Learn Swedish: Vi diskuterar byggandet av parken och bevarandet av träden. - We are discussing the building of the park and the preserving of the trees.

They are verbal nouns, formed by adding -ande to the verb stems (bygg- and bevar-). This turns the verbs into neuter (ett) nouns. The final -t is the definite article, making them mean 'the building' and 'the preserving' rather than just the general concepts of 'building' and 'preserving'.

Why do we use av parken and av träden instead of the Swedish s-genitive (like parkens byggande)?

This uses the objective genitive. The park and the trees are the objects receiving the action, not the owners doing the action. While parkens byggande is grammatically possible, using av (of) is much clearer and more common when expressing that a noun is being acted upon.

Are byggandet and bevarandet the same as present participles?

They share the same base form (-ande), but here they function entirely as nouns. You can tell because they take the definite noun suffix -t. A present participle functioning as an adjective or adverb in Swedish never takes a definite suffix like this.

Could we just use the infinitive here, like Vi diskuterar att bygga parken?

Yes, using att bygga and att bevara is completely natural and often preferred in everyday speech. Using the -ande nouns makes the sentence sound a bit more formal and treats the building and preserving as abstract concepts or official projects.