Disposing of felled trees in an environmentally friendly way

by Rebecca Priestley / 06 October, 2012
What is the most environmentally friendly way of disposing of felled trees?
Disposing of felled trees in an environmentally friendly way


Question: At this time of year I often cut down a few small trees to let more sunlight reach the vege garden. I then have the choice of cutting up the felled trees and burning them as firewood, letting them slowly decompose where they fall or taking them to the green waste station at the landfill to get turned into compost. I asked Lincoln University’s Professor Steve Wratten and Charles Merfield if any of these choices are better or worse in terms of the amount of greenhouse gases they release into the atmosphere.

Answer: ‘A tree is largely recently fixed – that is, photosynthesised – carbon from the atmosphere, not ‘fossil’ carbon like coal and oil,” they replied. “As long as New Zealand plants as many trees as it cuts down, burning trees is carbon-neutral. But the wood needs to be burnt when it is dry in an efficient burner; otherwise particulates – small particles of soot, or black carbon – can pose other atmospheric problems. “If a tree is left to decompose on the soil surface, some of its carbon will enter the soil where it will be stored, but much of it will oxidise and enter the atmosphere. Again, this is recent carbon, so with the same planting proviso as above, it is carbon-neutral. “Chipping a felled tree and then composting the chips is fine – apart from fossil energy used by the chipping machine – but it depends on what then happens to the compost. Cities can’t get rid of all their compost, and if it goes to a landfill, where the compost might be stored in low-oxygen conditions, the carbon may become methane (CH4), which is a much more potent greenhouse gas than CO2.”

Question: Martin Nield from New Plymouth is curious about how rainbows are formed. “I understand that each individual raindrop forms a prism and splits the sunlight from red through to violet. How do all these individual split lights form into one arched rainbow? Is a full rain circle formed, with the bottom half not visible against the Earth? Why do you occasionally have a double rainbow?

Answer: Erick Brenstrum, severe weather forecaster from MetService, says sunlight striking the curved side of a raindrop changes direction as it enters the drop, with the amount of refraction, or bending, depending on the wavelength of the light. Some of the refracted light is then reflected off the back of the raindrop, and then refracted again when it exits the drop. The different colours that reach your eye when you see a rainbow come from different raindrops, “the red from the raindrops higher in the sky and the blue from the raindrops lower in the sky. The raindrops are falling, so each drop is replaced by one from above, giving the illusion of a rainbow fixed in the sky.”

“The rainbow bends down to the ground because the angle between sunlight arriving and leaving each raindrop, on the way to your eye, is unique for each colour. So when you look to the left or the right of the bow’s highest point, the raindrops that fit that angle are closer to the ground.” But Brenstrum assures me the rain circle is not completed underground. “That would require raindrops under the ground and a way for sunlight to go through the Earth.” He says a double rainbow is formed “when light reflects twice off the inside wall of the raindrop”. The secondary rainbow is fainter because “each time the light reflects off the inside surface of the drop, some of the light escapes through the back of the drop”.

Send questions to science@listener.co.nz
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