Rainfall Gradient Research Articles

Weathering and mineral alteration of metamorphic rocks and genesis of Planosols along a rainfall gradient in Borborema Province, Northeast Brazil

Soils in Northeast Brazil have a correlation with soils in the Northwest Africa, as Planosols are predominant in these regions. However, the weathering and genesis of these soils are little understood, compromising their use and conservation. The present study aimed to investigate weathering and mineralogical changes as well as genesis of Planosols along a pluviometric gradient, considering their formation from gneiss rocks, and analyze climate influence on soil formation. We collected three profiles in different climatic regions of the Brazilian northeastern region and described and classified the samples morphologically. We also performed the physical, chemical, mineralogical, and micromorphological analyses. The results indicated that external material added to soil surface contributed to the formation of surface horizons rich in coarser fractions. Subsurface horizons however were rich in clay (e.g.: P1–2Bn: 433 g kg−1; P2-2B: 427 g kg−1; P3-2Bn1: 429 g kg−1) and were formed from changes of the parent material and the clay formation in situ. Biotite, plagioclases, K-feldspars, and hornblende are the main primary minerals sources for the formation of clay minerals (mainly smectite and kaolinite) from transformation processes and neoformation. Pedoplasmation, monosiallitization, bisiallitization, redoximorphism, and alkalization contributed to the formation of the Planosols studied. Despite incipient soil evolution, the Planosols displayed variation according to the climatic condition. Nevertheless, the incipient mineral alteration favored important soil attributes that could influence agricultural productivity, even in the region with higher precipitation rates.

Alluvial fan response to Alpine Fault earthquakes on the Westland piedmont, Whataroa, Aotearoa‐New Zealand

AbstractWe examined the stratigraphy of alluvial fans formed at the steep range front of the Southern Alps at Te Taho, on the north bank of the Whataroa River in central West Coast, South Island, New Zealand. The range front coincides with the Alpine Fault, an Australian‐Pacific plate boundary fault, which produces regular earthquakes. Our study of range front fans revealed aggradation at 100‐ to 300‐year intervals. Radiocarbon ages and soil residence times (SRTs) estimated by a quantitative profile development index allowed us to elucidate the characteristics of four episodes of aggradation since 1000 CE. We postulate a repeating mode of fan behaviour (fan response cycle [FRC]) linked to earthquake cycles via earthquake‐triggered landslides. FRCs are characterised by short response time (aggradation followed by incision) and a long phase when channels are entrenched and fan surfaces are stable (persistence time). Currently, the Te Taho and Whataroa River fans are in the latter phase. The four episodes of fan building we determined from an OxCal sequence model correlate to Alpine Fault earthquakes (or other subsidiary events) and support prior landscape evolution studies indicating ≥M7.5 earthquakes as the main driver of episodic sedimentation. Our findings are consistent with other historic non‐earthquake events on the West Coast but indicate faster responses than other earthquake sites in New Zealand and elsewhere where rainfall and stream gradients (the basis for stream power) are lower. Judging from the thickness of fan deposits and the short response times, we conclude that pastoral farming (current land‐use) on the fans and probably across much of the Whataroa River fan would be impossible for several decades after a major earthquake. The sustainability of regional tourism and agriculture is at risk, more so because of the vulnerability of the single through road in the region (State Highway 6).

Do populations of Festuca pallescens from a rainfall gradient differ in the expression of morpho-physiological traits under drought stress?

In arid environments, stochastic rainfall and high evapotranspiration force plants to optimize water resources. North Patagonia is characterized by a deep rainfall gradient that gives rise to environments with very contrasting water availability. Festuca pallescens is a key native forage species, growing widely in those environments. To explore morphological and physiological traits involved in the response to drought, we exposed plants from populations sampled along the rainfall gradient to different water availability conditions (Well-watered, Water-pulse, Water-drought). We evaluated morphological traits in all populations to assess inter-population variability and physiological traits between selected populations from sub-humid and arid environments to explore possible macro-environmental responses. Populations showed variation in survival after 45 days of drought conditions and differences in the expression of morphological traits. Also, populations from arid environments were less affected than those from humid environments, showing a longer recovery when they received water pulses. Although a population survival pattern related to the rainfall gradient was not evident, populations from arid environments exhibited local adaptation to their home environments, taking better advantage of water pulses. These results provide information about the response of a non-model species to environments with contrasting water availability and possibly, to changing rain patterns in arid environments under climate change. Data Availability StatementThe data that support the findings of this study are available from the corresponding author upon reasonable request.

