Foremost is the question of what is the truly wild native habitat of the species. This is the natural floristic assemblage at the geographical site that the plant would grow in even in the absence of human intervention. None of the sites that we had seen so far would meet this criterion. All of the trees that we had seen so far were clearly associated with human populations, and even though sometimes they were reproducing by themselves, this was always in very disturbed vegetation that was almost all invasive species, never in a more or less intact vegetation type. Therefore, it would seem unlikely that they were truly wild plants. Confusing the issue is that many people call a plant that is growing essentially untended as “wild.” As a result, we have heard many reports, and there are many in the literature as well, that Moringa oleifera grows “wild” in places where it is not wild in the sense that I mean here. The sense that I mean here, the geographical location and floristic assemblage that it grew in before humans, is important for biologists. It is important because knowing the wild range of a species tells us about its biology. Knowing where it lives and what its lifestyle is in its wild range helps us understand why the organism has the characteristics that it does. Moringa oleifera is an odd plant; it grows extremely quickly, resists virtually any drought, and flowers when just 6 months old, unique in the family. To unravel why it has these unsual features, it is necessary to figure out where the plant grows wild.
But what passes for “wild” in the dry tropics? Dry tropical habitats such as tropical dry forest, subedciduous tropical forest, thorn forest, or spiny forest, are tropical habitats, as is the tropical rain forest. However, with their good soils for growing crops, tropical dry habitats are much more inhabited than rainforests are. Whereas there are still wilderness rainforest areas, there are very few tropical dry forests of any great extension that don’t bear the mark of humans. Indian dry and subtropical dry forests are no exception. Most of them were cut down long ago to make way for towns and agricultural fields. Those patches that persist are almost without exception used constantly by people. So even in a large patch of forest, where you can’t see any people, you can usually see their mark. You can almost always find trees from which branches have been hacked off with a machete, or even the stumps of saplings that have been cut down for firewood. You can find trees whose bark has been scraped for medicine, or patties from cows left to graze in the woods. Sometimes you can find old or even recent stumps where a particularly straight tree was removed for building. You can even find small clearings or hollowed trees used as beautiful natural settings for shrines. This pervasive habitation means that it is usually impossible to rule out entirely the possible influence of people in bringing a plant to a given area.
Here are the criteria we used to identify potentially wild moringas. To be considered potentially wild, a moringa had to:
1. Grow at least 100 meters from any road, clearing, building, or other obvious human alteration.
2. Grow in a vegetation type made up entirely or mostly of wild native plants and not invasive speces or weeds. This vegetation must be at least a hectare in area and show no or only minimal anthropogenic degradation, e.g. occasional firewood collection but not extensive felling.
3. Be present in a population, not just one or two isolated trees. The population must make ecological sense, that is, have a clear and consistent habitat preference, consistent with its growth rate, growth form, and temperature and water regime tolerances.
4. Given that closely related species tend to resemble one another, and all other species of Moringa grow in lowland dry tropical habitat, putative M. oleifera habitat that is found in hot, seasonally dry lowlands is more likely wild than moister, higher, and cooler habitats.
5. Having found a population, based on the above information, it must be possible to predict where other populations should be found. This is because wild plants are usually consistently found in suitable habitat within their areas of occurrence. Plants moved around by people usually have more idiosyncratic and unpredictable distributions.
Using these criteria, we were able to rule out all of the localities we had visited so far. One locality did have us guessing for a while. We found on the floodplain of the Sutlej River in northwestern India a stand of huge M. oleifera trees—by far the biggest ones I have ever seen (see the photos below). They towered 50 feet high, not giant by tree standards but very tall for M. oleifera in dense floodplain forest. I had seen Moringa drouhardii do the same thing in Madagascar. It usually grows in dry tropical forest, but it can establish in what is known as gallery forest, the tall, semi-deciduous woods that make a green ribbon along rivers in places that otherwise turn brown in the dry season. These gallery forest M. drouhardii were growing among tall gallery forest trees and were relatively slender and the tallest I have ever seen. Standing on the cobbles of the Sutlej floodplain woods, I wondered if M. oleifera might do the same thing. Walking through the woods we came upon some very unusual species, including bald cypress from the US southeast—and then we came to the headquarters of the forestry station and met the foresters who had planted all of the unusual trees of the area, including the moringas. We visited many hillsides of degraded, eroded slopes with mesquite and Leucaena dominating the landscape. It was dry and hot, just like moringas usually like it, but no moringas in sight.