By 40-60 years after farm fields had been abandoned, the amount of herbaceous plants decreased. Give one reason why the amount of herbaceous plants decreased as the amount of trees increased.

Explanation:

Herbaceous plant are plants that lack woody stem, they include mostly annual crop and perennial crop.

The reduction in the population of herbaceous plant over the years is due to competition. The competition intensity is usually high due to limited supply of resource and the woody plant out compete the herbaceous plants leading to a gradual decline in the population.

Competition intensity is usually high during dry season when herbaceous biomass and soil water content were at the lowest level at this time the herbaceous plant finds it difficult to thrive well with the limited resource while the woody plant utilizes the little resource and keeps on thriving. All this reduce herbaceous population after several years of the field been abandoned.

to this answer it would be edge enhancement