Although crabapples and rowans are the best choice for attracting bees and birds to your garden, the trees that sustain the most wildlife are all native trees – oak, willow and birch – in our ancient forests.
Introduction To Attracting Wildlife
It is worth noting that different species within a genus can have wildly differing effects on wildlife. Native oaks (Q. robur and Q. petraea) support immutable species, but their European cousin Quercus cerris do not.
There is one theory first proposed in Southwood (1961) that native species support many more species than non-native species – species introduced by humans. It is worth noting that a species native to the United Kingdom isn’t necessarily native to Scotland. Likewise, trees are concentrated in certain areas.
It is worth noting the quantity of species a tree supports isn’t the same as its benefit to wildlife. It tells you nothing of the biomass supported (nor whether they are endangered species, although this is beyond the scope of this article). A tree may support one species in a huge number.
As trees age, the species they support change. An oak’s bark starts smooth but becomes rugged with age, creating a habitat for rough-bark species of lichen.
Different species support the same species at different times of year. While most species of angiosperms flower in spring, the species that flower in summer provide support for bees.
Young trees do not support the same number of species as ancient ones. Damage, wear creates habitats for many organisms. Our oldest trees have dead and decaying wood at their heart of which some of our rarest habitats exist.
Open-grown trees – trees grow in the open – are more beneficial to wildlife than closed-grown trees – trees grown in the forest. This is because as trees die they become unable to support their highest branches. In the forest they are quickly shaded out by competitors, which hastens their death, while in the open they take much longer to die, supporting these “rare habitats” for far longer.
Similarly, trees left to their natural life cycle will support more wildlife than managed trees and landscapes as more nutrients are recycled by organisms.
Pollution reduces the amount of species a tree can support. Lichen are famously pollution sensitive.
Considering The Whole Ecosystem
When people think of the best trees for attracting wildlife, they usually think of trees that attract pollinating insects and birds, but trees support a huge array of wildlife.
Alexander et al (2006) list eight types of organisms that trees support and are supported by:
- Mycorrhizal fungi and the organisms that feed on them
- Soil-inhabiting organisms associated with live and dead roots
- Decay communities within dead areas of wood
- Decay communications which exploit fallen dead leaves
- Epiphyte communities which exploit all surfaces (bark, wood and leaves) and the organisms that feed on them
- Animals which feed on pollen, nectar, fruits, foliage and seeds
- Animals that feed on fungi and animals that live on plants
The list is no means complete, but it gives you a sense of the larger ecosystem trees support and the array of organisms needed for such an ecosystem to exist.
Mycorrhizal fungi play a key role in supporting trees by improving their nutrient absorption capacity and it is questionable trees would exist without them.
Soil-organisms and decay communities break up complex substances (animal and plant matter) into soluble forms, available for uptake by plants. Separately, bacteria can also fix nitrogen directly from the atmosphere when in a mutualistic relationship with certain plant species.
Epiphytes grow on the surface of plants. You may remember being shown lichens in science class, which are composed of algae and fungi in a mutualistic relationship. The algae providing carbohydrates in return for protection. In turn, many species of mollusks (slugs and snails) feed on fungi.
Trees have evolved a mutualistic relationship with insects and birds, as to exchange genetic information and disperse seeds, in return for protein (pollen) and sugar (nectar/fruits).
Southwood (1961) provides data on the quantity of species associated with various tree species. He revisited his original paper and provided new data in 1984. Southwood concentrated on foliage eaters and omitted species feeding on a wide range of species.
Rose & Harding (1978) provides data on the quantity of lichen associated with various tree species.
Alexander et al (2006) rank how much value a species is to various organisms, giving a rating of 1-5.
I have reprinted the top ranking species as well as members of the Rosaceae family, but it’s worth looking through the original tables for perspective.
|Species||Insects (1961)||Insects (1984)||Lichen (1978)|
|Hawthorn||149||no data||no data|
It’s worth talking through these columns. While some columns are useful for policymakers deciding on the best species for reforestation projects, they are irrelevant to most gardeners who are merely interested in attracting birds and bees. Garden trees are likely to be kept small and leaves collected.
Trees For Attracting Birds & Bees
A tree’s value to birds is a product of its longevity, cavity provision and food resources. Cavities are used for shelter and nesting. If a tree supports insects and produces fruit, it supports the birds that feed on them. Seasonality is important. The seeds pine and larch release in March and April provide a vital lifeline for many species. Different species have a preference for different trees with the hawfinch and greenfinch specialising in hard seeds. Birds more than any species benefit from mixed planting as argued in Peck (1989).
It’s worth noting that wind-pollinated species (mainly gymnosperms) are of no use to bees. These species can be identified by cones (as opposed to colourful flowers). Species with long listless catkins are often wind-pollinated, but their are some exceptions to this.
Just like humans, bees are attracted to the most vivid flowers, which often have the highest concentrations of pollen. Plants also put out nectar, which is used to fuel flights. Seasonality is again important with the early flowering blackthorn and hawthorn sustaining spring-flying colonies. Come summer, the sycamore provides excellent value.
Top Varieties For Attracting Wildlife
All below are native species.
The best tree for supporting wildlife. You may have to wait till you’re three hundred to enjoy it!
The silver birch with its open canopy provides the perfect conditions for many bulbs, grasses and mosses to grow, creating those magical woodland glades. A magnet for insects, you can expect ladybirds and many species of moth. Woodpeckers commonly nest in the trunk and its seeds attract greenfinches and redpolls.
The goat willow provides an important early source of pollen. Now available in an architectural form small enough to fit in any garden.
laevigata and monogyna are both native to the UK, known as English and common hawthorn respectively. Some of the best varieties are hybrids including ‘Paul’s Scarlet’ and ‘Rosea Flore Pleno’. The dense thickets provide excellent shelter and the haws (berries) are consumed by a wide range of wildlife.
Long-living, beech provides habitats for many deadwood specialists, such as hole-nesting birds. You may have noticed a wide range of mushrooms in beech forests. These are mycorrhizal fungi that help the tree obtain nutrients.
Rowan’s berries were once used to catch birds in the middle ages. Known as mountain ash as it is found growing at high altitudes. Complete with gorgeous serrated pinnate leaves and creamy white flowers, ‘Mountain Ash’ is up there with the best ornamental trees.
The holly’s small flowers are extremely attractive to insects and you can expect a mistle thrush aggressively guarding its long lasting berries.
Jorge works in the Primrose marketing team. He is an avid reader, although struggles to stick to one topic!
His ideal afternoon would involve a long walk, before settling down for scones.
Jorge is a journeyman gardener with experience in growing crops.