Reducing low-growing plants from an area would likely lead to a decrease in food availability for tortoises that can only extend their necks a short distance. Consequently, tortoises with the ability to extend their necks a long distance would have an advantage in reaching food that is less accessible.
Therefore, the most likely impact would be:
The proportion of tortoises with the ability to extend their necks a long distance would increase.
This response reflects the idea that those tortoises with longer necks would be better adapted to find food in a habitat with fewer low-growing plants, leading to a potential increase in their population relative to those with shorter necks.