Partitioning carbon sources between wetland and well-drained ecosystems to a tropical first-order stream – implications for carbon cycling at the watershed scale (Nyong, Cameroon)

Tropical rivers emit large amounts of carbon dioxide (CO2) to the atmosphere, in particular due to large wetland-to-river carbon (C) inputs. Yet, tropical African rivers remain largely understudied, and little is known about the partitioning of C sources between wetland and well-drained ecosystems to rivers. In a first-order sub-catchment (0.6 km2) of the Nyong watershed (Cameroon 27 800 km2), we fortnightly measured C in all forms and ancillary parameters in groundwater in a well-drained forest (hereafter referred to as non-flooded forest groundwater) and in the stream. In the first-order catchment, the simple land use shared between wetland and well-drained forest, together with drainage data, allowed the partitioning of C sources between wetland and well-drained ecosystems to the stream. Also, we fortnightly measured dissolved and particulate C downstream of the first-order stream to the main stem of order 6, and we supplemented C measurements with measures of heterotrophic respiration in stream orders 1 and 5. In the first-order stream, dissolved organic and inorganic C and particulate organic C (POC) concentrations increased during rainy seasons when the hydrological connectivity with the riparian wetland increased, whereas the concentrations of the same parameters decreased during dry seasons when the wetland was shrinking. In larger streams (order > 1), the same seasonality was observed, showing that wetlands in headwaters were significant sources of organic and inorganic C for downstream rivers, even though higher POC concentration evidenced an additional source of POC in larger streams during rainy seasons that was most likely POC originating from floating macrophytes. During rainy seasons, the seasonal flush of organic matter from the wetland in the first-order catchment and from the macrophytes in higher-order rivers significantly affected downstream metabolism, as evidenced by higher respiration rates in stream order 5 (756 ± 333 gC-CO2 m−2 yr−1) compared to stream 1 (286 ± 228 gC-CO2 m−2 yr−1). In the first-order catchment, the sum of the C hydrologically exported from non-flooded forest groundwater (6.2 ± 3.0 MgC yr−1) and wetland (4.0 ± 1.5 MgC yr−1) to the stream represented 3 %–5 % of the local catchment net C sink. In the first-order catchment, non-flooded forest groundwater exported 1.6 times more C than wetland; however, when weighed by surface area, C inputs from non-flooded forest groundwater and wetland to the stream contributed to 27 % (13.0 ± 6.2 MgC yr−1) and 73 % (33.0 ± 12.4 MgC yr−1) of the total hydrological C inputs, respectively. At the Nyong watershed scale, the yearly integrated CO2 degassing from the entire river network was 652 ± 161 GgC-CO2 yr−1 (23.4 ± 5.8 MgC CO2 km−2 yr−1 when weighed by the Nyong watershed surface area), whereas average heterotrophic respiration in the river and CO2 degassing rates was 521 ± 403 and 5085 ± 2544 gC-CO2 m−2 yr−1, which implied that only ∼ 10 % of the CO2 degassing at the water–air interface was supported by heterotrophic respiration in the river. In addition, the total fluvial C export to the ocean of 191 ± 108 GgC yr−1 (10.3 ± 5.8 MgC km−2 yr−1 when weighed by the Nyong watershed surface area) plus the yearly integrated CO2 degassing from the entire river network represented ∼ 11 % of the net C sink estimated for the whole Nyong watershed. In tropical watersheds, we show that wetlands largely influence riverine C variations and budget. Thus, ignoring the river–wetland connectivity might lead to the misrepresentation of C dynamics in tropical watersheds.

