Litterfall Rates Research Articles

Higher temperature accelerates carbon cycling in a temperate montane forest without decreasing soil carbon stocks

Global warming is expected to accelerate the cycling of soil organic carbon (SOC) and the assimilation of new carbon, but the net effect of those counteracting accelerations and their ultimate effects on SOC are still uncertain. This hinders the prediction of long-term changes in biospheric carbon stocks and SOC-climate feedbacks. Here, we studied the long-term effect of temperature on carbon cycling across a 3.2 °C altitudinal temperature gradient in a temperate forest ecosystem in New Zealand. Across the gradient, soil respiration rates increased with increasing temperature from 9.0 to 10.4 tC ha−1 yr−1, but SOC stocks down to 85 cm depth also tended to increase, from 154 to 176 tC ha−1, albeit non-significantly (P = 0.06). This system was able to maintain higher soil respiration rates at higher temperatures without reducing SOC because the higher respiration rates were sustained by higher litterfall rates. Aboveground litterfall increased from 1.8 to 2.4 tC ha−1 yr−1 and estimated belowground C inputs increased from 7.2 to 8.0 tC ha−1 yr−1 along the temperature gradient. These higher fluxes were associated with significantly (P < 0.05) increased biomass at higher temperatures. As a direct measure of the effect of temperature on carbon cycling processes, we also calculated the turnover rate of forest litter which increased about 1.4-fold across the temperature gradient. This study demonstrates that higher temperatures along the thermal gradient increased plant carbon inputs through enhanced gross primary production, which counteracted SOC losses through temperature-enhanced soil respiration. These results suggest that temperature sensitivities of both plant carbon inputs and SOC losses must be considered for predicting SOC-climate feedbacks.

Improved process representation of leaf phenology significantly shifts climate sensitivity of ecosystem carbon balance

Abstract. Terrestrial carbon cycle models are routinely used to determine the response of the land carbon sink under expected future climate change, yet these predictions remain highly uncertain. Increasing the realism of processes in these models may help with predictive skill, but any such addition should be confronted with observations and evaluated in the context of the aggregate behavior of the carbon cycle. Here, two formulations for leaf area index (LAI) phenology are coupled to the same terrestrial biosphere model: one is climate agnostic, and the other incorporates direct environmental controls on both timing and growth. Each model is calibrated simultaneously to observations of LAI, net ecosystem exchange (NEE), and biomass using the CARbon DAta-MOdel fraMework (CARDAMOM) and validated against withheld data, including eddy covariance estimates of gross primary productivity (GPP) and ecosystem respiration (Re) across six ecosystems from the tropics to high latitudes. Both model formulations show similar predictive skill for LAI and NEE. However, with the addition of direct environmental controls on LAI, the integrated model explains 22 % more variability in GPP and Re and reduces biases in these fluxes by 58 % and 77 %, respectively, while also predicting more realistic annual litterfall rates due to changes in carbon allocation and turnover. We extend this analysis to evaluate the inferred climate sensitivity of LAI and NEE with the new model and show that the added complexity shifts the sign, magnitude, and seasonality of NEE sensitivity to precipitation and temperature. This highlights the benefit of process complexity when inferring underlying processes from Earth observations and representing the climate response of the terrestrial carbon cycle.

Adapting 3-PG foliar variables to deciduous trees in response to water restriction: poplar short rotation plantations under Mediterranean conditions

