(See notes below on how to use this program to
calculate mortality rates)
(What are "Bird Lines"? - Long (~ 2.5 km) forest
inventory transecs in the central valley at Hubbard Brook in an area used
for decades by the "Bird People" to study birds in the Northern Hardwood
forest. The usage "Bird Lines" has meaning to the Hubbard Brook scientific
community and we have preserved the notation as the identification for
this forest inventory data set.)
The diagram below illustrates
the types of information which can be extracted with this program. The
forest plots with tagged trees were set up in 1991. Each tree was uniquely
identified on 396 plots each 25 meters long and 10 meters wide. The plots
were on continuous contiguous transects in the central part of the Hubbard
Brook valley in a general area called the Bird Area - because it is the
area in which long term studies of bird populations is taking place. The
dbh and species of each tagged tree was taken in 1991 and each tree was
accessed in one of 4 states of vigor - healthy, sick, dead tree and snag.
In 1993, 1995, 1997, 1999 and 2001 each tree was revisited and its current
state of vigor again checked. Dbhes were not obtained in the two year intervals
until 2001 when all trees were again dbhed. From 1993 on two more categories
of vigor were added - trees which had fallen to the ground dead and new
trees in-growing at the 10 cm minimum starting diameter of the study.
From this data the program
allows the determination of the changes in the vigor of all the trees between
each of the sampling intervals. The program asks for the starting vigor
you would like to have analysed- such as asking what is the fate of all
the live trees in 1991 by the year 1993. The results of this
are illustrated in the accompanying diagram. There were 4649 live healthy
trees in 1991 (code 0) and the arrows with the numbers show the numbers
of these trees which continued as live healthy trees, which became sick
trees, which died and were standing dead trees in 1993, which had broken
off (died) and became snags or had fallen to the ground dead. So
if you run the program for the options of live trees to start, then all
species, then the starting year you should get the answers shown in the
figure. If you chose dead trees to start for the same all species and starting
in 1991 you would get the fates of those trees in the ensuing 2 year interval
- in 1993. This may be done for any of the 5 two year intervals for
any species. If you do not select a specific diameter range over which
to make the summaries, all the trees are used.
When running years after 1991 the program
also gives you the number of in-growth trees which were added because they
reached 10 cm dbh during the two years period. When running
the 1991 starting year the program also gives you the average radial growth
in mm of the selected species or for all trees. Since we only measured
the trees in 1991 and again in 2001 there is really only one set of
"growth". The radial growth increment is the average growth of all the
trees obtained by subtracting the initial diameter from the final diameter
and making the necessary conversions to radial growth in mm. There are
a number of odds and ends with this which will be added as the fine
print below.
5 - The transect line
number. This project was established initially to characterize
the forest in which the bird studies were taking place and in so doing
has used a pre existing "grid system" set up by the Bird People. This
grid system consists of a 50 x 50 m grid laid out by compass and pacing
and covers many square km of forest. We selected 4 East-West running lines
of this grid system to establish the tree study The Bird Line transects
are 50 meters apart and we selected their line #5 as the northerly of the
4 lines we used to establish plots along. See map. We then used lines
9, 13 and 17 each 200 meters apart to the south of the first used line.
Thus each set of plots is 25 meters long arranged in a continuous contiguous
set of plots along about 2.5 km of line. Each line is essentially
a transect through the forest. We call them lines and transects interchangeably
here.
1 - plot number- note there are only
three trees in plot 1 - this is because it crosses and runs along a stream
and swale.
25 - length of the plot in meters (plots
are 10 meters wide- 5 meters either side of the 25 m long center line.
Plot lengths will vary a little. The bird lines were established by compass
and pacing and thus the grid is not perfect. We thought it better to have
our plots start and end near the grid corners established by the bird people.
Sometimes this is a few meters longer or shorter than 25 meters.
0 - this is the compass bearing
of the plot- which was not recorded for the first plot, the second plot
is 290 degrees. Since the transccts were originally established by
compass and pacing over many years and are not perfect, the plot bearings
occasionally vary a little from a straight line.
5 - species number
ACSP - the first tree is a Mountain maple - Acer
spicatum - see species list below for all the species numbers.
0 - code for multiple stemmed
trees - if there were two stems there would be a number 2 for two trees
in a row,
if a 4 then 4 trees in a row would have a 4 in this location for 4 stems.
