Tree herders

14 de October 2017

Some giant trees, such as cedars and redwoods, are an example of great longevity and their populations depend much more on tendencies than on specific traumatic episodes. Climate change and human pressures can put their survival at risk.

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For Enric.

In the Jaffar mountain, in the High Atlas, monumental cedars form the skyline, showing their pruned branches, while a reduced canopy of leaves allows the tree to survive (see Picture 1). They are centennial trees, which have been periodically pruned by shepherds to provide food for their flocks. Transhumant shepherds climb up the trunk —we can see the notches that help them ascend— and drop the cut branches to the ground to supplement the sparse grass and the holm oaks that form a stratum at a great distance from the cedar canopy. Quite a large number of trees die as a result of this pruning regime, combined with a drought that has lasted for decades. For species of long-lived trees, such as cedars, death is not easy, unless they suffer a trauma —avalanches, lightnings, windstorms, wildfires— or the attack of some insect or pathogen. How much longer could these cedars have lived if they had not been systematically cut down for centuries? Although the grazing activity is very old, it has probably reached its peak since about 1200 years, when the Arab tribes arrived to the area.

In the Inyo Mountains of California inland, under a cold and desert climate grow the oldest trees in the world. They are specimens of Pinus longaeva (Great Basin Bristlecone pine), some of which have been dated 4,900 years ago from their growth rings (see Picture 2). Scientists attribute such longevity to the absence of fires due to lack of fuel and the low winter temperatures that limit the development of pests. More to the west, in the Sierra Nevada, giant redwoods grow, some of them 3,000 years old (see Picture 3). We humans have needed about 500 generations to travel from the last glaciation to our days. The sequoias have only seen 5 replacing generations since the ice covered the mountains.

Sierra redwood forests are a heritage that we want to preserve. That is why managers have monitored and studied them for long time. They know that new seedlings emerge from fallen pine cones after they are opened by to the heat of surface fires, which are not very intense, and also clear the soil of litter. But what does matter an annual event of recruitment of new seedlings in a species that lives millennia? Human life is much shorter than redwood one; so, we find it difficult to put ourselves in their place. We want to understand the life of these trees according with our cadences of years or decades, but that is surely inappropriate. Perhaps forestry scientists that study these species should look at larger temporal scales of centuries, such as paleoecologists do when analyzing pollen or sedimentary records. This involves extending the intervals at which the averages are obtained or focusing on particular moments in which events are accelerated.

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Picture 1. Forest of Cedrus atlantica with Holm-oak from High Atlas, subjected to intense cutting of their branches to provide forage for sheep and goat herds. Credit: Francisco Lloret

We can ask ourselves how these trees can be worried by climate change when a single individual has experienced numerous climatic fluctuations throughout his life. Maybe the current climatic trend is just one more or a long history of fluctuations, or maybe the current change is developing too fast for what they’re used to. To address this question, we must remember that the fate of populations is determined by three phases: the establishment of plants, their growth and their death. In short-lived species, the annual rates of these phases are important in determining the decline, persistence or increase of their populations. But in long-lived species the changes from one year to the next are less relevant and we must look at trends.

For example, in 2014 there was massive defoliation of Californian redwoods as a result of unprecedented drought. In other species, the episode would have probably caused significant tree mortality, particularly if it had coincided with other aggressive agents —as was the case of Pinus edulis in New Mexico and Arizona in 2001-2007 when drought was combined with infection by bark beetles. But in this case few redwoods died and many other trees later recovered. Therefore, to see relevant changes in populations of these long—lived species these traumatic episodes should be repeated over time.

This is what happened to the Atlas cedars when they experienced continuous pruning throughout their lives. In fact, as climate models point out, climate change is already exposing these long-lived species to chronic extreme drought episodes. It is also important to see what happens at key moments, such as after a fire when new individuals are established. North American scientists are finding that in recent decades the establishment of new coniferous plants after fires in the Rocky Mountains of North America is declining, most likely due to increased aridity associated with climate change. This example also illustrates that the combination of different extreme situations, such as fires and drought, affecting key demographic processes such as mortality and establishmen can lead to rapid changes —that is, a loss of resilience— that would be more difficult otherwise.

