Those who fear anthropogenerated climate change have long claimed that global warming will negatively impact Earth’s ecosystems, including old-growth forests, where it is hypothesized that these woodland titans of several hundred years age will suffer decreased growth and increased mortality as a consequence of predicted increases in temperature and drought. However, others see the situation as the opposite — one in which trees are enhanced by the aerial fertilization effect of rising atmospheric CO₂ concentrations, which is expected to increase growth and make trees less susceptible to the deleterious effects of drought.

So which vision of the future appears more likely to come about? According to the seven member research team of Urrutia-Jalabert et al. (2015), the much more optimistic future is not only coming, it is already here.

Working in the Andean Cordilleras region of southern Chile, Urrutia-Jalabert et al. performed a series of analyses on tree ring cores they obtained from long-lived Fitzroya cupressoides stands, which they say “may be the slowest-growing and longest-lived high biomass forest stands in the world.”

Focusing on two of the more pertinent findings of their study, as shown in Figure 1 below, both the basal area increment (a surrogate for aboveground woody biomass accumulation) and intrinsic water use efficiency (a measure of drought resistance) of Fitzroya dramatically increased over the past century. Commenting on these trends, the authors write “the sustained positive trend in tree growth is striking in this old stand, suggesting that the giant trees in this forest have been accumulating biomass at a faster rate since the beginning of the [20^th] century.” And coupling that finding with the 32 percent increase in water use efficiency over the same time period, Urrutia-Jalabert et al. conclude the trees “are actually responding to environmental change.” Indeed they are. Magnificently.

Figure 1. Basal area increment chronology (left panel, AD 1355–2010) and intrinsic water use efficiency (right panel, AD 1900–2010) of long-lived Fitzroya cupressoides trees in the Andean Cordilleras of southern Chile.

With respect to the cause of these favorable developments, the researchers state “we believe that this increasing growth trend…has likely been driven by some combination of CO₂ and/​or surface radiation increases,” adding that “pronounced changes in CO₂ have occurred in parallel with changes in climate, making it difficult to distinguish between both effects.” Thus, it is clear that of the two views predicting the future of old-growth forests, the one most likely to occur (and which is actually occurring in the southern Chilean Andes) is the one in which the benefits of CO₂ win out over model projections of climate-induced demise.

Reference

Urrutia-Jalabert, R., Malhi, Y., Barichivich, J., Lara, A., Delgado-Huertas, A., Rodríguez, C.G. and Cuq, E. 2015. Increased water use efficiency but contrasting tree growth patterns in Fitzroya cupressoides forests of southern Chile during recent decades. Journal of Geophysical Research, Biogeosciences 120: 2505–2524.