When the authors examined the pattern of xylem cell initiation, they found an interesting correlation with patterns in air temperatures in the two years. Across all three sites, xylem cell production in black spruce trees started earlier in 2006 than in 2005, corresponding with an earlier spring (and warmer May temperatures) in 2006indicating a positive relationship between temperature and onset of xylem production.
Temperature affects not only when cells begin to grow, but also the growth patterns of those cells. Xylem cells produced early in the seasonearlywoodare large in size with thin walls, while those produced later in the seasonlatewoodare smaller and have thicker walls.
Despite early warm temperatures in 2006, temperatures for the rest of the growing season were actually lower in June through August compared with 2005. And, correspondingly, Huang and co-authors found that in 2006 black spruce trees stopped producing both early and latewood earlier than in 2005. Consequently there were higher ratios of latewood cells to total xylem cells in 2006, and narrower, less-productive growth rings.
"Our study implies that despite the expected occurrence of earlier phenological development due to early spring climate warming, boreal trees like Picea mariana might not be producing wider rings if cold temperatures occur later in the growing season in June to August," Huang said. "These results may challenge the view that boreal trees could be benefiting from spring warming to enhance growth."
Thus, not only is the timing of the onset of spring important, but the amplitude of summer warming temperatures also plays a role in wood production.
Huang and his colleagues intend to further explore how intra-annual xylem formation of other boreal species, particularly broadleaf species, is responding to climate warming and varies across specie
|Contact: Richard Hund|
American Journal of Botany