Abstract
Intra-annual density fluctuations (IADFs) are anatomical features formed in response to changes in the environmental conditions within the growing season. These anatomical features are commonly observed in Mediterranean pines, being more frequent in younger and wider tree rings. However, the process behind IADF formation is still unknown. Weekly monitoring of cambial activity and wood formation would fill this void. Although studies describing cambial activity and wood formation have become frequent, this knowledge is still fragmentary in the Mediterranean region. Here we present data from the monitoring of cambial activity and wood formation in two diameter classes of maritime pine (Pinus pinaster Ait.), over two years, in order to test: (i) whether the differences in stem diameter in an even-aged stand were due to timings and/or rates of xylogenesis; (ii) if IADFs were more common in large trees; and (iii) if their formation is triggered by cambial resumption after the summer drought. Larger trees showed higher rates of cell production and longer growing seasons, due to an earlier start and later end of xylogenesis. When a drier winter occurs, larger trees were more affected, probably limiting xylogenesis in the summer months. In both diameter classes a latewood IADF was formed in 2012 in response to late-September precipitation, confirming that the timing of the precipitation event after the summer drought is crucial in determining the resumption of cambial activity and whether or not an IADF is formed. It was the first time that the formation of a latewood IADF was monitored at a weekly time scale in maritime pine. The capacity of maritime pine to adjust cambial activity to the current environmental conditions represents a valuable strategy under the future climate change conditions.
Highlights
As trees get older and/or taller, physiological processes such as hydraulic conductivity change [1], with consequences in secondary growth
This study investigated the timings of cambial activity and xylem formation over two years (2011 and 2012) in even-aged maritime pine trees belonging to two diameter classes growing under Mediterranean climate
Cambial activity, wood and Intra-annual density fluctuations (IADFs) formation were monitored in maritime pine trees with similar age but belonging to two diameter classes, growing under Mediterranean climate
Summary
As trees get older and/or taller, physiological processes such as hydraulic conductivity change [1], with consequences in secondary growth. A study comparing xylogenesis in timberline species of different age revealed that old trees (> 250 yr.) had a shorter and delayed period of cambial activity than younger trees (< 80 yr.) [2]. Older trees were larger, so the effect of age was not completely disentagled from size. With trees of different size and social status but similar age [3]. They determined that differences in tree size were due to a higher rate of cell production in dominant trees. In maritime pine (Pinus pinaster Ait.) it was found that in trees with similar size and age the rate of cell production was responsible for the different tree-ring widths observed in the last 15 years [4]. The timings of xylogenesis were linked with the rate of cell production, with a higher rate of cell division being responsible for a later end of xylogenesis [5,6]
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