More than a third of all the critically endangered plant species on Earth grow from a type of seed that can’t be dried, meaning we can’t preserve them in seed vaults as an insurance policy against extinction. Unless we can find other preservation techniques, once these plants—which include oak trees, mango trees, and horse chestnut trees—are gone, they’re gone, according to a new study published Friday in Nature Plants.

“Conventional seed banking is not suitable for all seed plants, with some species having recalcitrant seeds unable to survive the drying process and therefore incapable of being frozen,” the paper, written by scientists at the Royal Botanical Gardens, Kew, reads.

Recalcitrant seeds need to keep their water content in order to grow. Think of an avocado: if you keep a fresh avocado pit and root it in a container of water, it can still grow. But if you dry it out in your cupboard for a few weeks, it’s basically dead.

Though it doesn’t always get as much attention as the threats facing animals, plant diversity across the globe is also facing declines, with species being lost at an unprecedented rate. In an effort to combat the rapid loss of so many plants, conservationists set goals back in 2002 for things they could achieve to help preserve more plants. One of these goals was to preserve 75 percent of threatened plant species ex situ (as in, somewhere other than their native habitat) by 2020.

This works well for most of our crop plants, but for wild plants, particularly in tropical regions, it’s more common for species to have recalcitrant seeds that can’t be dried, and so can’t be kept in a seed vault. In addition to the critical species that can’t be preserved in this way, researchers found that 35 percent of vulnerable species, 27 percent of endangered species and 33 percent of all tree species produce recalcitrant seeds.

There are already other ex situ methods of conservation, but many of these techniques are too expensive or are only able to preserve seeds for a short period of time. As such, the researchers urged for more rapid development of new techniques so that we don’t risk losing more than a third of our most at-risk species. And they emphasized the need to protect the living species’ habitats while we still have them.