A foundation monograph of Ipomoea (Convolvulaceae) in the New World

Ipomoea species are tolerant of drought and several are recorded from desert. In South America I. incarnata is the best adapted to arid conditions occurring in the coastal deserts of Peru and the Colombian Guajira as well as the Caatinga of NE Brazil. Other indicators of very arid conditions in South America are I. nationis from Peru, I. verruculosa from Venezuela and I. sericosepala from Brazil and Bolivia. In North America, Felger et al. (2012) record some 13 species from the Sonora desert region of Mexico-Arizona, listing I. cardiophylla, I. costellata, I. cristulata and I. ternifolia as typical of this habitat. Most tree species from the Arborescens clade in both South and North America favour arid habitats but are more typical of dry deciduous forest than true desert.
Of some interest are morphological adaptations found in several species growing in dry habitats. One such occurs in the coastal lomas of Peru and the northern Atacama of Chile. Here forms of Ipomoea dumetorum, I. nil and I. purpurea occur with short, erect stems, very unlike the normal long twining stems found in other habitats. The Galapagos Islands comprise another arid habitat where there occur extreme forms of I. muricata and I. incarnata, once treated as distinct species under the names respectively of I. tubiflora and I. linearifolia. In the former the fleshy teeth of the stems are largely suppressed while the latter presents with very narrow leaves. In the Sonora Desert in Mexico, forms of I. cristuluta occur with erect, woody virgate stems, a facies very different from the normal herbaceous, twining stems. Perhaps the most remarkable is the dwarf form of the usually lowland I. platensis which grows in arid situations at over 2000 m in the Argentinian Andes. (Figure 83).
Desert merges into dry grassland, particularly in North America. Erect and, less commonly, trailing species of Ipomoea are characteristic of grassland habitats. There are relatively few examples from North America, I. leptophylla being the only widespread prairie species but several other North American species are clearly adapted to the grassland habitat, including I. longifolia and the Mexican endemic I. durangensis. However, it is in the South American cerrados that a great number of grassland species have evolved. Erect species occur in different clades and include I. hirsutissima, I. malvaeoides and I. cuneifolia and several others from Clade A1, I. argentea and I. paulistana from Clade A2 and I. squamisepala and I. pinifolia from Clade C. Trailing species are also common including I. descolei, I. psammophila and I. langsdorfii, I. burchellii, I. goyazensis and I. procumbens.
Thorn scrub merging into seasonally dry forest is another important semi-arid habitat, which is common throughout much of tropical America. Ipomoea is at its most diverse in this habitat. In South America the relatively widespread species I. amnicola, I. megapotamica, I. incarnata and I. abutiloides are good indicators of this habitat. However, each of these dry forest regions has its own set of localized species, I. argentinica, I. oranensis and I. schulziana where the chaco meets the Andes, I. brasiliana, I. longibracteolata, I. marcellia and others in NE Brazil. Ipomoea verruculosa in the dry coastal woodland of Venezuela, I. pauciflora and I. velardei in Ecuador and Peru. Dry forest species are also noted from the Caribbean Islands, I. carolina from Cuba, for example, but it is in Mexico and Central America that very large numbers are recorded as growing in dry forest, usually pine or oak woodland, either wholly deciduous or partially so. All the tree species (from both North and South America), lianas like I. bombycina and numerous other species are recorded from this habitat. The roll call of dry forest species from Mexico is long and includes such relatively common species as I. orizabensis, I. pedicellaris, I. praecana, I. seducta, I. lobata and many others.
Ipomoea species tend to avoid closed forest but occur along streams, by tracks and roads and often favour rock outcrops where the forest cover is broken. Species diversity is greatest in deciduous forest, possibly because there is more plentiful light during the dry season (McDonald 1991). This could be an explanation for why some dry forest species flower in the dry season at a time when they are leafless. This is a particular feature of the tree species in general, some Mexican species such as I. tehuantepecensis, I. pseudoracemosa, I. concolor and I. pruinosa, but of relatively few South American species wth the exception of I. juliagutierreziae and I. schulziana.
Rocks provide a specialized habitat for some species. In Mexico, cliffs or “crags” are often cited as the habitat for Ipomoea rupicola, I. chilopsidis, I. teotitlanica, I. seeania and I. concolor whereas in South America the only species cited from a similar habitat is I. killipiana. The geological composition of the cliffs is not usually recorded but volcanic rocks are mentioned for I. seeania and limestone for I. teotitlanica. Limestone, however, is often cited for plants from the Caribbean including I. montecristina, I. praecox and I. fuchsioides from Cuba, the last two characteristic of limestone towers locally known as mogotes. It is also cited for several species from Hispaniola including I. digitata and I. desrousseauxii. Ipomoea luteoviridis is recorded from serpentine outcrops in Hispaniola but we are unaware of any other American species with this habitat preference. A few species are noted from lava flows, notably I. tuboides from Hawaii, but several Mexican species are recorded on pedregales including I. orizabensis and I. dumetorum. In South America the most commonly recorded specialized rock habitat consists of granite domes and platforms, which outcrop sporadically in dry forest and cerrados on the pre-Cambrian shield. The commonest species of this habitat are I. bonariensis and I. maurandioides, but neither is restricted to granite. More restricted geographically and geologically and often very disjunct in their distribution are I. caloneura, I. chiquitensis and I. graniticola, the last being found in isolated locations in Bolivia, Brazil and Paraguay. Ipomoea leprieurii is locally frequent on granite outcrops in French Guiana and neighbouring parts of Brazil while I. marabaensis, I. scopulina and I. fasciculata are currently known only as pin-point endemics.
Ipomoea species are also frequent in secondary scrub and in disturbed places around settlements. This is the kind of habitat where the widespread pantropical species are often found. Ipomoea indica, I. nil, I. hederifolia, I. purpurea and I. cairica are rarely found far away from human habitation and I. alba, I. cairica, I. tricolor, I. indica, I. quamoclit and I. carnea subsp. fistulosa are sometimes clearly garden escapes. The same is true for many species of the Batatas clade. Ipomoea tiliacea, I. triloba, I. cordatotriloba, I. australis, I. leucantha, I. grandifolia and I. trifida are all recorded as characteristic of disturbed bushy ground and are rare in truly natural habitats.