Some Ecological Landscape Analysis & Other Updates from Nova Scotia Natural Resources & Renewables 24Apr2023

Thank you for your question.

The Development Stage layer are part of series of interpreted GIS layers to support Ecological Landscape Analysis. All of these layers are intended for broad-scale analysis across ecodistricts and the whole province. No decisions related to whether a forest is harvested or put into old-growth forest protection is based on these layers, these types of decisions are only based on ground-based field measurements.

The two development layers you are referring are based on two independent forestry inventories. The 2017 Development Layer is based on an inventory (from Halifax County) that used air photos from 1992 and 2003, and the 2023 Development Layer was based on air photos taken in 2017 (for Halifax County). Forest inventories are completed every approximately 10 years by professional air photo interpreters, using previous inventories, new photos, harvest data and other data to make new independent interpretations of over 1 million polygons across Nova Scotia. Since 1992 there have been some changes to the forest inventory process and techniques used, which results in some of changes we see between forest inventories (e.g. starting 2007 photography, the scale of the photographs changed from 1:10,000 to 1:12,500).

Development layers are just reclassifications of those independent interpretations. The forest inventory includes a series of attributes determined by the air photo interpreter that we can use to reclassify the forest into other data layers to support broad-scale landscape analysis. One of those attributes is age. Age is not a measured variable, but an estimated one, based on site class and stand height (both independent interpretation calls). The interpreters then use those attributes with a series of graphs (i.e. curves) to estimate age. Better site classes (good growing sites) generally allow trees to grow taller faster. Therefore, higher site class areas will have a lower estimated age for stands that are the same height compared to a lower site class.

The two areas you are referencing experience the following changes related the new forest inventory that was used to develop the new independent 2023 Development stage layer.

1) North of Sandy Lake – The interpreter classified using 2017 photos (2023 development layer) this stand as a single height level instead of multi-height in the previous forest inventory. This resulted in the stand only being considered either “early or late mature” and not “multi-aged”. There was also a change in the interpretation of the site class from Site Class 6 from Site Class 5. These issues resulted in classification of development stage being “Early Mature” (40-80 years) instead of Multi-age (over-120 years). This stand is an under prediction of the development stage based on your field work.

2) Peninsula – The interpreter also classified this stand as Site Class 6 from a Site Class 5 (which leads to lower predicted age). The older Development Stage layer and its associated forest inventory used the 1992 air photos for the interpretation of this stand instead of the 2003 air photos. The current age used in the classification categories in the development stage layer is calculated based on the estimated age at time of photo (from Site class and height) corrected to date of release of development stage. Therefore the 2017 Development Stage layer added 25 years to the estimate age (1992-2017). Whereas the 2023 Development Stage layer only added 5 years (2017-2023). These two issues resulted in the stand being classified as “early-mature” in the 2023 Development stage layer. This stand is an under prediction of Development Stage based on your field work.

How does this affect the broad-scale estimation of development stages across the province? On average these commission errors (wrong classification) will average out between inventories. However, as long as height/site is the method of age prediction there in an inherent bias to underestimate age in older stands where height growth is being limited by environmental factors. My initial assessment and based on your observations this layer underestimates some the late-mature and multi-age forests (i.e. some forest displayed as early-mature are likely late-mature on the ground). We will complete some additional analysis to better quantify this estimate of accuracy.

In conclusion, the 2023 Development Stage layer is different then previous development stage layers. Development stage layers are not the best geospatial datasets to use for changed detection at a stand-level, especially when looking for subtle changes within the mature classes (early/late mature/multi-aged-old) the inventory would perform better on detecting major changes (mature/establishment). That said Remote sensing using standard formats like Landsat, are likely a better tool for change detect. Historical air photos may also be used for change detection at a stand level. Development stage layer is an independent classification of interpreted forest inventory across the entire province (1 million polygons) which provides the best available tool for landscape analysis about categories of forest ages.

The department is committed to improving the forest inventory on a continual bases and will be incorporating new remote sensing (including lidar) into an Enhanced Forest Inventory in the near future.

The department will work with the Dr. Mathew Novak at Saint Mary’s University’s Wicked Problems Lab to explore ways to better quantify the commission errors in the Development Stage layer based on ground-truth measurement and ages (including your OGF assessments from Sandy Lake).