Farmer’s Perception of Associates Non-Cocoa Tree’s Leaf Litterfall Fertilizing Potential in Cocoa-Based Agroforestry System

Investigations to assess farmer’s perceptions on the fertilizing potential of associated trees species in cocoa agroforest of degraded forest ecology were carried out in southern Cameroon. The perception of the farmers was based on the ability of the trees to maintain or improve soil fertility of their farms. The verification of these perceptions was done through an evaluation of litter fall biomass nutrient content (N, P, K, Ca and Mg) of selected trees. The top 5 associates trees ranked by farmers was: Milicia excelsa, Ceiba pentandra, Ficus mucuso, Asltonia boonei, Terminalia superba. The chemical analysis of the leaf litter from the different tree species revealed a significant different between their chemical components. N appeared to have the highest concentrations varying from 2.82 to 5.57% with a mean value of 4.25 ± 1.065%, P had the lowest concentrations typically around 0.001%. The top 5 tree species based on the chemical analysis ranking were: C. pentandra, M. excelsa, Eribroma oblungum, Alstonia boonei, Zanthoxylum heitzi. Farmer’s perceptions thou holistic, are not completely different from scientific finding. Therefore, they should be taken in consideration in management plans for cocoa- based systems in order to enhance their ecological and economic performance.

Reactivation of nutrient cycling in an urban tropical dry forest after abandonment of agricultural activities

Introduction: Standing leaf litter represent an essential source of organic matter and nutrients to dynamize biogeochemical processes at the ecosystem level. Objectives: To characterize the accumulation and decomposition of organic materials and flow of nutrients from standing litter in an urban dry tropical forest in a successional stage, after 10 years of abandonment of agricultural activities, and to determine the potential use of three species in future active restoration activities. Materials and methods: Standing litter samples were collected from a forest fragment in Santa Marta, Colombia, separating leaves, reproductive material, woody material and other residues. Additionally, leaves of three species of interest for ecological restoration (Albizia niopoides Spruce ex Benth., Cordia alba [Jacq.] Roem. & Schult. and Machaerium milleflorum Dugand G. A.) were separated and Ca, Mg, K, N and P concentrations were determined. Results and discussion: Total standing litter was 8.3 Mg∙ha-1 with a mean residence time of two years. The leaves represented 20% of the standing litter, with a mean residence time of 1.4 years. Based on the decomposition constant (kj = 0.73) and the rate of leaf litterfall, organic matter returns accounted for 3.4 Mg∙ha-1∙year-1. Leaf decomposition rate decreased in the following order C. alba > M. milleflorum > A. niopoides. P represented the greatest limitation with low release rates (0.1 to 1.2 kg∙ha-1∙year-1). Conclusions: The passive restoration strategy allowed reactivation of biogeochemical cycle via fine leaf litter. Cordia alba showed potential for inclusion in restoration activities, with lower values for leaf N/P ratio, and higher rates for leaf litterfall, litter decomposition and nutrient release.

Drought effects on leaf fall, leaf flushing and stem growth in the Amazon forest: reconciling remote sensing data and field observations

