Tree health is affected by less than ideal causes such as non biological (abiotic), human, natural, or biological (biotic).
The 'human factor' includes habitat destruction, pollution and poor cultural practices. Planting too deeply, poor quality stock, lack of aftercare or space for roots creates unfavourable conditions for trees.
Natural abiotic factors include mineral deficiencies in the soil as well as climatic stresses. Examples are extreme temperature or moisture conditions, droughts and flooding.
Biotic factors are easily observed. These include fungi, viruses, bacteria, nematodes (parasitic worms), and wood and bark borer.