Climate effects on growth differ according to height and diameter along the stem in Pinus pinaster Ait.

Climate-growth relationships in forest trees are increasingly the focus of research aimed at understanding and assessing responses to climate change. Many studies have been confined to annual radial growth at breast height as an easy-to-measure dendrological standard variable, although its validity as a proxy for overall annual growth patterns in trees has scarcely been addressed. In this study, we test this hypothesis by exploring additional information on climate-growth relationships as well as analyzing both the radial growth at different stem heights and the height increment. For this purpose, past annual radial growth and shoot elongation were measured in 10 dominant Pinus pinaster Ait. trees in a 130-year-old stand. Radial increments were measured on disks taken from five different trunk heights up to 15 meters. Height increments were obtained by measuring the distance between consecutive branch whorls, which appear as knots after sawing a longitudinal section of the stem. The relationships between climate and both radial growth and height increment were analyzed through Pearson's correlations and the response to extreme climatic episodes was analyzed using resilience indices. Results revealed that the climatic variables affecting growth were different for height and stem diameter. Additionally, in the case of stem diameter, the climatic variables affecting growth also depended on the height at which the sample was taken. Precipitation prior to bud break, both in the year in which the studied shoot elongation takes place and in the previous year, has a positive effect on height increment. Radial growth in the upper part of the stem was mainly influenced by spring temperatures and precipitation, whereas in the case of basal radial growth it was the autumn and winter temperatures and precipitation of the previous year to growth which had the greatest influence. Similarly, severe droughts cause greater decline in height increment, while the decline in radial growth of the upper part of stem is smaller than that of radial growth at breast height. In conclusion, the analysis of height increment and upper radial growth provides important information to complement the dendroclimatology data for radial growth at breast height, thus improving our understanding of the impact of climate change on tree growth.