August 12, 2011 § Leave a comment
Maybe we need to rethink trees in our parking lots.
It’s a miserable life for trees in parking lots.
Yeah, I know, they shade our cars, and they reduce the “heat island” effect; but really, it’s miserable for trees – actually for us as well.
First of all, a parking lot really is hotter than hell; at 6 foot above the asphalt the temperature at mid-day can be 150 deg. F when the air temperature is just 105; secondly, they are crammed into planters surrounded by that hot impervious asphalt so no rain gets to their roots; thirdly, that hot asphalt bakes the undersides of the leaves where the stomata release the water that cools them and the air around and makes photosynthesis efficient; thirdly, again, those little roots that are supposed to be out at the perimeter (drip line), the ones that actually soak up the water, they need oxygen and relatively loose soil, but you know what happens with pavement – compaction, compaction, and then impervious toxic asphalt; fourthly, the sprinklers are right next to the trunk not out at the drip line where they should be; fourthly (again), of course the sprinkler heads get broken; fifthly, they are all alone, they are not part of a system so all the environmental assaults come full bore – the wind, the reflected heat, you name it; and lastly, they are never maintained correctly either because they are hacked out of ignorance, or the building owner doesn’t want his signs hidden.
So, what to do. Install PV panels as carports. A study done at ASU comparing trees to carports in terms of heat island reduction and water reduction noted the following:
- The shade from carports reduced the temp about 56 deg compared to 43 deg from tree shade;
- The canopies used no water;
- The canopies generate electricity.
Reduce the heat island effect and water usage – not bad for a sustainable practice. But what about aesthetics, you say. Who wants a harsh street environment of pavement and metal roofs, dark PV panels, and cars; at least trees bring some softness to our oversized parking lots.
Correct, but I think we could change city ordinances requiring shade so that they are favorable toward PV carports, not trees. Use the trees in ways that make them happy, shade what can’t be shaded by canopies and make a strong urban design statement.
- Use trees selectively to line the main drive aisles in generous continuous planters. (We can steal some width from the overly wide drive aisles.) This will make a stronger urban design statement and provide shade on driveways.
- Use permeable pavers or pervious concrete in the area around the trees (out to the drip line.) Some rain can get through these, and perhaps we can adjust the substrate to make the soil more root-friendly.
- Concentrate tree planting at important (pedestrian and urban design) locations so the trees create a bosque/microclimate, and have a strong visual impression. They also have a better chance to thrive because they provide mutual support, irrigation is more efficient, and store owners won’t be trying to kill them off.
- Use PV panel carports to cover cars. They generate electricity, and by reducing the need for more fossil fuel or nuclear generating plants, they reduce overall water usage. (It takes a lot of water to generate electricity. According to one Canadian study, it takes up to 30 gal of water per kWh for natural gas, and 50 gal per kWh for nuclear plants.)
Ref: “Photovoltaic Canopies: Thermodynamics to Achieve a Sustainable Systems Approach to Mitigate the Urban Heat Island Hysteresis Lag Effect.”
Jay S. Golden, Director
Sustainable Materials and Renewable Technologies Program
Arizona State University
P.O. Box 873211
Tempe, Arizona 85287-3211
Phone / Fax: +1-(480) 965-4951 / (480) 965-8087