A mature forest consists, minimally, of three layers: the upper (emergent) canopy trees; the smaller, middle layer, understorey trees; and the lower dwarf shrub and herb layer.
These are the forest giants and are genetically programmed to produce a dense canopy of leaves. They maximize their growth rate by getting above the other trees, capturing as much light as possible with their leaves and hence shading out their competitors. This is a winner-takes-all situation; you could call these trees the big bullies of the forest. It’s a jungle in there.
2. The Understorey Trees
These use different survival strategies. Their energy comes from above in the form of beams of light which squeeze between gaps in the upper canopy leaves. When a beam of light strikes a solid object it forms a circle of light called a sunfleck. The understorey trees are genetically slower growing and have to rely on sunflecks for their photosynthesis (sunflecks were illustrated with a slide of a fawn lying among vegetation).
2a. Deciduous Understorey Trees
These rely on sunflecks for their energy requirements but cannot predict where the arc that a sunfleck tracks will be as the sun moves across the sky. They therefore adopt a widely spreading shape with their branches as a shallow angle or almost horizontal attitude with their leaves scattered along the branches separated or barely overlapping. This is a statistical approach that maximizes the chance that at least some leaves will be illuminated by a sunfleck passing over them during the course of the day. The branching attitude of a tree is genetically controlled and while they grow more rapidly and denser in full sun the branch angle and general shape is retained.
2b. Evergreen Understorey Shrubs
These include laurels, hollies and rhododendrons and are promoting yet another philosophy. They invest in very long-lasting leaves – really well made products, thick, weatherproof, good wax job, prickly or full of poisons to repel grazers. They can afford to sit around for years collecting what little light falls on them spring, summer, fall or winter. These are slow-growing shrubs which hoard their energy gains producing a dense covering of leaves before they even think of flowering.
3. Subshrub, Herb and Bulb Layers
These plants form the backbone of many gardens but were not discussed.
The above is a highly condensed bit of academic forest ecology. Is it of use to gardeners? Well it would be if nurseries had labelled their trees ‘Forest Emergent’ or ‘Medium Understorey Tree’ etc. because then people would know what to expect.
A few years ago the Hardy Plant Group were given an amusing talk on ‘The Oops! Of Gardening’. This was illustrated with slides of gardens whose owners had planted small, innocent-looking emergent species near their homes with the result that some years later the house had become swallowed up by monster trees towering over them. Here in Victoria you only have to walk down Moss Street from the Art Gallery to see the magnificent California Redwoods, Sequoiadendron giganteum, planted initially as seedlings in a tiny front garden. People even ask for trouble: Customer, “I need a quick growing tree to screen off my neighbour.” Nursery Owner, “You can’t beat red cedar.” And you can’t!
Of our native trees the most notorious emergent is the Western Red Cedar, Thuja plicata. Listen to almost any phone-in radio show and someone will complain that anything they plant near their cedar dies, or that their neighbour has a cedar hedge along the property line and that nothing will grow by it. This well known effect is the result of cedars being extremely competitive: they cast a dense shade, their surface roots absorb nutrients and water and if you think the complete lack of herbs under cedars looks as if chemical weedkiller has been spread over the surface – it has.
Linda and I sometimes visit Butchart Gardens and walk through the Japanese Garden laid out, I think, in 1912 by Isauro Kushita (newly freed from planning Laura Dunsmuir’s Japanese garden at Hatley Park). In places the old Japanese Maples and the Wedding Cake Tree, Cornus controversa variegata, have blended together in a wonderful symphony of branches. We often pause to have tea from a thermos in the tea house on the island by the stepping stones. The Tea house is in the process of being absorbed by the adjacent red cedar eliminating the original plantings. I fantasize what I would do with regard to trimming the lower branches and small trees if I could be let loose with my chain saw for a few hours.
Among our other BC native trees, Garry Oaks cast a transparent shade allowing lower vegetation to thrive and this is seen at Playfair park where the Rhododendron Garden is gently guarded by the overstorey oaks.
Douglas Firs are pretty dominant emergents but of course can be limbed up to allow more light to penetrate. Our native maples are a contrast: the Big-leaf Maple is a deciduous emergent and casts a heavy shade in summer but nothing as complete as our cedar. The other common maple is the Vine Maple, Acer circinatum. This has a quintessential understorey type of growth. Related to the Japanese Maples it makes an excellent tree for small gardens permitting herbs to grow under it.
A. Ecological Balance
The problem I have with some Rhododendron gardens is that they have so many Rhododendrons that they become monotonous to the eye. So the question comes up of what makes a garden pleasing.
As a preliminary the emergent trees – the ones that take over – have no place on small suburban lots. On large acreages they add shape and solidity to the landscape but require care in siting and should be planned with at least 10m on all sides where nothing will be built or planted that would be endangered by subsequent growth. In practice most gardens are ‘landscaped’ with nursery trees with thought only to the immediate effect. Even professional landscapers are guilty of inappropriate choice of material although they can plead that the information is not available and that clients demand over-planting.
The ideal trees for suburban gardens are the understorey members. These include birches, Japanese cherries and maples, Styrax and Ginkgo among hundreds of choices. Their inherently open shape and slower growth combined with leaf and flower interest make them ideal choices for gardens of all sizes. They are compatible with a wide range of ground vegetation. So the first thing a gardener needs to consider is the ecological nature of the plants in their garden. This was something that Sir Peter Smithers brought up about 20 years ago.
B. Topographic Balance
Balance of ecological types is not the sole criterion for a good garden. When you examine the more pleasing gardens around Victoria you find that they have in addition topographic complexity – flat areas, slopes, hill, streams, ponds, swamps, rocks, pathways, structures, the latter including, in Victoria, Japanese lanterns.
One of the best gardens in Victoria for a successful combination of ecological and topographic balance is the Abkhazi Garden. The accident of the rocky site (builders don’t like rocks), combines with a careful layout, and selection of materials have matured to the perfect blend.
The second half of the talk considered the Japanese Maples which can provide a glorious combination of colour and texture of bark, spring foliage, summer leaf texture and fall brilliance. They are all understorey trees with an almost perfect combination of characteristics. They are used to maximum advantage in the Peet Garden on the Saanich Peninsula.
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But
that was not all. As the 5 minute
speaker, Margaret
deWeese treated us to a fine excursion
through the world of ferns. From her
garden we were shown a great array of some 18 ferns, ranging from the delicate
maidenhair which dies tragically in winter and reappears supernally in due
season with its coiled black stems; the Soft Shield ground-hugging rosette
fern; delicate and hardy tree ferns, and
the handsome Dryopteris wallichiana, shuttlecock-shaped in its
bold maturity of some 5 metres. In
addition, she presented selected slides from Edinburgh’s Fern House and the
Glass House at Kew.
It
was easy to conjure a feeling of walking through Joe Harvey’s three-storeyed
forest with ample sunflecks abounding among Margaret’s great fernery, thriving
in its cool moist bed of loamy soil and delicious organic mulch. We were
transformed.