Recent projections of climatic change have focused a great deal of scientific and public attention on patterns of carbon (C) cycling as well as its controls, particularly the factors that determine whether an ecosystem is a net source or sink of atmospheric carbon dioxide (CO 2 ). Net ecosystem production (NEP), a central concept in C-cycling research, has been used by scientists to represent two different concepts. We propose that NEP be restricted to just one of its two original definitions-the imbalance between gross primary production (GPP) and ecosystem respiration (ER). We further propose that a new term-net ecosystem carbon balance (NECB)-be applied to the net rate of C accumulation in (or loss from [negative sign]) ecosystems. Net ecosystem carbon balance differs from NEP when C fluxes other than C fixation and respiration occur, or when inorganic C enters or leaves in dissolved form. These fluxes include the leaching loss or lateral transfer of C from the ecosystem; the emission of volatile organic C, methane, and carbon monoxide; and the release of soot and CO 2 from fire. Carbon fluxes in addition to NEP are particularly important determinants of NECB over long time scales. However, even over short time scales, they are important in ecosystems such as streams, estuaries, wetlands, and cities. Recent technological advances have led to a diversity of approaches to the measurement of C fluxes at different temporal and spatial scales. These approaches frequently capture different components of NEP or NECB and can therefore be compared across scales only by carefully specifying the 1041 fluxes included in the measurements. By explicitly identifying the fluxes that comprise NECB and other components of the C cycle, such as net ecosystem exchange (NEE) and net biome production (NBP), we can provide a less ambiguous framework for understanding and communicating recent changes in the global C cycle.

To assess the diversity of tropical tree life histories, a conceptual framework is needed to guide quantitative comparative study of many species. We propose one such framework, which focuses on long-term performance through ontogeny and over the natural range of microsites. For 6 yr we annually evaluated survival, growth, and microsite conditions of six non-pioneer tree species in primary tropical wet forest at the La Selva Biological Station, Costa Rica. The species were: Lecythis amp/a, Hymenolobium mesoamericanum, Dipteryx panamensis, Pithecellobium elegans, Hyeronima alchorneoides (all emergents), and Minquartia guianensis (a canopy species). The study was based on long-term measurement of individuals from all post-seedling size classes. Trees were sampled from 150 ha of primary forest spanning several watersheds and soil types. To evaluate individuals' microsites we recorded the number of overtopping crowns, forest phase (gap, building, mature), and crown illumination index (an estimate of the tree's light environment). For comparison, we also evaluated the microsites of three species that have been categorized as pioneers (Cecropia ins ignis, C. obtusifolia) or high-light demanders (Simarouba amara).For the six species of non-pioneers, mortality rates declined with increasing juvenile size class. As a group, these emergent and canopy trees showed a much lower exponential annual mortality rate (0.44%/yr at> 10 em diameter) than has been found for the La Selva forest as a whole. Growth rates increased with juvenile size class for all six species. As adults (trees > 30 em in diameter), all five emergent species showed substantial annual diameter increments (medians of 5-14 mm/yr). Small saplings and adults of all species had significant year-to-year variation in diameter growth, with much greater growth occurring in the year of lowest rainfall. Passage time analysis suggests that all six species require> 150 yr for growth from small saplings to the canopy.Evaluation of all nine species revealed four patterns of microsite occupancy by juveniles. Among the non-pioneers, one species pair (Lecythis and Minquartia: Group A) was associated with low crown illumination and mature-phase forest in all juvenile stages. For two species (Dipteryx and Hymenolobium: Group B) the smallest saplings were in predominantly low-light, mature-forest sites, but crown illumination and association with gapor building-phase sites increased with juvenile size (Simarouba also showed this pattern). Two species (Pithecel/obium and Hyeronima: Group C) were strongly associated with gap or building phase as small juveniles (~4 em diameter) and again as subcanopy trees(> 10-20 em diameter), but were predominantly in mature-phase sites at intermediate sizes.Juveniles of the two pioneer species (Cecropia: Group D) showed the highest crown illumination and association with gap or building sites.Among the six non-pioneer species, only one aspect of juvenile performance clearly varied according to microsite group. The smallest saplings ( ~ 1 em di...

