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Canopy Studies -
Overview
The study of canopy environments in
wet-belt forests poses many challenges and opportunities. Traditionally,
studies of biological diversity have been ground-based, leaving many gaps in
our knowledge of canopy inhabitants and their life-cycles. This is particularly
true for the interior cedar-hemlock forests, where individual trees can reach
heights of 55 to 60 m, necessitating specialized canopy access techniques.
In coastal forests of the Pacific
Northwest, major infrastructure such as canopy cranes have previously been used
to facilitate canopy studies. Within interior wet-belt forests of northern B.C.
modified single-rope climbing techniques have provided a lower-cost
environmentally sensitive way of studying canopy environments.
 One of the major groups of organisms
that have been studied in the canopy of interior wet-belt forests are canopy
epiphytes, especially lichens and mosses. As Pacific storm systems move over
the interior mountain ranges in British Columbia, they create a zone of high
precipitation, which favors the growth and development of canopy epiphyte
communities. These epiphytes, mainly lichens, rely upon their host trees for
support, but are wholly dependent upon rainwater and snowmelt for moisture to
support growth and reproduction.
Left: Actively growing lichen thalli
of Alectoria sarmentosa during winter snowmelt event.
 The development of canopy lichen
communities is strongly influenced by the age of the forest stand within which
they reside and the continuity of forest cover on a given site. In even-aged
younger forest stands, the greater density of foliage in mid- to upper canopy
positions reduces light availability in the lower canopy, limiting growth of
most canopy lichens. The denser foliage in younger stands also intercepts more
rain and snowfall (much of which then evaporates before dripping through to the
lower canopy), creating drier conditions for canopy epiphytes.
Right:
Widely spaced trees in old-growth cedar-hemlock
forest encourage lichen
growth in lower canopy positions.
In contrast, within older wet-belt
forest stands, not only do individual trees attain greater stature, but the
forest stand is characterized by gaps, created by past treefall events, where
sun flecks can penetrate through the canopy to the humid environment near the
forest floor surface. This promotes the establishment and growth of a suite of
old-growth lichen indicator species, including species such as the canopy
cyanolichen (cyanolichens are lichens that contain a blue-green, sometimes
called cyanobacterial, algal component) Nephroma helveticum (see image
below).
The distribution of lichens within
the canopy of wetbelt forests is therefore closely linked to vertical gradients
of temperature, moisture and light availability. In the upper canopy of
cedar-hemlock forests, where exposure to sun and wind creates drier conditions
for growth, the more dessication tolerant hair lichens are abundant, providing
a valuable forage source for mountain caribou when blown to the forest floor
surface. In mid- to lower canopy positions, rich cyanolichen communities
develop, including large foliose lichens such as Lobaria pulmonaria.
 Left: The foliose lichen Lobaria
pulmonaria is common on branches in the lower canopy of old-growth wet-belt
cedar-hemlock forests.
Many canopy lichens in wetbelt
forests are limited to specific microhabitats. Large cottonwoods, for instance,
often grow as isolated individuals in conifer dominated wet-belt forest stands,
where they create dripzone effects, releasing nutrients to passing rainwater
that can enhance the growth of cyanolichens on surrounding trees. Additionally,
rainwater channels directly on the bark of large cottonwoods can support unique
communities of gelatinous cyanolichens, such as Collema. At the other
extreme are many of the pin-lichens, or stubble lichens, that only grow on the
underside of large snags, where they are protected from direct contact with
rainfall or meltwater.
Site continuity, or the length of time between major disturbance events, is also a major factor influencing the development of canopy lichen communities in wetbelt forests. Successful long-distance dispersal of many old-growth dependant lichens appears to occur only rarely. Thus forests that develop in wet or protected microsites, such as in toe-slope positions or in protected side-valleys where fire occurs less frequently, often contain more diverse canopy lichen communities, including many rare species, for some of the Calcicoid lichens, that normally have more oceanic affinities. These forests, a subset of old-growth forests, have been denoted as antique forests in pioneering studies by Trevor Goward. Current studies on the distribution of these forests within regional landscapes of BC’s inland rainforest (see Stand structural attributes and canopy lichen diversity) suggest that they represent an endangered landscape element and should be regarded as a high conservation biology priority. Many of the lichens found within these stands share affinities with lichen populations from coastal wet temperate rainforests worldwide (see South Temperate Rainforests).
 The knowledge that is now being
obtained from canopy studies in interior wet-belt forests will provide valuable
information on the ecological function and biodiversity of this little known
ecosystem and will help guide local communities in making land-use planning
decisions.
Above: Canopy biodiversity is being
evaluated within partial-cut harvesting trials at Lunate Creek, in wet-belt
forests of the upper Fraser River Valley. (photo M. Geertsema).
Among questions that are being posed
in this regard: Are some old-growth forests more valuable than others (eg.
antique forests), in terms of preserving canopy biodiversity and as potential
candidates for protection? Can forest harvesting be adapted using partial-cut
harvesting techniques to retain elements of canopy structure required by
old-growth dependant organisms and minimize edge effects in adjacent
unharvested stands?
 Many gaps remain in our knowledge of
canopy biology in wet-belt forests. Little is known, for instance, about the
diversity or ecological role of insects (eg. spider guilds) within wetbelt
forests. Further, basic information on many aspects of ecosystem function, such
as canopy nutrient cycling, for instance from lichenized nitrogen fixation,
remains largely unknown within the interior wet-belt forests.
Above: Antique forest dependant
lichen Nephroma helveticum (photo by Anna L. Roberts) |
