The warning came from John Oliver, HSE's south east area director, speaking at a tree work safety day organised in Kent.
Said Mr Oliver: 'The HSE is becoming concerned at the number of serious accidents in the forestry and allied industry sector.
'In the past three years 21 people have died as a result of this activity. The main risks are from falling trees, operators falling from trees and severe injuries from machines such as chainsaws.
Mr Oliver said that the actual number of persons earning a living in tree work was unknown. 'But what is known is that some of these people have to work alone deep and out of sight in woodland and in all weathers.
'The main dangers they face are being crushed by trees during felling work; severe injury resulting from chainsaw cuts and vehicles overturning trapping drivers,' he commented.
He spoke about the work of tree surgeons. 'There are too many cowboys in the south east pretending to be tree surgeons.
Nearly all of these take no precautions whatsoever. They are a risk not only to themselves but also to others assisting them as well as to their customers. The HSE recommends that only qualified tree surgeons are engaged as contractors.
He added: 'The HSE is concerned about the welfare of people who earn their living in forests or woodland. Our surveys reveal that some operators are going nearly deaf because of high noise levels from the machines.
'Some operators have the condition known as vibration white finger which is caused by over vibrating components of machinery, particularly the chainsaw.
'Inspections often reveal the use of dangerous machines such as circular saws with safety guards missing.
'Tractors are often used with the safety cab or frame missing and the driver is at high risk if the tractor should overturn.
'During this winter I have instructed agricultural inspectors working in Kent, Surrey and Sussex to crackdown on 'cowboys' and on employers who fail to meet basic health and safety requirements for their employees.'