Palaeocene high-latitude leaf flora of Antarctica Part 2: Tooth-margined angiosperms

The late Palaeocene (Thanetian) Cross Valley Formation on Seymour Island is one of the few floras of this age in Antarctica. Understanding the diversity is critical for comparisons with coeval high-latitude forests and their evolution across South America and Australasia. This macroflora has few conifers and ferns but a significant diversity of angiosperms. Taxonomic assessment of previously undescribed morphotypes, based on new and existing collections, facilitated the identification of twelve new tooth-margined angiosperms of uncertain affinities. Comparisons to Gondwanan families already established in the Cross Valley Flora from pollen and wood records include Nothofagaceae, Proteaceae and Cunoniaceae. A total of 37 angiosperm taxa, including 23 tooth-margined and 14 entire-margined morphotypes, have been described from the Cross Valley Flora. They provide evidence that the most diverse Palaeocene floral assemblage recorded from this region inhabited the east side of the Antarctic Peninsula. The new morphotypes enrich the cool-temperate nature of the angiosperms that dominated over the conifers in these high-latitude forests. They indicate dynamic forests that responded to frequent disturbance as they grew on the slopes of active volcanoes; a physical divide that created a rainfall and climatic gradient from east to west across the Antarctic Peninsula. Floral elements, such as Proteaceae that indicates drier climates and Nothofagaceae that indicates cooler climates, signify that these forests were on the lee side of volcanoes on the Antarctic Peninsula. These forests represent extirpation and retraction of the Valdivian cool-temperate rainforests, which extended further south than their modern Patagonian distribution, in southern South America. Thus, the new morphotypes suggest additional floral diversity was present in the late Palaeocene and highlight the diversity contrast between the east and west Antarctic Peninsula.

Microbial community shifts in pearl millet root zone soils with Guiera senegalensis intercropping along a rainfall and soil type gradient in the Sahel

AbstractThe Sahel of West Africa has vulnerable agroecosystems that threatens food security. A potential solution is intercropping with the indigenous shrub, Guiera senegalensis J.F. Gmel. Previous research of the Optimized Shrub‐intercropping System (OSS) (high density of ∼1,500 shrubs ha‐1 and coppiced residue incorporation) has been shown to dramatically improve pearl millet [Pennisetum glaucum (L.) R. Br.] yield, which is attributed to improved soil quality, nutrient and water availability, and harboring a distinct microbial community. Whether this response is consistent over a climate and soil type gradient in farmers’ fields has not been investigated. Therefore, the objective was to determine the impact of G. senegalenis on soil chemistry, enzyme activity, microbiomes, and metabolic pathways of millet root zone soils in farmers’ fields. The experiment was a three‐by‐two factorial with three rainfall and soil type sites along a north–south gradient in the Senegal Peanut Basin and two sampling locations (millet root zone soil within and outside the influence of the G. senegalensis). Guiera senegalensis shifted certain predicted bacterial metabolic pathways and enriched some bacterial and fungal genera. Notably, the increased crop growth due to G. senegalensis positively correlated with the abundance of genera having plant growth promoting properties (e.g., Enterobacter agglomerans and Paraburkholderia). Paucibacter, a genera that has deleterious and/or pathogenic properties, was highly abundant in non‐shrub soil but completely suppressed beneath the shrub. The results showed that G. senegalensis in farmers’ fields even at typical, low densities, where coppiced residues are annually burned, still increased soil chemical and microbial properties, suggesting that a more important factor than litter is the presence of shrub roots that provide root turnover and exudates, and water inputs through hydraulic lift.

