Text by Lauri Laanisto
Journal of Plant Research, which is the scientific outlet of The Botanical Society of Japan made some changes in the end of last year, including a annual special issue called JPR symposium (read more about it here). And now Ülo has published another meta-analysis in this special issue, titled: “Leaf age dependent changes in within‑canopy variation in leaf functional traits: a meta‑analysis” (link to full text).
The idea was to test, how much different functional leaf traits (nitrogen content, photosynthetic capacity, leaf mass per area etc.) would change in the same canopies when sampled during different time periods. To see if changes in canopy leaf age will have any effect on these traits. For that, Ülo managed to find data for 71 canopies sampled on different times. The results indicate that woody plants tend to become more plastic during their lifetime – intraspecific variability of functional traits increased during time. Ontogenetic changes in intraspecific variability is a rather interesting thing to study. We might know a thing or two about the mechanisms involved, but we have very little information about the actual dynamics and rates of these changes. How much impact plasticity has to plant/population/ecosystem functioning. And this is actually a topic which can really give insight to the potential vegetation changes due to climate warming. So, from this point of view – this meta-analysis is a great step forward.
Not yet full citation: Niinemets, Ü. (2016). Leaf age dependent changes in within-canopy variation in leaf functional traits: a meta-analysis. Journal of Plant Research, DOI 10.1007/s10265-016-0815-2(link to full text).
Within-canopy variation in leaf structural and photosynthetic characteristics is a major means by which whole canopy photosynthesis is maximized at given total canopy nitrogen. As key acclimatory modifications, leaf nitrogen content (N A) and photosynthetic capacity (A A) per unit area increase with increasing light availability in the canopy and these increases are associated with increases in leaf dry mass per unit area (M A) and/or nitrogen content per dry mass and/or allocation. However, leaf functional characteristics change with increasing leaf age during leaf development and aging, but the importance of these alterations for within-canopy trait gradients is unknown. I conducted a meta-analysis based on 71 canopies that were sampled at different time periods or, in evergreens, included measurements for different-aged leaves to understand how within-canopy variations in leaf traits (trait plasticity) depend on leaf age. The analysis demonstrated that in evergreen woody species, M A and N A plasticity decreased with increasing leaf age, but the change in A A plasticity was less suggesting a certain re-acclimation of A A to altered light. In deciduous woody species, M A and N A gradients in flush-type species increased during leaf development and were almost invariable through the rest of the season, while in continuously leaf-forming species, the trait gradients increased constantly with increasing leaf age. In forbs, N A plasticity increased, while in grasses, N A plasticity decreased with increasing leaf age, reflecting life form differences in age-dependent changes in light availability and in nitrogen resorption for growth of generative organs. Although more work is needed to improve the coverage of age-dependent plasticity changes in some plant life forms, I argue that the age-dependent variation in trait plasticity uncovered in this study is large enough to warrant incorporation in simulations of canopy photosynthesis through the growing period.