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System‐level adjustments to elevated CO 2 in model spruce ecosystems
Author(s) -
HÄTTENSCHWILER STEPHAN,
KÖRNER CHRISTIAN
Publication year - 1996
Publication title -
global change biology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 4.146
H-Index - 255
eISSN - 1365-2486
pISSN - 1354-1013
DOI - 10.1111/j.1365-2486.1996.tb00088.x
Subject(s) - photosynthesis , shoot , ecosystem , understory , seedling , plant litter , picea abies , carbon dioxide , deposition (geology) , herbaceous plant , leaf area index , environmental science , growing season , agronomy , photosynthetic capacity , botany , biology , ecology , canopy , paleontology , sediment
Atmospheric carbon dioxide enrichment and increasing nitrogen deposition are often predicted to increase forest productivity based on currently available data for isolated forest tree seedlings or their leaves. However, it is highly uncertain whether such seedling responses will scale to the stand level. Therefore, we studied the effects of increasing CO 2 (280, 420 and 560 μL L ‐1 ) and increasing rates of wet N deposition (0, 30 and 90 kg ha ‐1 y ‐1 ) on whole stands of 4‐year‐old spruce trees ( Picea abies ). One tree from each of six clones, together with two herbaceous understory species, were established in each of nine 0.7 m 2 model ecosystems in nutrient poor forest soil and grown in a simulated montane climate for two years. Shoot level light‐saturated net photosynthesis measured at growth CO 2 concentrations increased with increasing CO 2 , as well as with increasing N deposition. However, predawn shoot respiration was unaffected by treatments. When measured at a common CO 2 concentration of 420 μL L ‐1 37% down‐regulation of photosynthesis was observed in plants grown at 560 μL CO 2 L ‐1 . Length growth of shoots and stem diameter were not affected by CO 2 or N deposition. Bud burst was delayed, leaf area index (LAI) was lower, needle litter fall increased and soil CO 2 efflux increased with increasing CO 2 . N deposition had no effect on these traits. At the ecosystem level the rate of net CO 2 exchange was not significantly different between CO 2 and N treatments. Most of the responses to CO 2 studied here were nonlinear with the most significant differences between 280 and 420 μL CO 2 L ‐1 and relatively small changes between 420 and 560 μL CO 2 L ‐1 . Our results suggest that the lack of above‐ground growth responses to elevated CO 2 is due to the combined effects of physiological down‐regulation of photosynthesis at the leaf level, allometric adjustment at the canopy level (reduced LAI), and increasing strength of below‐ground carbon sinks. The non‐linearity of treatment effects further suggests that major responses of coniferous forests to atmospheric CO 2 enrichment might already be under way and that future responses may be comparatively smaller.