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The production of aboveground soft tissue represents an important share of total net primary production in tropical rain forests. Here we draw from a large number of published and unpublished datasets (n=81 sites) to assess the determinants of litterfall variation across South American tropical forests. We show that across old-growth tropical rainforests, litterfall averages 8.61±1.91 Mg ha−1 yr−1 (mean ± standard deviation, in dry mass units). Secondary forests have a lower annual litterfall than old-growth tropical forests with a mean of 8.01±3.41 Mg ha−1 yr−1. Annual litterfall shows no significant variation with total annual rainfall, either globally or within forest types. It does not vary consistently with soil type, except in the poorest soils (white sand soils), where litterfall is significantly lower than in other soil types (5.42±1.91 Mg ha−1 yr−1). We also study the determinants of litterfall seasonality, and find that it does not depend on annual rainfall or on soil type. However, litterfall seasonality is significantly positively correlated with rainfall seasonality. Finally, we assess how much carbon is stored in reproductive organs relative to photosynthetic organs. Mean leaf fall is 5.74±1.83 Mg ha−1 yr−1 (71% of total litterfall). Mean allocation into reproductive organs is 0.69±0.40 Mg ha−1 yr−1 (9% of total litterfall). The investment into reproductive organs divided by leaf litterfall increases with soil fertility, suggesting that on poor soils, the allocation to photosynthetic organs is prioritized over that to reproduction. Finally, we discuss the ecological and biogeochemical implications of these results.

Fossil fuels can be used with minimal atmospheric emissions of carbon dioxide by capturing and storing the CO2 away in geologic structures. However, stored CO2 can leak back to the atmosphere reducing the utility of this technology. To explore the trade-offs between discounting, leakage, the cost of sequestration and the energy penalty (the energy necessary to capture, transport and inject carbon underground), we derive analytic expressions for the value of leaky CO2 storage compared to perfect storage when storage is a marginal component of the energy system. If the annual leak rate is 1% and the discount rate is 4%, for example, then CO2 mitigation using leaky storage is worth 80% of mitigation with perfect storage. Using an integrated assessment numerical model (DIAM) to explore the role of leakage when CO2 storage is non-marginal, we find that a leakage rate of 0.1% is nearly the same as perfect storage while a leakage rate of 0.5% renders storage unattractive. The possibility of capturing CO2 from the air, not only from flue gases, makes storage with higher leakage rates interesting. Finally, we speculate about the role of imperfect carbon storage in carbon accounting and trading., Les combustibles fossiles peuvent être utilisés avec peu d'émissions en capturant le dioxyde de carbone et en le stockant dans des structures géologiques. Cependant, une fraction du carbone stocké pourrait fuir vers l'atmosphère, ce qui limiterait l'utilité de cette technologie. Afin d'explorer les compromis entre actualisation, étanchéité du réservoir, coût de séquestration et pénalité énergétique (l'énergie nécessaire pour capturer, transporter et injecter le carbone en sous-sol), nous développons un modèle analytique marginal simple de la valeur d'un stockage imparfait comparativement à stockage idéal. Si le taux de fuite annuel est de 1 pour cent et le taux d'actualisation de 4 pour cent, par exemple, alors le stockage temporaire) de la tonne de CO2 se compare à l'élimination (permanente) de 8 quintaux de ce même gaz.