1 - Tree tag number - the
first tree, the first tag.
10.7 - dbh in cm in 1991
0 - alive in 1991
0 - alive in 1993
0 - alive in 1995
3 - sick in 1997
-1 - down dead on the ground in 1999
-1 - still and forever dead and down
0.0 - (No dbh taken in 2001. We generally did not
take the diameters of down trees.
We did not tag dead trees or
snags in 1991. These plots were rather small so we could keep track of
the one or two dead trees or snags without tagging them. Thus the second
tree on the line has no tag number. It has a starting code of 5 which means
it was a snag in 1991 and remained a snag until 2001 when it had
fallen to the ground dead.
The next tree also without a tag was a standing
dead tree (code 4) in 1991 but had fallen to the ground as dead and down
tree by 1993 and obviously stayed as such forever.
The forth tree was alive to start as tag #2 and
stayed alive for the entire interval - all code 0 (zero). It had a starting
dbh of 13.3 in 1991 and in 2002 had a dbh of 16.0 cm.
You will note that some beeches (FAGR) about
15 trees down the list have large tag numbers - these trees were added
during the study as they became large enough to add to the inventory (10
cm dbh). There is some tricky coding here- we use a 7 as a place holder
for the years before a tree is added to the data set. In this case
the first added beech has a 7 in 1991 and 1993 and was added to the population
in 1995 with a 1 code. After that it continues as a code 0 or a live tree.
The next beech was added in 1999. The diameter at the time
it was added to the list appears on the left and the diameter in 2001 appears
on the right.
Mortality rates-
It is not practical to program
in mortality rates directly. However they can be calculated from the data
when you run Live or Sick trees as the starting vigor. (Clearly irrelevant
in relation to already dead trees). So if you run live trees say
for 1991 as a starting year - the program gives you the number of live
trees which died in three categories- fell to the ground (-1), died standing
and remain as standing dead trees (4) or died standing but broke off above
dbh (snags) (5). It also gives the total number of starting live trees.
It also gives the percent dead in each of these three categories. To get
total percent dead you need to add up the total number of trees dying and
then make the division with respect to the total number of
live trees starting the period - don't
forget to divide by two because this mortality is based on two years of
time!!!
Very Fine print---
Uniquely identified:
Since we did not tag standing dead trees or snags at the start of the project
in 1991, we clearly have trees which are not "uniquely" identified.
However these are one or two trees on each plot and thus can be easily
found and checked at each 2 year inventory interval. Many of these have
fallen to the ground over the 10 year period and any trees dying after
the being tagged as live are clearly identifiable.
"Missed" trees- In 1993 we
found a few trees which we apparently missed in 1991 when we established
the study. These trees were added to the study at that time. We even have
a few trees like this in succeeding years. If you study the data file itself
in detail you will find a 2 as a place holder in the vigor location. These
mean that the tree was added not as in-growth, but rather as a tree we
think we missed in the preceding years. There are very few of these and
when they were added the dbh was decreased by assuming a growth rate of
1 mm per year radial growth,
Definitions of dead trees
and snags; Dead trees are trees which are dead burt still alive most of
their main branches and even their fine branches. Snags are dead trees
broken off above DBH and without major branches. This is usually an easy
definition but clearly not a perfect one since the trees progressively
loose branches and slowly change from a standing dead tree to a snag.
But on the other hand most of the time it is obvious which is the case!
Trees getting smaller: You
will find that there are a few live trees which "get smaller" in diameter
between 1991 and 2001. This is a fact of field measurement. Trees such
as the birches which have loose curly bark which accts like a spring when
the dbh tale is pulled around it are the most likely to "get smaller".
This is why we don't bother to make the measures every two years - but
rather every 10 years. The average growth does not consider this problem
but just adds in negative growth- presumable this is offset by those which
were larger than the real growth for the same bark reason. We have checked
the average growth based on these dbh change measures with 1000's of tree
cores from the same area and the answer oe basically the same. So it averages
out.
Diameters on down trees not
taken- We did not take the diameters of the down trees- basically because
we were not sure which the were in some cases and also it was not possible
to be very sure of where dbh was on a down tree.
Ingrowth - live (Never
sick)??