>>11171900The areas you're seeing in this photograph are above the flood zone, or at least the regularly inundated zone.
To answer the question:
In natural ecosystems where there is an abrupt change it is usually due to one of two things: Fire, or abrupt edaphic changes (soil/subsurface changes).
Where you find these boundaries it usually implies that fire regularly burns to the margins of low-flammability forest, or it undergoes sharp changes in soil type (there's an image from California where a guy has one foot in grassland and one foot in what is essentially chapparal, not sure where to find it).
>>11171915I did not know this, so I am very grateful to read it.
Usually it is due to clearing, logging, changed fire regimes or a combination of both.
