Regionwide temporal gradients of carbon allocation allow for shoot growth and latewood formation in boreal black spruce

In boreal ecosystems, phenological events display seasonal patterns. These
patterns optimize the development of tissues during the short time window
available for growth in cold climates. Primary and secondary growth, two
expensive processes for plants, are supposedly modulated in time to
optimize allocation of carbon to bud and woody tissues. We aimed to assess
the phenology of primary and secondary meristems, testing their
relationship over the closed black spruce stands of the commercially
exploited forest region in Quebec, Canada. Location: Quebec, Canada Time
period: 2002-2016 Major taxa studied: Gymnospermae We combined weekly
scaled field observations with Moderate Resolution Imaging
Spectroradiometer (MODIS) time-series of the Normalized Difference
Vegetation Index (NDVI) to extract timings of photosynthesis and meristem
growth in five black spruce [Picea mariana (Mill.) B.S.P.] stands located
along a latitudinal gradient and to assess their relationship. We then
tested an empirical relationship based on geographical position and
seasonal temperatures to predict wood phenology i.e. the onset and ending
of earlywood and latewood formation, and its spatial patterns were
compared with existing predictions of bud phenology for the same study
area. Photosynthesis started at the beginning of May, three weeks before
bud reactivation and the onset of wood growth. Latewood formation started
in mid-July, after shoot elongation was completed. For wood phenology
models, residual standard error ranged from one week up to 12 days. Growth
dynamics spatialized across the boreal forest of Quebec varied with the
transition between the subarctic and humid continental climate. Shoot
elongation and latewood formation were temporally separated, providing
evidence of a trade-off in structural carbon allocation between primary
and secondary growth in trees. Wood phenology spatial patterns predicted
for the black spruce polygons are consistent with bud phenology spatial
patterns, demonstrating synchronized meristems temporal dynamics at region
scale.