Abstract Poplar plantations growing in short rotation are a crucial biomass source of raw material for bioenergy and/or bioproducts, making an important contribution towards achieving a low-carbon bioeconomy. To optimize yield predictions of poplar plantations, this study aims to adapt the foliar variables of the process-based model 3-PG (Physiological Principles Predicting Growth) to a deciduous species like poplar. A total of 138 trees were sampled from a poplar plantation of the highly productive hybrid P. x canadensis (‘AF2’) over a first rotation at a planting density of 10 000 trees ha−1. Two irrigation scenarios, full soil field capacity (FC) and 50% FC, were considered to take into account the impact of climate change in the context of irrigation restrictions. Based on this information, the objectives were not only to determine the required species-specific foliar parameters but also to adapt the 3-PG model architecture to a pattern of variation along each growing season and identify the foliar parameters which present a significant response to restrictive irrigation. For this, specific leaf area (SLA) changes were modelled and the litterfall rate (${\gamma}_F$), and maximum canopy quantum efficiency (${\alpha}_{Cx}$) were calculated. SLA follows a similar dynamic in terms of water availability and year, with SLA for mature leaves being 19.9 m2 kg−2 and the SLA at the beginning of the growing season 10.4 m2 kg2. Leaf litter season begins in late August and lasts until early December, with 26 per cent litterfall by October. Additionally, the highly sensitive parameter ${\alpha}_{Cx}$ was calibrated and a proposed value of 0.093 molC mol PAR−1 was used. The validation of the proposed parameterization showed realistic estimates of the changes of leaf biomass and LAI during the growing season. These results will enable improved 3-PG-based estimations of the real variation along the growing season of variables such as Net Primary Productivity, leaf litterfall or analysis of the soil–plant nutrient cycle.

Effects of a warmer climate and forest composition on soil carbon cycling, soil organic matter stability and stocks in a humid boreal region

Abstract. The maintenance of the large soil organic carbon (SOC) stocks of the boreal forest under climate change is a matter of concern. In this study, major soil carbon pools and fluxes were assessed in 22 closed-canopy forests located along an elevation and latitudinal climatic gradient expanding 4 ∘C in mean annual temperature (MAT) for two important boreal conifer forest stand types: balsam fir (Abies balsamea), a fire avoider, and black spruce (Picea mariana), a fire-tolerant species. SOC stocks were not influenced by a warmer climate or by forest type. However, carbon fluxes, including aboveground litterfall rates, as well as total soil respiration (Rs) and heterotrophic (Rh) and autotrophic soil respiration (Ra), were linearly related to temperature (cumulative degree days &gt;5 ∘C). The sensitivity of soil organic matter (SOM) degradation to temperature, assessed by comparing Q10 (rate of change for a T increase of 10 ∘C) of soil respiration and Rs10 (soil respiration rates corrected to 10 ∘C), did not vary along the temperature gradient, while the proportion of bioreactive carbon and nitrogen showed higher values for balsam fir and for warmer sites. Balsam fir forests showed a greater litterfall rate, a better litter quality (lower C : N ratio) and a higher Rs10 than black spruce ones, suggesting that their soils cycle a larger amount of C and N under a similar climate regime. Altogether, these results suggest that a warmer climate and a balsam fir forest composition induce a more rapid SOC turnover. Contrary to common soil organic matter stabilisation hypotheses, greater litter input rates did not lead to higher total SOC stocks, and a warmer climate did not lead to the depletion of bioreactive soil C and N. Positive effects of warming both on fluxes to and from the soil as well as a potential saturation of stabilised SOC could explain these results which apply to the context of this study: a cold and wet environment and a stable vegetation composition along the temperature gradient. While the entire study area is subject to a humid climate, a negative relationship was found between aridity and SOM stocks in the upper mineral soil layer for black spruce forests, suggesting that water balance is more critical than temperature to maintain SOM stocks.

Litter removal impacts on soil biodiversity and eucalypt plantation development in the seasonal tropics

