Phytotoxicity refers to the damage to plants caused by exposure to some toxic stressor. Phytotoxicity can be evidenced by acute injury such as obviously damaged, necrotic tissues. Chronic phytotoxicity does not present such obvious symtoms and may result in such "hidden injuries" as a decrease in productivity or a change in growth form. Plants can be poisoned by naturally occurring toxins, such as nickel in serpentine-influenced soils. Humans also release phytotoxic chemicals into the environment. Such emissions occur when, for example, gaseous and/or metal pollutants around smelters damage vegetation. Deliberate use of phytotoxic chemicals includes the use of herbicides in agriculture or forestry to decrease the abundance of unwanted plants or weeds.