The role of dominant prairie species ecotypes on plant diversity patterns of restored grasslands across a rainfall gradient in the US Great Plains

AbstractQuestionsA robust ecosystem requires a functionally heterogeneous community of organisms with ecological traits that permit broad resource partitioning. Understanding community diversity patterns can help investigate drivers of community assembly and assess restoration success. Do biodiversity patterns differ among grassland communities sown with different ecotypes of dominant species during restoration along a rainfall gradient in the tallgrass prairie of the central US Great Plains?LocationFour field sites across a rainfall gradient within the North American Great Plains: Colby, Kansas (39°23′17.8″N, 101°04′57.4″W), Hays, Kansas (38°51′13.2″N, 99°19′08.6″W), Manhattan, Kansas (39°08′22.3″N, 96°38′23.3″W), and Carbondale, Illinois (IL, 37°41′47.0″N, 89°14′19.2″W).MethodsWe applied linear mixed models to assess the effect of dominant species ecotype, year, and location on grassland taxonomic, phylogenetic, and functional diversity.ResultsThe non‐local grass ecotype (compared to the local ecotype) promoted species richness. In contrast, the effect of the dominant species ecotype on phylogenetic or functional diversity was site‐specific over the 10‐year restoration. Richness decreased across the rainfall gradient from dry to moist sites, and the wettest site had the highest phylogenetic and functional diversity.ConclusionsOur results suggest that abiotic filtering by rainfall is a key assembly mechanism that could predict grassland changes in biodiversity in the early restoration phases. Given the community response across the tallgrass prairie, restoration practitioners should consider the impact of regional sources of dominant species used in restoration when biodiversity is a restoration goal. It is recommended for future grassland restoration to detect gaps and limitations in evolutionary and trait structure that will reveal which diversity components to evaluate.

Herbivores disrupt the flow of food resources to termites in dryland ecosystems.

Irruption of herbivore populations due to the extirpation of predators has led to dramatic changes in ecosystem functioning worldwide. Herbivores compete with other species for their primary source of nutrition, plant biomass. Such competition is typically considered to occur between species in closely related clades and functional groups but could also occur with detritivores that consume senescent plant biomass. In this study, we tested predictions that in ecosystems where herbivores are not regulated by predators, their indirect impacts on dead vegetation increase with primary productivity and extend to termites that feed on senescent vegetation. We compared dead vegetation cover and termite activity in herbivore exclosures and associated grazed plots at three locations situated along a rainfall gradient in arid Australia where kangaroo populations have irrupted. Dead vegetation cover and termite activity increased with rainfall in ungrazed plots but showed a negligible response to rainfall in grazed plots. Our results suggest that grazing can disrupt the flow of energy to detritivores and decouple the relationship between termite activity and primary productivity. Such disruption could have far-reaching impacts on arid ecosystems because many organisms sit within "brown food webs" that are sustained by energy derived from the decomposition of senescent plant tissues.

Vessel dimorphism and wood traits in lianas and trees among three contrasting environments.

Determining how xylem vessel diameters vary among plants and across environments gives insights into different water-use strategies among species and ultimately their distributions. Here, we tested the vessel dimorphism hypothesis that the simultaneous occurrence of many narrow and a few wide vessels gives lianas an advantage over trees in seasonally dry environments. We measured the diameters of 13,958 vessels from 15 liana species and 10,430 vessels from 16 tree species in a tropical seasonal rainforest, savanna, and subtropical evergreen broadleaved forest. We compared differences in mean and hydraulically weighted vessel diameter (MVD and Dh ), vessel density (VD), theoretical hydraulic conductivity (Kt ), vessel area fraction (VAF), and wood density (WD) between lianas and trees and among three sites. Nine liana species and four tree species had dimorphic vessels. From the tropical seasonal rainforest to the savanna, liana MVD, Dh and Kt decreased, and VD and WD increased, while only tree WD increased. From the tropical seasonal rainforest to the subtropical forest, six wood traits remained unchanged for lianas, while tree MVD, Dh and Kt decreased and VD increased. Trait space for lianas and trees were more similar in the savanna and more divergent in the subtropical forest compared to the tropical seasonal rainforest. These results suggest that lianas tend to possess greater vessel dimorphism, which may explain how lianas grow well during seasonal drought, influencing their unique distribution across tropical rainfall gradients.

