caused a huge flow of sediment and forest debris to descend from the hills, smashing property fences, damaging buildings and exposing residents to serious injury.

Two months later, an even larger debris flow damaged a total of 61 bridges and culverts in the East Coast district. This, again, was a product of extremely high rainfall and vulnerable forestry land.

In the Mangakahia Valley, Northland, poorly planned planting in the ‘80s has created consistent issues with debris flow. In 2003, resident Bruce Alexander carried out a survey for Carter Holt of the damage caused to local waterways from planting on soft soil and steep slopes.

His photographs show pines planted right down to the riverbanks, toppling into the water, taking big chunks of the bank with them, turning the stream brown with silt and creating dams as waste timber built up behind them.

Overseas, debris flows have washed away entire villages, destroyed agricultural lands, resulted in fatalities and had environmental impacts like loss of habitat, increased siltation of waterways and changes in topography.

How can debris flows be minimised?

As the NZFOA states, debris flows are a natural process, and therefore they cannot and should not be prevented altogether. However, their increasing frequency – a product of plantation forestry – can be curbed, provided careful planning is put in place.

NZ Forestry identifies the following six measures as integral for reducing both the occurrence of debris flows and the environmental + economic damages they cause:

1) Hazard avoidance. Identify areas where debris flows are likely to occur and take note of infrastructure (roads, bridges, buildings) and people that may resultantly be affected.

2) Defence/channelling structures. In areas where risk to infrastructure and life is high, introduce structures (slash racks, driven railway irons) that may mitigate risk by trapping or channelling debris into storage areas for management and disposal.

3) Narrowing the window of opportunity. In order to reduce the 5-6 period in which forestry soils become vulnerable to water infiltration and erosion, replant at a higher stocking density. Additionally, look into planting species that maintain live roots after harvest.

4) Riparian management. Riparian planting on lower slopes has been known to trap and impede debris flows. It is important however, that careful logistical planning is undertaken to prevent riparian plants being entrained into the flow.

5) Longer rotation crops or retirement into permanent forest cover. Identify areas most vulnerable to landslides and debris flows and consider a change to rotation length or conversion to permanent forest cover.

6) Improved risk models. Current data on risks and mitigation options for forestry management is widely considered inadequate. Collection of standardised data on size, frequency and impact of debris flows as a part of routine forest monitoring operations will enable the improvement of risk models, and the more effective management and mitigation of the impact of debris flows.

The Labour government’s emissions-curbing plan to plant 1 billion trees over the next ten years will require careful planning to ensure the risk of debris flows, along with other afforestation-related hazards, are minimised.

The Eastland Wood Council’s 2018 publication “Our changing landscape” highlights the vulnerability of the Gisborne region to debris flows and emphasises the need to reduce their occurrence when further planting occurs there over the next decade:

“The Gisborne East Coast is part of a plate collision crush zone. It is geologically fragile, with our steep hillsides formed as a consequence of natural erosion and land uplift, something that has been going on for thousands of years and it is not stopping. Weak underlying geology, with recent thin soils on top, steep slopes, rapid down-cutting by rivers (due to the rate of uplift) and periodic high rainfall and high intensity storm events mean erosion rates are naturally high. The main form of erosion is mass movement — and under forests this will mean debris flow.”

 Photos taken in the Tapuaeroa Valley show the effects of erosion from forestry. Credit - Eastland Wood Council.

Photos taken in the Tapuaeroa Valley show the effects of erosion from forestry. Credit - Eastland Wood Council.

It mentions the measures previously stated for risk mitigation, and additionally notes species change:

“Under the Government’s 1 Billion Trees programme there is an opportunity to look at other species, particularly for red-zoned land, and to understand the benefits of them. Many forest owners are also investigating tree species along riparian margins.”

Strategies like these will continue to be studied and trialed by the industry as it works with researchers, regulatory bodies, communities, and Government agencies to develop and improve best practice and ensure that the future planting of forests, both exotic and native, is something that helps – not hinders - people and the environment



Q+A - Soil Carbon

The Pastoral Greenhouse Gas Research Consortium is dedicated to mitigating greenhouse gas emissions from the agriculture sector by educating farmers and providing them with the tools to work at reducing their own carbon footprint. The following information is taken from their fact sheets on soil carbon, which show us why it is important and how it can be measured and stored for the benefit of both the agriculture sector and the environment.


Soil carbon is an intrinsic part of everyday life on earth; it is the basis of soil fertility, releases nutrients for plant growth, promotes the structure, biological and physical health of soil, and is a buffer against harmful substances.

Soil carbon results from the interactions of ecological processes - namely the feeding of microbes on decomposing matter - and can be affected by human activities that disrupt these processes.

The important issue is how we humans limit our impacts on these processes and work to keep carbon stored in the soil, rather than let it be released into the atmosphere.


There is more carbon in soil than terrestrial plants and the atmosphere combined. Practices that increase the amount of carbon stored in soils cab offset some of the greenhouse gas emissions from agriculture. Conversely, practices that deplete soil carbon and release it back into the atmosphere can increase emissions from agriculture.

The total carbon content of New Zealand soils isn’t the issue, rather it’s how that total changes over time.


