Interview whit prof. Jacek Oleksyn (PAS Institute of Dendrology in Kórnik)
Academia: As an ecologist, how do you view aging in nature? After all, plants aren’t generally the first organisms that come to mind when we talk about old age.
Prof. Jacek Oleksyn: I look at aging through the prism of my own work. I study trees, which are long-lived, so the element of age is ubiquitous. I should add that plants and animals age differently, in that in the former individual organs age, die, and are replaced, while the latter can’t survive such a process. As such, aging in perennial plants is regarded as a more complex process than aging in animals, which have very few tissues capable of developmental plasticity. While a tree can live for hundreds of years, the leaves of many species survive for just one season and are then discarded. Before a plant drops its leaves, an entire sequence of events must take place, including reclaiming nutrients found in the leaves. I have always been fascinated by these issues, since they combine biological and economic concepts. Economic theories tend to be more reliable when it comes to trees and other living organisms than in the economy per se, since plants – unlike businesspeople – aren’t guided by emotions.
When trees discard their leaves, does that mean that they have been fully used up?
That’s right. We can imagine each tree as a kind of McDonald’s franchise, and branches carrying leaves as individual restaurants. The corporation provides each one with branding and company equipment, but every outlet keeps its own accounts, and if it goes bankrupt individually, it doesn’t bring down the company as a whole. Going back to trees, if one branch is permanently situated in the shade and its balance of photosynthesis products is negative, it will not receive assistance from the rest of the organism, because this would contradict economic principles. There’s no room for philanthropy – if part of a tree becomes insufficiently productive, it must be eliminated for the good of the whole plant. To survive, it must pay its “dues” back to the organism – i.e. non-structural carbohydrates – and if it doesn’t, there is no more rationale for its survival. Like companies, plants exist in an ever-changing environment, in which both short- and long-term planning is necessary. For example, plants no longer produce roots in those parts of the soil where the investment in their formation exceeds the potential gain they could provide as water and minerals. As such, it’s like an enterprise putting economic theories in practice every step of the way.
From a physiological perspective, aging of leaves is a process coordinated on several levels, from the molecular and cellular to physiological and biochemical. The process ends with the organ’s death. Aging and shedding of leaves can also be induced by stress factors. One such trigger is drought; shedding leaves allows plants to maintain the correct water balance. We could say this follows the principle of “dying to let others live.”
What is aging like in trees that don’t shed their leaves in winter?
The differences aren’t major. We should first consider the pros and cons of shedding leaves every year. Keeping leaves throughout the winter enables a tree to get going quicker in spring. Meanwhile, plants that do shed their leaves must accumulate capital – the non-structural sugars I mentioned earlier – which will be used in spring as fuel for making new leaves. In those plants, these “solar batteries” only start working after a while, once the young leaves are around half their full size.
So a tree must invest in production first, before it can reap the benefits?
Yes. I am currently studying trees in major transcontinental transects, focusing on the Scots pine. Beyond the Arctic circle, its growing season lasts just over three months, while here in Poland conditions supporting photosynthesis last around double that.
So different environments affect how long individual trees and their organs survive, even in the same species?
That’s what drew me to the subject in the first place. While I was living in the US, I worked at a few universities. Over there, online resources have made libraries almost redundant – they stand largely empty, and mainly serve as rest stops for students during exams. Today’s electronic databases include articles published in the last forty years, while earlier periodicals exist only as printed copies. The building which housed my office had a library on the ground floor, and its collection included vast volumes of forestry journals published since the start of the 19th century. When I had nothing better to do, I’d head to the library and look at different magazines – so I would have a Russian day, a Swedish day, and so on. And it was during my day of reading Swedish journals that I came across a paper published in 1915; the author was commissioned by the Swedish Academy of Sciences to investigate whether two distinct pine ecotypes – northern and southern – existed in the country. He wrote to forest keepers across Sweden asking them to send him sample branches of felled trees. Lumberjacks packed them into bags and posted them to the author’s address, and since it was winter, they didn’t have to worry about them drying out. The author analyzed the results following 19th-century conventions, so his article spanned 100 pages and featured scores of tables and graphs. Alongside each sample he also noted the name of the nearest village with a church, which meant I could determine their geographical coordinates. His data included the age of the pine needles, established on the basis of annual growth, clearly visible on pine branches. I fed the data into a computer, and used it in a few studies. I also conducted a meta-analysis of the literature, and compared the results with contemporary data. I discovered that pine needles now survive for far shorter periods than they did just a century ago. To avoid any accusations of methodological differences, I obtained a grant for conducting another collection of samples from forests near the villages listed in the original paper, and we confirmed a dramatic decrease in the survival of pine needles.
