Old growth forests and recruitment maps

Mapping priority stands for protection in southeastern British Columbia

The goal of these maps is to help people understand where old growth forests exist in the region, where the best forests may be, and to identify areas for long-term protection.

Old growth trees and forests play a vital role in maintaining biodiversity and genetic diversity in forest ecosystems, storing carbon and staving off climate change impacts.

Old forests are typically defined in British Columbia based on age alone, but many other stand attributes are important to distinguish old growth forests, such as tree size, carbon biomass and structural complexity, and the presence of particular species such as lichens or caribou.

The Inland Temperate Rainforest that covers a major portion of southeastern B.C. is globally unique. The wide diversity of forest types range from the dry Douglas fir and ponderosa pine forests of the south to the very wet cedar and hemlock forests further north. The amount of old forest in these ecosystems differs naturally, but maintaining sufficient old forest in all ecosystems is critical.

Old growth forests have been the focus of forest harvesting, and as a result are significantly more rare than historically, and the biggest old forests have long-since disappeared from many of these ecosystems.

The actual conservation value of the remaining old forest ecosystems depends on many factors, including the stand structures present, how much has been harvested, how much old forest should be present naturally, location of the stand on the landscape, amongst many others.

For effective conservation, higher overall levels of old forest protection are generally required in wetter ecosystems than in drier ecosystems. Where sufficient old forest no longer exists, it is important to identify younger forests that have the potential to develop old forest characteristics in the near future — or “recruitment” areas.

In this context, these maps show the largest remaining old and potential recruitment forest stands across the Kootenay-Boundary Region (Selkirk and Rocky Mountain Districts). Priority areas in each sub-region are identified on a five-class scale. Additional lower-rated old forest (older than 250 or 140 years in age, depending on the sub-region) is shown on the map for reference.

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Considerations when using the maps

• The maps identify the largest and most productive remaining older forests in the region, and do not differentiate between high elevation and low elevation forest types.
• The data used to create the map has limited accuracy. Forest stands identified as high value may have poor attributes on the ground, or may even be logged. Conversely, high quality large old forest may not appear as a priority on the map due to underlying inventory issues.
• The map identifies priority areas for protection at a minimal level (approx. 50% of what was estimated to occur historically). These forests are likely good candidates for the core of an effective conservation design, but may not be sufficient to meet all conservation needs.

The maps show National and Provincial Parks and protected areas, and existing Old Growth Management Areas (OGMAs). They also show caribou zones under the Government Actions Regulation (GAR) order, some of which do not allow any forest harvest, while others do allow some forest harvesting.

This information illustrates how well existing conservation zones protect highest priority old forest. There are other management designations that protect old growth forests such as Wildlife Habitat Areas and riparian reserves, but they are generally limited in area, and difficult to show at this regional scale.

The goal of these maps is to help people understand where old growth forests exist in the region, where the best forests may be, and to identify areas for long-term protection.

The maps and guide were prepared by Greg Utzig, Rachel Holt and Aita Bezzola in October 2021 to provide indications of where old growth forests still exist in the region, and to identify mature stands which have high suitability for recruitment of future old growth forests. The maps are an important first step in designating areas for conservation and long-term protection of old growth forests and the myriad of values associated with these forests. 

Technical background

The old growth maps produced here represent a summary of the best available information from the Province of B.C.’s 2020 B.C. Vegetation Resources Inventory (V.R.I.), using a combination of tree, stand and site characteristics (stand age, tree height, tree diameter, site index and stand volume). Overall, the maps aim to identify where the oldest and largest forests occur in the southeastern part of the province.

Each of the V.R.I. forest polygons were rated according to the tables shown below.

• Table 1 illustrates how stands were assessed using five attributes: age, height, diameter, site index and volume.
• Table 2 shows the rating values assigned to each attribute.
• Table 3 illustrates how the final priority was assigned to each stand based on the sum of the rating criteria. The priority rankings are rated from 1 (dark green) to 5 (lightest green), with the highest being 1 or dark green. All five classes combined include approximately 30% of the forest type polygons in the North Columbia, 20% in the West Kootenay and Mid Rockies, and 14% in the East Kootenay and Boundary.

