Passages taken from “A Botanical History of the Islands of Macaronesia” by Prof. Sir Mackenzie Fernsby, Fellow of the Royal Society. Published 1961.
Beyond the borders of the misty laurel forests, the next most common ecosystem on the Papagaios islands is grassland. If an area cannot support a forest due to lower rainfall, altitude, or poor soil, a grassland will likely be found. Many are species rich, being home to dozens of species of grass and sedge, as well as shrubs and bushes where conditions allow. Despite some endemic plant species, the Papagaian grasslands are in most respects the same as those found in Western Europe or North Africa. There is however one nominal grassland that is unique to the mountain foothills on the island of Milagres.
At first glance it looks like any other grassland: gently rolling slopes covered in numerous grasses and sedges striving for survival. Yet on closer inspection, one begins to notice unusual details. While there are many species growing here, there are nowhere near as many as in the islands’ other grasslands. The climate too is off. It’s not as wet as the densest areas of laurel forest, but the rainfall and humidity are high enough that one would expect to see patches of trees and thickets of woody plants.
There are trees here: tall, twisted and ancient, but also widely spaced from their neighbours, almost eerily so. They are not random in placement either, instead appearing a regular distance apart, almost to the point of being mathematically predictable. On closer inspection it can be noticed that the trees are all of the same species (walnut to a keen eye) and around the base of each is a perfect ring of bare soil with not a shoot growing. These are the walnut meadows, a biome like no other on this Earth.
The clue to what is happening in this strange meadow likes with the circle of barren soil around the tree’s roots. If a sample of the soil were analysed in a laboratory, the results would indicate the presence of high levels of juglone, also known as 5-hydroxy-1,4-naphthalenedione. That is a level of technical knowledge beyond this writer, but suffice it to say juglone is a chemical which occurs in the tissues of walnuts and related species and is toxic to most other plants.
The habit of plants to produce toxins to inhibit the growth of rival plants is known as allelopathy and has been known since ancient times. Pliny the Elder wrote “The shadow of walnut trees is poison to all plants”. How big a role allelopathy plays in benefiting the plant versus other methods of competition such as more efficient resource extraction is generally unknown. In the case of the Papagaian walnut (Juglans territorialis) however, it is undoubtedly the main factor in its success.
The Papagaian walnut produces juglone in all its tissues, as do all walnuts, but at far higher levels, the roots in particular releasing vast volumes into the soil. The tree’s drip zone – the region directly below the branches - bears the highest concentration, hence dead area around the tree, but the chemical’s effects are felt much further afield, turning almost all soil in the meadow toxic, to some degree. This results in a biome shaped entirely by the walnuts and their aggressive herbicide.
Given the ability of J. territorialis to suppress other plants, trees in particular, one might expect to find forests of walnuts, instead of sparsely-spread individuals. Indeed, walnuts of other species grow in the midst of forests. Once again, it is the trees’ toxicity that is to blame. While the juglone is most concentrated within an individual’s drip zone, when trees grow closer together the cumulative toxin levels are too great for even the walnuts themselves to survive. And so, they live in isolation from others of their kind.
They are not alone, however. A number of grasses have evolved to be resistant to juglone, at least to the reduced levels beyond the drip zones. The most common species are members of the Mithridatum genus, named for the ancient king who sipped poison to build his immunity.
Mithridatum species are not merely taking advantage of the walnut trees’ inability to grow more densely, but benefit the trees by fixing nitrogen in the soil. Their shorter life cycle compared to the tree also aids in keeping the soil rich in nutrients. The arrangement is beneficial to both parties. The trees gain resources, while the grasses thrive in the toxic soil free of rival species that might otherwise outcompete them. Some Mithridatum species take the juglone into their own tissues, making them in turn toxic to herbivores.
Not all relationships in the walnut meadows are mutualistic. Look closely at an individual tree and you will spot leaves that are…unusual. Their shape, size and colour is a little off; they are also growing in small clusters directly from the middle of older, thicker branches, not from the budding ends of newer shoots. They are Brachiflorus venenamans, a parasitic species related to ivy.
The Brachiflorus genus is endemic to Papagaios and can species be found throughout the forests. B. venenamans however, is a specialist, adapted to tolerate the high toxicity of the Papagaian walnut. As with all Brachiflorus, it releases enzymes from its roots to break down the wood of its host. It can then send roots into the dissolved branch to gain purchase, as well as access to its phloem and xylem to steal resources.
The only time B. venenamans draws attention to itself is during the flowering season. Over a few weeks, the plant grows a long vine with a flower at its end. Once the flower is mature, the vine drops down, hanging above the dead soil at its host’s roots. Doing so allows greater access to the flower which otherwise would be hidden between the thicker branches of the tree, away from pollinators. It is at this time that one realises just how many parasites a single tree may have, with some displaying up to fifty vines.
The walnut meadow was once far more widespread than it is today. Early colonists felled many trees to make way for farmland, though found to their dismay that the soil remained toxic to crops for years, sometimes decades. In time they turned to clearing forest instead, leaving the walnut meadows as untamed islands amidst an agricultural ocean. Today, they are a protected area, and attempts are being made to encourage new trees to grow, as previous generations, now hundreds of years old, fall victim to storms and old age.
So far, these efforts have been thwarted by the walnut’s own toxins. Clearly the wild trees propagated themselves, but how, mankind has yet to understand.