SAMPLING DESIGN
This study was performed at the Hubbard Brook Experimental Forest
(HBEF) in central New Hampshire, USA. Most of the HBEF is northern
hardwood forest, dominated by sugar maple (Acer saccharum Marsh.),
American beech (Fagus grandifolia Erhr.), and yellow birch (Betula
alleghaniensis Britt.). Together, these three species accounted for
86% of the above-ground living tree biomass in a similarly situated
stand at the HBEF when this study was initiated in 1991 (Siccama et
al. 2007). This work was conducted in the area of the south-facing
experimental watersheds 1-6 at the HBEF. In July 1990 and May 1991, 71
trees of the three dominant species were felled with a chain saw. The
trees were approximately 50-70 years old. From each felled tree, two
adjacent segments of the bole, each approximately 1.5-m long (range:
0.99 – 2.22 m), were isolated with a chain saw. After measuring the
length and the diameter at each end, one of these segments was then
placed on the forest floor under fully intact forest canopy. The
incubated bole segments were placed on sloping ground, approximately
perpendicular to contours, in two similar stands approximately 200 m
apart. The other ‘fresh’ segment was taken to the lab for sampling.
Each of the 71 samples incubated in situ was therefore paired with a
fresh sample from the same tree.
INITIAL LABORATORY WORK
After measuring the dimensions of the fresh bole-segments, disks about
8-10 cm thick were cut from one end of each segment. The bark was
separated from the wood of each disk, and both were dried at 80o C to
constant weight. Subsamples were collected from the fresh wood disks
by drilling from the side to the center with a 2.5-cm drill bit. The
dried bark samples and the wood shavings were ground in a Wiley mill.
Subsamples from the same log were composited for chemical analysis.
FIELD COLLECTION PROCEDURES
Incubated boles were collected from the field in April, 1993 (T1; 2Y);
May, 1997 (T2; 6Y); May-July, 2001 (T3; 10Y); July, 2007 (T4; 16Y) and
July of 2015 and 2016 (T5; 25Y). Three boles of each species (nine
total) were collected in 1993, 1997 and 2001. In 2007, three beech,
six maple and six birch boles were collected. In 2015/2016, all
identifiable remaining boles were collected. After removing any
surface litter the bole was gently rolled onto a sampling tarp, and
any loose material was collected in a bag. The log was placed in the
bag and returned to the laboratory for processing.
POST-INCUBATION SAMPLE PREPARATION
The incubated boles were dried at 80 oC to constant weight, which was
recorded. The dry boles were laid out on kraft paper, along with the
loose debris collected in the field. After measuring the dimensions of
each log, we then removed the bark from the bole and gathered the bark
fragments from the debris. The mass of the bark and the wood
(log+loose debris) was recorded. Subsamples of the log were collected
by drilling to the center of the log with a 2.5-cm bit. Log, loose
debris, and bark samples were ground in a Wiley mill for chemical
analysis. A wood sample for chemical analysis was created by mixing
loose debris and log samples proportionally by mass. Two beech boles
collected in 2001 (10Y) had so little bark remaining that it was not
sampled and judged to be completely decomposed. The samples collected
in 1997 (6Y) were inadvertently discarded after mass determination, so
we have no chemical data for those samples. In 2015/2016, there was so
much fine loose rotten wood and bark that a separate sample (mixed) is
included in the archive. In total, 42 incubated boles have been
collected from the field to date. Chemical measurements have been made
on 106 bark samples (54 fresh, 52 decomposed) and 108 wood samples (54
fresh, 54 decomposed) plus 21 mixed tissue samples from T5.
CHEMICAL ANALYSIS
Total carbon, hydrogen and nitrogen were measured on oven-dried
subsamples using a Carlo Erba EA1108 elemental analyzer. Other
elemental concentrations were measured on HNO3 digests of the samples.
A crucible containing approximately two grams of oven-dried sample was
placed in a muffle furnace (500 oC) overnight. The ash was then
dissolved into 8 mL of 6 M HNO3, filtered through a Whatman #40 paper
filter, and brought to a volume of 50 mL using deionized water. The
concentrations of Ca, Mg, K, P, and Mn were measured using inductively
coupled plasma optical emission spectrometry (ICP-OES). All values
were blank-corrected. Silica was analyzed on a sodium hydroxide
extract of wood and bark samples. 100 mg of sample was extracted with
40 mL of 0.5 M NaOH for 4 hours in an 85C warm water bath. Two
milliliters of extract were added to 4 mL of 0.25 M HCl. The
neutralized extract was analyzed colorimetrically for Si as SiO2.
CALCULATIONS
The disks were used to calculate the initial dry masses of wood and
bark for the paired sample left in the field using two different
calculation methods. The density method used the density calculated
for these disks and the measured bark/wood mass ratio for the fresh
bole were used with the initial dimensions of the incubated bole to
compute the initial masses of wood and bark. The moisture method used
the moisture calculated for these disks and the measured bark/wood
mass ratio to compute the mass of the in situ log. These initial log
masses can be compared to the incubated bole masses to determine the
mass loss during decomposition.
