Canopy Fuels - Wildland Fuel Fundamentals and Applications - page 55

Chapter 4
Canopy Fuels

A forest of these trees is a spectacle too much for one man to see
David Douglas Scottish Botanist

4.1 Introduction

The forest canopy is an amazing ecological entity consisting of a wide variety of
suspended and interacting plants, insects, pathogens, and many other life-forms,
essentially creating an aerial ecosystem within a terrestrial ecosystem (Nadkarni
1994 ). The canopy habitat provides important food, shelter, and shade for criti-
cal wildlife species. It consists of a wide variety of life-forms, species, and plants
(e.g., mosses, lichens, epiphytes, tree crowns) that have a wide variety of sizes,
shapes, and other biophysical characteristics (e.g., foliage, twigs, branches, and
boles). Interestingly, foliage is only a small fraction of the total canopy biomass;
the woody material in tree boles accounts for over 80 % of the total canopy biomass
(Table 4.1 ). Moreover, the forest canopy is constantly changing as climate, distur-
bance, and vegetation development processes interact to create unique tree crown
and forest canopy mosaics across the landscape. Yet, even with this remarkable
diversity of canopy species, structures, and conditions, wildland fuel science has
described canopy biomass using only a handful of variables that relate poorly to
many ecological processes. This chapter describes how canopy fuels are described,
measured, and summarized for fire management and presents possible limitations
of these approaches.
The concept of scale is again needed to fully understand current descriptions
and quantifications of canopy fuels. Early studies often called forest aerial biomass
“ crown ” fuels (van Wagner 1977 ). A crown is defined as the portion of a tree's
height that is composed of branches that support live foliage. However, tree crowns
may also have arboreal lichens and mosses, dead branches, and abnormal growths,
such as split boles and dwarf mistletoe infections. The height to the bottom of the
crown (HBC) and the height to the top of the crown (tree height; HT) (Fig. 4.1 ) are
often used to compute canopy fuel vertical distribution. The problem is that tree
crown characteristics describe fuels at the fine-scale tree-level, not at the broader
canopy or stand-level, which is the scale that best matches crown fire spread and
fuels management. Therefore, the term “ canopy ” fuels was introduced to capture