Effect of Seasonal Rains and Floods on Seedling Recruitment and Compositional Similarity in Two Lowland Tropical Forests

Research Highlights: Seasonally flooded and terra firme forests are characteristic ecosystems of the Colombian Orinoco Basin and of great importance in the maintenance of regional biodiversity and ecosystem function. These forests have a unimodal precipitation regime that can cause a temporal effect on the seedling regeneration niche. This could partly explain the high diversity and coexistence of plant species in these forests, as well as the similarity in composition of seedlings and trees. Background and Objectives: Seedlings are a key factor in the assembly of plant communities. We evaluated the effect of flooding and rains on the dissimilarity and compositional affinity between trees and seedlings of seasonally flooded and terra firme forests. Materials and Methods: the tree community of these forests in San Martín (Meta, Colombia) was characterized and compared with their respective seedling communities before (June) and after (December) rain and flooding (during the rainy season). We evaluated plant species diversity and abundance (Shannon diversity and Pielou eveness index), as well as the compositional dissimilarities of each tree community with their corresponding seedling community sampled at the beginning and end of rains and flooding (Bray–Curtis dissimilarity). We also compared sampling site composition using a NMDS analysis. Results: We found that the terra firme forest had higher diversity compared to the flooded forest. Seedling density in the seasonally flooded forest decreased significantly after the flood but not in the terra firme forest at the end of the rainy season. The compositional dissimilarity between trees and seedlings in the seasonally inundated forest also decreased after the flood. However, this pattern was not evident in the terra firme forest. Conclusions: These results indicate that seasonal flooding generates a strong ecological filter that affects the realized niche of plants in these forests. Our results can contribute valuable information for the effective development of assisted restoration and conservation programs.

Buds, Bugs and Bienniality: The Floral Biology of Eschweilera tenuifolia (O. Berg) Miers in a Black-Water Flooded Forest, Central Amazonia

Research Highlights: Our study establishes the biennial nature of flowering intensity as a life-time energy-conserving strategy; we show unexpectedly high flower:fruit ratios despite extensive predation of buds and flowers by insect larvae; ‘selective’ bud abortion may be a key annual energy-saving strategy. Background and Objectives: We aim to explain the strongly biennial flowering pattern of Eschweilera tenuifolia, an ecologically key tree species of Amazon blackwater-flooded forest, inundated for up to nine months annually, and with large flowers (6 cm in width). Materials and Methods: We quantified the insect infestation of central Amazonian Eschweilera tenuifolia buds and flowers; we measured nectar production from flower opening onwards, examined flower duration and monitored pollen theft. We tested the role of infestation in bud abortion, nectar production and fruit production initiation. Results: Our study shows extensive predation of buds and flowers by insect larvae, as well as selective abortion of heavily infested buds, and limited loss to pollen thieves which fed largely on infertile fodder pollen. Nectar production peaked in the morning, with no nocturnal nectar production recorded. Sucrose levels were similar to congeneric values (mean 37.4%), and near-constant during production. Flower duration (4–5 days) was longer than reported for other congenerics. Conclusions: Insect infestation of buds can play an important role in regulating flower:fruit ratios, thus setting limits on individual total seed set. Individual Eschweilera tenuifolia appear to invest highly in reproduction every second year. Extended flower duration may be a strategy to enhance pollination success, but increases overall reproductive investment. Abortion of heavily infested buds may minimize allocation of energy to malformed flowers, which have a lower chance of attracting pollinators, thus functioning as a short-term energy-saving strategy. Additionally, biennial flowering in E. tenuifolia is likely to be an energy-conserving response in a highly physiologically-challenging environment. Thus, E. tenuifolia exhibits energy-conservation strategies at two divergent temporal scales.

Integrated Use of AHP and GIS Techniques for Generating Forest Fire Risk Map in Karacabey Flooded Forest

