Why isn't there rain?
(Thank you for the inspiration, Willk...)
Your planet has rivers and oceans. Rain is rare or nonexistent. Why? Evaporation from the oceans should be generating humid air which forms into clouds (and rain will happen, over the deep ocean).
However, in the distant past, your plant life got into a biological arms race, not waiting for water to fall. Instead, your plants have become more and more efficient at filtering water out of the air. This leads to swathes of territory which are being rendered desert-like by the greedy plants.
But what about the humid air that floats too high above the plants, where the clouds should be forming? Well, some of these "plants", while solar-powered (as all plants are), have gone so far as to create, effectively, vine-tethered balloons, floating high up into the sky to filter out the water, depriving the land of lifegiving rain (and to get first dibs on the sunshine beaming down, too).
Seemingly paradoxically, after having harvested an overabundance of the water from the air, many of these massive plants dump their excess water, and life flourishes around the oasis-like streams and rivers resulting from the watery abundance in the midst of otherwise desert territory. (Except, some noxious strains of the great plants poison the water they pour out, like how black walnut trees excrete chemicals harmful to their competition, and only a few hardy creatures are able to eke out survival in the neighborhood.)