Dr. Pamela Wright
Maintaining or restoring connectivity for species and ecosystems is key to saving biodiversity. We often think of connectivity across geographies, like grey whales migrating from wintering grounds in Baja, Mexico, to summer feeding grounds in the Arctic. However, vertical connectivity is also crucial at large and small scales.
Large Scale: Elevational and Latitudinal Connectivity
Over large spatial scales, vertical connectivity is expressed as cross-elevational (or cross-latitudinal) connectivity. Many terrestrial species move up or down mountains seasonally. Endangered mountain caribou spend their summers primarily in alpine and upper subalpine areas. In the winter, some herds move down to old coniferous forests, with abundant arboreal (tree-growing) lichens, for shelter. Maintaining connectivity between lower-elevation old forest patches and undisturbed high-elevation areas is essential for many disturbance-sensitive species in BC. As climates change, many mountain-dwelling species may flee the warmer temperature by retreating to higher elevations or latitudes.
Researchers at UBC and the Cornell Lab of Ornithology have called this the ‘Escalator to Extinction’ where species, like birds, who are more able to move uphill to track changing climates will soon run out of room. Other species, like trees, will not be able to keep pace with the changing climate and those that already exist at high elevations, like the endangered Whitebark pine, may get pinched out.
Credit: FREEMAN ET AL, PNAS 2018
On the Southern Gulf Islands elevational or latitudinal connectivity is less of an issue in terrestrial environments, although it is a critical issue in marine environments. Larger provincial and national initiatives to designate additional protected areas and connect those areas, particularly those that cross elevations and latitudes, are our best response. However, on Pender, we can work effectively on maintaining and restoring vertical connectivity at smaller scales.
Small Scale: Connecting the Forest Floor to the Forest Canopy
On a daily basis, many species of birds, insects, and forest mammals need to move from the forest floor up to mid-canopy or to the upper reaches of the forest canopy for food or for nesting. But of course, tree trunks stretch from the ground to the canopy top so you might be wondering, particularly for flying things like insects and birds, why can’t they just fly upwards? Some, of course, can. But for many, in a forest that lacks vertical structural diversity preferred habitats for mid-canopy feeders like chickadees are missing or species face too much exposure from predators as they travel up and down.
So what do we mean by vertical structural diversity? A forest with healthy structural diversity has a mix of species at different ages and heights, downed woody debris, and standing dead trees for cavity-dependent species like woodpeckers, owls, and bats. On the SGI and coastal BC, we have drier forest ecosystems where Garry oak and Arbutus are present within the Douglas-fir stands and wetter forest areas where Western redcedar, Bigleaf maple, Red alder and others co-exist with Douglas-fir. But forests
are more than their trees. Older, undisturbed forests have greater structural diversity and coastal forests with old growth characteristics will typically have a healthy and diverse flora on the forest floor and in the shrubby understory. Tree canopies are also typically multi-height.
A Douglas fir forest with a healthy natural understory of salal and coarse woody debris, as well as some younger trees within the stand providing mid-canopy structure. Photo Credit: Pamela Wright
In the drier forest types, the grass and forb-rich forest floor are more open with less of a shrubby understory. In these forests, Arbutus and Garry oak are typically shorter in stature and their more convoluted form means they serve the function of a mid-canopy layer.
Garry oak forest with an open meadow understory. The growth form of the Garry oak provides good vertical structure. Photo credit: BC Government
So what’s happened to these structurally diverse forests?
Forestry, road development and land clearing for houses have simplified our forests. Most of Pender Island has previously been logged and in places where it has regrown it is beginning to develop the old-forest characteristics that contain good structural diversity. While we still have lots of trees on Pender, in our yards and neighbourhoods we have simplified the forest structure, removing much of the mid- and understory layers as well as the standing and fallen coarse woody debris. We’ve done this to improve visibility, expand our yards and garden areas, and reduce concerns about fire (https://firesmartbc.ca/). In addition, invasive species like Scotch broom and Daphne (Spurge laurel) reduce structural diversity in the forest by choking out native vegetation
and tree regeneration. Hyper-abundant deer populations also browse heavily on understory plants and deciduous trees, simplifying the forest structure to be only a tall canopy of trees.
So What Can We Do?
The great news is that at the Pender scale, in our homes and neighbourhoods we can make positive changes to improve vertical connectivity by increasing structural diversity in our yards.
1. Plant Mid-Canopy Trees
Consider adding in some mid-canopy trees (Red alder) or trees with vertical structure like Garry oak, Arbutus or Western redcedar. A simple deer enclosure will protect them until your tree gets tall enough.
2. Don’t Forget the Shrubby Understory
Except on those south-facing forest meadow slopes, our Pender forests should have a healthy tall-shrub layer. Most of these should be native species like Oceanspray, Vine maple, Black hawthorn, Nootka rose, Red osier dogwood, Salmonberry, Tall Oregon grape and others. Even if your preference is for more formal gardens you can often find a place to add vertical structure by incorporating one of these plants or non-invasive ornamental shrubs. Deer enclosures will be important for many of these species as well until the shrub gets tall enough.
This part of my yard has great structural diversity with a healthy overstory of Douglas fir and Western redcedar, and a shrubby understory of Oceanspray (foreground) as a mid-story deciduous. Photo Credit: Pamela Wright
3. There’s Life in Dead Trees!
Standing or fallen, there is a great deal of life in dead trees. Unless it’s a hazard tree, leave some standing dead trees on your property for cavity nesters and to serve as nutrient sources. Many times, hazard trees can just be topped and still serve as excellent vertical structure and habitat. Fallen coarse woody debris is equally important for structure providing habitat and a nursery area for young plants. The creative gardener can incorporate these structures as part of the architecture of their gardens and yards.
Standing tree trunk provides habitat and nutrients in your yard and enhances forest structure. Photo Credit: Pamela Wright
But What About Being FireSmart?
Fire risk, particularly with extended summer droughts is definitely a concern on the islands. Advice for fire smarting properties varies throughout the province but could be in conflict with some of the advice on increasing structural diversity. The good news here though is there are ways to meet both objectives. Many of the mid-canopy and understory plants recommended here are deciduous plants that have low fire risk and will retain moisture on the site. Coarse woody debris (standing or dead) can help retain moisture and slowly release it into the ecosystem.
Get Connected – Vertically
Check out Pender Islands Conservancy plant sales in the fall and spring and other nurseries for some of the plants that can help restore vertical connectivity in your yards and encourage some structural (and species) diversity. Not only will you provide great habitat for critters but a structurally diverse landscape – particularly one with drought-tolerant native plants - will keep some cool, green areas in your yard during the dry summers.