Successional shifts in tree demographic strategies in wet and dry Neotropical forests

AbstractAimTropical forest succession and associated changes in community composition are driven by species demographic rates, but how demographic strategies shift during succession remains unclear. Our goal was to identify generalities in demographic trade‐offs and successional shifts in demographic strategies across Neotropical forests that cover a large rainfall gradient and to test whether the current conceptual model of tropical forest succession applies to wet and dry forests.LocationMexico and Central America.Time period1985–2018.Major taxa studiedTrees.MethodsWe used repeated forest inventory data from two wet and two dry forests to quantify demographic rates of 781 tree species. For each forest, we explored the main demographic trade‐offs and assigned tree species to five demographic groups by performing a weighted principal components analysis to account for differences in sample size. We aggregated the basal area and abundance across demographic groups to identify successional shifts in demographic strategies over the entire successional gradient from very young (<5 years) to old‐growth forests.ResultsAcross all forests, we found two demographic trade‐offs, namely the growth–survival trade‐off and the stature–recruitment trade‐off, enabling the data‐driven assignment of species to five demographic strategies. Fast species dominated early in succession and were then replaced by long‐lived pioneers in three forests. Intermediate and slow species increased in basal area over succession in all forests, but, in contrast to the current conceptual model, long‐lived pioneers continued to dominate until the old‐growth stage in all forests. The basal area of short‐lived breeders was low across all successional stages.Main conclusionsThe current conceptual model of Neotropical forest succession should be revised to incorporate the dominance of long‐lived pioneers in late‐successional and old‐growth forests. Moreover, the definition of consistent demographic strategies that show clear dominance shifts across succession substantially improves the mechanistic understanding and predictability of Neotropical forest succession.

Economic aspects of urban greenness along a dryland rainfall gradient: A time-series analysis

Studies at the global scale show that urban greenness is not equally distributed across and within cities. Yet, quantification of urban greening trends in drylands is still lacking. We have modeled urban greenness dynamics and its determinants, using Landsat-based time-series analyses of NDVI and census data, of 23 localities along a dryland climatic gradient in southern Israel, between 1997 and 2019. NDVI trends and their associated temporal changes in local average wages, age of locality, and average annual rainfall were analyzed by exploiting a panel structure and model parameters estimation with fixed effects to control for unobserved differences between localities, and to strengthen causal interpretation. Results show positive NDVI trends in all localities—indicating that urban construction increases greenness in drylands. However, the localities varied greatly in the slopes of their respective linear NDVI trends (0.05 <α < 0.161). Specifically, we found that, ceteris paribus, the differences in average wages between localities is the most important factor in explaining spatio-temporal differences between the respective NDVI of localities. Given the well-known high correlation between individuals’ wages and other characteristic such as education and income (all important determinants of the Socio-Economic classification of local authorities), this leads us to conclude that residents’ economic characteristics are an important predictor of the level of greenness. Harlin’s Granger causality test for wages and NDVI panel data indicate that wage is a Granger cause of urban greenness (p < 0.001)—however, greenness was not found to be the driving force of wage.

Contributions of hypolithic communities to surface soil organic carbon across a hyperarid-to-arid climate gradient

Carbon (C) in soils accounts for a substantial and dynamic portion of the global C cycle, with concentrations of soil organic carbon (SOC) typically closely linked to vascular plant productivity. However, C fixed by non-vascular photosynthetic organisms may account for a sizeable proportion of SOC in locations where vascular plants are not abundant. In the hyperarid Namib Desert, vascular plant growth is very limited and largely ephemeral. Extreme abiotic conditions even limit establishment of cyanobacterial soil crusts, with photoautotrophic cyanobacteria largely restricted to the undersides of translucent quartz clasts that buffer environmental extremes. The importance of these ‘hypoliths’ in enhancing SOC pools, and how this may vary with climate and clast physical characteristics, remains unknown. We worked across a rainfall and fog gradient in the central Namib to assess quartz clast size and distribution, factors affecting the probability of hypolithic colonization, and the landscape-level influence of hypoliths on surface SOC pools. Clast colonization increased with clast thickness and with increasing mean annual rainfall. SOC and chlorophyll a (a proxy for cyanobacterial biomass) concentrations were greater under colonized quartz clasts than under non-quartz clasts or in bare soil. Landscape-level SOC, estimated by combining SOC concentration with the distribution and colonization of quartz clasts, was greatest at the site with the highest rainfall and lowest at a mid-gradient site with moderate rainfall and fog. Climate change scenarios that promote quartz colonization have the potential to double SOC pools at the mid-gradient site, although SOC changes would be more muted at other sites. Low vascular plant and biocrust cover in the Namib Desert allows hypolithic communities to play an outsized role in SOC pools; a loss of all hypolithic cyanobacteria would lead to 10–20% declines in surface SOC pools. Future climate change has the potential to shift surface SOC if it alters cyanobacterial colonization of quartz.

