A land manager's guide to conserving habitat for forest birds in ...

Mid-tolerant
Young forest — Photo: OMNR
Shade and moisture conditions shape tree
species composition
Tree species differ in their abilities to survive under varying moisture
and light conditions. Some can tolerate extreme dryness (drought)
and others, excessive flooding. Some species survive while living in
the shady conditions of a mature forest floor. These trees are
considered to be shade-tolerant. The Shade and Moisture
Tolerance of Various Tree Species table (opposite) are used to
classify southern Ontario tree species based on their ability to adapt
to these environmental conditions. This will help guide land
managers in deciding what silviculture system will be most
appropriate for a given mix of species.
Shade and Moisture Tolerance of Various Tree Species
Shade-tolerant
Species
(Climax – Late
Successional)
Shade tolerance
Tolerant
Shade-tolerant (low light)
Shade-tolerant species can germinate and survive in the shade of the
forest canopy. The most tolerant species can live as long as
40 years in the shade. When an opening is created from a tree dying
or falling, the small shade-tolerant trees will quickly grow up to fill the
gap and take their place in the canopy. Forests canopies comprised
of primarily large shade-tolerant species such as sugar maple, beech,
and hemlock are often considered to be at the end stage of
succession known as the climax or late successional forest.
Mid-
Tolerant
Species
Mid-tolerant (medium light)
Mid-tolerant species can germinate in shady conditions but will die
within 1–3 years if they do not receive some light, for at least part of
the day. Mid-tolerant species such as oaks and white pine evolved
from a history of heavy ground fires. These fires kill all the low shade
and some of the canopy trees to provide a partially shaded
environment. During Ontario’s settlement phase the heavy
disturbances associated with land clearing provided ideal conditions
for many mid-tolerant forests to become established. However, the
past 70 years of minor disturbance (such as single tree selection) and
fire suppression has allowed succession to continue towards more
tolerant species, favouring the replacement of mid-tolerant canopy
trees in many remnant woodlots.
Intolerant
Species
(Pioneer – Early
Successional)
Shade-intolerant (full sunlight)
Intolerant tree species need full sunlight to germinate and survive. In
open conditions they out-compete both the tolerant and midtolerant
trees. Species like white birch, red pine, trembling aspen,
tulip tree, and sassafras are considered to be intolerant species.
These species are colonizers or early successional species that
dominate even-aged forests where large disturbances such as fires or
big wind events have occurred.
Artwork: Peter Burke
Intolerant
full sunlight
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Forest Harvesting