The most likely impact of reducing low-growing plants from an area would be:
The proportion of tortoises with the ability to extend their necks a long distance would increase.
This is because, with fewer low-growing plants available for food, tortoises that can extend their necks longer would have a competitive advantage in reaching food sources that are further away. As a result, they would be more likely to survive and reproduce compared to those that can only extend their necks a short distance.