Age-dependent variations of radial growth, intra-annual density fluctuations and pointer years in Pinus pinaster under Mediterranean climate

Abstract

Dendrochronology generally assumes that climate-growth relationships are age independent once the biological growth trend has been removed. However, tree physiology changes with age, namely photosynthetic capacity and hydraulic conductivity. Additionally, in a scenario involving climate change it is important to verify if the relationship between tree growth and climate is also changing. We tested whether the radial growth response to climate and the frequency of intra-annual density fluctuations (IADFs) and pointer years (PY) of Pinus pinaster Ait. varied with age. It was also investigated whether the climatic factors triggering IADFs formation were stable over time. Trees were sampled in Pinhal de Leiria (Portugal), and were divided in two age-classes: young (<65 years-old) and old (>115 year-old). Earlywood and tree-ring width of young P. pinaster trees were more sensitive to climate influence while the response of latewood width to climate was stronger in old trees. Young trees start the growing season earlier, thus a time window delay occurs between young and old trees during which wood cells of young trees integrate environmental signals. Intra-annual density fluctuations (IADFs) in tree-rings are produced as a response to variations in climatic conditions during the growing season, particularly water availability. Young trees usually have a longer growing season and respond faster to climate conditions, thus young P. pinaster trees presented a higher frequency of IADFs compared with old trees. Most of the IADFs were located in latewood and were positively correlated to autumn precipitation. The radial-growth response of P. pinaster to climate and the IADFs frequency were age dependent. The use of trees with different age to create a tree ring chronology for climate studies can increase the resolution of climatic signals. Age-dependent responses to climate can also give important clues to predict how young and old trees react to climate change. If IADFs frequency was higher in young P. pinaster, how was the frequency of IADFs when old trees were young? The frequency of IADFs in old P. pinaster trees was analyzed from 1900 to 2006. The high frequency of latewood IADFs during the beginning of the 20th century was probably age related, since IADFs are more prone to occur in younger and wider tree-rings. From the 40s to the 70s IADFs frequency decreased in parallel with a general increase of the tree-ring width. From 1970 onward, it was observed an increase of IADFs frequency and a decrease of tree-ring width. The increase of IADFs frequency in narrower rings suggests that climate conditions favoring IADFs formation were becoming more frequent. Latewood IADFs were positively correlated with September to December precipitation and with November and December temperature, but this correlation changed over time. In the last 30 years, the precipitation in October and the temperatures of November and December increased, partly explaining the higher frequency of IADFs since the 70s. Our results suggest that the frequency of IADFs and the change of their frequency over time can help us understand how trees are being affected by climatic changes. The frequency of extreme climatic events is increasing in the last 50 years. These climatic extremes are recorded in tree-rings width, with the formation of wide and narrow tree-rings, named pointer years (PY). In P. pinaster narrow PY were formed in exceptionally dry years and wide PY in years of abundant precipitation. Narrow PY were more frequent than the wide ones and young trees presented more PY than old trees, showing that young trees are more sensitive to drought conditions. Additionally, most of the narrow PY occurred after the 70s, confirming that the frequency of extreme climatic events is increasing in the last decades. The climatic signal extracted from the PY chronologies was stronger for specific months suggesting that these chronologies are suited for climatic reconstructions. In conclusion, there are age-dependent responses to climate in P. pinaster trees growing in the Mediterranean region. It is also important to notice that climate changes in the Mediterranean are already being recorded in tree-rings. The study of trees with different age, IADFs and PY frequencies can bring additional climatic information to tree-ring chronologies, important to study past climate conditions.