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Fours years after thinning, there is grass gains in the Cranbrook Community Forest-August 7th, 2022

If you have been walking the Gateway Trail in The Cranbrook Community Forest over the past four years you will have noted many changes. The College of the Rockies has added five new student residences and a fine new bicycle track, but the most startling had to be the thinning of 36 hectares of College and Crown land from December 2017 to February 2018. This thinning was carried out by a local logging company under a contract funded by the Forest Enhancement Society and College of the Rockies. The project was overseen by the Rocky Mountain Trench Natural Resources Society and supported by the Rocky Mountain Natural Resource District, the City of Cranbrook, the College of the Rockies and the Cranbrook Community Forest Society.


The Cranbrook Community Forest is actually a Government Recreation Site with a mandate to act as a Demonstration Forest; not just trails and picnic tables. It includes forest management such as setting up innovative forest management and education opportunities for the community. The thinning treatment from 2018 was an opportunity to show the effect of opening the forest canopy for the benefits of grass, forb (non woody, non grass annual plant) and shrub growth.

The thinning treatment was set up as per the map below to show five levels of thinning. Unit 3 started with 7000 trees per hectare; (very spindly growth and virtually no growth in the last 40 years) and this stand was reduced to 150 stems per hectare, which would be the historic stocking for this dry Douglas-fir forest.


But 12 hectares near the old garbage dump presented a very different opportunity. In 1998 this unit had been reduced from 3500 trees per hectare to about 800 stems per hectare. After twenty-five years examinations, the stand showed no increase in growth in trees, or grass or shrubs. This 12 hectare unit was divided into 4 units, Unit 1-D was left at 800 stems/ hectare, 1C reduced to 200 stems/ hectare, 1B to 100 stems per hectare and 1A as 50 stems/ hectare. Two Wildlife Tree Patches (WTP 1 and WTP 2) were left untreated. It is possible to walk these sites and see changes, but it was preferred to do proper plots to measure changes.


A total of 14 plots were established in 2017, 2 in every stocking class and two in the untreated units. By measuring untreated units, we can see what the effect of treatment is. Good weather can improve growth with better water and heat; we need to make sure the change in growth is due to the thinning. The Cranbrook Community Forest Society was able to rehire a local biologist to remeasure the plots in 2019 and we hope to remeasure these plots in 2023 and 2028 to see the progress of grass and forb growth.


The growth has been very spectacular this year 2022. As the photos show these forests started out with a dominant understory growth of dry moss and 1.5 to 9% pine grass with a fraction of bunch grass. This year grasses cover 50 to 60% of the ground. The dominant grass is chest high Idaho fescue and hip high needle grasses, both very high value high protein winter food for ungulates. Pine grass has increased from 1 to 9% to about 1/3 of this grass cover (about 20% of total cover) but it flourished to about knee height in 2020 and 2021 but this year it is barely half this previous height. It is a low protein, low value winter food for deer but it forms a significant barrier to invasive plants and sets up a succession of forbs.

We will carry on with remeasurements on this site and we will add these plots to the annual fall tour that the Cranbrook Community Forest runs with the Environmental Sciences class at the College of the Rockies.




Map of the treatment units and plot location of a thinning project in the Cranbrook Community Forest just uphill of the College of the Rockies.



Both photos show the east aspect of plot 3294 in unit 1A of the Cranbrook Community Forest 2018 thinning project. The upper photo shows the forest in 2017 prior to the thinning when grass cover was 9%, the lower photo shows August 2022 when grass cover grew to is 60%





Both photos above show the west aspect of plot 3297 in unit 1A of the Cranbrook Community Forest 2018 thinning project. The upper photo shows the forest in 2017 prior to the thinning when grass cover was 1.5%, the lower photo shows August 2022 when grass cover grew to is 50%

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