What reasons did you find for this?
We considered two hypotheses. The first involved the influx of nitrogen pollutants from Eastern Europe – these compounds can cover great distances. We rejected this theory, though, since in the entire zone of northern boreal forests there is just 3 kg nitrogen deposition from local and transboundary sources per hectare, so not nearly enough to have such an effect.
The second theory involves changing climate. We discovered that Sweden has records of all climate data from the last 150 years, showing that springtime has shifted considerably during that period. What does this mean? Well, the growth of pines experiencing cold weather is regulated by the photoperiod. They can’t rely on temperature, because it’s an unreliable factor – a warm week during spring can easily be followed by frost. However, evergreens have no way to halt photosynthesis processes once the temperature is sufficiently high and their roots have access to water. And so when spring starts earlier, the vegetation period is shifted and extended, providing a good explanation for the reduced lifespan of pine needles. It is also likely that an increased concentration of carbon dioxide in the atmosphere has an effect by increasing the efficacy of photosynthesis. All this means that trees can get by without the oldest, less productive needle cohorts. Before they shed them in the autumn, they extract minerals and sugars from them, which they use during the next vegetation period to support growth processes.
This second theory is confirmed through provenance studies, dating back to the crisis of cultivated forests at the turn of the 20th century, when seeds obtained from France and Germany were planted too far north. At the time, no one realized that the source of seeds affects how the trees they grow into adapt to new conditions. Seedlings originating from seeds imported from warmer regions initially grew well in Scandinavia, until the arrival of devastatingly cold winters, when just a few trees survived. In order to explain this, in the early 20th century provenance studies were set up, in which seedlings grown from seeds originating from different locations were planted in a single location in order to establish the effect of transporting seeds over long distances. We now know that pine trees grown in Poland from seedlings from different regions of the species’ range maintain their needles as long as local trees, since the duration is not regulated by genetic factors but by climate. This means we better understand the effect of long-term environmental changes on trees.
So far, we have been discussing the aging of individual organs. How do trees age as complete organisms? They do die eventually, even though they are long-lived.
We usually talk about three types of aging in plants: chronological, describing the period between germination and plant death; physiological, describing loss of vitality culminating with the death of individual organs or the whole plant; and ontogenetic, describing a genetically-driven process of developmental phases from the embryo to the mature plant. Trees often live for very long periods of time. For example, Pinus longaeva earned its Latin name for its famous longevity.
There are also clonal plants, which propagate via offshoots; one example is an aspen tree in Colorado dated at over ten thousand years old. Clonal organisms share the same genome; they reborn as a series of “individuals”, providing them with a kind of genetic immortality.
But we should remember that the main form of reproduction is generative. Trees use a range of different strategies; for example, Pinus banksiana is fire-adapted, with the cones remaining closed until the high temperature of a forest fire triggers their opening. They can remain attached to their branches and closed for over twenty years, until the right moment comes for releasing seeds. Poplars use a different tactic, with individual trees producing up to 25 million seeds.
Aging of trees as a whole is not unlike Aesop’s fable about the tortoise and the hare. Certain of victory, the hare races ahead, but he doesn’t cross the finish line immediately so he can do so smugly once the tortoise finally gets there. He falls asleep as he waits; in the meantime the tortoise arrives, and crosses the line first. The tale shows that victory can be achieved in different ways – fast, like the hare, or slowly, doggedly, like the tortoise. Plants are a good example of such behavior – the finish line is always successful reproduction, in order to pass on genetic material. And so it doesn’t matter whether a tree is gnarled or straight, scruffy or elegant – all that matters is whether it produces progeny.