Substrate type and discovery govern decomposition along a savanna rainfall gradient

Substrate type and discovery govern decomposition along a savanna rainfall gradient

Phytoplankton and macroinvertebrate diversity and eco-exergy responses to rainfall diverge in semiarid reservoirs

Rainfall is a variable that drives community structure in aquatic ecosystems and is even more important in dry regions. Most studies have evaluated how the seasonal variation of rainfall influences aquatic communities. However, studies that evaluate the influence of spatial gradients of rainfall on aquatic organisms are scarce, especially regarding biological diversity and thermodynamic information. This study evaluated the response from diversity metrics and thermodynamic indices (eco-exergy and specific eco-exergy) in phytoplankton and benthic macroinvertebrate communities in 13 reservoirs distributed along a spatial gradient of rainfall in the Brazilian semiarid region. We hypothesized that the phytoplankton and benthic macroinvertebrate communities are congruent in response to the spatial gradient of rainfall in terms of diversity and thermodynamic indices in semiarid reservoirs. We expect to find higher values of biomass and the eco-exergy index under lower rainfall. To test our hypothesis, generalized linear models (GLMs) were used to relate the effects of rainfall changes on community structure. The eco-exergy and specific eco-exergy indices of the phytoplankton community were positively related to rainfall and reflected a greater investment in biomass production and genetic information (e.g., Chlorophyceae). As for the macroinvertebrate community, the reduction in rainfall promoted a greater investment in energy storage by generalist organisms (e.g., Melanoides tuberculata and some genera of Chironomidae). Thus, for a holistic evaluation of ecosystem health, the joint analysis of thermodynamic indices associated with an assessment of taxonomic composition, diversity and taxonomic richness is important, providing a more complete and multifaceted response. Additionally, thermodynamic indices better reflected the changes in both aquatic communities in response to rainfall, and were more sensitive than the diversity and richness indices. Consequently, the thermodynamic indices represent an efficient tool in the assessment of aquatic community responses to climate change, even when these communities respond in an opposite manner.

Contrasting dynamics of seed banks and standing vegetation of annuals and perennials along a rainfall gradient

The soil seed bank is a major component of plant communities. However, long-term analyses of the dynamics of the seed bank and the ensuing vegetation are rare. Here, we studied the dynamics in plant communities with high dominance of annuals in Mediterranean, semiarid, and arid ecosystems for nine consecutive years. For annuals, we hypothesized that the density of the seed bank would be more stable than the density of the standing herbaceous vegetation. Moreover, we predicted that differences in temporal variability between the seed bank and the vegetation would increase with aridity, where year-to-year rainfall variability is higher. We found that the temporal variability at the population level (assessed as the standard deviation of the loge-transformed density) of the nine dominant annuals in each site did not differ between the seed bank and the ensuing vegetation in any of the sites. For the total density of annuals, patterns depended on aridity. In the Mediterranean site, the temporal variability was similar in the seed bank and the vegetation (0.40 vs. 0.40). Still, in the semiarid and arid sites, variability in the seed bank was lower than in the vegetation (0.49 vs. 1.01 and 0.63 vs. 1.38, respectively). This difference between the population-level patterns and the total density of annuals can be related to the lower population synchrony in their seed bank. In contrast, for the herbaceous perennials (all species combined), the seed bank variability was higher than in the vegetation. Overall, our results highlight the role of the seed bank in buffering the annual vegetation density with increasing climatic uncertainty typical in aridity gradients. This role is crucial under the increasing uncertainty imposed by climatic change in the region.

