What we can learn from Canada’s oldest forest sample plots

Background

Petawawa Research Forest (PRF) located near Chalk River, Ontario is home to Canada’s oldest continuously measured permanent sample plots (PSPs). These plots were set up in 1918 (PSP 1) and 1926 (PSP 2) in young natural white and red pine dominated stands of fire origin, stands that are now 144 years old. These permanent sample plots had tagged trees and were remeasured at least 18 times since their inception. This includes spatial mapping of all trees in both plots and following rigorous scientific measurement protocols. The plots were set up as paired plots. PSP 1 was setup to record the impacts of forest management. PSP 2 was set up to monitor conditions in a stand that is influenced by natural processes with no harvesting taking place. PSP 1 has been harvested seven times using partial harvest techniques commonly used in uniform shelterwood silviculture, cutting the smaller trees, especially those with defects. PSP 2 has had no human interventions. This overview contains some interesting observations that are based on measurements and analysis done in 2016 by Margaret Scott. It is understood that the following observations are based on a small sample size and represent a case study. Nonetheless, they provide some interesting trend through time observations that for such long-time frames, are rarely available in Canada’s forests.

 

 

 

Partial Harvesting Captures Mortality

The total observed mortality since 1918 in PSP 1 (harvested) is 50 m³/ha. Meanwhile, in the unharvested plot (PSP 2) the total observed mortality is 420 m³/ha. PSP 1 has been harvested seven times capturing a total accumulated harvest volume of 445m³/ha. Many of the harvest operations were thinning and improvement cuts, with a regeneration cut occurring in 1941. This cut was partially successful, and another regeneration harvest was required in 2015. In 2015, PSP 1 had a current standing volume of 335 m³/ha. This is in comparison to PSP 2’s standing volume of 604 m³/ha. The natural mortality in the unharvested plot was 8.4 times higher than in the harvested plot. It appears that much of the volume harvested and converted to wood products would otherwise have been mortality. Additionally, many of the wood products produced from pine forest are of high value and long lasting, thus some of the wood products from the seven earlier harvests might still be storing carbon.

PSP 1 Partial Harvest Record

Stand Characteristics of PSP 1 and 2 Based Upon Measurements in 2016

The Harvested Stand Continues to Exhibit Positive Net Growth, The Un-Harvested Stand Exhibits Negative Net Growth

It appears the unharvested plot has reached the age of senescence where mortality exceeds growth since 1999, when the stand was approximately 120 years old. The periodic annual increment (PAI) from 1999-2016 was negative 2.8 m³/ha/year for PSP 2. Conversely, in PSP 1, the plot partially harvested 7 times, exhibited positive net growth during this same time period with a PAI of 5.7 m³/ha/year from 1999 to 2016. Based only on growth rates, the trees of PSP 1 remain a carbon sink since 1999 while PSP 2 has become a carbon source.

 

Harvesting Influences Structure and Diameter

There are vast differences in diameter and forest structure observed between these two plots. The harvested plot exhibits two distinct cohorts of diameter classes. These have resulted from the partial harvest treatments; these treatments stimulated an understory of white pine regeneration that currently has an average diameter at breast height (DBH) of 15 cm. Meanwhile, the overstory in PSP 1 has a very large average DBH of 59 cm with a range of 55-75 cm DBH. The unharvested plot (PSP 2) has just one structure cohort of overstory trees with an average DBH of 40cm and a range of 25-55 cm DBH. The much larger average tree size in the harvested plot can be attributed to a common partial harvest silvicultural practice of thinning from below. This practice targets the removal of small diameter trees with each partial harvest intervention.

Diameter Distributions Red and White Pine (PSP 1 and PSP 2)

Differences in Downed Wood

There were significant differences in observed downed wood between the harvested and unharvested plots. The unharvested plot had 182 m³/ha of downed wood. As well, the unharvested plot contained all the decay classes including advanced decay classes. After mechanical site preparation to prepare a seed bed for white pine, the harvested plot had 23 m³/ha of downed wood present. Additionally, there was no downed wood in the advanced decay classes of 4 and 5. The downed wood in the harvested stand post-harvest (but pre-site preparation) was 40 m³/ha. Downed wood is a reservoir for arthropod, fungal and plant biodiversity, the unharvested sample plot has 7.9 times more downed wood than the harvested PSP, and it is in a more advanced decay condition.

