Modelling reveals how fire and water shape the life strategies of Pinus halepensis

Is it better to grow fast, or to grow slowly and invest in the future? Structural Equation Modeling reveals that Pinus halepensis cannot decide.

If you travel around the Mediterranean, then you’re likely to run into Pinus halepensis, the Aleppo Pine. The shade can come as a relief if you’re feeling the heat of the sun. This heat stress isn’t something that the tree itself can avoid. So what is the best way to grow in a stressful environment? A new paper by Filippo Santini and colleagues uses Structural Equation Modeling to find why that the pines try two life strategies. One is to grow quickly to take advantage of opportunities. The other trees grow slowly, investing more in reproduction and reserves.

Aleppo Pine
Aleppo Pine. Photo: Canva.

Filippo Santini said that this paper represents the final output of wider work on genetic variation in phenotypic traits among populations of P. halepensis. “Several studies have been carried out since the early 2000s in the study site considered in our paper. These studies provided the data that we used for building our model and summarize patterns of adaptation in this species.”

One of the problems the authors have tackled is how the Aleppo Pine responds to both fire and drought. The challenge for any plant facing the threat of fire and drought is that the traits needed to succeed might be in opposition to each other.

Santini said that the Aleppo pine is particularly suitable for studying adaptation in Mediterranean forest species. “Pines are very plastic species that can grow under very contrasting environmental conditions, and Aleppo pine does not represent an exception. Across the Mediterranean basin, Aleppo pine can be found from semi-arid conditions (i.e. less than 400 mm of rain per year; Iberian peninsula and North Africa) to much more humid environments (up to 1000 mm of precipitation per year in Greece). This huge variability in growing condition across the species range makes P. halepensis an ideal model to study different adaptations to contrasting environmental conditions.”

“In the case of Mediterranean low-land pine species such as P. halepensis and P. pinaster, the main stresses are represented by summer water deficit and frequent forest fires. To cope with these stresses, these species have developed strategies which differs between and, as we show in this study, within species in relations to the disturbance intensities.”

Santini believes that the pines are likely to come under increasing stress in the coming years as the climate changes. “Despite the adaptation developed by these pines to Mediterranean conditions, all the models indicate that, due to the human-induced climate change, Mediterranean forests are likely to expect an increase in frequency and intensities of drought events in the near future. This changes will probably be much faster than the capacity of forest species to adapt to new environmental conditions. Indeed, drought-induced decline of Mediterranean pine forests has been already observed in several areas of the Iberian Peninsula.”

Santini said that while their work did not have a specific focus on conservation, it did provide a deep insight into the stresses that Aleppo pine populations face across the species range and into the adaptive strategies developed to cope with these stresses. This information will be necessary to predict the fate of Aleppo pine forests under climate change.

One problem that the authors think is important is the speed of response by the pines. “Indeed, environmental changes induced by human activities occur in a time scale that is much shorter than the time required to forest species to adapt to new environmental conditions. Several models predict that environmental modifications generated by climate change will result in the shift of species ranges and in local extinction.”

“As a solution, the possibilities of performing assisted-migration of forest species is being debated (among others) in the scientific community. Assisted migration consists in the artificial introduction in a determined area of species or populations which show adaptations to the predicted future conditions for that area in the context of climate change. However, this (and other) conservation procedures require prior knowledge on the adaptive strategies of forest species and population that is still largely lacking.”

The reason the work is possible now is partly due to increasingly accessible technology. The team gathered the data for this paper using drones, and Santini sees plenty of opportunity for more work. “More studies like the one that we carried out are needed to characterize the adaptive potential of Mediterranean forest species to the on-going climate change, to forecast the future performance of these species and to implement the correct conservation measures. There is an urgent need of detailed information on integrating adaptive responses of forest populations to environmental constraints, and we believe that our model could be applied in future research aiming at filling the knowledge gaps still existing for many species with fundamental ecological roles.”

Further reading

García de la Serrana, R., Vilagrosa, A., & Alloza, J. A. (2015). Pine mortality in southeast Spain after an extreme dry and warm year: interactions among drought stress, carbohydrates and bark beetle attack. Trees, 29(6), 1791–1804. https://doi.org/10.1007/s00468-015-1261-9

Hernández-Serrano, A., Verdú, M., Santos-del-Blanco, L., Climent, J., González-Martínez, S. C., & Pausas, J. G. (2014). Heritability and quantitative genetic divergence of serotiny, a fire-persistence plant trait. Annals of Botany, 114(3), 571–577. https://doi.org/10.1093/aob/mcu142

Resco de Dios, V., Arteaga, C., Hedo, J., Gil-Pelegrín, E., & Voltas, J. (2018). A trade-off between embolism resistance and bark thickness in conifers: are drought and fire adaptations antagonistic? Plant Ecology & Diversity, 11(3), 253–258. https://doi.org/10.1080/17550874.2018.1504238

Santini, F., Climent, J. M., & Voltas, J. (2019). Phenotypic integration and life history strategies among populations of Pinus halepensis: an insight through Structural Equation Modeling. Annals of Botany. https://doi.org/10.1093/aob/mcz088

Santini, F., Kefauver, S. C., Resco de Dios, V., Araus, J. L., & Voltas, J. (2019). Using unmanned aerial vehicle-based multispectral, RGB and thermal imagery for phenotyping of forest genetic trials: A case study in Pinus halepensis. Annals of Applied Biology, 174(2), 262–276. https://doi.org/10.1111/aab.12484

Voltas, J., Lucabaugh, D., Chambel, M. R., & Ferrio, J. P. (2015). Intraspecific variation in the use of water sources by the circum-Mediterranean coniferPinus halepensis. New Phytologist, 208(4), 1031–1041. https://doi.org/10.1111/nph.13569