Neighboring trees growing under identical
environmental conditions can exhibit different dynamics
and periods of growth. Despite the recent advances in
cambial biology, the exogenous and endogenous factors
generating asynchronous xylem growths still remain
undetermined. This study investigated timings and duration
of xylem formation in maritime pine (Pinus pinaster Ait.)
from an even-aged plantation in Portugal growing under
Mediterranean climate. Cambial phenology and stem
diameter were monitored weekly, from March to December
2010, on two classes of trees divided according to the tree
ring widths of the last 15 years, but similar age and size:
fast- and slow-growing trees. We tested the hypothesis that
differences in tree ring widths result from cell production
which in turn affects timings of xylogenesis and that the
bimodal growth pattern, typical of the Mediterranean,
originates from a double reactivation of the cambium: in
spring and autumn. Cambial activity started earlier and
ended later in fast-growing trees, confirming that cell
production is a key factor determining the duration of xylogenesis.
Intra-annual variations in stem diameter recorded
by band dendrometers revealed two peaks of increment
occurring in spring and late summer. However, the number
of cambial cells did not increase in late summer, which
suggested that the second peak of increment was caused by
stem rehydration, rather than by a reactivation of cell
division. These results demonstrated that the variability in
the timings of xylem phenology observed among trees of
the same age and size and growing under similar environmental
conditions was closely related to cell production
and not to age or size per se.This study was supported by the Fundac¸a˜o para
a Cieˆncia e a Tecnologia, Ministe´rio da Educac¸a˜o e Cieˆncia (FCT) cofinanced
by Compete, through the project PTDC/AAC-AMB/111675/
2009. Joana Vieira was supported by a Ph.D. Grant (SFRH/BD/
48089/2008) and Filipe Campelo by a postdoctoral research grant
(SFRH/BPD/47822/2008), both grants from FCT with funds from
POPH (Portuguese Operational Human Potential Program) and
QREN Portugal (Portuguese National Strategic Reference Framework)
