A point that seems to be lost on most people who advocate for a plant based diet is that not all plants have the same environmental impact. Among plants as a class, there is a 60x range in the amount of calories provided per unit of resources required. So it matters a lot which plants you replace meat consumption with.

Generally speaking, beans, grains, and plant oils provide a lot of calories per input, whereas leafy greens and berries provide less calories per input than meat. So, if you replace your meat consumption with a bunch of kale salads and berry smoothies, your diet probably has a similar environmental impact to a diet that includes meat. But if you replace your meat consumption with beans, rice, olive oil, etc. then you're probably reducing the environmental impact of your diet.

I wrote a blog post about this:

http://www.richardjones.org/why-kale-is-actually-terrible-fo...