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Carbon storage and sequestration in tropical mountain forests and their dependence on elevation and temperature are not well understood. In an altitudinal transect study in the South Ecuadorian Andes, we tested the hypotheses that (i) aboveground net primary production (ANPP) decreases continuously with elevation due to decreasing temperatures, whereas (ii) belowground productivity (BNPP) remains constant or even increases with elevation due to a shift from light to nutrient limitation of tree growth. In five tropical mountain forests between 1050 and 3060 m a.s.l., we investigated all major above‐ and belowground biomass and productivity components, and the stocks of soil organic carbon (SOC). Leaf biomass, stemwood mass and total aboveground biomass (AGB) decreased by 50% to 70%, ANPP by about 70% between 1050 and 3060 m, while stem wood production decreased 20‐fold. Coarse and large root biomass increased slightly, fine root biomass fourfold, while fine root production (minirhizotron study) roughly doubled between 1050 and 3060 m. The total tree biomass (above‐ and belowground) decreased from about 320 to 175 Mg dry mass ha−1, total NPP from ca. 13.0 to 8.2 Mg ha−1 yr−1. The belowground/aboveground ratio of biomass and productivity increased with elevation indicating a shift from light to nutrient limitation of tree growth. We propose that, with increasing elevation, an increasing nitrogen limitation combined with decreasing temperatures causes a large reduction in stand leaf area resulting in a substantial reduction of canopy carbon gain toward the alpine tree line. We conclude that the marked decrease in tree height, AGB and ANPP with elevation in these mountain forests is caused by both a belowground shift of C allocation and a reduction in C source strength, while a temperature‐induced reduction in C sink strength (lowered meristematic activity) seems to be of secondary importance.

Aim This study investigates how estimated tree aboveground biomass (AGB) of tropical montane rain forests varies with elevation, and how this variation is related to elevational change in floristic composition, phylogenetic community structure and the biogeography of the dominant tree taxa.Location Lore Lindu National Park, Sulawesi, Indonesia.Methods Floristic inventories and stand structural analyses were conducted on 13 plots (each 0.24 ha) in four old-growth forest stands at 1050, 1400, 1800 and 2400 m a.s.l. (submontane to upper montane elevations). Tree AGB estimates were based on d.b.h., height and wood specific gravity. Phylogenetic diversity and biogeographical patterns were analysed based on tree family composition weighted by AGB. Elevational trends in AGB were compared with other Southeast Asian and Neotropical transect studies (n = 7).Results AGB was invariant from sub-to mid-montane elevation (309-301 Mg ha )1 ) and increased slightly to 323 Mg ha )1 at upper montane elevation. While tree and canopy height decreased, wood specific gravity increased. Magnoliids accounted for most of the AGB at submontane elevations, while eurosids I (including Fagaceae) contributed substantially to AGB at all elevations. Phylogenetic diversity was highest at upper montane elevations, with co-dominance of tree ferns, Podocarpaceae, Trimeniaceae and asterids/euasterids II, and was lowest at lower/mid-montane elevations, where Fagaceae contributed > 50% of AGB. Biogeographical patterns showed a progression from dominant tropical families at submontane to tropical Fagaceae (Castanopsis, Lithocarpus) at lower/ mid-montane, and to conifers and Australasian endemics at upper montane elevations. Cross-continental comparisons revealed an elevational AGB decrease in transects with low/no presence of Fagaceae, but relatively high AGB in montane forests with moderate to high abundance of this family.Main conclusions AGB is determined by both changes in forest structure and shifts in species composition. In our study, these two factors traded off so that there was no net change in AGB, even though there were large changes in forest structure and composition along the elevational gradient. Southeast Asian montane rain forests dominated by Fagaceae constitute important carbon stocks. The importance of biogeography and species traits for biomass estimation should be considered by initiatives to reduce emissions from deforestation and forest degradation (REDD) and in taxon choice in reforestation for carbon offsetting.

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Drained peat soils are a significant source of greenhouse gas (GHG) emissions to the atmosphere. Rewetting these soils is considered an important climate change mitigation tool to reduce emissions and create suitable conditions for carbon sequestration. Long-term monitoring is essential to capture interannual variations in GHG emissions and associated environmental variables and to reduce the uncertainty linked with GHG emission factor calculations. In this study, we present GHG balances: carbon dioxide (CO ), methane (CH ) and nitrous oxide (N O) calculated for a 5-year period at a rewetted industrial cutaway peatland in Ireland (rewetted 7 years prior to the start of the study); and compare the results with an adjacent drained area (2-year data set), and with ten long-term data sets from intact (i.e. undrained) peatlands in temperate and boreal regions. In the rewetted site, CO exchange (or net ecosystem exchange (NEE)) was strongly influenced by ecosystem respiration (R ) rather than gross primary production (GPP). CH emissions were related to soil temperature and either water table level or plant biomass. N O emissions were not detected in either drained or rewetted sites. Rewetting reduced CO emissions in unvegetated areas by approximately 50%. When upscaled to the ecosystem level, the emission factors (calculated as 5-year mean of annual balances) for the rewetted site were (±SD) -104 ± 80 g CO -C m yr (i.e. CO sink) and 9 ± 2 g CH -C m yr (i.e. CH source). Nearly a decade after rewetting, the GHG balance (100-year global warming potential) had reduced noticeably (i.e. less warming) in comparison with the drained site but was still higher than comparative intact sites. Our results indicate that rewetted sites may be more sensitive to interannual changes in weather conditions than their more resilient intact counterparts and may switch from an annual CO sink to a source if triggered by slightly drier conditions.

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