The little layer of tree plantations provides primary nutrients for uptake, buffers changes in soil moisture, and provides habitat and substrate to soil epigeic fauna. However, this layer in eucalypt plantations is often removed to reduce fuel load during the fire season in the Brazilian savanna (Cerrado). Therefore, it is necessary to quantify the effects of changes in litter dynamics on the function of these plantations, on key nutrient cycling processes and on epigeic fauna diversity and abundance. In two adjacent stands (one juvenile and one mature), the consequences of two years of litter removal were quantified as monthly litterfall, leaf and fine wood litter decomposition, epigeic fauna abundance and diversity, soil biogeochemical variables, and tree diameter and basal area increments. Monthly litterfall rates in juvenile and mature stands did not change with litter removal over the study period. Annual litterfall ranged from 4.1 to 4.9 Mg ha−1a−1 in litter removal plots and from 3.9 to 4.8 Mg ha−1a−1 in control plots. Fine wood litter decomposition was slower in litter removal plots compared to controls, while leaf decomposition rates were similar in both. Two years of litter removal in the juvenile stand did not affect topsoil biogeochemical parameters but decreased available phosphorus at 20–40 cm depth relative to controls. In the mature stand, total cation exchange capacity (0–20 cm) was higher in controls (6.4 cmolc dm−3) relative to litter removal plots (6.3 cmolc dm−3), while soil moisture (0–40 cm depth) was lower in litter removal (25.45 m3 m−3) compared to control plots (26 m3 m−3) in the dry season. A non-metric multidimensional scaling ordination revealed an increased homogeneity in epigeic fauna where litter was removed. Litterfall, decomposition, diameter increment, four soil physical parameters and fourteen chemical parameters at 0–20 and 20–40 cm depth explained the differences in soil epigeic fauna composition between litter removal and control plots. Diameter increment decreased with litter removal only in the juvenile stand, which had reached its growth peak. The results indicate that removing excess litter to decrease fuel volume can alter soil biodiversity and edaphic conditions that negatively affect nutrient cycling and tree growth.

Male ant reproductive investment in a seasonal wet tropical forest: Consequences of future climate change.

Tropical forests sustain many ant species whose mating events often involve conspicuous flying swarms of winged gynes and males. The success of these reproductive flights depends on environmental variables and determines the maintenance of local ant diversity. However, we lack a strong understanding of the role of environmental variables in shaping the phenology of these flights. Using a combination of community-level analyses and a time-series model on male abundance, we studied male ant phenology in a seasonally wet lowland rainforest in the Panama Canal. The male flights of 161 ant species, sampled with 10 Malaise traps during 58 consecutive weeks (from August 2014 to September 2015), varied widely in number (mean = 9.8 weeks, median = 4, range = 1 to 58). Those species abundant enough for analysis (n = 97) flew mainly towards the end of the dry season and at the start of the rainy season. While litterfall, rain, temperature, and air humidity explained community composition, the time-series model estimators elucidated more complex patterns of reproductive investment across the entire year. For example, male abundance increased in weeks when maximum daily temperature increased and in wet weeks during the dry season. On the contrary, male abundance decreased in periods when rain receded (e.g., at the start of the dry season), in periods when rain fell daily (e.g., right after the beginning of the wet season), or when there was an increase in the short-term rate of litterfall (e.g., at the end of the dry season). Together, these results suggest that the BCI ant community is adapted to the dry/wet transition as the best timing of reproductive investment. We hypothesize that current climate change scenarios for tropical regions with higher average temperature, but lower rainfall, may generate phenological mismatches between reproductive flights and the adequate conditions needed for a successful start of the colony.

Benefits of Short-Duration, High-Stocking Rate Opportunistic Grazing on Arid Rangelands During Favorable Conditions