Large amounts of carbon flow through tropical ecosystems every year, from which a part is sequestered in biomass through tree growth. However, the effects of ongoing warming and drying on tree growth and carbon sequestration in tropical forest is still highly uncertain. Field observations are sparse and limited to a few sites, while remote sensing analysis shows diverging growth responses to past droughts that cannot be interpreted with confidence. To reconcile data from field observations and remote sensing, we collated in situ measurements of stem growth and leaf litterfall from inventory plots across the Amazon region and other neotropical ecosystems. These data were used to train two machine-learning models and to evaluate model performance on reproducing stem growth and litterfall rates. The models utilized multiple climatological variables and other geospatial datasets (terrain, soil and vegetation properties) as explanatory variables. The output consisted of monthly estimates of leaf litterfall (R2= 0.71, NRMSE = 9.4 %) and stem growth (R2= 0.54, NRMSE = 10.6 %) across the neotropics from 1982 to 2019 at a high spatial resolution (0.1∘). Modelled time series allow us to assess the impacts of the 2005 and 2015 droughts in the Amazon basin on regional scales. The more severe 2015 drought was estimated to have caused widespread declines in stem growth (−1.8σ), coinciding with enhanced leaf fall (+1.4σ), which were only locally apparent in 2005. Regions in the Amazon basin that flushed leaves at the onset of both droughts (+0.9σ∼+2.0σ) showed positive anomalies in remotely sensed enhanced vegetation index, while sun-induced fluorescence and vegetation optical depth were reduced. The previously observed counterintuitive response of canopy green-up during drought in the Amazon basin detected by many remote sensing analyses can therefore be a result of enhanced leaf flushing at the onset of a drought. The long-term estimates of leaf litterfall and stem growth point to a decline in stem growth and a simultaneous increase in leaf litterfall in the Amazon basin since 1982. These trends are associated with increased warming and drying of the Amazonian climate and could point to a further decline in the Amazon carbon sink strength.

Cycling and retention of nitrogen in European beech (<i>Fagus sylvatica</i> L.) ecosystems under elevated fructification frequency

Atmospheric deposition of nitrogen (N) has exceeded its demand for plant increment in forest ecosystems in Germany. High N inputs increased plant growth, the internal N cycling within the ecosystem, the retention of N in soil and plant compartments, and the N output by seepage water. But the processes involved are not fully understood, notably the effect of fructification in European beech (Fagus sylvatica L.) on N fluxes. The frequency of fructification has increased together with air temperature and N deposition, but its impact on N fluxes and the sequestration of carbon (C) and N in soils have been hardly studied. A field experiment using 15N-labeled leaf litter exchange was carried out over a 5.5-year period at seven long-term European beech (Fagus sylvatica L.) monitoring sites to study the impact of current mast frequency on N cycling. Mean annual leaf litterfall contained 35 kg N ha−1, but about one-half of that was recovered in the soil 5.5 years after the establishment of the leaf litter 15N exchange experiment. In these forests, fructification occurred commonly at intervals of 5 to 10 years, which has now changed to every 2 years as observed during this study period. Seed cupules contributed 51 % to the additional litterfall in mast years, which creates a high nutrient demand during their decomposition due to the very high ratios of C to N and C to phosphorus (P). Retention of leaf litter 15N in the soil was more closely related to the production of total litterfall than to the leaf litterfall, indicating the role of seed cupules in the amount of leaf N retained in the soil. Higher mast frequency increased the mass of mean annual litterfall by about 0.5 Mg ha−1 and of litterfall N by 8.7 kg ha−1. Mean net primary production (NPP) increased by about 4 %. Mean total N retention in soils calculated by input and output fluxes was unrelated to total litterfall, indicating that mast events were not the primary factor controlling total N retention in soils. Despite reduced N deposition since the 1990s, about 5.7 out of 20.7 kg N ha−1 deposited annually between 1994 and 2008 was retained in soils, notably at acid sites with high N/P and C/P ratios in the organic layers and mineral soils, indicating P limitation for litter decomposition. Trees retained twice as much N compared to soils by biomass increment, particularly in less acidic stands where the mineral soils had low C/N ratios. These results have major implications for our understanding of the C and N cycling and N retention in forest ecosystems. In particular the role of mast products in N retention needs more research in the future.

Litterfall Production and Decomposition in Three Types of Land Use in Bengkulu Protection Forest

