It was during the XV century when Leonardo da Vinci (1452 — 1519) mentioned that “trees form rings annually and their thickness is determined by the conditions under which they grew”. Since then, the relationship between climate and tree growth is a matter of great interest for dendrochronologists. Most attention has been focused on the effects of local climate (temperature and/or rainfall) on forest productivity. However, we can also infer information on the influence of large-scale climatic patterns on tree growth; and whether tree species are all similarly influenced or different responses can be detected among them.
Large-scale climatic patterns —also referred as teleconnections— are changes of weather linked together between areas located far away. Depending on their origin and source, they are defined by the quantification of diverse atmospheric elements such as Sea Level Pressure or Sea Surface Temperature. El Niño (ENSO) is probably one of the most famed climatic patterns in the world for its strong incidence on both sides of the Pacific Ocean in the equatorial latitude. During El Niño events, the warmer temperatures of eastern Pacific Ocean stimulate the formation of convective clouds that lead to precipitations. Meanwhile, in Indonesia and eastern Australia, sea surface temperature turns cooler than normal; precipitation decreases, and more droughts can be experienced, especially in the east coast of Australia. The North Atlantic Oscillation (NAO) is not so famous, but it plays a key role in the winter weather of Western Europe. In this sense, rainfall increases in Iberian Peninsula when NAO turns to its negative phase, what coincides with the weakening of the Azores High that allows the arrival of wet wind from the Atlantic Ocean (Fig 1b). The opposite is true when NAO moves to a positive phase. Azores High blocks wet air streams and move to northern parts of Europe (Fig 1a). There are more large-scale climatic patterns affecting western Europe: the East-Atlantic pattern (EA) mediates the intensity and recurrence of NAO phases and affects winter temperatures; the Atlantic Multidecadal Oscillation (AMO) controls summer temperatures; and the Western Mediterranean Oscillation (WeMO) is related to summer and autumn temperatures. Hence, local weather may appear subordinated to one or various climatic teleconnections at the same time- A thorough understanding of these teleconnections might help us to answer the eternal holidays dilemma: beach or mountain.
Figure 1: positive (a) and negative (b) phases of the North Atlantic Oscillation and its climatic consequences for Europe.
In our recent research, published in the journal Forest Ecology and Management, we aimed to explore the link between large-scale climatic patterns and tree growth in the pine forests in Sierra de Gredos. This area is part of Central System range, the mountainous system that splits the principal Iberian Peninsula plateau. We chose this mountain range because represents the point where three important pine species (Pinus nigra, Pinus pinaster and Pinus sylvestris), coexist near the limit of the species’ distribution. This area covers the rear —and therefore dry— edge of the range distribution of P. sylvestris; and at the same time represents the western-edge of P. nigra, where it needs to cope with the handicap of growing under acidic soils. P. pinaster also finds one of its continental distribution ranges limits due to the occurrence of frosts and low temperatures during most days of the cold season. As such, Sierra de Gredos range supposes an interesting and constrained habitat for such three contrasting pine species that are facing global change challenges.
Figure 2: Image of the Sierra de Gredos mountain range acquired by the satellite Landsat 8 (29-03-2017). In this false-colour picture we can perceive how the snowy massif (white-blue and brown areas) separate the Iberian plateau (light green areas). Conifers are represented by the dark green areas close to the slopes, some of them covered by snow. Authors: Cambium Research Group.
We detected how two large-scale climatic patterns —NAO and EA— regulate weather conditions in the Sierra de Gredos range. As expected, negative NAO phases concurred with wet winters while positive EA phases contributed to mild or warmer winters. EA had the largest influence on tree growth for the three pine species —even for Eurasian P. sylvestris—, yet P. pinaster received the strongest benefit from warm winters and springs from positive EA phases. NAO was also a relevant driver of tree growth, but its role was secondary. NAO effects were mainly patent in the species under stronger hydric demands, that is P. sylvestris, but also on the subhumid P. nigra, while this effect was non-existent in the drought tolerant P. pinaster. These results demonstrate that coexisting pine species respond differently to large-scale patterns, with their response patterns reflecting their climatic demands.
The ongoing climate change will bring warmer and drier conditions that may challenge the hydric demands of P. nigra and P. sylvestris. This scenario may lead to changes in the composition of mountainous mixed forests, altitudinal shifts in the abundance of pines, or even local extinctions of the pines most vulnerable to drought spells.
Figure 3: In Gredos, we can find the three pine species living together. In the image, a look of one of the various sampled forests where P. pinaster and P. sylvestris coexist. Author: Héctor Hernández-Alonso.
