Here's why:
* Deciduous is already a complete word describing a type of plant or tree that loses its leaves seasonally.
* Suffixes are word parts added to the end of a word to modify its meaning.
Instead of a suffix, you might use related words or phrases to describe the characteristic of being deciduous:
* Deciduous forest
* Deciduous tree
* Deciduous foliage
* Deciduous plants
* Deciduous habit
If you're looking to modify the meaning of "deciduous", you'd likely need to use a different word altogether. For example, you could say "semi-deciduous" or "evergreen" to describe a plant that keeps some or all of its leaves year-round.