We’re taught that trees grow annually, close to the bark, and each year an extra ring of growth is added. Count the rings and you know how old a tree is. Mediterranean trees have patterns of cambial activity leading to intra-annual density fluctuations (IADFs), called false rings. Climate change is expected to alter wood formation and xylogenesis, affecting forest productivity. Balzano et al. have followed xylogenesis and IADF formation in the softwood Pinus pinea (stone pine, Pinaceae) and hardwood Arbutus unedo (strawberry tree, Ericaceae), growing at the same site in southern Italy.
Putting samples from the trees under the microscope, they found cambium in P. pinea was productive throughout the calendar year. From January to March 2015, post-cambial (enlarging) earlywood-like tracheids were observed, which were similar to transition tracheids. The beginning of the tree ring was therefore not marked by a sharp boundary between latewood of the previous year and the new xylem produced. The tree formed true earlywood tracheids in April.
But what would an IADF look like? It has to be a variation in density, so an E-type IADF is growth that has latewood-like cells in earlywood. In contrast an L-type IADF is growth that looks like earlywood in latewood. Balzano and colleagues note: “L-IADFs, characterized by the presence of conducting cells with larger lumina than in true latewood, have been related to an extended period of wood production due to the return of favourable conditions for tree growth after the onset of latewood formation. L-IADFs are therefore primed by a bimodal pattern of wood formation, with two peaks of cambial activity: in spring and autumn.” And sure enough, they found L-IADFs in the autumn growth in P. pinea. They dated the the starts of L-IADFs to mid-October, and tracked them to mid-December, when they could see true latewood cells growing again.
In A. unedo, a double pause in cell production was observed, in summer and winter, leading to L-IADFs in autumn as well. A. unedo started growing as normal in April, and started laying down latewood cells in mid-July. This continued to mid-October when they’d have another bout of earlywood production to mid-December. Oddly, in half the samples they found another L-IADF in between August and September.
The team found that growth was species-specific, but also that the production of earlywood, latewood and these IADFs is very strongly linked to climate. To some extent you’d expect that, people know the difference between thick rings and thin rings for tree growth. But, by getting closer the actual cells where wood forms, Balzano and colleagues are finding the tools to create a more detailed reconstruction of past climates.