With the historical drought in California causing the death of something like 102 Million trees in the Sierra Nevada, the topic of tree mortality is hotter than ever. It is usually easy to see if a tree is standing dead or alive. However, understanding the physiological mechanisms that lead to tree mortality and predicting mortality patterns over space and time remains a challenge. Read more what a large pool of researchers recently found using to tree ring records.

Why and how a tree dies?

There are many possible elements that can cause the death of a tree. If we want to simplify, there are two main factors that may cause tree death: (i) intense or long-lasting stress (e.g., drought, shade, low temperature) and (ii) natural disturbances (e.g., wind, pathogen outbreak, fire). The first strongly affect tree vigor, and often this can be detected in the growth pattern of the tree for years – or decades – before mortality. The second, lead to a sudden death of the tree. This is hard – if not impossible – to identify in the growth pattern prior death, as it can occur irrespective of the vitality and growth of the tree (e.g., an unexpected disturbance such as a wind storm). In both cases, mortality can be caused by either biotic (e.g., competition, pathogens) or abiotic factors (e.g., volcanic eruptions, ice storm, fire, flooding, etc.). Sometimes is very difficult to identify the precise cause, as most of tree deaths originate from a complex of these factors that occur simultaneously or consecutively in time. In those cases where death is preceded by a decline in tree vigor, tree rings have been successfully used to predict mortality probability.

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A group of researchers led by Maxime Cailleret at ETH Zürich (Switzerland), compiled a large tree-ring width database, which include both living and dead trees conjointly growing at 190 sites located across several continents. Their synthesis study was the result of a large collaborative effort done in the context of the COST Action STReESS (Studying Tree Responses to extreme Events: a SynthesiS). The project aimed of making use of dendro-based data from individual trees (i.e., dendrochronology, wood anatomy and ecophysiology) to investigate effects of extreme events such as drought, heat waves, or late frost on tree performance.
In the present study, the scientists compared the temporal change in radial growth rates of trees that died and others that survived stress events. They focused on mortality induced by stress, thus identifiable in the tree-ring chronology, and differentiated the different causes of mortality into drought, biotic agents, drought + biotic and others.

Maxime Cailleret said about their results published in the journal Global Change Biology:

“We found that tree rings can be used to successfully predict stress-induced mortality events as most of them were preceded by long-term decrease in radial growth rates. However, in case of intense drought or bark beetle outbreak, growth may either decline abruptly or may even increase before death highlighting the need for considering other ecophysiological data (e.g. xylem traits)”

Growth patterns before mortality

Results showed that in more than 80% of the cases, growth of the dying trees the year before death was lower than surviving ones, and that this growth reduction averaged 60%. In most of the cases, the duration of the decline period ranged between 5 and 50 years, confirming that trees can survive a long time with low growth, accumulating stress until a fatal lack of resources. However, in some cases (bit less than 20%) trees died after a quick growth decline (<5 years). This may be the case when trees experienced intense stress within a short window of few years (e.g., similar case as for the latest intense drought in the Sierra Nevada).

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Outcomes of this study also highlighted that growth patterns prior mortality vary among sources of mortality and among tree species. The most apparent difference is between gymnosperms and angiosperms. The first usually showed long-term and slow growth reductions before death, while the second rather died after a fast decline. This because gymnosperms live slow but long, and are usually able to survive a long period with low growth but longer legacy effects (lower resilience) and lower growth rate in good conditions. Differently, angiosperms live fast and die fast. They typically show higher growth potential, they have more reserves (parenchyma cells), higher resilience but usually die abruptly. Similar results were found if species were grouped according to stress-tolerance classes. As expected, drought and shade-tolerant species exhibited a longer and stronger reduction in growth before death than intolerant ones.

Read also: What can tree rings tell us about Earth’s past?

This synthesis study clearly confirms that changes in growth levels before death provides useful insides to disentangle mechanisms triggering tree mortality. Forest ecologist and modellers should take advantages of these growth data to better implement mortality functions that can be integrated in forest scenario models. It is worth to note that this study was made possible thanks to the availability of many scientists around the world to share their tree-ring datasets. Thus, keep coring and measuring trees but be open to share your data. Only this way science can advance!

Source: Cailleret, M., Jansen, S., Robert, E. M. R., Desoto, L., Aakala, T., Antos, J. A., Beikircher, B., Bigler, C., Bugmann, H., Caccianiga, M., Čada, V., Camarero, J. J., Cherubini, P., Cochard, H., Coyea, M. R., Čufar, K., Das, A. J., Davi, H., Delzon, S., Dorman, M., Gea-Izquierdo, G., Gillner, S., Haavik, L. J., Hartmann, H., Hereş, A.-M., Hultine, K. R., Janda, P., Kane, J. M., Kharuk, V. I., Kitzberger, T., Klein, T., Kramer, K., Lens, F., Levanic, T., Linares Calderon, J. C., Lloret, F., Lobo-Do-Vale, R., Lombardi, F., López Rodríguez, R., Mäkinen, H., Mayr, S., Mészáros, I., Metsaranta, J. M., Minunno, F., Oberhuber, W., Papadopoulos, A., Peltoniemi, M., Petritan, A. M., Rohner, B., Sangüesa-Barreda, G., Sarris, D., Smith, J. M., Stan, A. B., Sterck, F., Stojanović, D. B., Suarez, M. L., Svoboda, M., Tognetti, R., Torres-Ruiz, J. M., Trotsiuk, V., Villalba, R., Vodde, F., Westwood, A. R., Wyckoff, P. H., Zafirov, N. and Martínez-Vilalta, J. (2016), A synthesis of radial growth patterns preceding tree mortality. Global Change Biology. doi: 10.1111/gcb.13535

Author of the post:

Marco Mina – Forest ecologist. PostDoc at the Swiss Federal Research Institute WSL. PhD in Forest Ecology at ETH Zürich, after graduating in Forestry and Environmental Science from Padua University. The focus of his research is on better understanding the impacts of climate change on forests and ecosystem services using process-based, statistical models and forest inventory data. He is also interested in dendroecology, silviculture and wood biomass production. Passionate traveller, nordic skier and homebrewer.

Main photo credit: Rafal Chudy