Most of the social forestry program plantations in Bengkulu are in the form of mixed planting of coffee or rubber trees. The type of land use affects the production and decomposition of litterfall, which play an important role in nutrient cycle. The aim of the research was to determine the production and decomposition rate of litterfall in coffee (Coffee robusta) monoculture, coffee and Gliricidia sepium (gliricidia) agroforestry, and rubber (Hevea brasiliensis) monoculture. The research was arranged in a systematic design with three treatments and fifteen replications. The variables measured included production, composition, and decomposition rate of litterfall. The collected data were analyzed using T-test. According to the results, the litter production in coffee monoculture, agroforestry of coffee and gliricidia, and rubber monoculture was 1051.5, 1001.5, and 662.5 Kg ha-1 4 months-1 with the decomposition rate about 5.13, 4.25, and 5.28 gr m-2 4 months-1, consecutively. The litterfall composition in the three types of land use consisted of leaf, twig, fruit, and flower. Leaf was the highest component of litterfall in coffee monoculture, agroforestry of coffee and gliricidia, and rubber monoculture, reaching 830.2 (78.99%), 646.7 (64.73%), and 391.0 (59.01%) kg ha-1, respectively. Nutrition analysis of leaf litterfall indicated that the highest content of C, N, P, and K was observed in agroforestry of coffee and gliricidia compared to other plantation types.

Drought effects on leaf fall, leaf flushing and stem growth in Neotropical forest; reconciling remote sensing data and field observations

Large amounts of carbon flow through tropical ecosystems every year, from which a part is sequestered in biomass through tree growth. However, the effects of ongoing warming and drying on tree growth and carbon sequestration in tropical forest is still highly uncertain. Field observations are sparse and limited to a few sites while remote sensing analysis shows diverging growth responses to past droughts that cannot be interpreted with confidence. To reconcile data from field observations and remote sensing, we collated in situ measurements of stem growth and leaf litterfall from inventory plots across the Neotropics. This data was used to train two machine learning models and to evaluate model performance on reproducing stem growth and litterfall rates. The models utilized multiple climatological variables and other geospatial datasets as explanatory variables. The output consisted of monthly estimates of leaf litterfall (R2 = 0.67, NRMSE = 9.5 %) and stem growth (R2 = 0.51, NRMSE = 11.2 %) across the neotropics from 1982 to 2019 at a high spatial resolution (0.1°). Modelled time series allowed to assess the impacts of the 2005 and 2015 droughts in the Amazon basin on regional scales. Both droughts were estimated to have caused widespread declines in stem growth (−0.6σ ~ −1.8σ), coinciding with enhanced leaf fall (+0.7σ ~ +0.9σ). Regions in the Amazon basin that flushed leaves at the onset of both droughts (+1.1σ ~ +1.9σ), showed positive anomalies in remotely sensed enhanced vegetation index, while sun-induced fluorescence and vegetation optical depth were reduced. The previously observed counterintuitive response of canopy green-up during drought in the Amazon basin detected by many remote sensing analyses can therefore be explained by enhanced leaf flushing at the onset of a drought. The long-term estimates of leaf litterfall and stem growth point to a decline of stem growth and a simultaneous but weaker increase in leaf litterfall in the Amazon basin since 1982 that is not observed in long-term inventory plots. These trends are associated with increased warming and drying of the Amazonian climate.

Cycling and retention of nitrogen in European beech (Fagus sylvatica L.) ecosystems under elevated fructification frequency

Atmospheric deposition of nitrogen (N) has exceeded its demand for plant increment in forest ecosystems in Germany. High N inputs increased plant growth, the internal N cycling within the ecosystem, the retention of N in soils and plant compartments, and the N output by seepage water. But the processes involved are not fully understood, especially the role of fructification which has increased in its frequency. A field experiment using 15N labelled leaf litter exchange was carried out over a 5.5 years' period at seven long-term monitoring sites with European beech (Fagus sylvatica L.) ecosystems to study the impact of current mast frequency on N cycling. Mean annual leaf litterfall contained 35 kg N ha−1, but about one half of that was recovered in the soil 5.5 years after the establishment of the leaf litter 15N exchange experiment. Retention of leaf litter N in the soil was more closely related to the production of total litterfall than to the leaf litterfall indicating the role of fructification of beech trees in the amount of leaf N retained in the soil. In these forests fructification occurred commonly in intervals of 5 to 10 years, which has now changed to every two to three years as observed during this study period. Seed cupules contributed 51 % to the additional litterfall in mast years which caused a high nutrient demand during their decomposition due to their very high carbon (C) to N and C to phosphorus (P) ratios. Higher mast frequency increased the mass of mean annual litterfall by about 0.5 Mg ha−1 and of litterfall N by 8.7 kg ha−1. Mean net primary production (NPP) increased by about 4 %. Mean total N retention in soils calculated by input and output fluxes was unrelated to total litterfall indicating that mast events were not the primary factor controlling total N retention in soils. Despite reduced N deposition since the 1990s about 5.7 kg N ha−1 out of 20.7 kg N ha−1 deposited annually between 1994 and 2008 were retained in soils notably at acid sites with high N / P and C / P ratios in the organic layers and mineral soils. Ongoing N retention increased the N / P ratios in acid soils with moder type humus forms and reduced the availability of P for plant growth and litter decomposition. Trees retained twice as much N compared to soils by biomass increment particularly in less acid stands where the mineral soils had low C / N ratios.

