Canadian Forest Service Publications
Growth decline and divergent tree ring isotopic composition (δ13C and δ18O) contradict predictions of CO2 stimulation in high altitudinal forests. 2013. Gómez-Guerrero, A.; Silva, L.C.R.; Barrera-Reyes, M.; Kishchuk, B.; Velázquez-Martínez, A.; Martínez-Trinidad, T.; Plascencia-Escalante, F.O.; Horwath, W.R. Global Change Biology 19(6):1748-1758.
Issued by: Northern Forestry Centre
Catalog ID: 34758
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Human-induced changes in atmospheric composition are expected to affect primary productivity across terrestrial biomes. Recent changes in productivity have been observed in many forest ecosystems, but low-latitude upper tree line forests remain to be investigated. Here, we use dendrochronological methods and isotopic analysis to examine changes in productivity, and their physiological basis, in Abies religiosa (Ar) and Pinus hartwegii (Ph) trees growing in high-elevation forests of central Mexico. Six sites were selected across a longitudinal transect (Transverse Volcanic Axis), from the Pacific Ocean toward the Gulf of Mexico, where mature dominant trees were sampled at altitudes ranging from 3200 to 4000 m asl. A total of 60 Ar and 84 Ph trees were analyzed to describe changes in growth (annual-resolution) and isotopic composition (decadal-resolution) since the early 1900s. Our results show an initial widespread increase in basal area increment (BAI) during the first half of the past century. However, BAI has decreased significantly since the 1950s with accentuated decline after the 1980s in both species and across sites. We found a consistent reduction in atmosphere to wood 13C discrimination, resulting from increasing water use efficiency (20–60%), coinciding with rising atmospheric CO2. Changes in 13C discrimination were not followed, however, by shifts in tree ring δ18O, indicating site- and species-specific differences in water source or uptake strategy. Our results indicate that CO2 stimulation has not been enough to counteract warming-induced drought stress, but other stressors, such as progressive nutrient limitation, could also have contributed to growth decline. Future studies should explore the distinct role of resource limitation (water vs. nutrients) in modulating the response of high-elevation ecosystems to atmospheric change.
Plain Language Summary
Controlled exposure of trees to high carbon dioxide levels may increase growth. This has not been shown in natural forests. The complex relationships among higher carbon dioxide levels, water use, nutrients, and growth are not well understood. This study used long-term growth trends and wood composition to determine how growth has changed under rising carbon dioxide levels in forests in central Mexico. However, the study showed that drought stress worked against potential growth stimulation from higher carbon dioxide levels, and that the effects vary with site, soil, and tree species. The study provides evidence of changes occurring under higher carbon dioxide levels in natural forests, which can be used to better understand the mechanisms responsible for environmental effects of a changing climate.