The aim of this study was to assess the impact of short grazing periods with high-stocking density on vegetation during weather conditions favorable to plant growth. Continuous grazing is widely practiced in Tunisian arid rangelands and across most drylands and deserts of the Middle East and North Africa. In the early 1990s, the Tunisian government combined a variety of incentives and restrictions on rangeland grazing practices and initiated a national strategy for rangeland improvement. The strategy emphasizes grazing exclusion for three consecutive years, a practice known locally as gdel. At the end of this period, grazing is allowed with no restrictions or with some guidelines. However, these regulations created discontent among pastoral communities, mainly when local rainfall conditions result in a considerable quantity of green biomass. High stocking-density grazing for a short period would help satisfy pastoralist concerns and achieve the government’s strategic goal. This study was implemented in three arid rangeland types of southern Tunisia that have been under restricted grazing for 2 years. Each area in the study was grazed for short periods in late May (about 7 days) with a flock of 150 head of sheep. Measurements of vegetation cover, forage productivity, density, and species richness were taken before and after grazing. Our results suggest that perennial vegetation is more stable under grazing than annual vegetation. The presence of annual species would enhance rangeland vegetation cover and diversity and at the same time offers an opportunity for livestock to select a high-quality diet rich in protein. Opportunistic grazing, applied to heterogeneous rangelands in late spring and the beginning of the annual temperature increase, encouraged animals to select annual plants due to their higher palatability, higher digestibility, and water content rather than heavily grazing perennial species. Perennial biomass materials that die and do not fall through rapid biological decay tended to decline with increased period of placement, resulting in further oxidation to CO2 which affects photosynthetic performance negatively and may eventually cause plant death. Trampling from high-density stocking enhances the litterfall rates and removes the oxidized plant material. These findings may contribute to strategies for addressing the extreme climatic variations that threaten rangeland and livestock sustainability.

Linking soil biodiversity and ecosystem function in a Neotropical savanna

Conserving the remaining savanna ecosystems in the Brazilian savanna (Cerrado) — a global biodiversity hotspot that stores carbon and provides water to a large portion of South America — requires understanding the ecological processes maintaining their function. Nutrient cycling supports savanna function via plant litter production and decomposition by soil fauna, releasing nutrients for plant and soil organism uptake. Soil biodiversity and biogeochemistry linkages with litter dynamics in Neotropical savannas under a changing climate are poorly understood. Here, we combined two years of rainfall seasonality, leaf and wood litter production and decomposition with soil epigeic fauna abundance — the number of ground-surface dwelling invertebrates collected through pitfall traps — taxa richness, Shannon's diversity and Pielou's evenness, and 16 soil biogeochemical variables measured in 12 plots of preserved savanna. Rainfall seasonality modulated the mean soil epigeic fauna diversity and evenness across all plots, which were highest in the rainy season, in contrast to litterfall rates, which peaked in the dry season. In the dry season (April to September), the Formicidae family was the most abundant with 50% of all individuals, while in the rainy season (October to March), the Isoptera order was the most abundant with approximately 39% of individuals. Wood litter decomposition grouped with annual Hemiptera abundance, co-varying with soil epigeic fauna diversity and evenness per plot and against soil fertility variables. Leaf litter decomposition co-varied with the total epigeic fauna abundance and soil pH. We speculate that the specific need to decompose wood litter may be associated with a greater need for diversity than an abundance of soil epigeic fauna. Our work highlights the role of rainfall seasonality on soil biodiversity and physicochemistry, which is also tightly linked with litter production and decomposition. This study advances our understanding of the mechanisms governing nutrient cycling in savanna ecosystems on nutrient-impoverished soils, with implications for achieving sustainable conservation and restoration goals.

Reactivación del ciclo de nutrientes en un bosque seco tropical urbano tras el abandono de actividades agrícolas

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. &amp; 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 &gt; M. milleflorum &gt; 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

Abstract. 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.

Spatio-Temporal Variability of Peat CH4 and N2O Fluxes and Their Contribution to Peat GHG Budgets in Indonesian Forests and Oil Palm Plantations