Fotografía 2. Ejemplares de Pinus longaeva del oeste de Norteamérica con miles de años de edad

Picture 2. Pinus longaeva trees from western North America thousands of years old. Credit: Francisco Lloret

But below the sequoias that stand out towards the sky, we find other species of trees and shrubs of smaller size and with a less longevity. Their growth is conditioned by the resources that the giants leave them. However, the dynamics of their populations —death, recruitment— seem to be governed by rhythms different from that of redwoods. It looks like the redwoods and the vegetation with which they coexist are decoupled, each at its own pace. Intuitively we can understand that the persistence of populations will occur if in the approximate time of a generation the deaths are compensated by the establishment of new plants that grow until they reach adults. Obviously, from this simple model, things can be quite complicated and the variability between species is very large. In any case, this coexistence of species with different demographic rates shows that there is no single ecological solution to the same environmental conditions. It also indicates that the longevity of species is a way of adapting to the temporal changes of the environment. As you survive, you expect better times to come. Animals can move around looking for better conditions; plants, which can barely scan a few meters around, play with their demographic cycle —also called life cycle.

Perhaps the best known case of demographic typology is that proposed by MacArthur and Wilson in 1967, completed by Pianka in 1970. These authors described a variety of possibilities, ranging from species in which natural selection would have favored high population growth rates associated to a high reproductive effort (selection by r, corresponding to the maximum rate of population growth), to species with low growth rates, but able to accumulate size and resources over a long life (selection by K, corresponding to the population carrying capacity, which would indicate high densities of individuals, exploiting to the maximum the available resources).

We are amazed by the longevity of the trees. But we see that biological solutions to the challenges of the environment are multiple and often coexist —it is often more useful to think why a species is not in a place than to think the reasons why it is there. This multiplicity of solutions allows to maintain the functioning of the ecosystem, which by nature is changing. It also helps to redo that functionality after a deep alteration, as described by the concept of resilience. Thus, what is lost with the sudden death of long-lived species may be soon recovered thanks to fast-growing species.

Fotografia 3. Bosc de sequoies (Sequoiodendron giganteum) de Califòrnia dominat per gegants mil·lenaris.

Picture 3. Sierra Redwood Forests (Sequoiodendron giganteum) with a canopy dominated by millennial giants. Credit: Francisco Lloret

Some ecologists think that it is not very relevant what species are in an ecosystem to explain their balances of water, energy or nutrients. The important thing would be the species’ way of working, not his identity. They may be right, but species often have unique combinations of functional features that cannot be separated. Species that apparently perform a similar function play different roles in another function. This may be important for ecosystem resilience, because recovery of the different functionalities may depend on the species involved, and how their demographic rates —such as mortality and settlement— are compensated over time.

Lindenmayer y Laurance han puesto recientemente en valor a estos gigantes —ellos los llaman ‘Large Old Trees‘, LOT—, señalando las dificultades de conservarlos debido a sus bajas densidades y a sus lentos ritmos vitales. Para eso, deberíamos aprender de los “ents”, pastores de árboles, de la obra de Tolkien, que supieron adaptarse a esos ritmos y así persistir con ellos.

Lindenmayer and Laurance have recently recognized the value to conserve these giants —they call them ‘Large Old Trees‘, LOT—, pointing out the difficulties due to their low densities and their slow, vital rhythms. For that, we should learn from the “ents“, tree herders, from Tolkien‘s writtings, who knew how to adapt to those rhythms and thus persist with them.

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Francisco Lloret
Professor d’Ecologia de la UAB. Investigador del CREAF en temes d’ecologia de la vegetació i canvi global. President de l’Asociación Española de Ecología Terrestre.
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