Carbon is constantly being moved between the atmosphere, plants, and soil.

During photosynthesis, plants algae and some micro-organisms use sunlight to convert CO2 and water into sugars that are incorporated into their cells – some of which are consumed by animals.

Decomposing animal material, plant matter, fungi, worms and an array of micro-organisms contribute carbon to the soil. Most of this matter quickly decomposes as microbes feed on it and release the CO2 back into the atmosphere as they respire.

However, a small proportion of it becomes tightly bound to the mineral surfaces of soil particles, or tightly trapped in soil clumps, where it is protected and less accessible to microbes. It can remain locked away in this ‘stabilised’ state for hundreds of years.

The question is how you increase the amount of carbon stored in soils, therefore reducing the amount released into the atmosphere.


There aren’t yet any robust rules around reliably storing carbon in New Zealand’s pasture soils.

It is important, first, to maintain current stocks, as carbon can be lost quickly and recovered only slowly.

Beyond that, increasing soil carbon breaks down into two categories:

  •      Add more carbon and stabilise that carbon in the soil.
  •      Stabilise more of the existing input and reduce carbon turnover.

Research has shown that overgrazing reduces soil carbon, as it reduces overall plant cover and carbon inputs via roots. However, under grazing may equally contribute to soil carbon loss.

There are several options for increasing soil carbon currently in practice, though all of them need further research to determine their effectiveness under NZ conditions, and whether they are restricted to certain soil types, climatic conditions or management practices.

The First:

Add Nitrogen - The addition of nitrogen fertilizer or clover fixation might increase soil carbon in the short term, but in the longer term, it reaches a plateau. This option needs further testing, and it must be noted that it may create undesirable side-effects like the production of nitrous oxide.

The Second:

Optimise Irrigation - This should increase soil carbon due to increased plant growth and greater inputs of carbon into the soil, and hence remove carbon dioxide from the atmosphere. However, irrigation also encourages greater soil microbial activity, which in turn would convert this soil carbon into C02 and release it back into the atmosphere. Again, more research is needed.

The Third:

Increase Root Inputs of Carbon - Increasing the amount and turnover of roots should deposit more carbon into the soil, where some of it would be incorporated into stable soil organic matter. This method is currently being researched with tests on different pastures to see which is most effective. Mixed swards and plant species have previously shown to have the greatest root biomass and turnover, but more research is needed, particularly into how different pastures affect milk and meat production.

The Fourth:

Add Biochar - There is strong evidence that Biochar represents a very stable form of carbon, so it could be used to store more carbon in soils. The main challenge at present is the cost of the material, and the wide areas it would need to cover, which make Biochar an economically unfeasible solution for NZ.


Scientists estimate that an increase in soil organic carbon stocks of 0.4% per annum would compensate human-induced greenhouse gas emissions on a global basis. Soil carbon also provides a source of nutrients, helps particle aggregation, increases water storage and protects from soil erosion and compaction. These factors can significantly improve food production.


Measuring soil carbon and how its levels change over time, is a costly and labour-intensive exercise. This is because soil carbon can vary significantly from paddock to paddock and year to year.

Traditionally it is done by extracting soil cores and analysing their carbon content in a lab. However, this method limits the ability to make a credible farm-scale measurement.

If data points are too sparse, they could give a misleading picture of the average or total soil carbon stocks across an entire farm.

If we are to increase our ability to characterise and monitor these changes in soil organic carbon stocks, we need to develop rapid, practical, accurate, and cost-effective methods that combine spot measurements with robust tools to interpolate between data points across the diverse landscapes spanned by typical New Zealand farms. Recent research is opening exciting and cost-effective opportunities to make farm- and paddock-scale sampling and estimates of soil carbon more affordable and accurate.

For example:

  • Soil Spectroscopy: Soil Spectroscopy is a sensing technology developed to accelerate the prediction of soil properties. It relies on the fact that the reflectance of light from a soil surface is related to the bonding and stretching vibrations of molecules in the soil. The issue with this method is that it’s time and cost-effective. However, Internationally, soil spectroscopy is acknowledged as a major advance in the estimation of soil carbon, and other soil properties, allowing many more values to be estimated for the same time and cost as traditional analytical methods.
  • Digital Soil Mapping: Digital Soil Mapping uses environmental datasets along with advanced modeling methods (e.g. geostatistics, data mining) to develop spatial models of soil properties such as soil carbon. This method has been employed successfully in the Hawkes Bay Region and is being evaluated by regional councils throughout the North Island.


  • Soil carbon supports healthy and productive farm systems, and greater understanding of the distribution and changes in soil carbon across farms can help farmers adjust and improve management of this essential resource.
  • Soil carbon sequestration could offset some greenhouse gas emissions that are currently difficult to reduce otherwise
  • New soil spectroscopy and digital soil mapping technologies help reduce the uncertainty of soil carbon stock and stock change predictions, enabling soil carbon stock changes to be monitored at farm scales

As global warming continues to intensify, it is essential that emission-reducing strategies like these are studied, tested and employed.

For more information on soil carbon, visit www.nzagrc.org.nz