Vegetation-type is determined by temperature and precipitation. You can't un-desertify a true desert unless you irrigate. Most of these projects that "succeed"

Vegetation-type is determined by temperature and precipitation. You can't un-desertify a true desert unless you irrigate. Most of these projects that "succeed" - miraculously correlate with good rainfall experience. Most of the Sahel revegetated all by itself, when the decadal drought ended in the 90's. Re-vegetation projects can help put vegetation back when it's been removed by shitty management practises like severe over-grazing. It can also help speed revegetation if there are no nearby seed sources and dispersion speeds for those species are low. But dry-adapted vegetation seeds can usually persist for very long periods of time waiting for water.