[permaculture] Fwd: The Overstory #250: Trees and diversity for promotion of agroecological functions

Lawrence F. London, Jr. lflj at bellsouth.net
Tue Oct 30 09:48:27 EDT 2012

-------- Original Message --------
Subject: 	The Overstory #250: Trees and diversity for promotion of 
agroecological functions
Date: 	Tue, 30 Oct 2012 07:37:55 -0600
From: 	The Overstory <overstory at agroforestry.net>

The Overstory #250: Trees and diversity for promotion of agroecological
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/Traditional Yapese farming system with a diversity of crops including
tree crops./

**/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.



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