It’s that leafy time of year again

The overwinter decay of leaves: I know graphs are not to everyone's taste but this one shows how leaves submerged overwinter 50 cm down in the margins of a Swedish lake decayed, and did it quite quickly too. Despite being the coldest time of year, the oak leaves (the upper line) dropped in weight by about 80% in one winter (the reference is at the end of the post)

Leaves are one of the great conundrums for pond wildlife enthusiasts.

Should we love them or hate them (the leaves that is) and are some leaves better or worse than others (as Flying Ant asks)?

One source I found on the web – a wildlife organisation you would assume to be reliable – says:

“…some species, such as oak and sycamore, have a more adverse effect on on the quality of the water than finer-leaved species such as willow.”

Another says: “…some pond-keepers insist that you should never site a pond beside sycamore.

 So is there something to these suggestions or are they just more myths?

In this case it’s perhaps more a matter of misunderstanding.

First, are leaves good or bad? They are definitely good, but its fair to say you can have too much of a good thing if they completely fill your pond. When you’ve got a small pond completely full of leaves it will usually have a rather small range of animals – though there will still be animals. Watch out for newt larvae, for example. I’ve found newts – only one or two admittedly – in a pond completely stacked to the brim with leaves.

Are different kinds of leaves better or worse? This is a more difficult question to answer: some leaves, like willows, break down more quickly than others but that doesn’t necessarily mean they’re better. Oak and sycamore leaves which decay more slowly might release more tannins or other chemicals than willows, which could help suppress the growth of algae.

Are sycamores particularly bad? Probably not – it’s just that sycamore leaves are quite big and if a lot fall into a small pond they could well fill it up. A few falling into the pond are probably going to disappear fairly quickly in a well-oxygenated shallow pond.

But really, the jury is still out on what constitutes exactly the ‘right’ amount of leafy material in your pond.

At the moment, the best way to tell whether you’re getting it right is to dip the pond with a sieve and check what the animals are telling you.     

[The graph comes from this report: The timing of winter-growing shredder species and leaf litter turnover rate in an oligotrophic lake, SE Sweden by Irene Bohman and Jan Herrmann}.


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