@twinkletoesimnot growing vegan food takes 93% land than animal farming, so I am sure there will be more than enough space for trees and food growth.
Re fertiliser, quoted from here:
www.quora.com/How-would-vegan-farms-fertilise-their-crops-Would-this-be-possible-on-a-worldwide-scale
"There are a few Vegan friendly techniques ready to fertilize crops, that are already practiced worldwide. Growing polycultures with nitrogen-fixing crops in the mix is a good idea for so many reasons, it just happens to be vegan as well. An example of this system is well known - corn (not the sweet stuff, but the kind for storage and making tortillas) grown with dry beans (the nitrogen fixer, it helps feed the crop and uses corn as a scaffold) grown with squash (has big leaves which act as weed suppressant). This system is called the 'Three sisters', and was practiced by native Americans centuries ago. It is still relevant today. In fact, any polyculture will have nitrogen fixers to feed the crops. Black locusts grown with chestnuts, clovers grown between rows of grapes, and alfalfa with tomatoes are all interesting examples of how to feed your crops, using other crops.
The second technique, and far more widely practiced, is the use of cover crops to build organic matter and fertility in the soil. Tilling results in huge losses of rodents and other animals that Vegans are fond of not killing, so I think a no-till cover crop approach is best for this situation. There are very few plant deficiencies that will arise if organic matter is high. Earthworms and countless microbes as well as mycorrhizae will feed the plants just fine as they break down residues from previous crops. How do you keep from killing little animals? Run your dairy cows in your cover crop when it is time to kill tge crop and plant new. They will eat 50% of the above ground plant matter, push over and trample the rest, and little mice will escape, as they always do. Of course, you need an extremely high density of cows to make this work; anywhere from 500,000 to 1million pounds per acre in small paddocks on rotation. All that urine and manure will cycle nutrients around your field, as well, so no need to spread manure or fertilizer with a tractor. You don't have to drink milk to see that cows who live in this manner are humanely treated, and love working on the farm fertilizing the crops for your navy bean hummus and naan."
"https://www.quora.com/How-would-vegan-farms-fertilise-their-crops-Would-this-be-possible-on-a-worldwide-scale