Age was given the highest weighting as it is the best indicator of “oldness”.

Height and diameter were second as these directly reflect the size of trees in the stand.

Site index and volume were weighted lowest as they are indirect measures of stand biomass and productivity, and likely the least reliable measures.

The combination of these attributes is an attempt to compensate for the lack of reliability in any individual factor. If there were multiple layers described for a stand, the layer with maximum values was utilized. The result of applying the criteria to a stand was summarized by adding the ratings for each of the five attributes.

We note that some of these attributes are correlated with one another, but examination of preliminary maps suggested that using multiple attributes helped to differentiate between different types of older forest present in the region.

Sub-regions of the study area

Southeastern B.C. was subdivided into five sub-regions (G. Utzig from previous work) based on natural disturbance regimes.

The North Columbia sub-region is part of the epicenter of the Inland Temperate Rainforest. Due to a wet climate and lack of wildfires, the area was dominated by extensive old growth forests prior to fires associated with railroad construction and forest harvesting.

The East Kootenay and the Boundary sub-regions have hotter and drier climates and more extensive wildfire histories. These more frequent natural disturbance regimes have resulted in fewer old growth stands, and more open stands with some grasslands.

The West Kootenay and Mid Rockies fall in between those two extremes. The B.C. government generally defines “old” as stands greater than 250 years old in the wetter areas and greater than 140 years old in the drier areas.

To reflect ecological variation, we used different rating criteria in each sub-region, with the goal of identifying approximately half of the area of old growth stands expected to occur in each sub-region under historic conditions. In the North Columbia the priority rated stands are all old (> 250 years).

In the drier and intermediate areas, there is not enough old (>140 or >250 years) remaining to fill old growth priorities, and here younger large forest was identified for recruitment, along with the existing old.

In addition, there are other areas of mapped old forest (based on age), that were not prioritized in this mapping exercise (shaded in lavender and tan). These forests may have high value attributes, or be important for other functions (landscape resilience) or values (such as caribou). These are shown on the map based on their age.

Three categories are identified:

1. Ancient forests, mapped as forests older than 350 years in age;
2. Old forest between 250 – 349 years old in wetter and intermediate ecosystems; and
3. Old forest between 140 – 249 years old in drier ecosystems.

Table 1. Stand attribute classes and defining criteria

Attribute	Very high	High	Medium	Low	Very low
Age (years)	≥350	250-349	140-249	100-139	80-99
Height (m)	≥35	30-34	28-29	25-27	22-24
Diameter (cm)	≥50	40-49	30-39	25-29	20-24
Site index (m@50)	≥25	19-24	15-18	10-14	5-9
Volume (m3)	≥400	360-399	300-359	250-299	200-249

Table 2. Stand attribute class ratings

Attribute	Very high	High	Medium	Low	Very low
Age	16	12	10	8	4
Height	8	6	4	2	1
Diameter	8	6	4	2	1
Site index	6	4	3	2	1
Volume	6	4	3	2	1

Table 3. Priority classes and defining values by sub-region

Priority rating	North Columbia	West Kootenay	Mid Rockies	East Kootenay	Boundary
1	≥33	≥32	≥34	≥29	≥31
2	31-32	30-31	32-33	26-28	29-30
3	29-30	28-29	30-31	24-25	27-28
4	27-28	27	28-29	23	26
5	24-26	25-26	27	21-22	25

Caution: This mapping assesses and prioritizes forest stands based on their V.R.I. mapped age, tree size and estimated productivity. The stands are prioritized for their potential contribution to conservation and recruitment of old growth forests. The ranking process does not consider landscape context (patch size/shape/connectivity), ecosystem type, stand structural components, or overlap with old growth-dependent species’ habitat (e.g. caribou).

Not all old forest stands meet the minimum ranking criteria. Lower or non-ranked stands may also have high values that are not reflected in the V.R.I. data, and ranked stands may have lower values on the ground, or may be logged. Final application of the rankings will require field verification.

Additional resources

Access the backgrounder document as a PDF.

Header photo: Doug Thorburn