The cambium clock of climate change

The cambium is the secondary meristem of plants that produces layers of phloem and xylem cells that envelope the wood cylinders of stem, branches and roots, and results in seasonal radial growth. In temperate, boreal and some tropical ecosystems, the cambium undergoes winter dormancy, producing annual tree rings. During development, the xylem undergoes several different biochemical processes throughout its sequential stages of maturation. Secondary growth represents an intriguing model of complex processes of single cells that gradually and successively proliferate during a growing season. In contrast to the primary meristems (leaf and flower buds), cambial activity (e.g. cell division and differentiation) occurs within the plants and cannot be directly observed during the growing season.

Species native to colder climates are associated with a short growing season and low growth and productivity. At higher latitudes and altitudes, growth has to be completed within a limited time period and in less favourable conditions than in temperate climates. Species adjust their phenology with shifted or compressed growth and reproduction phases, according to specific regional environmental drivers, local adaptations and individual plasticity to climate. It is unclear if and how this pattern applies to the cambium and its phenology across taxonomic groups and locations.

A recent paper in Annals of Botany  examines whether phases of xylem production and differentiation occur independently of each other. If any relationships exist among the cambial phenological timings, what is their form (e.g. linear or not) and what does this imply about cambial and growth processes? Answers to these questions could contribute to a more complete understanding of the growth dynamics of forest ecosystems and their possible large-scale responses to climate change.

A meta-analysis of cambium phenology and growth: linear and non-linear patterns in conifers of the northern hemisphere. (2013) Annals of Botany, 112(9), 1911-1920.
Background: Ongoing global warming has been implicated in shifting phenological patterns such as the timing and duration of the growing season across a wide variety of ecosystems. Linear models are routinely used to extrapolate these observed shifts in phenology into the future and to estimate changes in associated ecosystem properties such as net primary productivity. Yet, in nature, linear relationships may be special cases. Biological processes frequently follow more complex, non-linear patterns according to limiting factors that generate shifts and discontinuities, or contain thresholds beyond which responses change abruptly. This study investigates to what extent cambium phenology is associated with xylem growth and differentiation across conifer species of the northern hemisphere.
Methods: Xylem cell production is compared with the periods of cambial activity and cell differentiation assessed on a weekly time scale on histological sections of cambium and wood tissue collected from the stems of nine species in Canada and Europe over 1–9 years per site from 1998 to 2011.
Results: The dynamics of xylogenesis were surprisingly homogeneous among conifer species, although dispersions from the average were obviously observed. Within the range analysed, the relationships between the phenological timings were linear, with several slopes showing values close to or not statistically different from 1. The relationships between the phenological timings and cell production were distinctly non-linear, and involved an exponential pattern
Conclusions: The trees adjust their phenological timings according to linear patterns. Thus, shifts of one phenological phase are associated with synchronous and comparable shifts of the successive phases. However, small increases in the duration of xylogenesis could correspond to a substantial increase in cell production. The findings suggest that the length of the growing season and the resulting amount of growth could respond differently to changes in environmental conditions.