A new approach to identify the climatic drivers of leaf production reconstructed from the past yearly variation in annual shoot lengths in an evergreen conifer (Picea mariana)

Key Message Annual shoot length is useful for analyzing the climate influence on tree level leaf production because it is proportional to leaf mass, and their yearly variations are synchronized within trees. Recent global warming could cause boreal forests to become carbon sources instead of large carbon sinks. A robust prediction of carbon uptake capacity of such forests is, therefore, necessary. However, even though leaf production is a determinant of forest carbon sink capacity, reconstructing annual leaf production in evergreen conifers is still challenging. A new method was proposed to reconstruct the past yearly variation in leaf production of evergreen conifers and was applied to an open stand of Picea mariana in Canada. A clear linear relationship was obtained between annual shoot length and leaf dry mass. Annual shoot lengths on primary branches were measured for over twenty years on ten mature trees. Yearly variations in these lengths were synchronized within trees, which suggests that the measured variation could be scaled up from the branch to the tree level. They were also synchronized among most of the sampled trees; however, two trees showed their own variation. These differences must be considered before scaling up from the tree to the stand level. Neither tree-ring width nor radial area increment of the stem at breast height predicted leaf production. Temperature and rainfall during the growing season were the dominant climatic drivers of leaf production reconstructed from annual shoot lengths. Leaf production may increase under future global warming if the temperature continues to rise and precipitation during the growing season does not decrease, which would otherwise negate the growth benefits of a warmer climate.

Automated summary

This study focused on the impact of climate change on leaf production in evergreen conifers, proposing a new method to reconstruct annual leaf production of trees. By measuring annual shoot length to estimate leaf mass, it found that temperature and rainfall are the key climatic drivers of leaf production.