| Age distribution of trees on Watershed 5 at the time of harvest: |
The harvesting of W5 provided a one time opportunity to obtain cross sections of trees. These cross sections record the tree's age, and in the growth rings, considerable information about the stand history. Chemical information on a year by year basis may also be obtained by analysis of the wood. All of the tree cross-sections referred to in this summary are stored in the Hubbard Brook archive building. Four separate sets of cross sections were obtained as follows:
1.
All white ash trees
2.
Most of the trees on the "biomass" plots on the lower third of the watershed
3.
Most of the trees on one of the upper biomass plots
4.
A set of 25 fir trees harvested for dimension analysis/allometry
White ash is a shade intolerant species that can only come into a stand in large natural gaps or following major harvesting activities. Its initial success in establishment is probably also enhanced by the availability of mineral soil. In the forest inventory completed before the cut ( in the summer of 1982), all the trees > 10-cm dbh were measured on all the 360 grid units which comprise the whole of the watershed. (See Forest plot inventory). Based on the trees recorded in this inventory, we were able to locate all the white ash trees. For most of them, we were able to cut a section off the remaining stumps, or in a few cases in which the stump was cut too low, count the rings on the stump in the field. A total of 93 ash sections were obtained and returned to the lab. Sections were allowed to air dry and then were sanded and the rings counted. The results are summarized in the figure below. It is quite obvious that the ash initiated in large numbers starting around 1915, although there were some which initiated in the decade before that. Since these trees were basically cut at ground level, the ages we measured are very close to tree age, in contrast to tree cores which may be taken higher in the lower bole. There were also a few ash which initiated at intervals back to 1865. It is interesting that there were very few white ash which established after the 1920's. The 1938 hurricane, which shows up as major release in this area, did not result in any ash establishing in this stand. It is also interesting that back before the 1915 cutting of the stand, there was enough disturbance or gaps forming for an occasional white ash to get started.
2. Trees on the lower "biomass" plots
When the forest was cut and chipped we took the opportunity to test the reliability of the allometric equations prepared by Whittaker et al. (1965) for Hubbard Brook. We selected three one quarter hectare areas (each comprised of four contiguous 25 x 25-m plots which were established on the watershed). The biomass of these plots was estimated by using the allometric equations and then the trees were very carefully cut and weighed in total for each plot (See Biomass plots for further details (future link)). After the cut, sections were taken from most of the stems remaining on these plots. These sections were dried and sanded and their ages determined as for the ash trees discussed above. The results for yellow birch, sugar maple and beech are shown in the figure below. It appears that there was a fairly sharp increase in the number of yellow birch coinciding with the 1915 -1920 period. Sugar maple and beech, both shade tolerant species, have a much broader age distribution. This broader distribution probably reflects the release of seedlings and saplings in the understory, whereas the yellow birch may have begun from seed at that time. Regardless, the center of the initiation of all these species is still in the 1915-1920 period.
