Fish and Wildlife Service (FWS) affects a few elements of the natural world.

Fish will reproduce: as long as at least two fish are near each other (but are not overcrowded), they may reproduce. The overcrowding penalty is based roughly on vanilla fish generation settings.

Trees will regrow. Initial map conditions are considered to be the "natural equilibrium" tree density, however, so you won't see forests popping up where none were before. Trees will regrow much more aggressively in areas where a few natural forests have been thinned. However, completely deforesting an area will remove trees forever (unless replanted). Trees also won't regrow on artificial pavement.

Pollution kills trees. In vanilla, trees can become 'withered' by pollution but will remain. Under FWS, in areas of high pollution trees will actually die and be removed from the map.

Wood can be processed for a (configurable up to 100%) chance of yielding spores, which can be used to plant a new tree wherever you like. Spores will not grow on stone brick / concrete and tree growth will be slowed significantly in areas not naturally predisposed to forests.

Soil enrichment, which requires Chemical science pack (blue), enables you to create "fertile soil," a placeable tile that helps with pollution absorption and encourages tree growth. Trees growing on fertile soil will also be collected by construction bots, if available, enabling wood production to be semi-automated.

To give trees more usefulness, a wood -> charcoal -> coal production chain is also introduced. Wood is pyrolysed into charcoal in a furnace. Charcoal is converted to coal in a chemical plant after researching Advanced material processing 2 (electric furnaces). Each of the steps represents a loss of overall fuel energy (but increase in energy per stack size), but coal becomes effectively a renewable resource.

Note that when first loading a large savefile with this mod, you will likely experience a long delay as it initializes the map. I am exploring ways to speed this up.

Updates
0.1.2: removed some debug display information.
0.1.3: fixed bug on config change (like loading into an existing map), increased crowding restrictions.
0.1.4: fixed bug related to mod API syntax change
0.1.5: made tree health at update time unrelated to growth rate, FIXED MULTIPLAYER DESYNC BUG
0.1.6: should have increased mod compatibility, made growth retain graphic variation (cosmetic)
0.1.7: updated for Factorio 0.16
0.1.8: added recipe to convert unwanted spores to wood and precedence compatibility for Whistle Stop, updated to handle new "dead" trees, edited description to note pollution tree death (which has been there all along, not sure why I forgot to mention it before)
0.1.9: fixed bug with PvP scenario, tweaked growth math, increased range and fire rate of spores, added fertile soil and tree autoharvesting. There will be at least one more update with more math tweaking and possibly some graphics changes for fertile soil, but apart from that these are all the features I currently intend to include in FWS; any further updates will be for compatibility and bugfixes.
0.1.10: increased mod compatibility, removed duplicate recipe unlock, re-hid "invisible" counters, and adjusted the subgroup for fertile soil.
0.1.11: fixed bug with tree death pollution
0.2.0: updated for 0.17 compatibility. Changed equilibrium storage to avoid using dummy resources (plays nicer with other mods now, at the cost of a slightly larger savefile). Added a few settings; will add more eventually.
0.2.1: fixed crash
0.2.3: properly adjusts HR sprites