Even if a surge in Wood magic leads to a relatively fast eruption of forest or jungle where before there was no dense vegetation, this could have some disastrous effects for the species and people who had adapted to its previous situation. When a large contiguous area of land changes its biome type, it will tend to be a different kind of habitat loss for whatever lived there before, even as expanding tree cover can deny needed light for underbrush, etc.
For example, if there are large migratory herd animals adapted to a certain path that gets overgrown with forest, the herd may not follow its old path, which might be a huge problem for them, or if not for them, for whatever was used to them appearing where they used to appear. Displaced predators and prey may roam to new areas, disrupting farms and other species, or grazing away plants needed to prevent erosion elsewhere.
Plants also have large effects on landscape erosion and possibly river water, possibly causing diversion of rivers away from cities which depended on them, and leading to flooding elsewhere, or subtler effects such as reduced erosion leading to less minerals in water leading to affected adapted plants downstream, and so possible mysterious crop failures as conditions change.
Bird migrations may also be greatly affected.
Those are all just relatively subtle effects of biome changes. If the Wood magic can cause mutations or even the appearance new or old species somehow, depending on what the nature of this magic is, you might have the appearance of great beasts or "monsters". There might also be great witherings of existing plants, or a great eruption of mold and fungus spores making practically all food impossible to preserve. There might also be high levels of effective toxins, hallucinogens, or magic in all the plants (or even all living matter) in the area, disrupting, changing, or wiping out pre-existing life in the area, as well as creating a lot of potential spell and potion ingredients.
Perhaps there could be a perverse interaction between berserk Wood and Fire ley lines, where fast eruptions of flammable plants were followed by sudden fires, leading to an awful series of large-scale landscape fires. Perhaps an Air ley line might do something awful with all the resulting smoke, as well. There could also be an awful mixture somewhere of crazy swamp life producing lots of methane, directed by Air magic to wash over and choke out air-breathers somewhere, and/or be suddenly ignited by fire, leading to huge methane explosions far away from the magic swamp that generated the methane.
Greatly increased fertility in any species can have awful effects - locusts, mosquitoes, giant wasps... even on Earth there are some terrifying giant wasps - if boosted to great numbers and aggression by magic - yikes! Water creatures might also suddenly be a great danger if hit by life magic - perhaps plagues of piranha, aggressive sharks, giant kraken, or super-fast-growing giant barnacles eating through ship hulls.
