If this pattern really is the product of natural selection, then several implicit predictions immediately emerge, and these predictions have never been tested. First, natural selection acts on variation within populations, so we would expect that variation in the vessel diameter-stem length relationship should be observable. What is more, this variation should impact performance. Plants with vessels that are “too wide” for their trunk height should be vulnerable to embolism in their vessels. Plants with vessels that are “too narrow” for their height should suffer because of low conduction and low productivity. Goldilocks style, there should be a sweet spot in the middle where vulnerability and productivity just balance out, and this is where the majority of plants should lie. Finally, to be the target of natural selection, this variation with peformance consequences must be heritable.
Because most wood plants grow so slowly, these aspects, especially heritability, are hard to test. Thanks to moringa’s spectacular growth rate, UNAM PhD student Alberto Echeverría can use the tree to test all of these predictions. He planted a very genetically diverse selection of moringa seeds a month ago, and the biggest trees are already chest high. In a couple of months, once they get to about 3 meters, Alberto will be able to start harvesting them and trying to detect that variation in vessel-plant size proportionality that is the entire basis of his project. Good luck Alberto!