Search

Ecosystems where carbon fluxes are being monitored on a global scale are strongly biased toward temperate Northern Hemisphere latitudes. However, forest and moorland ecosystems in the Southern Hemisphere may contribute significantly to the global and regional C balance and are affected by different climate systems. Here, we present the first data from an old‐growth forest representative of temperate, broad‐leaved rainforests from southern South America. Carbon fluxes monitored over two years using the eddy covariance technique showed that this rainforest acts as an annual sink (−238 ± 31 g C/m2). However, there were significant pulses of carbon emission associated with dry episodes during the summer months (i.e., peak of the growing season) and periods of significant carbon fixation during the cold austral winter, indicating that the carbon balance in this forest is very sensitive to climate fluctuations. The carbon fixation surges in winter seem to be related to the mild temperatures recorded during this period of the year under the prevailing oceanic climate. Winter carbon gain was more relevant in determining the annual carbon balance than summer pulse emissions. Regarding the annual carbon balance, this southern forest resembles the patterns observed in montane tropical forests more than the behavior of narrow‐leaved evergreen temperate forests from the Northern Hemisphere. These patterns make this southern forest type relevant to understanding the mechanisms and thresholds that control ecosystem shifts from carbon sinks and sources and will provide key data to improve global dynamic vegetation models., This work was supported by FONDEQUIP AIC-37, Iniciativa Científica Milenio grant P05-002 and CONI-CYT grant PFB-23 to the Institute of Ecology and Biodiversity-Chile (IEB), FONDECYT grant 1130935 to JPQ, and the LINCGlobal research program funded by CSIC-Spain and Catholic University of Chile. The authors thank Robert Cook and Alison Boyer, who kindly provided the FLUXNET database 2015. The authors are grateful to Paul Reich (USDA) for sharing the Biome database, Sebastiaan Luyssaert for providing them with the database of CO2 balance of forest ecosystems, and an anonymous reviewer whose comments helped them to improve the manuscript. This is a contribution to the Research Program of Senda Darwin Biological Station and the Chilean Long-Term Socio-Ecological Research Network (LTSER-Chile), affiliated with ILTER, AMERIFLUX, and FLUXNET.

BACKGROUND: Quillaja saponaria Mol., Cryptocarya alba Mol. Looser, and Lithraea caustica Molina Hook et Arn., are common sclerophyllous species in Mediterranean Central Chile. Mesophyll conductance, g m, may strongly limit photosynthesis in these semiarid environments. RESULTS: Simultaneous measurements of gas exchange and chlorophyll fluorescence were carried out in 45 nursery plants from these species to determine diffusional and biochemical limitations to photosynthesis. Values of stomatal conductance, g s, were greater than those of mesophyll conductance, g m, while their ratio (g m/g s) was not influenced by species being on average 0.47. Relative limitations posed by mesophyll conductance to photosynthesis, Lm, (0.40 ± 0.02) were high compared to those imposed by stomata, Ls (0.07 ± 0.01). The average CO2 concentration in the intercellular air spaces (Ci) was 32 µmol mol-1 lower than in the atmosphere (Ca), while the average CO2 concentration in the chloroplasts (Cc) was 131 µmol mol-1 lower than Ci independent of species. Maximal rates of Rubisco carboxylation, Vcmax, and maximal electron transport rates driving regeneration of RuBP, Jmax, ranged from 13 to 66 µmol CO2 m-2 s-1 and from 33 to 148 µmol electrons m-2 s-1, respectively, and compare well to averages for C3 plants. CONCLUSIONS: Photosynthetic performance was in the series: Q. saponaria > C. alba ≥ L. caustica, which can be attributed first to mesophyll conductance limitations, probably mediated by leaf anatomical traits and then to species specific foliage N partitioning strategies.