Giant Sequoia is the world's largest tree in terms of total volume. They grow to
an average height of 50-85 m (150-280 ft) and 5-7 m (16-23 ft) in diameter.
Record trees have been reported to be 93.6 m (307 ft) in height and 8.85 m (29
ft) in diameter. The oldest known Giant Sequoia based on ring count is 3,200
years old. The Giant Sequoias are having difficulty reproducing in their
original habitat (and very rarely reproduce in cultivation) due to the seeds
only being able to grow successfully in mineral soils in full sunlight, free
from competing vegetation. Although the seeds can germinate in moist needle
humus in the spring, these seedlings will die as the duff dries in the summer.
They therefore require periodic wildfire to clear competing vegetation and soil
humus before successful regeneration can occur. Without fire, shade-loving
species will crowd out young sequoia seedlings, and sequoia seeds will not
germinate. When full grown, these trees typically require large amounts of
water and are therefore often concentrated near streams.
Below are two Giant Sequoias, Sequoia National Park. Note the large fire scar
at the base of the right-hand tree; fires do not kill the trees but do remove
competing thin-barked species, and aid Giant Sequoia regeneration.
Sequoia Trees
Fires also bring hot air high into the canopy via convection, which in turn
dries and opens the cones. The subsequent release of large quantities of seeds
coincides with the optimal post-fire seedbed conditions. Loose ground ash may
also act as a cover to protect the fallen seeds from ultraviolet radiation
damage.
So, you ask, what does this have to do with growing Protea. Well, fire and
germination off course.
The Proteaceae family is found in a vast variety of vegetations, from savannas
and grassland to heavily overgrown hillsides. They thrive in areas that are
prone to periodic veldt fires. Many of the Proteaceae have evolved specialized
survival techniques in response to these fires. Their strong root systems and
thick bark on some varieties, have given these extraordinary plants the ability
survive the heat and smoke generated by fires. In fact, fire and smoke provide
the major trigger in the wild for the Protea seeds to start the germination
process. Seed can lie dormant for years waiting for the right conditions,
usually brought on by wild fires. For these fire-sensitive species, habitat
burning is the single most important cue for triggering germination of the
dormant seed.
In growing fire-prone floras commercially, particularly those of Mediterranean
zones, a solution had to be found to germinate the Proteaceae seed, without
burning your growing fields every year. Following the discovery that smoke
stimulated germination of this rare South African Proteaceae, the exploration
of the benefits of smoke-mediated germination has expanded to different parts
of the world and has been applied in nurseries and in rare flora conservation.
It became clear that seed dormancy of Protea seed is broken when 1) the seeds
absorb water; 2) the seed are subject to relatively low temperatures; and 3)
the seeds have been exposed to smoke. Various methods have been tried; aerosol
smoke, smoke dissolved in water and solids (activated clays or sand particles)
that have been smoked have all been effective in promoting Proteaceae seed
germination. Various chemicals have also been found to break dormancy in the
seed, and even soaking the seed in green tea have been reported to trigger
germination in some varieties.
