Here's why:

* Chemical weathering involves the breakdown of rocks through chemical reactions.

* Physical weathering involves the breakdown of rocks into smaller pieces without changing their chemical composition.

Plants produce various chemicals that can alter the composition of rocks. Some examples include:

* Organic acids: Released by decaying plant matter, these acids can dissolve certain minerals in rocks.

* Lichen acids: Lichens, a symbiotic partnership of fungi and algae, produce acids that can etch and break down rock surfaces.

* Root growth: As roots grow, they can exert pressure on rocks, creating cracks. These cracks can then be further eroded by chemical weathering agents.

Therefore, the process of breaking down rocks by chemicals produced by plants is considered chemical weathering.