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.