[aeroman]

The increase in humidity is actually a different effect - evapotranspiration from trees is included in climate models (along with their response to increasing CO2 concentrations) [0].

The effect in this article is more to do with the particulates that form from the chemical emitted from trees. The article doesn't make it clear, but an increase in tree particulates (known as aerosols) would actually cause less rain.

Almost all cloud droplets form on an aerosol particle, so the cloud droplets in a cloud with more aerosols are on average smaller (as the water is spread out over more droplets). These smaller droplets take longer to grow large enough to form rain, an effect which is thought to decrease the amount of precipitation in some regions (although by a small amount).

This effect is also included in climate models, but the sources of aerosol (such as from trees) are more uncertain [1], producing the uncertainty in future climate projections.

[0] - https://gmd.copernicus.org/articles/4/677/2011/gmd-4-677-201...

[1] - https://www.nature.com/articles/nature12674