For carbon, diversity is essential

Over the last hundred years, the commercial forestry sector has made great strides in perfecting the art of growing trees. When it comes to pure speed of growth, nothing can compete with a high-yielding commercial monoculture  - in large part due to decades of research and artificial selection to narrow in on a set of extremely high-performing genotypes (often clones). It isn’t surprising, therefore,  carbon projects adopt a commercial monoculture approach in the hope of maximising their carbon yields - particularly in the first few years of establishment.

Of course, there are many downsides to planting monocultures if your goal is anything other than timber production. Relative to diverse natural forests, commercial monocultures usually support less wildlife, have weaker resilience to climate change, and can have negative impacts on soils and water- especially when they are planted in unsuitable places. Most important of all, however, is that they aren’t even the best way of maximising carbon returns –even over short timescales.¹

Commercial monocultures grow fast, but their manner of growth has been carefully optimised to produce timber – not carbon. To produce wood products to sell, you ideally want to grow even stands of identical trees that all grow at the same speed, with uniformly straight, branchless boles that allow for efficient harvesting and processing. In turn, you want trees that areas genetically similar as possible – often achieved by using clones- so you can have confidence that your timber products will all be similar. As soon as the trees reach the right size for timber, the trees are harvested, and a new round of clones can be planted in their place.

If the goal is to store carbon, however, it is better to leave trees in the ground to keep growing. This is where things start to get problematic under the monoculture approach. As trees get larger, they compete evermore intensely with their neighbours for essential resources - light, water and nutrients. As this competition increases, commercial plantations often need to be ‘thinned’ by removing some stems and leaving others to mature, lest the battle for resources gets so bad it starts to cause mortality. The result is an increasingly sparse forest made up of widely spaced but otherwise identical trees.

If you measure the carbon stored in an old-growth plantation managed in this way, you will typically find carbon stocks that are only a fraction of an equivalent natural forest. The reason for this is simple–natural forests have diversity. Scientists have long known that there is a fundamental relationship between species diversity and productivity (i.e. biomass growth) that transcends most ecosystems, with more diverse plant communities producing more biomass on average than those composed of just a few species. The reason for this relationship is straightforward - and again, it comes down to that ultimate enemy of growth: competition.

In a commercial monoculture, each stem responds to its environment in the same way as its neighbours, striving for the same resources in the same ways. Their leaves are all of similar shape, size and colour, so they capture light in exactly the same way. Crucially, the timing of their annual growth programs is also perfectly aligned, meaning they reach their peak water and nutrient requirements at the same times of year. They are all perfectly matched– and evenly matched fights invariably leave both protagonists bruised and bloodied.

Now imagine a natural tropical forest, where trees typically grow in a neighbourhood composed of entirely different species (NB there are exceptions to this!).  In these forests, neighbouring species will all be utilising the resources within the environment in a different way – differences shaped by competitive battles that have played out over millennia, driving each to carve out its own unique niche within the forest. Their different branch and leaf structures enable them to capture light in different ways, making sure that every last bit of the sun’s energy is captured and converted into growth.²‍

Crucially, they may also hit peaks in their requirements for water and nutrients at different times of year - when some are growing rapidly, others may be dormant; while some are fruiting, others may be putting all their energy into new leaf growth.³ The result is that collectively, the neighbouring trees are making optimal use of the resources available for growth by each targeting slightly different components at different times. Imagine a buffet restaurant where the customers diligently take turns to fill their plates, each selecting slightly different things – and contrast that with a scenario where they all rush at the same time to grab for the same meal. Thanks to this strategic diversity, trees in natural forests can be packed much more tightly together than those in monocultures, without experiencing the same degree of harmful competition.  In turn, the trees can grow ever larger, locking away more carbon by maximising their collective use of the resources available in the environment.

At Rainforest Builder, we prioritise planting diverse native forests for more reasons than just the carbon advantage. Our core aim is to restore forests to as close to primary conditions as we can get, including recovering the full complement of biodiversity that that entails. In simple carbon terms, however, the high-diversity approach stands up in its own right. By maximising tree diversity, we minimise competition and ultimately achieve far higher rates of long-term biomass growth than if we just planted fast-growing monocultures. By learning what we can from the successes of commercial forestry – particularly when it comes to managing weed species and invasives, and optimising nursery production– we are able to blend the best of both worlds. The outcome is a scalable ‘recipe’ for reforestation that allows us to generate large quantities of carbon credits, while also making significant contributions towards biodiversity protection and ecosystem health. Diversity is at the heart of this approach – and it remains our core mantra every time we set about restoring a new part of the landscape.