Quantification of Canopy Interception and Cloud Interception in the
Elevated Forests of Norfolk Island
Abstract
The higher elevation forests of Norfolk Island are regularly immersed in
the clouds and scientific and anecdotal evidence suggests that in
addition to rainfall, water is likely to be collected as cloud droplets
are intercepted by the forest canopy. This water is likely to be
important for the local hydrology and ecology, yet it has never been
quantified. To address this, a field measurement campaign was
established to measure hydrological inputs to the forest floor at two
elevated forest sites in the Norfolk Island National Park.
Instrumentation included throughfall and stemflow systems and
measurements of rainfall in the open in nearby clearings. Sites
exhibited very high stem density and basal area by international
standards and delivery of water to the forest floor was dominated by
stemflow because of the funnelling characteristics of the dominant palm
and pine trees. Both sites showed similar hydrological behaviour with
stemflow and throughfall of around 48% and 32%, respectively. Stemflow
contributions of 48% far exceed observations from the literature which
are typically less than 10%. Rainfall rarely occurred in the absence of
low-level cloud and some cloud immersion events lasted for many days
with hydrologic inputs continuing for extended periods despite rainfall
not being observed in the open. Cloud interception accounted for
approximately 20% of total water input at both sites which is
equivalent to 25% extra water on top of rainfall measured in the open.
From an island-wide perspective the calculated extra hydrological input
is only small due to the limited spatial extent of elevated forest,
however, the additional water is likely to be very important to local
hydrological processes and the unique plants, insects and animals which
inhabit the higher elevation forests of Norfolk Island.