The Overstory #250

Trees and diversity for promotion of agroecological functions

by Roger R.B. Leakey

October 29, 2012

http://agroforestry.net/overstory/index.html

Introduction

It has been estimated that approximately 1.2 billion farmers practise agroforestry, while about 1.5 billion people (over 20% of the world’s population) use agroforestry products. From my travels seeing a wide range of different agroforestry systems, I realized that agroforestry is more than just an agronomic practice that restores soil fertility and produces tree products in farmers’ fields. It is also applied ecology or, more accurately, applied agroecology – the ecology of farming systems. This means, therefore, that it could be expected to also deliver ecological functions over and above such environmental services as erosion control, water infiltration, provision of shade, etc. that we saw in the last chapter. Environmental services are basically physical processes, while ecological functions have to do with the biological processes that make ecosystems dynamic and that regulate the balance between different organisms. This ecological balancing act is all about regulating the interactions between organisms throughout their life cycles and along their food chains. So, this process is an altogether higher order of magnitude in the way life self-regulates and creates a balance between species. It is this balance that confers ecological sustainability in different types of vegetation, landscapes or land uses.

The reason that I was excited about this realization is that modern intensive agriculture is notoriously destructive of all these processes. First of all because it typically reduces the diversity of species and cuts the dominant plant species to one – a monocultural crop. Secondly, it uses agrichemicals to replace some of the key agroecological functions by the use of pesticides to prevent pathogens, pests and weeds from taking over control of this dominant invader. In other words, the agrichemicals try to stop the natural food chains in their tracks, so that the crop is unaffected by other organisms. This means that conventional high-input agriculture is always fighting nature, and in the tropics this fight can be a fierce one as there are so many organisms struggling to impose some natural ‘law and order’. This would not be too serious if it were happening on a small scale, but agriculture occupies nearly 40% of the land surface – so it has a huge ‘footprint’ on the global environment.

Some agriculturalists are very critical of the idea of introducing ecology into agriculture. The argument seems to be that ecology is not ‘good’ science. However, a more serious examination of the complexity of ecological interactions reveals that agroecology is really the next big scientific frontier and a massive challenge to modern science. A really good understanding of agroecology, and in particular the role that trees and diversity play in the promotion of agroecological function, could certainly revolutionize how we produce our food. Unfortunately, we are a long way from this level of understanding at the moment. In the meantime agroforestry seems to be a good way to deliver some ecological and environmental sustainability in agricultural landscapes.

Nutrient cycling in tropical environments

If we consider a tropical rainforest that has a very high biomass, it can only get sufficient nutrients for growth and survival by very rapidly recycling the nutrients held in its biomass. So as leaves, twigs, branches and tree trunks fall to the ground, they are rapidly invaded by the unplanned biodiversity – the numerous worms, termites, bugs, beasties and microorganisms that gnaw, chew and digest the biomass, absorb the nutrients, defecate, die and rot down, so that the nutrients are made soluble and can be drawn back up into the forest plants for their continued growth. While this is going on at the forest floor, there are also insects, birds and mammals up in the forest canopy that are also eating the leaves and fruits, as well as each other, and again defecating and dying, and so making nutrients available again even more rapidly. In addition there is also a network of roots and fungal filaments below ground to trap and recycle the nutrients back into the vegetation, so preventing them from being washed out of the soil by heavy rain. Some of these fungal filaments have special relationships with the roots of the plants they colonize. They are known as mycorrhizas and the relationship is symbiotic – in other words, beneficial to both the plant and the fungus. The fungal filaments help the plants to scavenge for nutrients and water. In exchange the fungi can benefit from the sugars coming down from the leaves to feed the roots. These processes are the driving forces of the nutrient and carbon cycles – the foundations of soil fertility and the reduction of carbon dioxide emissions to the atmosphere.

