Responses to Elevated Carbon Dioxide in Artificial Tropical Ecosystems

Abstract

Carbon, nutrient, and water balance as well as key plant and soil processes were simultaneously monitored for humid tropical plant communities treated with CO(2)-enriched atmospheres. Despite vigorous growth, no significant differences in stand biomass (of both the understory and overstory), leaf area index, nitrogen or water consumption, or leaf stomatal behavior were detected between ambient and elevated CO(2) treatments. Major responses under elevated CO(2) included massive starch accumulation in the tops of canopies, increased fine-root production, and a doubling of CO(2) evolution from the soil. Stimulated rhizosphere activity was accompanied by increased loss of soil carbon and increased mineral nutrient leaching. This study points at the inadequacy of scaling-up from physiological baselines to ecosystems without accounting for interactions among components, and it emphasizes the urgent need for whole-system experimental approaches in global-change research.

Fingerprinting the impacts of global change on tropical forests

“…We suggest three possible explanations that do not invoke measurement error or bias. First, the global dimming paper compares the years 1958, 1965, 1975, 1985and 1992(Stanhill & Cohen 2001, whereas the satellite TOA data cover 1979(Wielicki et al 2002. The TOA measurements show very large variations, and comparing 1985 and 1992-the years covered by both sets of measurements-the two datasets are in qualitative agreement, both showing a decline in solar radiation.…”

“…If the above-mentioned changes occur, more rapid increases in growth, recruitment and mortality rates may be expected than the changes in stand-level biomass and stem density, which may change more modestly. This general scenario has been theoretically predicted by Norby et al (1999) for temperate forests and by Kö rner (1998Kö rner ( , 2003 for tropical forests.…”

“…Experiments where tropical seedlings of several species were grown together in competition under contemporary and higher CO 2 concentrations have shown no net increase (Lovelock et al 1998), and small non-significant increases in growth for all species combined (Reekie & Bazzaz 1989;Kö rner & Arnone 1992;Arnone & Körner 1995;Rosenthal 1998). However, these experiments have all been of low replication and short duration, and follow the standard practice of using ambient (highest for 20 million years) and higher CO 2 concentrations.…”

“…Levels of NSCs are very hard to deplete: even a major fruiting eventa severe drain on resources-does not change levels. But extreme exogenous changes, such as major defoliation, can cause trees to reduce their NSCs levels, as presumably these NSCs are used to produce new leaves (Kö rner 2003). Perhaps, then, NSCs are a plant's 'insurance policy' against severe structural damage, and investment in this is a better strategy for increasing overall fecundity than higher investment in immediate growth with no 'insurance policy'.…”

Nitrogen deposition: a component of global change analyses

“…Even Picea rubens seedlings, which would normally be expected to assimilate very little nitrate in their needles, had sufficient constitutive levels of nitrate reductase to assimilate the amount of HNO $ vapour or NO # that occurs in forests (Norby et al, 1989). There has been some speculation that CO # enrichment adds enough labile C to the soil to stimulate microbial activity (Ko$ rner & Arnone, 1992 ;Zak et al, 1993), which could lead to an increase in nitrification rates and leaching if the system is already close to N saturation. However, there are so many positive and negative feedbacks on nitrification that this speculation seems untenable as a general conclusion.…”