Perennial vegetables: A neglected resource for biodiversity, carbon sequestration, and nutrition

In order to be considered perennial, vegetables must provide more than one year of harvest. We exclude crops that live for several years but are killed by harvest, such as with some heart- of-palm vegetables like coconut [23]. Some PVs have edible vegetative parts. This includes leaves, shoots, and other vegetative structures like petioles and cactus cladodes. Culinary herbs are excluded based on the assump- tion that they are consumed in smaller quantities than vegetables due to strong flavors. Some species are used as culinary herbs in one region, and vegetables in another. In such cases, the crop is included in this study’s analysis. Other PVs have edible reproductive structures, including flowerbuds, flowers, unripe fruit, ripe fruit, and unripe seeds. Classification of fruits as vegetables is challenging. Many annual fruits are commonly used as vegetables, such as tomato (Solanum lycopersicum), string beans (Phaseolus vulgaris), and winter squash (Cucurbita spp.). The distinction is not botanical but rather based on how the fruit is used and how it tastes. Use also varies to some degree between cultures. This study follows [24] in distinguishing between dessert fruits which are sweet or tart, and vegetable fruits which are eaten in salads, cooked dishes, and appetizers. Dessert fruits are excluded from the study. PV fruits are used in salads, cooked in soups and stews, or other- wise a central part of a meal. For example, the fruit of chayote (Sechium edule) is widely con- sumed as a vegetable in Mesoamerican cuisines and throughout the tropics. In some cases a

fruit is a vegetable when unripe, and a dessert fruit when ripe, as in the papaya (Carica papaya). Roots crops like sweet potato (Ipomoea batatas) and starchy fruits like breadfruit (Artocar- pus altilis) are excluded on the grounds that nutritionally they are more properly seen as staple crops primarily producing carbohydrates, with lower vitamin and mineral content than other vegetables. Root crops are also incompatible with no-till perennial production systems as they require excavation for harvest [13]. Biodiversity This study aims to quantify for the first time how many PV species are in cultivation, and iden- tify their agroecological niches as to climate, shade tolerance, form and parts used. In the absence of a well-integrated literature, several approaches were used to identify and character- ize the global extent of cultivated perennial vegetable crop species. The first approach analyzed species from [25], which provides a global list of cultivated veg- etables. Though incomplete, it is international and can be used as a proxy for the world’s vege- table crop biodiversity. After the elimination of root crops (as above), the remaining species were categorized into perennial, perennial grown as annual, and annual crops. Second, we reviewed 6,000 species profiled in [26], a six-volume compendium of cultivated crops (excluding ornamentals), identifying those cultivated as vegetables. All species that are cultivated for the purpose of vegetable production, whether or not it is their primary use, were

considered for this study. Note that cultivation does not necessarily indicate domestication. The third approach was a search for cultivated PVs in global listings of crops and vegetables, as well as regional and specialty resources. The resources used in developing this listing were [4, 5, 13, 26–53]. These crops were classified by part used, form, cultivation status, and adapt- ability to climate, moisture, and shade. This information is important in assessing the PV spe- cies available to fit various agroecological niches. All cultivated PVs identified by the study were characterized by climate suitability and shade tolerance. Thermal climate analysis divided the crops into tropical lowlands (USDA har- diness zones 10–11, elevation under 1500m), tropical highlands (USDA hardiness zones 10– 11, elevation over 1500m), subtropical (USDA zone 9), warm temperate (USDA zones 7–8), PLOS ONE Perennial vegetables: A neglected resource for biodiversity, carbon sequestration, and nutrition PLOS ONE | https://doi.org/10.1371/journal.pone.0234611 July 10, 2020 4 / 19