Structure plays a pivotal role in agroforestry systems. By intentionally integrating different layers – including trees, shrubs, and crops – agroforestry systems generate diverse structure that enhances biodiversity by fostering various microhabitats and thus supporting a range of plant and animal species. Agroforestry systems are particularly unique because due to their perennial nature, they can provide more permanent microsites.
To the left is a diagram describing a general agroforestry system in a tropical ecosystem. Trees at different heights – which define different layers within the site – often provide different attributes to the system, ranging from taller trees geared towards production to shorter, edible root crops. While this changes across ecosystems, increased structural diversity in an agroforestry system allows for more functional diversity and structural stability.
Vertical and horizontal complexity
For easy conceptualization, we can break down the structural design of agroforestry systems into its vertical complexity, which speaks to diversity in tree height , and horizontal complexity, which speaks to diversity of plant species. Find a diagram mapping the vertical and horizontal complexities of primary agroforestry practices below.
Let’s take a look at the orchard as an example to better understand this diagram. Orchards are usually managed as monocultures, meaning they often only focus on one type of tree. This is why they would have a low horizontal complexity, similar to the classic annual system – there isn’t much species diversity. Yet, because the trees in orchards have some variance in height, they would have more vertical complexity than annual crops.
Now, let’s compare the orchard to a food forest. In a food forest, we have more diversity in plant species – as we are seeking to have a range of edible plants available – and therefore we have more horizontal complexity. We also have plants of more varied heights in the food forest, as we may have fruit bushes and ground herbs in addition to fruit and nut trees, rather than there only being trees in an orchard.
Ranges in plant density
In addition to considering use of space when it comes to horizontal and vertical distribution, it is also fundamental to consider the density at which something is planted at every level. Having several clustered trees would make for a different ecosystem than having few, dispersed trees.
Take, for instance, the example of forest farming, in which one would have a diverse vertical distribution, including tall trees and ground herbs. If you have several clustered trees, there would be little light reaching the ground plants. On the contrary, if you have few dispersed trees, there would be significant light reaching the ground plants. One would want to determine the light demands of their plants before deciding on a design.
Belowground Root Structures
Belowground structural diversity is also significant when considering ones agroforestry system, particularly relating to root structures. Tree roots are very good at penetrating through deep soil layers, and are able to pump nutrients and water up towards more shallow layers, which is particularly helpful during seasons of limited water and nutrients. This process is called hydraulic lift. Trees have several different types of roots, beneficial for different purposes, illustrated in the image below.
- Tap root: Starting root to provide stability and absorption. Eventually outgrown by other roots.
- Lateral roots: Grow right under soil surface; anchors tree and absorbs water and nutrients.
- Oblique/heart roots: grow at diagonal with same function as lateral roots
- Sinker roots: Grow downward from lateral roots. Absorb water and nutrients, and provide structures.
- Fine roots: Present in all of the above roots; directly absorb water and nutrients, and house mycorrhizae.1
Longer, stronger roots are especially beneficial in agricultural systems, where there is often a plow pan; a plow pan being the depth of the soil under which the soil is far firmer because it has not been plowed or tilled for agriculture. Because of the plow pan, it is more difficult for plants with shorter, weaker roots – fibrous-rooted species – to access these deep-seated nutrients.
Many traditional agricultural plants, such as corn, carrots, or wheat, cannot penetrate the plow pan. With the addition of tap-rooted species, nutrients trapped in the plow pan can circulate and reach all crops!
Smaller, fine roots have a high turnover, meaning they die and regenerate quickly, and can therefore quickly add SOC back to the system. They have different depositions of sugars and other enzymes that are beneficial to microbial communities, and they can help access portions of the soil where there may be nutrients. They are also helpful in forming secure clumps of organic matter, nutrients, rock and substrate, which increases their ability to stabilize their environments – for instance, by slowing and absorbing rushing water, or by holding onto carbon in the system.
Establishing microclimates
Additionally, the strategic arrangement of vegetation helps regulate the microclimate, providing shade, windbreaks, and influencing temperature and humidity. This microclimate regulation proves essential in safeguarding crops and livestock from extreme weather conditions, ultimately optimizing the overall productivity of agroforestry landscapes.
All of these components, along with the goals of your site, and species selection, will influence how you may consider integrating structure and spatially arranging your agroforestry system.
- https://aplustree.com/3-types-of-tree-root-systems/ ↩︎