SAMPLING BACKGROUND
The forest was first sampled in 1965. The hydrology has been monitored
since 1963. Surveys were initiated in 1965, repeated in 1977, and
repeated at 5 year intervals after that. These data were collected in
the summer of 2012. The stream water chemistry has been monitored
since 1963. The watershed is forested by typical northern hardwood
species (sugar maple, beech and yellow birch) on the lower 90% of its
area and by a montane boreal transition forest of red spruce, balsam
fir and white birch on the highest 10%. Surveying techniques were used
in 1965 to install a grid system of 208 grid cells, each 25 x 25 m
square. These grid units serve as the plot system. See the following
links for more information:
http://hubbardbrook.org/overview/maps/w6_zone_map.shtml
http://hubbardbrook.org/watersheds/w6_grid_elevations.shtml
SAMPLING DESIGN
The inventory consists of a total inventory of all trees >=10 cm
dbh on the whole of the watershed (13.23 ha), as measured in each of
the 208 25 m x 25 m plots. Trees >=2 to <=10 cm dbh were
subsampled using a 3 meter wide strip along one edge of each 25 m x 25
m plot (see plot layout). The lengths of these strips are not always
exactly 25 meters, due to drift or imprecise replacement of the corner
stakes. The actual measured length of the strip is used to calculate
the area of the plot (as given in the data) for the smaller trees. For
the larger trees, plot area was not adjusted for drift and is assumed
to be 625 sq. meters.
DATA DESCRIPTION
The data consist of the diameters (dbh) of all trees >=10 cm dbh,
live and dead, in the whole of the watershed (10714 individual stems).
Dead trees were recorded as standing dead trees (trees dead but
retaining most of their fine branches) and as snags (trees broken off
above dbh or with only their major large branches still intact). Live
trees were noted "sick" if they had very sparse foliage or
yellowing foliage in the mid summer sampling. A subsample of trees
>=2 cm dbh and <10cm dbh were measured at the same time (3870
individual stems). All stems that met the size criteria were measured
individually, and in some years it was noted whether a stem was part
of a group of stems from the same tree (i.e. multiple stems that split
from a single trunk below breast height). In 2002, uniquely numbered
aluminum tags were nailed to each stem >10 cm dbh to allow for
tracking of individual stems from survey to survey.
CALCULATIONS
Aboveground and belowground biomass was estimated for each stem in the
inventory and is included in the data. Estimates were based on
species-specific allometric equations developed at Hubbard Brook.
Phytosociology or biomass summaries can be obtained by using our
interactive calculators. Phytosociology summaries include basal area,
density and frequency of each species for the watershed as a whole or
for smaller units as chosen by the user. Diameter distributions for a
specific species can also be selected. Biomass summaries include
biomass estimates by plant part for each species for the watershed as
a whole or for smaller units as chosen by the user. Options to
estimate productivity and total nutrient pools in the vegetation are
also available. To access the calculators or for extensive details
about how these calculations are made, see link in ADDITIONAL
RESOURCES below.
NOTES
For surveys prior to 2002 (when stems where first tagged), it is not
possible to compare the data between sampling years on a plot by plot
basis, since trees were not individually tagged and plot corners have
shifted slightly over time. At the time of the 1977 and 1982
inventories, many of the aluminum rods marking plot corners had come
out and were replaced but not resurveyed. Also, trees on the lines
between plots can "drift" between plots between years
depending on the observers line of sight or decision as to whether the
center of the base of a tree is in one plot or the adjoining plot. In
1989 we prepared maps of each plot on which we outlined edge trees and
assigned them to one plot or another. These maps have been copied on
the backs of the field data collection sheets in the more recent years
such that problem trees can be assigned consistently. For extensive
notes about the details of these surveys and subsequent calculations,
see link in ADDITIONAL RESOURCES below.
With the addition of tree tags in 2002, tracking of individual trees
is now possible. Subsequent surveys of tagged trees allow for
corrections to be made. For example, it is possible to tag trees that
had clearly been missed in the previous survey (trees >19.5 cm dbh
as measured in 2012 – the cutoff was based on observed maximum 5-year
increment from tree rings). Another benefit of tagged tree inventories
is that it is possible to catch field blunders as defined by
extraordinary negative or positive diameter growth. Such examples are
coded in the dataset under the variable AnalysisCode, where an “I”
signifies that the dbh from that year was interpolated by regression
from subsequent survey data. It is not recommended that these trees be
used for demographic analyses since the growth has been estimated. DBH
note: Tagged trees <10cm dbh were kept in the inventory as long as
they were >10cm at the time of the initial tagging.
Vigor codes 1 and 2 (beech bark disease assessment) are deprecated for
2002 and later data with overall beech health recorded using codes 3 -
6.