It can develop slowly to grow big and solid, or live fast and die young. Trees following the tortoise’s example include the previously mentioned Pinus longaeva from Colorado. They grow at high altitudes, where they have little to no competition; their growth is slow, but they can live for over five thousand years. The “hares” of the tree world are poplars and willows – they invest far more in fast growth, development and production of seeds, but they live for short periods. Returning to our economic analogy, it’s like comparing someone who saves slowly, a penny at a time, with a broker trading high bids and staking everything on a single bet.
What are the symptoms of aging in trees? After all, it takes a long time for a tree to die and turn into a pile of rotting wood.
Generative processes, such as seed production, halt before the tree’s death. As the distance between the trunk and the finest roots supplying the organism with minerals and water increases, nutrients become more scarce, which means the tree doesn’t have sufficient carbohydrates to support the functioning of all of its organs. The process can take several years. Many trees employ special tricks to delay death. They can maintain their buds in a dormant condition; shoots formed by such epicormic buds have all the characteristics of a juvenile organism, but from a genetic and functional perspective, their age differs from that of the rest of the tree.
The famous ancient oaks in Poland’s Rogalin National Park exhibit this behavior. When I first came to Poznań in 1976, the trees were still green, but their reproduction was severely diminished. Today they are in an even poorer condition; however, at our phytotron we are growing seedlings germinated from dormant buds collected from the trees. They would be unlikely to survive in the wild, but the Institute of Dendrology wanted to give them a second chance, partly as a way of supporting these symbols of longevity and constancy.
We have been hearing recently about the role played by dead trees, their “afterlife.”
The cascades leading to an organism’s death are predictable processes; we can say what the general timeframe is for individual species on the basis of information we have about their longevity. Animals and plants have a role to play in the environment throughout their lives and after their death. In mountainous regions, tree stands need a base of old, rotten wood to be replenished, since there is a chronic shortage of organic matter. This is due to generally high levels of rainfall; additionally, the sparse soil makes it difficult for seedlings to take root. Dead logs can take decades to decompose, and since different sections decay at different rates, they form a sort of trough which gradually becomes a habitat for other plants and fungi. Dead wood also plays an important role in the circulation of carbon in the ecosystem.
So by collecting timber, we are also removing valuable matter…
We are shortening the time during which the carbon is bound up in organic matter. In fact, this is a significant political and economic issue, since we are obliged to keep tabs on carbon quotas. However, the problem is that the carbon balance is never closed, because of other processes in the ecosystem we know little about. After a tree is cut down, its trunk and main branches are taken away, but the fate of the roots is largely unknown: how long do they remain in the soil? How long do they take to decompose? Does the process occur in the same way just under the surface as at a depth of several meters?
There are around 4 billion hectares of forests around the globe, with just over 9 million in Poland. It is generally assumed that approx. 20% of forest biomass is found underground, but we don’t know whether its decomposition follows the same processes in rainforests as in taiga. And our recent meta-analysis indicates that this figure isn’t even very reliable. In rainforests, roots generally don’t exceed 15% of the total biomass, while in northern regions this can reach 35%. What would be useful is a reliable, systematic study; however, since the roots of just a single tree can weigh several tons, measurements would require heavy equipment, and as such it would be difficult to secure funding for such a project.
We started with small elements, organs, moving on to trees. What about aging of entire ecosystems, forests as a whole?
This is driven by a number of external factors such as wind, drought, human activity, and so on. Different species tolerate these conditions to different degrees. If a heliophytic plant takes root in a beech wood, where the canopies shade the forest floor, it won’t be able to develop. In turn, a tree stand mainly comprising shade-loving species is able to eliminate competition until such time when it starts to age and decompose. Conditions under which beech or lime trees have an advantage are fairly common – one example is the northern part of the Niepołomice Forest near Kraków. If trees in such a woodland were seeded at a similar time, they are also likely to start dying at the same time. This process also affects clonal organisms, such as clumps of willows. Finally, trees in natural forests tend to die slowly and gradually, while commercial forests are managed by lumberjacks who have a rather different approach. Interview by Agnieszka Kloch Photos Jakub Ostałowski
Interview by Agnieszka Kloch
© Academia nr 3 (43) 2014