Final answer:
The primary impact on deforestation absent of aerial ash fallout is largely on local climate changes and environmental degradation such as altered rainfall patterns and biodiversity loss, primarily due to human activities like agriculture and urban expansion.
Step-by-step explanation:
The primary impact of no aerial ash fallout on the degree of deforestation is related to the changes that deforestation itself induces in the climate and the environment. Without the ash fallout, the major deforesting factors remain human activities such as farming, cattle ranching, and urbanization which cause alterations in rainfall patterns, loss of biodiversity, and soil erosion, among other effects. For instance, the clearing of forests, as seen for oil palm plantations in Borneo, not only reduces the number of trees but also impacts agricultural productivity due to changes in local weather patterns, like rainfall consistency and temperature variability.