Mycorrhizal fungi are very important for the tree establishment, survival and growth. They are also very vulnerable to environmental disturbance. For example, when a forest is cleared there is an almost instant crash of the populations of forest fungi and they are rapidly replaced by fungi associated with the pioneer plants and weeds. It can then take many years under a forest plantation before these pioneer fungi are once again fully replaced by populations of the forest fungi. The absence of the appropriate fungi makes it more difficult to establish forest trees on cleared sites. The appropriate mycorrhizal populations can be extremely important for tree establishment in degraded arid land sites. To overcome these problems tree seedlings can be deliberately inoculated with the appropriate fungus in the tree nursery.

All of the above is much more important in the tropics than in the temperate zone. This is because tropical ecosystems are much more complex. In addition, the soils in cool temperate climates are more fertile. This is because in cool climates the organic matter breaks down more slowly and so accumulates in the soil. In contrast, the soils of tropical environments are geologically old and low in mineral fertility. This is exacerbated by the combination of high temperatures, moisture and the high biodiversity of tropical soil organisms, which together lead to rapid breakdown of organic matter so the soils are shallow. Actually most of the nutrient stock in tropical ecosystems is in the plants – the biomass – and not in the soils. These differences between the tropical and temperate zones make agroforestry more important in the tropics.

Scale of the system

There are several other aspects of agroecology that we need to consider. First, scale is important, as within natural ecosystems there is a hierarchy of organisms living at different scales. So, a bacterium in the soil may never move more than a few centimetres. It may be eaten by a nematode that travels a few metres, which will itself be eaten by a small mammal running around on the forest floor covering several kilometres. Although these food chains function reasonably well at scales as small as a hectare, the most efficient function only occurs when the top predators, such as an eagle or a jaguar can play their part. This requires a population of individual top predators each with a territory of many square kilometres if they are to breed satisfactorily.

Most plants, the bigger ones at least, are of course anchored to the spot by their roots. However, their populations can travel as seeds, often in the intestines of birds and animals, or in rivers. In addition, plant genes are carried around the landscape as pollen on the wind, or on insects, birds and mammals. Both seeds and pollen transport can be relatively local or long distance. So, plant species vary in the area required to support a viable population. The importance of this is that, if we are to find out the true impacts of agroecological factors on productivity and profitability, it is critical that the work is done at the appropriate scale.

In an agricultural landscape, the achievement of sufficient scale for top predators can probably be provided by a landscape mosaic that includes food crops, tree crops and natural vegetation; especially if there are some corridors of perennial vegetation providing connectivity between the mature components of the agroecosystem. In practical terms, landscape mosaics provide diversity in time and space – due to the location, configuration and duration of different species in the landscape. Part of this variability results from farmers applying different farming systems and management practices in accordance with their personal preferences. These will be influenced by: (i) differences in farm size; (ii) the wealth of the farmer; (iii) access to market; (iv) the tenure systems; (v) the availability and price of labour; and (vi) the availability of other sources of income. In ecological terms, this additional source of variability is desirable.

In the context of climate change, we perhaps need to recognize the impact of agriculturally induced land degradation and ecosystem dysfunction. When land is cleared for agriculture and cultivated, two of the repositories of stored carbon are adversely affected: (i) the woody vegetation, which sequesters carbon dioxide as carbohydrates and cellulose in woody perennial tissues; and (ii) the organic matter in the soil. The decomposition of vegetation and soil organic matter as a result of aeration and the burning of cut vegetation releases many different GHGs – most notably carbon dioxide – altogether contributing about 15% of global atmospheric emissions attributed to agriculture. Much of this could be prevented by the large-scale integration of trees into farming systems. Estimates by the World Agroforestry Centre suggest that carbon could be increased from about 2 t/ha in severely degraded land up to 90–150 t/ha in a dense agroforest over an area of about 900 million ha worldwide.

Outlook

The problem at the moment is that we do not have enough hard scientific data to provide adequate knowledge of all the ecological, hydrological and environmental processes at play to be able to convince the sceptics of the value of this ecological approach to agriculture. This research has, however, been started and many of the complex relationships are becoming better understood. Nevertheless we need the science community to do much more to unravel the complexities of how agro ecosystems function.

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