While Southeast Asia is rich in flora and fauna, Southeast Asia is facing severe deforestation which causes habitat loss for various endangered species such as orangutan and the Sumatran tiger. Predictions have been made that more than 40% of the animal and plant species in Southeast Asia could be wiped out in the 21st century. At the same time, haze has been a regular occurrence. The two worst regional hazes were in 1997 and 2006 in which multiple countries were covered with thick haze, mostly caused by "slash and burn" activities in Sumatra and Borneo. In reaction, several countries in Southeast Asia signed the ASEAN Agreement on Transboundary Haze Pollution to combat haze pollution.

Answer this question, if possible (if impossible, reply "unanswerable"): What caused the 19th century reginal hazes?
unanswerable