Chestnut musings

Whilst driving along a road in the province of Laguna, fellow field botanists and I spotted a lone fruiting Castanopsis. Only one Castanopsis is known from Luzon, C. philipensis, and what we had before us wasn't that species. My suspicion was that it belonged to C. psilophylla, a species of Bornean origin but listed for the Philippines, apparently in error.* In 2016, it was found on Mindanao, and on August 4, 2023, we found the first tree from the island of Luzon. The identity of the tree has since been confirmed by Pieter Pelser from the University of Canterbury.

For the benefit of the uninitiated, the genus Castanopsis differs from Castanea only by very minor suite of characters. Castanea is the genus where the chestnut belongs. Some species of Castanopsis bear seeds of comparable sizes to the chestnut, and these are also consumed in much the same way as the chestnuts of commerce. But ruminating on the tree, I am much more interested with its distribution. You see, Castanopsis do bear hundreds of seeds. But the seeds do not disperse well. They are large and heavy and therefore cannot be carried by winds very far, even the cyclonic ones. They do roll, and a tree growing on a ravine or ridge can have its seeds rolling down and settling somewhere where they can form new populations. However, the tree we found was on flat land with no higher point anywhere near it. It isn't something like there are trees higher up and sending its seeds down below. Also, these seeds often have low germination rates which might explain the tremendous energy expenditures a tree spends: the more seeds, the higher the likelihood of an enough number of recruits. Think of how an orchid is capable of manufacturing thousands to perhaps even millions of seeds yet the planet isn't covered with them because the success rate of its seeds settling where conditions are right is very low. Like orchids (at least during the early stages of most species), all members of the Fagaceae, or the oak family, are in symbiosis with mycorrhizae. The seeds, for reasons unknown to me, routinely attract fungi, some of which aren't exactly friendly. A perfectly viable seed can end up rotting from saprophytic fungi just days after it has fallen to the ground; one seed I attempted for germination was invaded by fungi less than two weeks after sowing. Additionally, the seeds are rich in oils and are quite nutritious; along with other genera and species in the Fagaceae, chestnuts and acorns form a prominent bulk in the diet of herbivorous mammals. Locals in many parts of the Philippines have claimed that the acorns of the stone oaks (Lithocarpus spp.) are relished by both deer and wild pigs, and it would not be far-fetched to believe that they do also have a taste for chestnuts. The fruits don't float whether in freshwater or seawater and acorns found on the beach near river mouths are barren (Van Steenis, 1940). The paltry germination rate, (out of about 200 seeds attempted three months ago, about 12 have only germinated, so far), high food value of the seeds both for fungi and vertebrates, and its incapacity to disperse in a short time now begs the question: why the Borneo-Mindanao-Luzon distribution? How did these trees manage to travel and form populations that are very far from one another? Birds are known dispersers of seeds, and when they ingest fruits, they deposit their droppings and its load of seeds quite a distance away from where they consumed it. But Castanopsis fruits are spiny. There are no birds large enough not to be bothered by the projections. Indeed, it makes much more sense to visualize that seed-eaters, notably hornbills and parrots, visit trees that are in fruit and consume the seeds right then and there. They don't fly away with the seeds and inadvertently drop them as they swoop through the trees. They perch on branches and eat the seeds there through mastication, effectively destroying the embryo inside. Preest (1963) in his study on Nothofagus- also a member of the Fagaceae- of New Zealand found out that the maximum dispersal distance of the trees was 2-3 km, and that long-distance dispersal by wind, birds, and ice bergs were not possible. In New Guinea, Kalkman and Vink (1970) found that dispersal is confined only to the stands. Not surprisingly, members of the Fagaceae have the inclination to occur in dense stands over large areas, but due to the very slow dispersion, such tracts would be very, very old indeed. Full stop. Wrap your mind around that, if you can. And then go back to the question of why the Borneo-Mindanao-Luzon distribution. The mind demands that this chestnut should be found only on just one island, but it does not. Nature revolts against that thought. Perhaps C. psilophylla first appeared on Luzon and then traveled south to Mindanao and then to Borneo, or it's the other way around. When did they start moving away from their origin? How? And why are there no existing records of this species on Palawan and Mindoro which are wedged in between Borneo and Luzon? Did the trees leap from Borneo to Luzon without passing through Palawan and Mindoro? Why are there no records in the Visayas? It just doesn't make sense that it exists in both Luzon and Mindanao but not on the intervening islands. Then again, maybe the trees are present there but no one has found them yet. Another possibility is that the populations have since died off whether from a changing climate or rising sea levels, leaving the disjunct populations that we know today. Whatever the answers may be, the extremely slow dispersal capabilities of these trees and the fact that it occurs across considerable inter-island distances likely points to a dispersal event that first took place thousands upon thousands of years ago.

Scarab larvae after emerging from the seeds. Note the holes. Such seeds are noticeably lighter than ones that are viable.

Some words for the reforesters and the self-proclaimed 'experts'

Maybe I know you as Camille or Fernando, but that does not necessarily mean that I know who you are. Those may not even be your name. My point here is that I personally do not believe that there is anyone out there, whether since or in the future, who really knows plants. Most people tend to think that just because someone knows the plant names then he is an 'expert', an authoritative figure. Or maybe one is, if we limit our criteria only to plant names. But plant names are just the beginning and only serves to provide a point of reference so that everybody knows what someone is talking about. True knowledge, in my view, is having the answers to questions such as how a plant arrived in one specific place and not nowhere else, how it has survived to this day where its other populations have perished elsewhere, what pollinates it and how does it attract its pollinators, how does it spread. Formulate your own theories, but train yourself how to discern the plausible to the outlandish so you do not waste your time thinking about ludicrous ideas and making an idiot out of yourself for making these public. And perhaps equally important is to possess enough intellectual humility to admit that you are wrong. The sooner you do, the sooner you can start over again. 

Also, you may opt not to believe it, but plants do evolve. I am not going into the reasons why I believe that, but if there are two separate populations of the same species and the climatic and/or soil conditions are not similar and/or there is a different set of pollinators in the second location, then environmental pressures will compel the plants in that population to change. Given enough time, the plants in that second population may appear slightly different from their ancestral stock, and in a few thousand years they may be both morphologically and genetically distinct enough to be considered a separate species. Two populations of the same species separated by vast distances may look the same, but one or both may already be diverging and heading towards another identity. We just are not capable of perceiving it because of the limitations of our senses and our very short lifespans. This is why real conservation measures, undertaken by reputable botanical institutions, repopulate and boost plant numbers in situ using materials taken from the area they are working on. If you take planting materials from another location, even if from the same species you are working on, then you risk contaminating the local population by the introduction of a possibly distinct ecotype**. A Pinus merkusii from China may be ecotypically distinct from a P. merkusii from Sumatra, and a P. merkusii from Zambales may not be exactly the same as a P. merkusii from Mindoro. So, if your noble intentions dictate to your conscience that a badly deforested area needs your intervention, then please, consider that. You can disrupt the evolutionary trajectory of a population if you do it just for the sake of planting trees. 

*The Philippine record was based on the specimen Ebalo 408 which was said to be from Banguey Island off Palawan. However, there is no island of that name near Palawan or indeed anywhere in the Philippines, but there is one near Sabah.

**An ecotype is a subpopulation whose individuals exhibit traits that are more or less consistently different, and this is often the result of environmental pressures. The differences may either be genetic or morphologic, or both. Genetically distinct populations are always candidates for full species recognition, though to the untrained eyes, these may appear no different from already known species.