The natural disturbance regime for this forest type is fire. The fire is usually a low intensity fire that predominantly burns in the understory leaving many of the thick-bark overstory trees alive after the fire. The typical fire cycle for this forest type is between 80 to 100 years. Fire has been excluded in both sample plots; however, fuel loading is much higher in the unharvested PSP 2. In this plot there is currently 182 m³ of downed wood per hectare. The fuel loading in the harvested PSP 1 is much lower at 23 m³ per hectare. This would lead to a much more intense fire crown fire in the unharvested plot if fire were to be introduced.

Stocking and Quality Differences

The unharvested plot had a much higher number of stems per ha (10 cm DBH and larger), at 400 stems per ha., while the harvested plot had 190 stems per ha. after harvest in 2015. The basal area was also very different between the two plots. The harvested plot had a post-harvest basal area of 24 ^m2/ha while the unharvested plot had a basal area of 45.5 m²/ha. The quality of the trees present in 2015 was also vastly different between the two plots. Quality is defined by the percentage of trees that are considered acceptable growing stock (AGS). AGS trees are high quality trees that are in a healthy condition. In the unharvested plot, 62% of trees were identified as AGS. Meanwhile, in the harvested stand, 81% of trees present after harvest in 2015 were identified as AGS. The stocking and quality differences can be attributed to the partial harvest silvicultural tree marking practices. The harvested stand had a trained and certified silvicultural tree marker select trees for removal to target a residual crown closure conducive to regenerating white pine, thus resulting in a targeted lower residual stocking. In addition, where possible, partial harvest tree marking targets retention of AGS trees. Therefore, this would contribute to the higher quality in the residual trees in the harvested plot.

Economic Values

In PSP 1 there have been seven harvest treatment since 1918. These treatments captured a volume of 445 m³/ha. Additionally, the standing volume of remaining trees in 2015 was 335 m³/ha. In total, the accrued volume was 800^m3/ha. The residual trees on PSP 1 are also very large with the over story diameters (DBH) ranging from 55-75 cm. Conversely, PSP 2, where there has been no harvesting, had a current standing volume of 604 m³/ha. Additionally, the overstory trees in this plot have a much smaller diameter ranging from 25-55 cm DBH. The mean tree sawlog volume in PSP 1 is 5.1 ^m3 and in PSP 2 the mean tree sawlog volume is 1.6 m³. Large trees have a much higher economic value that small trees. This is because as tree size increases logging costs are reduced, and lumber recovery increases with tree size. Based on the increased volume and tree size from PSP 1 it is estimated that the existing standing volume and accrued volume would be worth approximately $56,000/ha on the stump. PSP 2 with smaller diameters and a lower total volume per ha produced would be worth an estimated $24,000/ha. These estimates are in 2022 dollars and do not account for the fact that some additional value from PSP 1 was earned decades earlier.

Social Values

One of the most important values provided by old forests are the social values that they represent. This is a difficult value to quantify, as the definition of old forests and the important values they represent vary widely. For some, the definition of old growth is simply an age. For others, it is a forest condition. Many associate the old forest definition with large, tall trees. Regardless of one’s definition of old growth, PSP 1 represents a forest with larger, straighter, trees that have large crowns that might stimulate feelings of awe compared to PSP 2. The overall state of PSP 2 has declining wood volume since 1998. However, PSP 1 continues to produce positive net growth and has 81% healthy AGS trees that were continuing to store carbon, while PSP 2 had become a carbon source. PSP 2, however, has more downed woody material which will lead to higher biodiversity of species that thrive on decaying wood.

Summary

PSP 1 and 2 in the Petawawa Research Forest allow a long-term case study of different management of an eastern pine forest.  The rigorous scientific methodology, following growth and mortality of tagged trees, over a 104-year span, allowing us to report on two very different paths of stand development.

Acknowledgments:

This information bulletin has been made possible by analyses done by Margaret Scott in her capstone paper for the University of Toronto Master of Forest Conservation program. To collect and analyze data in a forest setting for 104 years is an incredible accomplishment. Many generations of researchers, forest technicians, and students have contributed to this effort. The original plots were established in 1918 by W.M. Stiell, others that maintained the analysis and measurements are Darwin Burgess, Craig Robinson, Steve Deon, Peter Arbour, Margaret Scott, Elizabeth Cobb, Jay Malcolm, and Mike Hoepting. Also, there are many unnamed forestry staff that deserve recognition for their dedication and efforts in the maintenance, measurement, and analysis, of these plots for the past 104 years.

 

Authors: A. Stinson, M. Scott, and Petawawa Research Forest Staff
Reviewed by: CIF-IFC Forest Advocacy Committee

March 31, 2023

 

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