Land-use change in tropical peatlands substantially impacts peat emissions of methane (CH4) and nitrous oxide (N2O) in addition to emissions of carbon dioxide (CO2). However, assessments of full peat greenhouse gas (GHG) budgets are scarce and CH4 and N2O contributions remain highly uncertain. The objective of our research was to assess changes in peat GHG flux and budget associated with peat swamp forest disturbance and conversion to oil palm plantation and to evaluate drivers of variation in trace gas fluxes. Over a period of one and a half year, we monitored monthly CH4 and N2O fluxes together with environmental variables in three undrained peat swamp forests and three oil palm plantations on peat in Central Kalimantan. The forests included two primary forests and one 30-year-old secondary forest. We calculated the peat GHG budget in both ecosystems using soil respiration and litterfall rates measured concurrently with CH4 and N2O fluxes, site-specific soil respiration partitioning ratios, and literature-based values of root inputs and dissolved organic carbon export. Peat CH4 fluxes (kg CH4 ha−1 year−1) were insignificant in oil palm (0.3 ± 0.4) while emissions in forest were high (14.0 ± 2.8), and larger in wet than in dry months. N2O emissions (kg N2O ha−1 year−1) were highly variable spatially and temporally and similar across land-uses (5.0 ± 3.9 and 5.2 ± 3.7 in oil palm and forest). Temporal variation of CH4 was controlled by water table level and soil water-filled pore space in forest and oil palm, respectively. Monthly fluctuations of N2O were linked to water table level in forest. The peat GHG budget (Mg CO2 equivalent ha−1 year−1) in oil palm (31.7 ± 8.6) was nearly eight times the budget in forest (4.0 ± 4.8) owing mainly to decreased peat C inputs and increased peat C outputs. The GHG budget was also ten times higher in the secondary forest (10.2 ± 4.5) than in the primary forests (0.9 ± 3.9) on the account of a larger peat C budget and N2O emission rate. In oil palm 96% of emissions were released as CO2 whereas in forest CH4 and N2O together contributed 65% to the budget. Our study highlights the disastrous atmospheric impact associated with forest degradation and conversion to oil palm in tropical peatlands and stresses the need to investigate GHG fluxes in disturbed undrained lands.

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.

Overhead tree canopy species has limited effect on leaf litter decomposition and decomposer communities in a floristically diverse, southern temperate rainforest

To decipher the <30% of unexplained variation in leaf litter decomposition, a process important for nutrient cycling yet vulnerable to rising CO2 levels, numerous hypotheses have been proposed. The home-field advantage (HFA) hypothesis states that leaf litter decomposes more rapidly beneath canopies of tree species from which the leaves originate (home environment), than beneath other tree species. Most HFA studies typically involve reciprocal litter transplants between habitat types (i.e. forest vs. grassland) or successional stages (i.e. early vs. late). The mixed and often conflicting results pertaining to this hypothesis, mostly assessed at the landscape-level, merit a finer scale analysis of decomposition variability, i.e. at the tree species level. Despite homogenous litter build up close to any given tree individual in a mixed forest, along with evidence of varying decomposition rates under different tree species, we still lack basic understanding of forest floor decomposition dynamics at the tree species level, strongly driven by host selection by fungi as well as arthropod detritivores. To address these gaps, we first determined differences in decomposition rates between leaves of three dominant forest tree species. We then compared decomposition rates between leaf litter placed beneath the canopies of source species vs. non-source species in a mixed-tree natural, temperate rainforest, accounting for the activity of both fungi (microbiota) and arthropod detritivores (both meso- and macrofauna). We found that leaf-litter decomposition rates can differ significantly depending on the source-tree species. Despite this, we found no effect of overhead tree species on decomposition rates of leaf litter, irrespective whether under source species or non-source species. Only one species' leaf litter showed differences in arthropod diversity between leaf litter placed beneath source tree species vs. non-source species, with higher arthropod diversity in leaf litter placed beneath source trees. Also, arthropods here contributed significantly more than fungi to overall decomposition. However, bacteria (not measured here) may also have decomposed litter greatly, with litter loss not solely due to our focal groups. We conclude that, in a single, diverse forest, perhaps due to differences in litter fall rate and nutrient content of leaf litter, only some plant species might reveal small-scale patterns for detritivore diversity, with yet unknown effects overall decomposition. As such, landscape-level studies on decomposition should incorporate and account for the effects of tree species-level decomposition variability.

