Annual biomass production

Does anybody know what the impact on biomass production of a long hot dry
summer such as we've just had in the UK (and Europe)?  Presumably the
drought doesn't affect trees too much (unless they get burned) because their
roots will be finding moisture - or does it?  On the other hand, all those
sunny days should mean plants with access to water get more
photosynthesising done.  I wonder because here in London there seems to be
fewer leaves fallen by this time of year for me to make leaf mould for my
garden.

Tim Lund