Intro

Wood is produced without any trees planted/cut. Trees in range are engaged into this process. So trees count determines the efficiency of all forester functions.

Forester functions

1. Produces wood and seedlings. First tier requires water for production, second tier needs liquid fertilizer.
2. Plants new trees with some low chance only while producing wood and only if seedlings are available in the output inventory. No wood production -> no growth. No seedlings -> nothing to plant, so inserters must have simple logics set to keep a few dozens of seedlings.
3. Cuts pollution poisoned trees. The more tree is hurt the faster it will be cut. Unpowered forester doesn't cut trees. What a surprise - factorio generates trees at game start with some chance of poisoning them, so even if one didn't massively produce pollution, a few trees will be poisoned and have lowered pollution absorption.
4. Decreases pollution.

Greenhouse and trees manual planting

This building generates saplings out of seedlings and other components.
Resulting saplings can be planted anywhere, but will turn into trees only if planted on fertile tiles. Saplings require some space for tree growth. And rarely they die without a reason.

Trees growth around forester

Only live trees are counted for forester efficiency, not dead or dry.
When upgrading to second tier it is worth to get some sapplings sometimes to quickly enlarge forest mass.
Seedling-provided growth happens with some low probability, which is dependent on trees number (not only live ones) - the peak is about 60%-70% of max efficiency values, thus dry trees nearby are good at start, when total trees number is small.

If several foresters are near, then each tree in range can be engaged with only one of the them. A very dense forest can be used by 2-3 buildings with good efficiency.