Flooded forests are very important ecosystems that are rich in terms of their diverse flora and fauna. However, they are mostly degraded in many parts of the world, and the remaining fragmented areas are in a critical condition. Forest fires are one of the major environmental disasters that cause serious damage to forest ecosystems, and negatively affect the sustainability of forest resources. In order to minimize the potential effects of fires on forest ecosystems, forest fire risk maps should be generated, and thereby the necessary precautionary measures can be taken in these areas, according to fire risk levels. Geographical information system (GIS) techniques, integrated with multi-criteria decision analysis (MCDA) methods, can be effectively used to develop risk maps for natural hazards, such as forest fires, winter storms, floods, etc. In this study, GIS techniques integrated with an AHP (analytic hierarchy process) method were used to generate a forest fire risk map. The study was implemented in the Karacabey flooded forest, located in the city of Bursa in Turkey. In the solution process, the forest fire risk was evaluated considering two major risk factors, including stand structures (tree species, crown closure, and tree stage) and topographic factors (slope and aspect). The vegetation factor under climate control was considered, instead of directly using data of climatic elements such as temperature and humidity. The results indicated that 25.28% of the forest area was of high fire risk, while 53.17% and 21.55% was of medium and low fire risk, respectively. It was found that the most effective criterion was tree species, followed by tree stage. This aspect had the least effective criterion on forest fire risk. It was revealed that GIS techniques integrated with MCDA methods can be used effectively to estimate forest fire risk zones.

Organic Carbon in Wetland Soil: Seasonal Flooded Forest, Northeastern Thailand

Seasonal flooded forest is one of the most important wetlands in northeastern Thailand, not only for its abundant biodiversity, but also as a source of carbon sequestration. Organic carbon plays an specially important role in the soil carbon cycle. To reinforce comprehension on soil organic carbon, five profiles in a northeast plateau were observed and determined. The most common trees were Albizzia Odoratissima, Combretum quadrangulare Kurz, and Streblusasper Lour. The contents of Soil Organic Carbon (SOC) varied from 3.52 g/kgto 5.90 g/kg in top soil and varied from 4.01 g/kg to 4.60 g/kg in sub soil. There was a close relationship between SOC content and basic soil properties, especially the bulk density of both top soil layer and sub soil layer. The distribution of SOC content was harmonized with distribution of plants. In comparative analysis, the flooded forest that composted with a high percentage of vegetation coverage (Khud Tew, Khud Chi Tao) had a significantly higher SOC content. The SOC storage varied from 2.65 kg/m2 to 4.18 kg/m2. Khud Chi Tao contained the maximum amount of SOC storage, whereas Kwo Chi Yai had the minimum. Limitation of flooded forest survival concerned over landscape change, particularly plant disappearance and waterlogged shortage. Therefore, vegetation and hydrology management have to be implemented practically to retain the existing organic carbon in wetlands and allow the soil to sequester additional carbon.

Aboveground Carbon Storage and Cycling of Flooded and Upland Forests of the Brazilian Pantanal

Tropical forests and savanna (cerrado) are important carbon (C) sinks; however, few data exist for seasonally flooded forests. We quantified the annual rates of aboveground net primary production (ANPP) over a five-year period for two forests, an upland mixed forest and a seasonally flooded cerrado forest, located in the northern Pantanal region of Brazil. We hypothesized that rates of ANPP would be higher for the mixed forest than the cerrado forest because seasonal flooding can limit rates of tree growth. ANPP was calculated as the sum of the annual litter production measured from litterfall traps and the stem growth increment measured from dendrometers and tree density. ANPP varied between 3.8–5.5 MgC ha−1 y−1 for the flooded forest and 1.6–4.6 MgC ha−1 y−1 for the upland forest. Litter production accounted for 57% of the ANPP, and the difference in ANPP between the upland and flooded forests was due to consistently higher litter production in the flooded forest. Annual variations in ANPP were not correlated with annual precipitation, presumably because the hydrology of these sites is driven more by the flood stage of the Cuiaba River than by local precipitation. However, consistent declines in forest floor litter mass occurred at both sites, suggesting that C storage may be responding to some unknown disturbance that occurred prior to our sampling campaign. Seasonal variation in rainfall exerted an important control on litter production dynamics, with leaf litter production increasing during the dry season and stem and reproductive litter production increasing during the wet season. While there are few studies of seasonally flooded tropical forests, our data suggest that the seasonally flooded and upland forests of the northern Pantanal can act as appreciable aboveground C sinks.