Impacts of prescribed burnings on litter production, nitrogen concentration, δ13C and δ15N in a suburban eucalypt natural forest of subtropical Australia

Prescribed burning is projected to be adopted more frequently with intensifying climate change; thus, a long-term study is necessary to understand the burning impacts on forest productivity and carbon (C) and nitrogen (N) cycling. Litter fall production rate can be used to indicate burning impacts on forest productivity, whereas N concentration, and C and N isotope composition (δ13C and δ15N) can be used to infer burning impacts on C and N cycling in plant-soil system. In this study, the impacts of low-intensity prescribed burning on litter production, N concentration, and C and N isotope compositions were continuously investigated for 6 years at five study sites in a natural eucalypt forest of subtropical Australia. Higher leaf litter production rate, N concentration and δ15N, and lower δ13C could be seen shortly after prescribed burning. The higher leaf litter N concentration and lower δ13C were likely due to the ease of competition for soil N and moisture from understory vegetation in the short term by prescribed burning. Leaf δ15N and N concentration were closely correlated, and seasonal changes in leaf litter production rate, δ13C and δ15N were observed. Burning season and related severity might determine the suppression degree of understory vegetation. Time since fire (TSF) was a significant impact factor influencing the litter fall production rate, N concentration, δ13C and δ15N of leaf litter fall for a decade following prescribed burning. However, monthly rainfall and temperature were less consistent in their impacts. Nitrogen limitation was enhanced by prescribed burning through the removal of litter and understory vegetation in the N poor forest and might be responsible for the long-term burning impacts. Low-intensity prescribed burning might have a long-lasting impact on forest litter productivity in nutrient poor forests in subtropical Australia.

Competing effects of soil fertility and toxicity on tropical greening

Tropical forests are expected to green up with increasing atmospheric CO2 concentrations, but primary productivity may be limited by soil nutrient availability. However, rarely have canopy-scale measurements been assessed against soil measurements in the tropics. Here, we sought to assess remotely sensed canopy greenness against steep soil nutrient gradients across 50 1-ha mature forest plots in Panama. Contrary to expectations, increases in in situ extractable soil phosphorus (P) and base cations (K, Mg) corresponded to declines in remotely sensed mean annual canopy greenness (r2 = 0.77–0.85; p < 0.1), controlling for precipitation. The reason for this inverse relationship appears to be that litterfall also increased with increasing soil P and cation availability (r2 = 0.88–0.98; p < 0.1), resulting in a decline in greenness with increasing annual litterfall (r2 = 0.94; p < 0.1). As such, greater soil nutrient availability corresponded to greater leaf turnover, resulting in decreased greenness. However, these decreases in greenness with increasing soil P and cations were countered by increases in greenness with increasing soil nitrogen (N) (r2 = 0.14; p < 0.1), which had no significant relationship with litterfall, likely reflecting a direct effect of soil N on leaf chlorophyll content, but not on litterfall rates. In addition, greenness increased with extractable soil aluminum (Al) (r2 = 0.97; p < 0.1), but Al had no significant relationship with litterfall, suggesting a physiological adaptation of plants to high levels of toxic metals. Thus, spatial gradients in canopy greenness are not necessarily positive indicators of soil nutrient scarcity. Using a novel remote sensing index of canopy greenness limitation, we assessed how observed greenness compares with potential greenness. We found a strong relationship with soil N only (r2 = 0.65; p < 0.1), suggesting that tropical canopy greenness in Panama is predominantly limited by soil N, even if plant productivity (e.g., litterfall) responds to rock-derived nutrients. Moreover, greenness limitation was also significantly correlated with fine root biomass and soil carbon stocks (r2 = 0.62–0.71; p < 0.1), suggesting a feedback from soil N to canopy greenness to soil carbon storage. Overall, these data point to the potential utility of a remote sensing product for assessing belowground properties in tropical ecosystems.

