I want to insert a nice drawing of a forest for example, to show
ecological processes between different parts of the ecosystem.
If this is the level of complexity you're looking at, I'd say it is best handled by an illustrator. First, it'd be hard to find a set of cliparts that feature ecological processes; and if you put a bunch of cliparts with different styles together it's going to look like an 8th grade science poster project. (Which is fine if that's the level you are happy with.)
I know that you said you don't have money, but have you checked out some alternative way to hire artists? There are artists that draw a logo for a couple hundred thousands dollars, and there are many more artists who charge much, must less than that. One possible place to look is the Hungry Artist sub-Reddit. And here what the local community consider a "good and clear" posting. You can also mention to the artist that it's likely to be featured in a publication, which can help them strengthen their portfolio.
I have looked on deviantart for a while and google but couldn't find
anything that fits and that is copyright free.
I'd suggest by all means e-mail the owners of the work you like and see what would be the process of using their work.
And lastly, have you considered drawing them yourself? Free software like Inkscape can do a lot of amazing things. Ecological drawings are also very suitable for a particular style known as isometric projection. They can be drawn using modules and you can paste a single module to form a landscape... the process is quite fun.
Even you're not planning to become a self-trained illustrator, It'd still be good to get familiar with a graphical software. Let's say if you have a background and a set of cliparts from different sources, sometimes little tricks can make them more artistically coherent. For example, unify the hue of the cliparts (e.g. turn them to sepia), passing through the same filter (e.g. through pen sketch filter), or frame them uniformly before placing them on the big forest (e.g. frame them all in circles) may help improve the quality of the art work.
Sorry, I know this is not what you're asking for. I feel that it may be nice to think outside the frame of the constraints once a while.