PRECIPITATION DEFICITS AND HIGH TEMPERATURE INCREASE LEAF LITTERFALL IN OPEN RESTINGA VEGETATION, IN SOUTHERN BRAZIL

Climate is an important driver of litterfall along different ecosystems. However, little is known about how climate affects litter production in plant communities of Restinga. The aim of this study is to characterize the temporal variation in leaf litter production in an Open Clusia Formation in Restinga de Jurubatiba National Park, from 2001 to 2018, and assess how local variability in litter production reflects the local climatic conditions and its variability. We tested the hypothesis that monthly leaf litterfall increases in dry months and in drier and warmer than average months; we also evaluated if annual leaf litterfall increases in wetter and warmer years following leaf production patterns, that increases in these conditions. We found that litterfall peaks in the drier months. This pattern is consistent for many tropical ecosystems, even evergreen ecosystems, and may have evolved as a strategy that result in reduced water stress by plants, during drier and warmer periods, or may simply be a stress symptom. However, we also found that decreases in monthly rainfall and increases in monthly ∆Temperature (temperature observed minus estimated based on 1970-2000 interval) stimulate leaf litter production. Hot and warmer than average years also seem to stimulate leaf litterfall. It suggests that annual leaf litterfall and leaf production are less affected by precipitation regimes than variations in temperature (or radiation, which is directly related to temperature). It may result from the fact that Clusia hilariana, the dominant species in this ecosystem which accounts to 80 % of leaf litterfall, is a CAM photosynthesis species, a characteristic commonly associated with avoidance of water stress by plants. Although leaf litterfall seems to be predominantly driven by climate at annual scale, only 15 % of its variation was associated to climate at seasonal scale, suggesting that local factors control litterfall at lower temporal scales in Open Clusia Formation.

Drought resistance increases from the individual to the ecosystem level in highly diverse Neotropical rainforest: a meta-analysis of leaf, tree and ecosystem responses to drought

The effects of future warming and drying on tropical forest functioning remain largely unresolved. Here, we conduct a meta-analysis of observed drought responses in Neotropical humid forests, focusing on carbon and water exchange. Measures of leaf-, tree- and ecosystem-scale performance were retrieved from 145 published studies conducted across 232 sites in Neotropical forests. Differentiating between seasonal and episodic drought, we find that (1) during seasonal drought the increase in atmospheric evaporative demand and a decrease in soil matric potential result in a decline in leaf water potential, stomatal conductance, leaf photosynthesis and stem diameter growth while leaf litterfall and leaf flushing increase. (2) During episodic drought, we observe a further decline in stomatal conductance, photosynthesis, stem growth and, in contrast to seasonal drought, a decline also in daily tree transpiration. Responses of ecosystem-scale processes, productivity and evapotranspiration are of a smaller magnitude and often not significant. Furthermore, we find that the magnitude and direction of a drought-induced change in photosynthesis, stomatal conductance and transpiration reported in a study is correlated to study-averaged wood density. Although wood density is often not functionally related to plant hydraulic properties, we find that it is a good proxy for hydraulic behaviour and can be used to predict leaf- and tree-scale responses to drought. We present new insights into the functioning of tropical forest in response to drought and present novel relationships between wood density and tropical-tree responses to drought.