There are pressing reasons for developing a better understanding of net primary production (NPP) in the world's forests. These ecosystems play a large role in the world's carbon budget, and their dynamics, which are likely to be responding to global changes in climate and atmospheric composition, have major economic implications and impacts on global biodiversity. Although there is a long history of forest NPP studies in the ecological literature, current understanding of ecosystem‐level production remains limited. Forest NPP cannot be directly measured; it must be approached by indirect methods. To date, field measurements have been largely restricted to a few aspects of NPP; methods are still lacking for field assessment of others, and past studies have involved confusion about the types of measurements needed. As a result, existing field‐based estimates of forest NPP are likely to be significant underestimates. In this paper we provide a conceptual framework to guide efforts toward improved estimates of forest NPP. We define the quantity NPP* as the summed classes of organic material that should be measured or estimated in field studies for an estimate of total NPP. We discuss the above‐ and belowground components of NPP* and the available methods for measuring them in the field. We then assess the implications of the limitations of past studies for current understanding of NPP in forest ecosystems, discuss how field NPP* measurements can be used to complement tower‐based studies of forest carbon flux, and recommend design criteria for future field studies of forest NPP.

During 1984 -2000, canopy tree growth in old-growth tropical rain forest at La Selva, Costa Rica, varied >2-fold among years. The trees' annual diameter increments in this 16-yr period were negatively correlated with annual means of daily minimum temperatures. The tree growth variations also negatively covaried with the net carbon exchange of the terrestrial tropics as a whole, as inferred from nearly pole-to-pole measurements of atmospheric carbon dioxide (CO 2) interpreted by an inverse tracer-transport model. Strong reductions in tree growth and large inferred tropical releases of CO 2 to the atmosphere occurred during the record-hot 1997-1998 El Niñ o. These and other recent findings are consistent with decreased net primary production in tropical forests in the warmer years of the last two decades. As has been projected by recent process model studies, such a sensitivity of tropical forest productivity to on-going climate change would accelerate the rate of atmospheric CO 2 accumulation. A lthough human activities are rapidly increasing atmospheric levels of the greenhouse gas CO 2 (1), understanding of the global carbon budget and how it is affected by climatic change remains approximate and evolving (2). Current knowledge of plant function, however, raises the likelihood that continued warming will alter the net carbon balance of global vegetation (3,4). Plant respiration increases exponentially with increasing temperature, whereas photosynthetic rates increase to a temperature optimum and then decline (5). At the ecosystem level, the balance between these two processes determines net primary production (NPP). Decreasing terrestrial NPP with rising temperatures would constitute a biotic positive feedback to the increase in atmospheric CO 2 (3, 4).Tropical rain forests, among the warmest terrestrial ecosystems, might be expected to be among the first to show negative temperature responses (6). Because these forests account for a third of global terrestrial NPP (7), any such responses could strongly affect atmospheric CO 2 levels. Studies at the leaf (8, 9) and stand level (10, 11) in this biome already suggest reduced carbon uptake with even small temperature increases. Measured respiration rates of tree boles in one tropical rain forest show an 8% increase with a 1°C temperature rise (12); compared with that of boles, the respiration of other plant parts tends to be even more sensitive to temperature changes (13). In addition, carbon losses by tropical trees in the form of volatile organic compounds increase exponentially over current temperature ranges (8,9). Quantifying responses of these processes at the ecosystem level, however, remains elusive because of limitations of both data and methods (14,15). Estimating forest carbon exchange with eddy covariance techniques has been found to be problematic for tropical forests, because of the prevalent still-air conditions at night (Ͼ80% of nights; ref. 11). In addition, short-term data may be poor indicators of longer-term trends, given the possibilities of c...