Tag Archives: urban forestry

tree canopy volume

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I had never thought about modeling tree canopy volume in 3D before. I’ve played around with simple algorithms to place trees on a map, assume a mature canopy area per tree, and estimate the total canopy area. This is useful because cities sometimes set targets and metrics in terms of number of trees, and sometimes in terms of tree canopy. The latter is better because it is more relatable to other goals a city might have related to the hydrologic cycle, carbon, heat, air quality, aesthetics and property values, biodiversity and habitat, and the financial cost to public offers of achieving these goals. Once you have an algorithm relating number of trees to canopy area, you can add more variables like type of tree, growth over time, and some assumed attrition rate or half life. Come to think of it, I have played around with leaf area index which is a quasi-3D concept. Anyway, without further ado here is the article that prompted my line of thought:

Local Impact of Tree Volume on Nocturnal Urban Heat Island: A Case Study in Amsterdam

The aim of this research is to quantify the local impacts of tree volumes on the nocturnal urban heat island intensity (UHI). Volume of each individual tree is estimated through a 3D tree model dataset derived from LIDAR data and modelled with geospatial technology. Air temperature is measured on 103 different locations of the city on a relatively warm summer night. We tested an empirical model, using multi-linear regression analysis, to explain the contribution of tree volume to UHI while also taking into account urbanization degree and sky view factor at each location. We also explored the scale effect by testing variant radii for the aggregated tree volume to uncover the highest impact on UHI. The results of this study indicate that, in our case study area, tree volume has the highest impact on UHI within 40 meters and that a one degree temperature reduction is predicted for an increase of 60,000 m3 tree canopy volume in this 40 meter buffer. In addition, we present how geospatial technology is used in automating data extraction procedures to enable scalability (data availability for large extents) for efficient analysis of the UHI relation with urban elements.

a million trees in New York

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New York City has managed to get a million new trees in the ground. Planting a bunch of trees seems like a no-brainer to many of us who are familiar with the logic and evidence in favor of green infrastructure. But this can still be hard for cities. There is a vocal minority of citizens who hate trees. They’re a minority, but did I mention they’re vocal? Then, trees are not a huge expense in the big picture of all the things cities have to pay for, like police, courts, prisons and pensions for example, but their planting and especially maintenance sometimes falls to city departments who are under-funded in good times and the first to get hit by budget cuts in bad times.

New York seems to have gotten past these challenges with strong planning, strong leadership to actually implement the plan, and partnering with a non-profit entity which could really focus on this one mission.

A collaboration between New York City’s parks department and conservation nonprofit New York Restoration Project (NYRP), the initiative just succeeded in planting 1 million new trees in the city this decade. The final tree was planted last month, two years ahead of schedule. While cities like Los Angeles, Boston and Denver have all set the same goal, New York is the first to meet it.

Beyond 220,000 new street trees, MillionTreesNYC planted in parks, on public and private property, and in all five boroughs, increasing the city’s urban canopy by 20 percent.

While the city planted 70 percent of the trees in parks and on streets, NYRP was tasked with getting the remainder into public and private spaces, including hospitals, libraries, churches, public housing developments and private yards.

I do have to point out that “a million trees planted” almost certainly does not mean a net gain of a million trees. While the program was being implemented, some trees must have died of “natural” causes (air pollution, heat stress, poor soil, lack of water). Some also must have been removed for legitimate reasons in the course of construction and infrastructure projects, and if my personal experience in Philadelphia is any guide, not all of those got replanted (the vocal minority of citizens having something to do with this). But all this is exactly why focusing on tree canopy is exactly the right way to look at it. By setting a tree canopy goal and periodically measuring where you are relative to it, you should know if you are replacing the trees lost to attrition at the right rate to keep your overall canopy from dropping.

growing the urban forest

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This abstract in Restoration Ecology contains an interesting result: planting shrubs along with urban trees helps the trees. You might think the opposite, due to competition, but I have heard this before. One theory I’ve heard is that shrubs help establish beneficial fungi in the soil that pave the way for healthy trees. It shouldn’t be too surprising, when this is exactly the succession that will occur in an abandoned field over time, given enough rainfall and not too much fire.

Compost also helps trees, which might be surprising to some professional engineers but not to any amateur gardener (luckily, some of us are both!). Still, in urban stormwater management we engineers are often encouraged to plant trees and other vegetation, but to minimize organic matter because the same nutrients that trees need can become water pollutants if they find their way downstream. It’s a delicate balance. Civil, “environmental”, and geotechnical engineers aren’t good at finding it because it is not part of our typical training. We need the agriculture, forestry, and soil science types to help us with this.

Forests are vital components of the urban landscape because they provide ecosystem services such as carbon sequestration, storm-water mitigation, and air-quality improvement. To enhance these services, cities are investing in programs to create urban forests. A major unknown, however, is whether planted trees will grow into the mature, closed-canopied forest on which ecosystem service provision depends. We assessed the influence of biotic and abiotic land management on planted tree performance as part of urban forest restoration in New York City, U.S.A. Biotic treatments were designed to improve tree growth, with the expectation that higher tree species composition (six vs. two) and greater stand complexity (with shrubs vs. without) would facilitate tree performance. Similarly, the abiotic treatment (compost amendment vs. without) was expected to increase tree performance by improving soil conditions. Growth and survival was measured for approximately 1,300 native saplings across three growing seasons. The biotic and abiotic treatments significantly improved tree performance, where shrub presence increased tree height for five of the six tree species, and compost increased basal area and stem volume of all species. Species-specific responses, however, highlighted the difficulty of achieving rapid growth with limited mortality. Pioneer species had the highest growth in stem volume over 3 years (up to 3,500%), but also the highest mortality (up to 40%). Mid-successional species had lower mortality (<16%), but also the slowest growth in volume (approximately 500% in volume). Our results suggest that there will be trade-offs between optimizing tree growth versus survival when implementing urban tree planting initiatives.