Hydrological fluxes of dissolved organic carbon and total dissolved nitrogen in subtropical forests at three restoration stages in southern China

In the restoration of subtropical forests, the hydrological fluxes of dissolved organic carbon (DOC) and total dissolved nitrogen (TDN) have not been fully investigated in different stages dominated by different tree species. The concentrations of DOC and TDN in rainfall, throughfall, stemflow, and surface runoff were measured for two years in each of the three forests at different restoration stages: early coniferous forest (Pinus massoniana - Lithocarpus glaber), intermediate deciduous broadleaved forest (Choerospondias axillaris), and late evergreen broadleaved forest (L. glaber - Cyclobalanopsis glauca). Input and output fluxes as well as leaching and migration coefficients of DOC and TDN were calculated to examine the nutrient cycling status in the processes of forest restoration. In stemflow, DOC concentrations in the evergreen broadleaved forest differed significantly from those in the coniferous and deciduous broadleaved forests, whereas TDN concentrations differed significantly between the coniferous forest and other two forests. In throughfall, DOC and TDN concentrations were not significantly different among the three forests. The amount of DOC leached was primarily influenced by rainfall and was significantly related to fluxes in throughfall, with the highest value in the deciduous broadleaved forest (57.78 kg C ha−1). However, the amount of TDN leached was affected by rainfall and the absorption of vegetation and was not significantly related to fluxes in throughfall, except for the deciduous broadleaved forest. The highest value of TDN leached was in the coniferous forest (11.90 kg N ha−1). Mean TDN concentration in surface runoff tended to increase but annual TDN output fluxes decreased with forest restoration. However, both mean concentration and annual output fluxes of DOC in surface runoff were the highest in the deciduous broadleaved forest at the intermediate restoration stage. The concentrations were closely related to surface runoff, decomposition rate of litterfall, and soil organic matter content. The annual output fluxes of DOC and TDN in surface runoff were less than 30% of the input in throughfall, indicating nutrient accumulation in subtropical secondary forests.

Spatial variability of mangrove primary productivity in the neotropics

AbstractMangroves are considered one of the most productive ecosystems in the world with significant contributions as carbon sinks in the biosphere. Yet few attempts have been made to assess global patterns in mangrove net primary productivity, except for a few assumptions relating litterfall rates to variation in latitude. We combined geophysical and climatic variables to predict mangrove litterfall rates at continental scale. On a per‐area basis, carbon flux in litterfall in the neotropics is estimated at 5 MgC·ha−1·yr−1, between 20% and 50% higher than previous estimates. Annual carbon fixed in mangrove litterfall in the neotropics is estimated at 11.5 TgC, which suggests that current global litterfall estimates extrapolated from mean reference values may have been underestimated by at least 5%. About 5.8 TgC of this total carbon fixed in the neotropics is exported to estuaries and coastal oceans, which is nearly 30% of global carbon export by tides. We provide the first attempt to quantify and map the spatial variability of carbon fixed in litterfall in mangrove forests at continental scale in response to geophysical and climatic environmental drivers. Our results strengthen the global carbon budget for coastal wetlands, providing blue carbon scientists and coastal policy makers with a more accurate representation of the potential of mangroves to offset carbon dioxide emissions.

Pervasively high mangrove productivity in a major tropical delta throughout an ENSO cycle (Southern Caribbean, Colombia)

