Competition for light can drive adverse species-composition shifts in the Amazon Forest under elevated CO2

The resilience of biodiverse forests to climate change depends on an interplay of adaptive processes operating at multiple temporal and organizational scales. These include short-term acclimation of physiological processes like photosynthesis and respiration, mid-term changes in forest structure due to competition, and long-term changes in community composition arising from competitive exclusion and genetic trait evolution. To investigate the roles of diversity and adaptation for forest resilience, we present Plant-FATE, a parsimonious eco-evolutionary vegetation model. Tested with data from a hyperdiverse Amazonian terra-firme forest, our model accurately predicts multiple emergent ecosystem properties characterizing forest structure and function. Under elevated CO2 conditions, we predict an increase in productivity, leaf area, and aboveground biomass, with the magnitude of this increase declining in nutrient-deprived soils if trees allocate more carbon to the rhizosphere to overcome nutrient limitation. Furthermore, increased aboveground productivity leads to greater competition for light and drives a shift in community composition towards fast-growing but short-lived species characterized by lower wood densities. Such a transition reduces the carbon residence time of woody biomass, dampening carbon-sink strength and potentially rendering the Amazon Forest more vulnerable to future climatic extreme events

Lagging Response of Belowground Functional Traits to Environmental Cues in a Mature Amazonian Tropical Rainforest

Context/Purpose: The stress-dominance hypothesis (SDH) is a model of community assembly predicting that the relative importance of environmental filtering increases and competition decreases along a gradient of increasing environmental stress. Therefore, trait variation at the community level should increase as resources are more available. Although the SDH was designed to explain spatial changes in plant communities based on aboveground traits, it is possible that root communities show similar switches in strategies at temporal scales in response to pulses in resource availability. Methods: To test this hypothesis we sampled for two years the morphological changes in root systems in a mature tropical forest in Central Amazon. Thirty-six samples along a 500 m transect were taken each three months from February 2016 to February 2018, separating the uppermost organic layer (0-5 cm) from the mineral soil (5-15 cm). Besides root biomass, we scanned approximately 20% of the total root systems to calculate specific root length (SRL), average diameter (D), root tissue density (RTD), and branching index (BI). Spatially, we expected shifts from acquisitive to conservative syndromes as roots penetrate in the mineral soil. Temporarily, we hypothesized that traits associated with resource acquisition (SRL, SRTA, BI) will increase with soil moisture. Moreover, we expected that trait range will increase as resources become more available. Results: We found significant differences in biomass and morphological traits between the organic and mineral soils. We found no patterns between biomass increases in seasonality, but mean community traits change significantly with seasonal rain patterns. More interestingly, changes in mean and range values were more strongly associated with rain events three months before the collecting date, suggesting a lagging between rain events and belowground community responses. Conclusions: Belowground dynamics are structured spatially and temporarily in tropical forests, in synchrony with the availability of resources, as predicted by the SHD. Our results suggest that species tend to show similar traits during stressful times but diverge during acquisition periods. The results suggest a belowground dimension to niche segregation little explored in tropical biomes to date

Fine roots stimulate nutrient release during early stages of leaf litter decomposition in a Central Amazon rainforest

Purpose Large parts of the Amazon rainforest grow on weathered soils depleted in phosphorus and rock-derived cations. We tested the hypothesis that in this ecosystem, fine roots stimulate decomposition and nutrient release from leaf litter biochemically by releasing enzymes, and by exuding labile carbon stimulating microbial decomposers. Methods We monitored leaf litter decomposition in a Central Amazon tropical rainforest, where fine roots were either present or excluded, over 188 days and added labile carbon substrates (glucose and citric acid) in a fully factorial design. We tracked litter mass loss, remaining carbon, nitrogen, phosphorus and cation concentrations, extracellular enzyme activity and microbial carbon and nutrient concentrations. Results Fine root presence did not affect litter mass loss but significantly increased the loss of phosphorus and cations from leaf litter. In the presence of fine roots, acid phosphatase activity was 43.2% higher, while neither microbial stoichiometry, nor extracellular enzyme activities targeting carbon- and nitrogen-containing compounds changed. Glucose additions increased phosphorus loss from litter when fine roots were present, and enhanced phosphatase activity in root exclusions. Citric acid additions reduced litter mass loss, microbial biomass nitrogen and phosphorus, regardless of fine root presence or exclusion. Conclusions We conclude that plant roots release significant amounts of acid phosphatases into the litter layer and mobilize phosphorus without affecting litter mass loss. Our results further indicate that added labile carbon inputs (i.e. glucose) can stimulate acid phosphatase production by microbial decomposers, highlighting the potential importance of plant-microbial feedbacks in tropical forest ecosystems

Amazonian forest degradation must be incorporated into the COP26 agenda

Nations will reaffirm their commitment to reducing greenhouse gas (GHG) emissions during the 26th United Nations Climate Change Conference (COP26; www.ukcop26.org), in Glasgow, Scotland, in November 2021. Revision of the national commitments will play a key role in defining the future of Earth’s climate. In past conferences, the main target of Amazonian nations was to reduce emissions resulting from land-use change and land management by committing to decrease deforestation rates, a well-known and efficient strategy1,2. However, human-induced forest degradation caused by fires, selective logging, and edge effects can also result in large carbon dioxide (CO2) emissions1,2,3,4,5, which are not yet explicitly reported by Amazonian countries. Despite its considerable impact, forest degradation has been largely overlooked in previous policy discussions5. It is vital that forest degradation is considered in the upcoming COP26 discussions and incorporated into future commitments to reduce GHG emissions