Conservation Manager, Tim Hill has been monitoring leaf fall for the last 15 years - read on to see what he’s learnt along the way.

Since 2008 I have been part of a nationwide project monitoring leaf fall from eight of our most common trees on behalf of the Met Office – Oak, Sycamore, Ash, Horse Chestnut, Beech, Lime, Birch and Poplar. The project involves estimating the percentage of leaves that have fallen from trees three times a week, from September through to the time when no leaves remain. 

Over the years I have found a general pattern which is repeated from year-to-year. Horse Chestnuts seem to be the only trees to lose leaves during September. In recent years, this has clearly been exaggerated by some trees succumbing to the Horse Chestnut Leaf Miner which results in premature browning and fall. Even last year when we experienced serious drought conditions and there was great concern that a lack of water would result in trees defoliating, I didn’t see any significant leaf fall from any trees apart from Chestnuts. The scale of fall can be impressive and sometimes the Chestnuts have been bald by mid-November, with Limes and Ash not far behind.

By the middle of October Poplars and Silver Birches will be starting to drop their leaves but it is the last week of the month when the fall really gathers pace, often coinciding with the first frosts of the year. In my experience, Silver Birches vary enormously from tree to tree with some holding on to leaves until the end of November. Oak and Sycamore are the last species to start losing leaves and it is usually the last week in October before they begin to drop. This is good news for late migrant warblers such as Chiffchaffs, which feast on the abundant aphids which are associated with Sycamore leaves. Small flocks of House Martins can often be seen feeding around Sycamores as they migrate south, also feeding on the numerous insects associated with the trees. Bats preparing for hibernation are also able to fatten up on this rich food resource. Oaks are always the last trees to become defoliated and there have been a few years when their crowns still held a scattering of leaves at Christmas.

Watching trees drop their leaves has become something of an obsession for me and I now find myself assessing percentage leaf fall on trees wherever I go through the autumn months. Why not have a go in your local area? If nothing else you will enjoy the autumn colours and get to know your Limes from your Poplars.

There are many ways to enjoy the autumn leaves, falling or otherwise! As you take a leisurely seasonal stroll, how about encouraging younger nature lovers to become an autumn tree and leaf detective by downloading our spotter guide here? Or find a local walk and try to walk it weekly, noting the changes in colour and form. You could even document your experiences on camera and share your photos with us @hertswildlifetrust on Instagram and Facebook and @HMWTBadger on Twitter…we look forward to seeing them!