Tree rings provide no evidence of a CO2 fertilization effect in old-growth subalpine forests of western Canada.

Atmospheric CO2 concentrations are now 1.7 times higher than the preindustrial values. Although photosynthetic rates are hypothesized to increase in response to rising atmospheric CO2 concentrations, results from in situ experiments are inconsistent in supporting a CO2 fertilization effect of tree growth. Tree-ring data provide a historical record of tree-level productivity that can be used to evaluate long-term responses of tree growth. We use tree-ring data from old-growth, subalpine forests of western Canada that have not had a stand-replacing disturbance for hundreds of years to determine if growth has increased over 19th and 20th centuries. Our sample consisted of 5,858 trees belonging to five species distributed over two sites in the coastal zone and two in the continental climate of the interior. We calculated annual increments in tree basal area, adjusted these increments for tree size and age, and tested whether there was a detectable temporal trend in tree growth over the 19th and 20th centuries. We found a similar pattern in 20th century growth trends among all species at all sites. Growth during the 19th century was mostly stable or increasing, with the exception of one of the coastal sites, where tree growth was slightly decreasing; whereas growth during the 20th century consistently decreased. The unexpected decrease in growth during the 20th century indicates that there was no CO2 fertilization effect on photosynthesis. We compared the growth trends from our four sites to the trends simulated by seven Earth System Models, and saw that most of the models did not predict these growth declines. Overall, our results indicate that these old-growth forests are unlikely to increase their carbon storage capacity in response to rising atmospheric CO2 , and thus are unlikely to contribute substantially to offsetting future carbon emissions.

Structure and dynamics of a nearly steady-state subalpine forest in south-central British Columbia, Canada.

We examined the size, age, and spatial structure of trees in an old Engelmann spruce (Picea engelmannii)-subalpine fir (Abies lasiocarpa) forest based on four stem-mapped, 0.25 ha plots. Dendrochronological techniques were used on basal discs of 1,190 trees to reconstruct age and growth pattern, including dates of rapid growth increases. There were no obvious age cohorts or other evidence of past major disturbance. The abundance of both subalpine fir and spruce decreased rapidly with age, especially beyond the ages of 150 years. Very old trees were present, but rare. The best evidence from tree-ring width patterns for past disturbance was a period of release 100 years ago. However, few of the released trees grew into the canopy, which suggests a disturbance of low intensity. Patch dynamics and gap processes were not pronounced in the stand. Clumping was generally weak and only present at small spatial scales (<5 m) for live trees, and largely non-existent for dead trees; mortality was spatially random in this forest. Although spruce were sparse (5.1% of trees) in the forest relative to fir, which is consistent with predictions that fir will ultimately replace spruce in the absence of disturbance, coexistence seems more likely judging from the age structure and numbers of dead trees. In contrast to almost all spruce-fir forests studied previously, the stand we examined showed no record of major disturbances. Thus this stand falls at the limit of the range of dynamics - from disturbance-structured to near steady-state - encompassed in current thinking about forest ecosystems.

Structure and dynamics of an ancient montane forest in coastal British Columbia.

Old-growth forests are common in the snowy, montane environments of coastal western North America. To examine dynamics of a stand containing four canopy tree species (Abies amabilis, Chamaecyparis nootkatensis, Tsuga mertensiana and T. heterophylla), we used four stem-mapped, 50 m x 50 m plots. From measurements of annual rings, we obtained ages from basal discs of 1,336 live trees, developed master chronologies for each species, reconstructed early growth rates, and delineated periods of release. The stand was ancient; individuals of all four species exceeded 900 years in age, and the oldest tree exceeded 1,400 years. The four plots differed in the timing of events, and we found no evidence of major, stand-level disturbance. Instead the stand was structured by small-scale patch dynamics, resulting from events that affected one to several trees and initiated episodes of release and relatively rapid early growth. The species differed in age structure and dynamics. A. amabilis and T. heterophylla had a classical reverse-J age structure indicative of stable populations, whereas C. nootkatensis and T. mertensiana appeared to rely on local episodes of increased recruitment, which were often separated by centuries, and were probably related to multiple-tree gaps that occurred infrequently. However, such gaps could be considered normal in the long-term history of the stand, and thus these species with their long life spans can persist. Most individuals of all four species grew extremely slowly, with trees typically spending centuries in the understory before reaching the canopy, where they were able to persist for additional centuries. Thus, the key features of this forest are the very slow dynamics dominated by small-scale events, and the slow growth of stress-tolerant trees.