Wetland Biogeochemistry Techniques - Wetland Techniques - page 364

necromass obtained in the measuring period. Hertel and Leuschner ( 2002 )
evaluated four methods that assessed fine root production in a Fagus spp. and
Quercus spp. forest and compared the results with C budget data. Twenty samples
each were collected at 4 week intervals over 1 year. The sequential coring or
minimum-maximum approach showed the best agreement with the C budget data
with an overestimation of 25 %.

7.4.2.7 Quantifying Litterfall

Leaf cages are used to catch abscised leaves from deciduous woody plants so that an
estimate of foliage biomass or C can be obtained. The cages can also be utilized to
collect abscised flowers and fruit. Techniques for quantifying branchfall are
presented by Bernier et al. ( 2008 ). It should be understood that senescing leaves
typically export simple carbohydrates prior to abscission, so that biomass or C
estimates based on abscised leaves will not necessarily equate to foliage biomass or
C. Foliage biomass can also be estimated for trees by using allometric equations
(see Tree Biomass above). Litterfall can also be estimated using the cohort layered
screen method (see below). However, the leaf cage approach will provide an
estimate of C returned to the forest floor via leaf abscission annually. Commercially
available laundry baskets can be used as leaf cages. Holes are drilled into the
bottom of the basket and the basket is elevated above the soil surface to promote
drainage. In addition, depending on the target species, fine mesh screening may
need to be used to cover openings on the sides of the basket. An alternative design
consists of an open wooden frame holding window screening. Screening is stapled
to the frame to form a box shape open at the top and with four wooden legs. The legs
are hammered into the ground to keep the cage in place. The screen bottom is
positioned above the soil to allow for air flow and to prevent the leaves from picking
up moisture from the soil. The height of the sides should be great enough to prevent
leaves from moving out of the frame via wind. Rainfall can leach out soluble
carbohydrates from the leaves so that samples should be collected at least weekly
or before significant rainfall events. Variability in leaf fall increases with distance
from the cage to the target canopy. For shrubs, two cages each halfway between the
main stem and the drip line are usually sufficient. For trees, more cages are needed;
the appropriate number depends on canopy diameter, and the contents of each cage
should be analyzed separately to determine a measure of variability. Sampling
designs are addressed by Bernier et al. ( 2008 ). The samples are placed in
pre-weighed paper or mesh bags and dried to a constant weight at 80 C.
Pre-weighing the bags allows sample dry weight to be obtained without removing
the sample from the bag.
Implicit to this technique is the assumption that leaves collected from the cage
represent the portion of the canopy equivalent to the surface area of the cage
bottom. Therefore, total canopy biomass can be estimated by: (biomass of collected
leaves) (surface area of cage bottom).

A major limitation to this method is the error associated with extrapolating to a

(horizontal surface area of canopy)