[Sdpg] The Overstory #250 Trees and diversity for promotion of agroecological functions by Roger R.B. Leakey October 29, 2012

Wesley Roe and Santa Barbara Permaculture Network lakinroe at silcom.com
Tue Oct 30 06:16:35 PDT 2012

**/The Overstory/* #250/

*Trees and diversity for promotion of agroecological functions*

by Roger R.B. Leakey

October 29, 2012



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 

*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.


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.



This article was excerpted from the original with the kind permission of 
the author and publisher from:

Leakey, Roger R.B. 2012. Living with the Trees of Life: Towards the 
Transformation of Tropical Agriculture. CABI.

More information about the book from the publisher 

Search on Facebook for "Living with the Trees of Life"



Roger Leakey <http://www.rogerleakey.com> was Professor of Agroecology 
and Sustainable Development of James Cook University, in Cairns, 
Australia (2001-2006); Head of Tropical Ecology at the Centre for 
Ecology and Hydrology in Edinburgh, UK (1997-2001) and Director of 
Research at the International Centre for Research in Agroforestry (now 
the World Agroforestry Centre 1993-1997). Currently he is Vice Chairman 
of the International Tree Foundation 
<http://www.internationaltreefoundation.org/>, a UK registered charity 
and Vice President of the International Society of Tropical Foresters.

Between 2006-2008, he was a Coordinating Lead Author in the 
International Assessment of Agricultural Science and Technology for 
Development (IAASTD). This Assessment examined the impact of 
agricultural knowledge, science and technology on environmentally, 
socially and economically sustainable development worldwide over the 
last 50 years. It and suggested that to meet these challenges 
agriculture has to become more multidisciplinary and embrace food 
production within a more integrated approach to achieving environmental, 
social and economic goals.



The Overstory #239: The Benefits of Tropical Homegardens
<http://www.agroforestry.net/overstory/overstory239.html>The Overstory 
#234: Trees on farms to enhance agricultural sustainability, resilience 
to climate change
<http://www.agroforestry.net/overstory/overstory234.html>The Overstory 
#216--Introduction to temperate edible forest gardens 
<http://www.agroforestry.net/overstory/overstory216.html>The Overstory 
#201--Indigenous Fruit Tree Domestication
The Overstory #162--Agroforestry Tree Products (AFTPs): Markets
The Overstory #145--Wild Foods and Food Security
The Overstory #139--"Hungry season" food from the forests
The Overstory #136--Underutilised Indigenous Fruit Trees
The Overstory #117--Between Wildcrafting and Monocultures
The Overstory #113--Forest Biodiversity
The Overstory #109--Cultural Landscapes
The Overstory #105--Complex Agroforests
The Overstory #93--Trees, Forests and Sacred Groves 
The Overstory #76--Ethnoforestry 
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