Pollution and Disease
Pollution is the release of chemical, physical, biological or radioactive contaminants to the environment. Because it is sometimes not visible to the naked eye, and it disperses through the mediums it is emitted into (usually air or water), its direct effects of can sometimes be hard to spot. This is especially true for plants.
Pollution is a big problem for plants. For example, Pesticides are designed to allow farmers to increase their yields (and gardeners to keep out weeds), but their misuse and over-application can adversely affect any wild plants in the area. Pesticides are also entering our water supplies, which are then contaminated. The misuse of insecticides also affects plants, as it can impact the populations of pollinators (such as bees) that some plants need to reproduce. In addition, light pollution is reputed to affect the navigational ability of pollinators such as moths.
Just as increased acidity in water threatens corals, in soil this causes shifts in the species which are best adapted to tolerate the situation. This can occur by atmospheric deposition, leaked chemicals, or acid rain. For example, in some areas deciduous forest may be replaced by coniferous forest. Soils can also be contaminated with heavy metals, which few plants can tolerate (although a few plants are so tolerant to certain pollutions, they are used to clean up polluted areas, a technique called 'phytoremediation'). At the very least contamination will cause reduced plant production and ecosystems may be more vulnerbale to other threats (whilst in agricultural land, reduced crop yields).