Biochar to Improve Tree Growth and Survival In Woodlands, Forestry and Agroforestry

Photo credit: Laurie Campbell / WTML

We are working with A Healthier Earth and the Blenheim Estate to increase tree survival and growth rates as they plant 500,000 trees on their estate. Alongside their commitments to the woodland environment, maintaining biodiversity, and using native broadleaf species, this project also focuses on mitigating climate change through carbon locked into the tree growth. A relatively small percentage increase in tree survival rate and annual growth rate will have a huge impact over a 30-50-year period.

Last year was a tough time for newly planted trees. Usually, woodland and forestry tree planting expects around 30-40% sapling loss in the first couple of years, but in 2023 losses on some projects were as high as 70%.  Drought-tolerant trees were high on the project team's agenda. 

Biochar For Tree Planting

Biochar increases water retention and reduces drought stress -  a positive place to start.

Biochar also:

• Improves nutrient supply to roots
  
• Reduces nutrient losses due to 'run-off'
• Supports microbes and AMF (mycorrhizal fungi)
• Supports the formation and stability of colloidal humus
• Improves soil aggregation (i.e. tilth)

Our core task was to bring together our existing knowledge of “biochar for trees” and build a ‘best practice tree planting method that would work for hundreds of thousands of trees.

We have a knowledge bank to call on having helped several tree clients with Miyawaki urban woodlands, specimen Tree recovery at Oxford University, Air spade root decompaction, general woodland planting, and of course, supply of Stockholm-type structural tree mixes with additional biochar. (see reference links at end of blog).

Even so, when looking at large new projects it is good to take a step back and look at the science and update on the latest best practice for tree planting. This project has additional complexity around seeking grant funding and approval for carbon offsetting credits. We must look at soil disturbance, mitigating future climate change, and a host of other issues. We undertook a review of the top specialist tree-websites, and Forestry Research guidelines (add hyper).  For good measure. we asked ‘AI’ (ChatGPT) to help!

Tree Planting Best Practice (click title for full article)

This best practice is wonderful – if you are planting a few specimen trees in a garden. However, it is a mile away from the logistical and commercial realities of large-scale forestry and agroforestry plantations. A more typical mass planting method is: to spade-split the soil, drop in a bare root sapling (30-60cm), heel down, stake, and add a plastic tree guard. Rarely if ever does large-scale tree planting receive ongoing watering. We noted some websites reporting professional planters planting as many as 2,000 saplings per person per day - an incredible 4 saplings per minute! The typical planted cost is £2-£3/tree. 

When we sought advice about the causes of the high rate of tree losses, we and others jumped immediately to ‘what do expect if you do not water and provide tree aftercare?’.  But this is too simplistic - it is not 'bad practice'; it is the reality that the cost of piping water and labour to irrigate would be huge. Even if it was affordable, often there is little or no chance of being granted a water abstraction licence from a river/ground source.   

Our first ‘best of best’ planting method was as follows: run a backhoe digger up and down the site to remove a 1m channel of soil. Mix the extracted soil 50/50 with biochar tree mix. Add drainage and watering, drop in cell-grown trees, and backfill. Mulch over top the whole 1m wide channel. Stake and protect with new compostable guards. This method was quickly dismissed. An increase in cost from £2/tree (£1m project cost) to £10/tree (£50m project) was not credible to present to the client.  

Planting Method - Re-think

We undertook a significant species analysis looking at the local soil, trees already growing at the site, and mixing things up to provide more biodiversity security. With access to very experienced carbon forestry experts, we could also look to the future in terms of climate impacts. One thing we did spot is the UK tree nurseries have invested a lot of time looking at drought-tolerant species in readiness for dryer, hotter Summers. We did not find a lot of discussion around the fact that most large-scale planting will be in poor, clayey soils.  The UK is also going to have significantly wetter and soggy clayey soils in Winter. Species like Alder (Italian) came high up for both drought and soggy!

Armed with 5 species, and a 5X5 randomized test plot of the first test of 1,000 trees, it was time to crack on with the decide the best economic planting. 

We have ditched the t-split planting of bare root saplings in favour of using cell/pot trees of 30-60cm in height. (More on this will come later when we outline the full “carbon forest factory” model.   We have automated the hole digging to use a 150mm tractor-mounted auger drill to a 30cm depth, then backfill with the SoilFixer Biochar-Woody Compost tree mix (aka SF40). 

Why This Mix?

Knowing the major cause of young tree loss is lack of water, adding more water retention is good. But too much water retention can also lead to all the void spaces in the soil filling with water or swelled clays/gels. This is equally bad news as the roots cannot breathe (respire) and they quickly die back and rot. Dense soils restrict fast root development. Low-density growing substrates like peat, coir, and compost have no weight to hold the roots and hence trees suffer from rocking and poor root development. The mixture is a balance seeking strong growth in years 1 and 2 and then allowing development of the roots into the surrounding soil.  When planting into pastureland, we will dip-inoculate the cell tree roots with AMF powder/solution. Where the planting is back into recent forestry soil we will skip the AMF. 

Sidestep - AMF Fungi

AMF are the symbiotic fungi that link roots to plant nutrients in the wider soil structure

We are still searching for compelling evidence that confirms that pasture (grass, crop) soil microbiology and AMF differ from those in a forest ecosystem. As far as we can find, all the commercial AMF powders are all based on 12 major AMF species and there is no specific plant or tree mix. Based on our composting knowledge there is also a strong hint that the white fungi filaments often mentioned as being present in forest litter (and by implication not in crop soils) are in fact ‘lignin (wood!) decomposing fungi’ filaments. Academic papers indicate the decomposing fungi are obligate (ie decomposer only) and do not participate in the symbiotic relationship – ie they are not AMF! The leaf litter (surface) layer is not the root rhizome layer.

Soil Type: 

There are online resources that map UK-wide general soil profiles for example the Landis database.  Even within a few miles of our site, there are at least 4 distinct soil types. It is always worth doing local soil tests just to check. pH testing is straightforward and a good pH meter will last years and cost around £30. The first planting site is pH 7.45. Although very rocky, it is otherwise a ‘heavy clayey’ soil. The biochar-compost tree mix we are using is pH 7.0.

Tree Guards

The site has problems with voles, rabbits, and deer. They can wipe out the young saplings within weeks. We have to use tree guards, and for voles, we need to ensure the guard goes into the surrounding soil so they cannot get under. 

Going forward environmental concerns may bring in compostable guards. But the sheer scale of planting means that at present the old plastic tree guards are being used multiple times. Eventually, they will be collected and hopefully be reprocessed into biochar!

Growth Rate Testing

We have set up some reasonably sophisticated ongoing tests to look at health and growth over 4 years. We will be monitoring leaf chlorophyll, stem diameter, and removing some saplings to analyse root growth.

We will be announcing the client name in due course when PR departments have approved the content. Do come back for updates.

If you are looking for advice on large-scale, carbon credit forestry with a higher payback please email: [email protected]. 

Other Tree, Woodland, and Forestry Blogs and Reference Materials

Biochar in woodlands – blog 

Our Best Practice Tree Planting Guidelines (for specimen trees) is now published

Stockholm Structured Soil for Urban Planting with Biochar 

Our blog on the Miyawaki urban woodland method is here

The Forestry Research Guide is here. 

The Forest Research tree specimen database is  here

UK-wide soil profiles Landis database