Paving paradise: Low-density housing linked to tree loss, rise in impervious surfaces in Metro Vancouver

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      With their gardens and landscaped yards, low-density housing, particularly the single-family home, is often seen as green.

      By contrast, high-density residential developments, like condo towers, are typically associated with the so-called concrete jungle.

      While this may have been true in the past, it is no longer the case.

      It’s the opposite that now prevails, based on an analysis of tree canopy and impervious surfaces in the Lower Mainland.

      “Perhaps surprisingly, high density housing (e.g. condos and towers) has accommodated increasingly more trees in recent decades, with a corresponding decrease in impervious surfaces,” regional planner Josephine Clark wrote in a report to the climate action committee of the Metro Vancouver regional district government.

      In comparison, according to Clark, lower density housing “appears to have shifted from a housing model that accommodated many trees to one that accommodates increasingly fewer trees and more impervious surface due to expanding home sizes and lot-splitting”.

      “These trends are likely to continue into the future,” Clark wrote.

      Tree cover and impervious surfaces are measures of the ecological health of the region.

      As Clark explained, trees provide many benefits. They give shade, suck carbon dioxide from the atmosphere, and absorb storm water.

      Impervious surfaces, like driveways and parking spots, do not take in rainwater, and are associated with heat.

      The two are inversely related to one another. An increase in one means a decrease in the other.

      Attached to Clark’s report is a study she co-wrote, and it is entitled ‘Regional Tree Canopy Cover and Impervious Surfaces: Analysis of Tree Canopy Cover and Impervious Surfaces in Metro Vancouver’.

      The analysis points to a decline in tree canopy in low-density housing developments in recent years.

      According to the study, the decline in average percentage of tree cover in these developments has been “consistent”, from 36 percent for those built in 1970 to 18 percent for those constructed in 2000.

      “This decline indicates that fewer, or smaller, trees are being retained or planted during construction of low density housing over time,” the document states.

      High-density housing development in recent years have greater tree canopy.

      “Although the relationship is less strongly linear, the data indicates that there has been an overall increase in the number of trees planted or retained for high density housing over time,” the study notes.

      The document also states that for “almost every year since 1970”, the average low-density housing parcel has more percentage of impervious surfaces “today than the average parcel for the previous year”.

      “The analysis shows that there has been a consistent increase in average” percentage of impervious surfaces “within the low density housing stock, from 49% for parcels built in 1970 to 75% for parcels built in 2012”.

      By comparison, the “average” percentage of impervious surfaces has been “decreasing over time within the high density housing stock”.

      “As average tree canopy cover has decreased over time within low density housing there has been a corresponding increase in impervious surface,” the study notes. “For high density housing this relationship is reversed, and as average tree canopy cover has increased, levels of impervious surface have decreased over time.”

      The paper explains that rapid urbanization starting in the 1960s “resulted in the subdivision of parcels in urban areas to accommodate more housing growth”.

      “While lot sizes shrunk, demand for bigger homes increased, resulting in increased lot coverage,” the study states. “This has resulted in less space for trees and an increase in impervious surfaces on low density housing parcels. If these housing trends continue (which seems likely), they may result in ongoing declines in tree canopy and increases in impervious surface.”

      Meanwhile, economic growth and advances in technology created a boom in skyscrapers in the 1960s and onwards.

      The new buildings are tall and slender, and “used up little lot coverage and allowed abundant greenspace, street trees and other public space at the bottom”.

      According to the study, this may explain why tree canopy increased and impervious surfaces declined in these developments.

      “The West End neighborhood in the City of Vancouver is a good example of this phenomenon, where the majority of its residential high rises were constructed between 1960 and 1980,” the study points out.

      The paper notes that tree canopy in high-density housing showed a “slight decline” after 1980, but this was “not matched” with a corresponding increase in impervious surfaces, which have “remained relatively steady”.

      “This suggests that since 1980, trees have been replaced by other types of vegetation (e.g. grass, shrubs) rather than increased lot coverage by buildings or other impervious surface,” the study notes.

      In her report to the climate action committee, anticipated growth in Metro Vancouver may likely reduce tree canopy from 32 percent of the urban containment boundary of the region to 28 percent over the next 20-30 years.