To elaborate - Ecosia is basically just a frontend for Bing. They split the revenue from advertisements with Bing, and donate a portion of their share to a variety is charities to support tree planting initiatives
It looks like the most recent month’s revenue split was low on trees (16.6%), but looking back at previous months they tend to average in the 25% to projects range. I wouldn’t be surprised if the increase in March advertising spending was all Earth Month related.
If you don’t mind Bing, and you don’t mind ads, Ecosia is a fantastic way to make a little impact every day with something you’d be doing anyways. If you do mind those things, then consider donating to one or more of the charities Ecosia supports, because at minimum they have a better project vetting budget than I do.