Xylogenesis in the early life stages of maritime pine

Trees change throughout their life stages, and although age-dependent changes are reported in the literature, the early life stages of trees are often excluded from these studies. The sapling/pole life stage corresponds to the establishment phase of a tree in the forest. To understand how wood formation changes in the early growing stages we compared xylogenesis in saplings/pole (10-15 years old) and young trees (50-55 years old). Trees were selected from two adjacent areas in different regeneration phases from a maritime pine (Pinus pinaster Aiton) plantation located on the west coast of Portugal. The cambial and differentiating xylem cells were monitored from March 2015 to March 2017. The climatic conditions in these years were contrasting: 2015 was hot and dry and 2016 wet. Xylogenesis started around the same time in both age-classes but ended later in young trees in both years. Despite the shorter duration, sapling/pole trees formed more tracheids than young trees in both years, presenting an intensive growth strategy. Tracheids in young trees did not present differences between years, but sapling/pole trees presented fewer tracheids with a higher latewood proportion and smaller lumen area in the driest year (2015). Our results show that sapling/pole trees have an intensive growth strategy, whereas older trees (50-55 years) present a conservative growth strategy with a longer growing season. If the frequency and intensity of droughts increases, sapling/pole trees will reduce growth immediately, whereas older trees will reduce productivity in the following years. Thus, it is expected that forest productivity will decrease under a scenario of increasing drought.