The Overstory Forest Layer
The overstory is comprised of trees that make up the canopy of the layered food forest system. They often provide beneficial shade for sub-canopy trees, shrubs, and other plants below them. Canopy trees are often nut trees or sometimes, in smaller designs, top fruits. As overstory trees fill out and the canopy closes, this can be a key factor in the evolution of a food forest.
The highest tier of trees in a forest garden or food forest, these trees form the canopy of the ecologically functioning system.
Overstory Layer: Canopy Trees
The overstory layer is made up of the tallest trees in a woodland or forest-like ecosystem. These trees define the height of the vegetation and the characteristics of the canopy.
When we talk about the canopy, we are talking about the term in the context of forest ecology. Biologically speaking, canopy is a term that can refer to all aboveground portions of a plant cropping or crop formed by the crowns of the plants. But in forest ecology, it refers specifically to the upper habitat zone formed by the crowns of mature tree species.
Communities of organisms within the canopy layer of a tree-based ecosystem are incredibly important in maintaining natural biodiversity, resilience and forest function.
In food forests or forest gardens, we are of course aiming to mimic a natural woodland or forest as closely as possible, while replacing at least some of the key species with plants of more use to us for food and/or other resources.
The overstory layer of taller tree species helps us mimic a natural forest or woodland canopy, deriving many of the same functions as those in natural ecological systems.
The Functions of Upper Canopy Trees
The position that upper canopy trees hold within a forest or woodland, or a system mimicking a natural forest or woodland like a food forest or forest garden, dictates their important functions within the system as a whole.
Upper canopy trees have a number of important roles. They are important in maintaining environmental stability both locally and globally.
Locally, forest canopies are crucial to controlling and buffering climate conditions within the forest microclimate.
These tallest trees within the system that form the canopy are able to photosynthesise relatively quickly because they can access more sunlight than those in, or partially in, their shade. Of course, the characteristics of the trees used, and the degree of canopy cover, dictate how much light will filter down to reach lower tiers of plants within the system.
These tall trees also intercept precipitation (rainfall and snowfall where this occurs), sometimes providing protection from weather extremes for plants in the lower tiers. Also often protecting other plants within the ecosystem from strong winds and storms.
In various forest or woodland systems, canopy trees produce feedback loops where the species within the system, their growth traits and the composition of the stand determine and are determined by forest microclimate.
Globally speaking, forest canopies are involved to a great degree in carbon sequestration and play a vital role, of course, in maintaining stability within the global climate.
When and Where Upper Canopy Trees Are Used
Typically, forest gardens or food forests follow the seven layered system. The overstory layer, or canopy trees, are the tallest layer within the system. Below them there may also be smaller trees, known sometimes as sub-canopy trees, or the understory layer.
The trees within this category are used within food forests or forest gardens as the tallest tier. These trees are typically, though not necessarily exclusively, taller fruit and nut trees within typical permaculture systems.
Though other taller trees may be used for their other characteristics rather than their yields – when they are used as nitrogen fixing species, for example.
Canopy trees are typically among the first plants to be selected for a permaculture design. And their placement will of course be a very important factor in the design.
In my own small forest garden, the upper canopy trees were already in place when I moved to the property– a series of mature apple, plum and cherry trees. Originally within a walled, grassed orchard, I have slowly turned this area into a much denser, more biodiverse and more productive food forest by filling in the lower tiers below these canopy species.
The Degree of Canopy Cover
One important factor that must be determined during the design phases of forest garden creation is the degree of canopy cover desired.
Broadly speaking, and as a general guideline, the warmer the climate, the greater the degree of canopy cover that can be beneficial.
In the tropics and subtropics, canopy trees are particularly important in protecting lower layers of planting from extremes of sunlight, heat and precipitation. Trees in these climates can typically be placed much more closely to create a denser canopy and a greater degree of shade.
In Mediterranean climates and particularly arid regions, canopy trees are also important in casting shade and reducing moisture losses. Shade is a beneficial thing in such climates in a way that it is not always quite so beneficial in cooler temperate climes.
In cooler temperate climates, shade can sometimes be excessive if we create a closed canopy with our tallest tree species, and too much shade limits the range of plants that we can successfully grow in the layers below them.
A closed canopy deciduous forest in temperate regions may have little understory growth though summer and autumn, since most growth and flowering below a more closed canopy tends to occur in spring, before the full foliage shades out the area.
So in most temperate climates, shade has to be managed carefully and the degree of canopy cover is a very important thing to think about. It is generally a good idea to create a more open woodland feel, rather than creating a dense forest, when planning a forest garden in cooler climes.
Managing Shade in Temperate Food Forests
Broadly speaking, the further north you live in the northern hemisphere, or the further south you are in the southern hemisphere, the more you are likely to need to think about how we can let the sunlight reach lower tiers and maximize photosynthesis within the system.
Managing the shade created by canopy trees in temperate food forests might include;
- Creating open, sunny glades within the area of planting.
- Making southward-facing bowls, with the tallest species to the north (in the northern hemisphere).
- The use of north-south access paths or alleys to ensure lower tiers of planting get a little more light than they would in a less open forest.
- Judicious pruning of trees and other taller species to let the sunlight through.
- And, of course, simply selecting more shade-tolerant species for our lower layers of planting.
Sunlight won't be as strong, of course, in temperate climes as it is in the tropics, so when designing our forest gardens that is an important thing to take into account.
Will You Always Have More than One Layer of Trees in a Food Forest?
When choosing trees for a food forest or forest garden, one important thing to think about is, of course, the size to which chosen trees can be expected to grow. Larger, taller trees are typically included in the overstory layer. But some may be deliberately coppiced or pruned to keep their size in check as a lower layer element.
Another thing to think about is that, while in many food forests, we will have trees within two layers, sometimes, there may only really be one layer of trees.
Just as forests and woodlands differ greatly in their composition and species mix, so too a food forest can vary depending on the site and the project goals.
Note also that in a much smaller area, we can still follow the ideas of food forest design and creation, even when large shrub or very small tree species make up the tallest layer of the system.
Choosing Upper Canopy Species
When choosing which species to include to make up the upper canopy within your food forest, it is important to consider:
- Which species might be best suited to the climate, microclimate and soil on a particular site.
- The size to which the species considered will grow and the space available on the land.
- How much sunlight and rain particular species will let through to lower layers within the system. (Think about the density of the foliage, whether the species in evergreen or deciduous, the positioning of the branches and the size and shape of the crown.)
- How the above will change throughout the seasons and over time as the system evolves and the canopy trees or other canopy species make it to maturity.
- The yields that particular species can provide (fruit, nuts, seeds, edible leaves, crafting materials, mulch & biomass etc...).
These are just a few of the main things to think about when you are selecting upper canopy species. Remember, however, that choosing your species is just one of the steps and placement and positioning is equally important, especially in a temperate climate, where managing shade is so much more important.