As autumn, or fall, arrives the environment we hike through and fly above changes dramatically – in fact for many, late summer and early autumn is their favourite time of year to fly. But things are shifting out there, and pilots see it and comment on it year to year. Michael Lavelle explains what’s going on in the forests in the northern hemisphere at this time of year…
You may have noticed it too: it is becoming more common for leaves to drop later in the season. A 2015 review of 64 studies, with observations ranging from 1931 to 2010 across Asia, Europe and North America, found that rising global temperatures had delayed leaves falling in the northern hemisphere.
To understand what is happening, we need to remember that trees perceive the world differently to us. They follow a combination of environmental cues to time the shedding of their leaves. Foliage tends to fall gradually, starting in October and sometimes taking until early December.
Most trees native to the UK, for example, hold on to their leaves until it gets cold, which usually happens by November. In essence, it either gets too cold, or too dark, and eventually, the plant becomes fully dormant.
Bedding down
While the mechanisms involved are complex, deciduous trees bed down for winter in three stages. The first of these starts in early autumn and is linked to a chemical produced by the buds called auxin, which slows the tree’s growth. This is produced in growing tissue and suppresses...
