They have conquered the treetops of the tropical rainforest in a fascinating way: epiphytes, so-called epiphytes. The Oldenburg ecologist Gerhard Zotz describes how they withstand the extreme solar radiation - and why their prospects are nevertheless not good.
Rainforests and the destruction of tropical forests: these are issues that the media have brought to the attention of the German public over the years. The repeated description of what we are losing in terms of biological treasures with the ten million hectares of tropical forest that are cut down every year has meant that every layperson now knows about the abundance of species in these ecosystems. It is an abundance of species that is fascinating: one hectare of Amazonian lowland forest contains more tree species than the whole of Europe, and a single giant jungle harbours more ant species than the whole of Great Britain.
In addition to the enormous abundance of species, there is a whole range of plant growth forms in the tropics that are rare or even completely absent in our latitudes. These include tree-shaped ferns, tree-spreading figs, climbing lianas and so-called epiphytes, literally translated as epiphytes. This last group of plants in particular is extremely species-rich: epiphytes account for almost ten per cent of all flowering plant species worldwide, including more than ten thousand orchids, thousands of ferns and more than a thousand pineapple plants.
Regular trips to the tropics
The current species record impressively documents this local diversity: more than 100 such epiphyte species have been recorded on a single tree in a mountain rainforest in Costa Rica. In my working group "Functional Ecology of Plants", we are investigating the group of vascular epiphytes in particular. We regularly travel to the tropics for field research, especially to Panama, where the research stations of the Smithsonian Tropical Research Institute offer ideal conditions for scientists.
Our studies cover a very broad spectrum: from comparative physiological analyses and documentation of the dynamics of entire epiphyte communities to decades of observation using a construction crane set up in the rainforest. The possibility of collecting and exporting plants on site also allows detailed physiological experiments to be carried out in the Oldenburg climate chambers. These are experiments that can, for example, provide answers to the question of how well epiphytes will cope with the predicted environmental conditions of the year 2100 - when temperatures and CO2 levels will be much higher.
Although Central Europeans tend to think of tropical forests as permanently humid, the epiphytes growing in the treetops are paradoxically more at risk of drought stress. As they grow directly on the bark or in moss cushions without "soil" in the treetops, even less than a day without rain at temperatures of around 30 degrees Celsius means an enormous water shortage.
A tank bromeliad can store up to 20 litres of water
To cope with this drought, the plants have developed a variety of adaptations. In addition to thick, fleshy leaves, these include the formation of a cistern, which is often called a tank. The tank bromeliads form their own "flowerpot", so to speak, through overlapping leaf bases, in which water and soil are collected, allowing them to easily survive for several days without rain. In extreme cases, a single tank can store up to 20 litres of water. These tanks are veritable wet biotopes in the treetops and in turn serve as a habitat for many animals. The most spectacular are certainly the tree climbing frogs, which lay their tadpoles in these small "pools". But you can also find many other animals that you wouldn't necessarily expect to find in a treetop, from dragonfly larvae and woodlice to crustaceans and earthworms.
Special photosynthesis pathway of epiphytes
However, the described tanks only occur - with a few exceptions - in bromeliads; they are therefore by no means typical of the many other epiphytes. Some of the epiphytes that do not have a tank have internal water reservoirs. Many orchids, for example, have so-called pseudobulbs, i.e. thickened stems, which, like the external reservoirs of many bromeliads, can compensate for the unreliable supply of water.
However, at least as important as efficient uptake and storage is the economical use of water as a resource. In extreme cases, some species simply shed their leaves temporarily, similar to many plant species that we know from areas with long periods of drought, such as the countries around the Mediterranean. This is particularly useful when forests have a regular season with little rainfall, as is the case in many Central American countries.
After all, the majority of epiphytes utilise a special photosynthetic pathway, the Crassulacean acid metabolism (CAM). This water-saving photosynthetic pathway was originally described for succulent plants of semi-deserts, for columnar cacti, opuntias, agaves or aloes - which is why the first reports on the occurrence of CAM in tropical epiphytes in the 1960s were still dismissed as exceptional observations. In the meantime, however, CAM has been found so frequently in epiphytes that it seems as if the majority of all CAM species worldwide grow epiphytically on trees.
Our research has shown that we probably need to replace our previous image of the "typical" CAM plant, a terrestrial cactus, with that of a thick-leaved orchid growing on a tropical jungle giant. So what's the trick with CAM plants? Normal plants have to keep their leaf pores, the "stomata", wide open during the day in order to absorb the carbon dioxide (CO2) in the air, which is converted into sugar using solar energy in the process of photosynthesis.
"Starve" or "die of thirst"? How CAM plants solve the dilemma
This process is fundamental to plant life and therefore also the basis of all other life on our planet. However, opening the leaf pores also has a price: the plants inevitably lose a lot of water. Of course, water loss could be avoided if the leaf pores remained closed. But then the plants would not be able to feed themselves. So the unattractive alternatives are "starvation" or "dying of thirst".
CAM plants have now elegantly solved this dilemma for plants in locations with little rainfall. They open their pores at night, when much less water is lost due to the higher humidity, store CO2 in the form of an organic acid and then use this store during the day to carry out photosynthesis as normal despite the closed pores. This reduces water loss by a factor of 10 to 100 compared to normal plants - one of many adaptations to drought that alone justifies the description of the "desert in the rainforest".
Climate change: weather extremes on the rise even in the furthest corners of the earth
So what future do these plants, which are adapted to regular drought, have in a world increasingly dominated by humans? Despite the ongoing destruction of tropical forests, we can hope that extensive forest areas can be preserved in the future. But do these also provide a suitable habitat? Unfortunately, this is by no means certain, as climate change means that weather extremes are increasing even in the remotest corners of the earth, for example.
For species that are less well adapted to drought, an exceptional drought like the one observed in the Amazon region in 2010 could spell the end. However, no one knows whether this has already happened, as there is a lack of long-term studies. In view of the large-scale conversion of primary forests into agricultural land, scientists are increasingly asking themselves what is happening to the flora and fauna in the secondary vegetation, i.e. in spontaneously emerging new forests, tree plantations or individual trees that are (still) found in large numbers on the pastures of Latin America. Loose secondary forests and, even more pronounced, solitary trees offer much drier growing conditions for epiphytes than natural forests.
A large part of the fauna living in tree crowns depends on epiphytes
A series of studies by my working group and colleagues from all over the world have now shown that the epiphyte communities there differ significantly from those in undisturbed forests. For example, the results of a Diplom thesis carried out a few years ago in the lowlands of Panama show that species that occur in rather moist areas of the forest largely fail, while species that normally grow in exposed areas sometimes even develop better than in the natural habitat. This applies in particular to species with CAM. Overall, however, there is a sometimes drastic reduction in species richness. It is not unlikely that this will worsen over time.
Whether this is really the case is currently being investigated in Oldenburg in a dissertation funded by the DAAD. In view of the fact that a significant proportion of the fauna living in the treetops depends on these epiphytes, the current loss alone should lead to a cascade of corresponding losses in the animal world. Not good prospects for the inhabitants of the "desert in the rainforest".
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The author
Prof Dr Gerhard Zotz has been a university lecturer in "Functional Ecology" in Oldenburg since 2006. He studied biology in Würzburg, where he gained his doctorate in 1993. After research stays at the Smithsonian Tropical Research Institute in Panama and at the State University of Vermont, Burlington, USA, he completed his habilitation in Würzburg in 1998. Zotz is currently summarising the results of his more than 20 years of research on epiphytes in a large monograph.