We present a model of gaps in the vertical structure of forest vegetat
ion. The traditional model of a forest gap assumes the existence of a
'hole' in the uppermost canopy layer, often extending down to near the
ground. The present model extends the concept to gaps at any level, i
ncluding those in lower layers below an intact canopy or subcanopy. It
assumes that gaps at any level represent spaces with unused resources
, especially favourable for plant growth and survival. Evidence from t
emperate and tropical forests indicates that gaps in the subcanopy and
understorey layers below intact canopies are common, and that plants
have higher growth rates in them than in non-gap sites. We also extend
this model to below-ground gaps in the root zone.