Rhizophora mangle is a mangrove forest foundation species that can create vegetated habitat, support entire animal and plant communities, and modulate ecosystem processes. Changes in freshwater subsidies over space and time may alter the performance of mangrove foundation species, presumably resulting in disproportionally large effects on energy flow. Yet the linkages between above-ground biomass and below-ground carbon reservoirs across freshwater-subsidy gradients remain poorly understood in many regions. We assessed the dynamics of litterfall production and breakdown of leaf material in a monospecific Rhizophora mangle forest thriving in a small bay in the Atrato River Delta (Southern Caribbean coast of Colombia), during a major El Niño to La Niña transition (from July 2015 to October 2016). Although parameters varied at different spatial scales within the bay, the mean annual total litter production (55 ± 15 g m−2 day−1 or 20 ± 5 Mg ha−1 yr−1) and the mean mass loss of leaf litter (1.0% and 1.7% day−1, above- and below-ground, respectively) are the highest reported for R. mangle-dominated forests worldwide. Total litter production and mass loss rates did not vary significantly over time. The lack of temporal effect was counterintuitive, because these rates typically show strong seasonal variation in mangroves elsewhere in the tropics. We concluded that mangroves in this area constitute a primary productivity hotspot (leaf fall carbon spatio-temporal range: 0.81–1.30 g C m−2 d−1). Based on a comparison of our results and data from the literature, we hypothesize that high freshwater input from the Atrato River subsidizes mangrove plants settled on the deltaic front, which sustain pervasively high litterfall rates. The high mass loss rates of leaf litter observed in this study demonstrate the importance of carbon vertical fluxes across the forest floor in fringing mangroves for carbon sinking.

Impact of ecohydrological fluctuations on iron-redox cycling

Because of its interaction with plants, carbon decomposition, and soil properties, soil iron cycle has been recognized as an important player in the carbon and other biogeochemical cycles. The impact of the hydrologic cycle and plants on it, however, remains largely unexplored. Here we focus on terrestrial ecosystems and develop a dynamical system to analyze the coupling between iron and carbon dynamics in the soil root zone as driven by fluctuations in soil moisture. Particular emphasis is placed on the modeling of the soil organic matter, the Fe-reducing population, the iron Fe2+-Fe3+ cycling, and the soil moisture level, which controls the redox rates. Informed by laboratory and field measurements from a tropical forest, our results suggest that soil moisture and litterfall rates are the primary drivers of iron fluctuations at daily-to-seasonal temporal scales. We also emphasize the important role of soil iron cycle in the decomposition of the organic matter.

Seasonal litterfall composition and carbon and nitrogen returns in New Zealand shrubland

Mānuka–kānuka shrubland is an important carbon (C) sink in New Zealand, yet little is known about C cycling within these systems. The objective of our work was to assess seasonal litterfall rates, composition, and C and nitrogen (N) inputs in mixed mānuka (Leptospermum scoparium J.R. Forst &amp;amp; G. Forst.) and kānuka (Kunzea ericoides var. ericoides (A.Rich) J.Thompson) stands. Litterfall was collected for 2years at Tongariro National Park (NP) and Stoney Creek, Wairarapa (SC), separated into leaf, twig, bark, seed and ‘other’, and the C and N content of each component measured. Total litterfall was between 3557 and 4443kgha–1year–1, of which leaf material contributed 46–67%. Litterfall peaked during spring–summer months at both sites, and the overall litterfall rate was greater (P &amp;lt; 0.001) at SC than NP. Litterfall at SC contained greater (P &amp;lt; 0.001) amounts of ‘other’ due to higher undergrowth contributions, and also greater seed fall (P &amp;lt; 0.001), possibility due to the lower altitude at SC. The proportion of leaf material in litterfall also peaked during summer (P &amp;lt; 0.001). C inputs in the total litter were 1941–2448kgCha–1year–1 and N inputs ranged between 28 and 37kgNha–1year–1. There was little seasonal difference in C and N contents and the majority of both C and N inputs in litterfall were in the leaf material (P &amp;lt; 0.001). C inputs peaked during summer, but N inputs were closely aligned with total litterfall maximums during spring–summer. The leaf:wood ratio was 1.9 at both sites, indicating litter quality was consistent at both stands, regardless of differences in composition. Although the sites had similar rainfall and shrub ages, the rate of total litterfall differed, reflecting the potentially site-specific nature of litterfall in mānuka–kānuka shrubland. Further work is needed assessing litterfall and degradation rates across New Zealand to establish if mānuka–kānuka shrublands would remain carbon sinks under climate change.