Local‐ and landscape‐scale drivers of terrestrial herbaceous plant diversity along a tropical rainfall gradient in Western Ghats, India

Abstract In tropical forests, understory herbaceous angiosperms (herbs), which can comprise up to 40% of plant species richness, have received relatively little attention compared with trees, and their diversity patterns and drivers remain poorly understood. While tropical tree diversity has been shown to be driven primarily by water availability, we hypothesized that herb diversity may be equally or more limited by light availability. To test the importance of water and light in shaping herb diversity, we surveyed herb communities in 13 one‐ha plots along a rainfall gradient in a seasonally dry tropical forest landscape in India. In each plot, herbs were censused thrice during the year in 47–50 one‐m2 subplots. We examined drivers of herb diversity and ground cover at the landscape scale (across plots) and local scale (among subplots) using simple linear regression and linear mixed models, respectively. At the landscape scale, herb diversity and ground cover were negatively related to rainfall and soil moisture, in contrast with previous studies of tropical forest trees. This indicates that water was not limiting for herbs. Instead, light availability, which was negatively correlated with rainfall, appears to drive patterns of herb diversity and ground cover across the gradient. Herb diversity and ground cover were uniformly low across the gradient during the dry season, indicating that water limitation does have a seasonal impact. When water was abundant (rainy season), herbs took advantage of higher light availability at more open (lower‐rainfall) plots. Consistent with landscape‐scale patterns, herb richness at the subplot level was positively related to light availability, with its effect stronger at higher‐rainfall (more closed‐canopy) sites. Herb species richness declined with increasing subplot soil moisture in all sites and seasons, suggesting negative impacts of water logging and high moisture. Synthesis. Diversity of understory herbs is limited by light availability at both local and landscape scales in this region, with the impact of water limitation restricted to the dry season. Our study shows that tree diversity patterns cannot be assumed to hold for other plant lifeforms, and conservation and restoration efforts must consider unique strategies for different lifeforms.

Moisture Sources and Pathways Determine Stable Isotope Signature of Himalayan Waters in Nepal

AbstractThe Himalayan mountain range produces one of the steepest and largest rainfall gradients on Earth, with &gt;3 m/yr rainfall difference over a ∼100 km distance. The Indian Summer Monsoon (ISM) contributes more than 80% to the annual precipitation budget of the central Himalayas. The remaining 20% falls mainly during pre‐ISM season. Understanding the seasonal cycle and the transfer pathways of moisture from precipitation to the rivers is crucial for constraining water availability in a warming climate. However, the partitioning of moisture into the different storage systems such as snow, glacier, and groundwater and their relative contribution to river discharge throughout the year remains under‐constrained. Here, we present novel field data from the Kali Gandaki, a trans‐Himalayan river, and use 4‐year time series of river and rain water stable isotope composition (δ18O and δ2H values) as well as river discharge, satellite Global Precipitation Measurement amounts, and moisture source trajectories to constrain hydrological variability. We find that rainfall before the onset of the ISM is isotopically distinct and that ISM rain and groundwater have similar isotopic values. Our study lays the groundwork for using isotopic measurements to track changes in precipitation sources during the pre‐ISM to ISM transition in this key region of orographic precipitation. Specifically, we highlight the role of pre‐ISM precipitation, derived from the Gangetic plain, to define the seasonal river isotopic variability across the central Himalayas. Lastly, isotopic values across the catchment document the importance of a large well‐mixed groundwater reservoir supplying river discharge, especially during the non‐ISM season.

Distribution of soil bacteria involved in C cycling across extensive environmental and pedogenic gradients

AbstractMicroorganisms play pivotal roles in soil processes. Metabolically related microorganisms constitute functional groups, and diverse microbial functional groups control nutrient cycling in soils. This study explored environmental (i.e., rainfall, temperature) and soil factors driving the distribution of bacterial functional groups involved in soil carbon (C) cycling in paired natural and agricultural ecosystems. Soil samples were collected at a regional scale covering gradients of temperature and rainfall across two orthogonal transects (~1000 km) in New South Wales, Australia. Putative functions of bacteria were linked to two soil C fractions: particulate organic carbon (POC) and mineral‐associated organic carbon (MAOC). We found: (i) temperature and rainfall were important drivers of bacterial functional groups, while soil properties, such as pH, soil C and nitrogen (N), also presented significant contributions; (ii) community structure of bacteria involved in C cycling was mainly related to POC content but not to MAOC; (iii) paired sampling showed that agricultural practices had significant impacts on the composition and responses of soil bacterial functional groups. This study demonstrated the environmental regulation (e.g., temperature and rainfall) of soil microbial functional groups at large scales, which was altered by agricultural practices.Highlights Soil bacteria involved in C cycling were investigated across two ~1000 km transects. Temperature and rainfall were important drivers of bacterial functional groups at large scale. Paired sampling showed that agriculture led to a significant shift in bacterial functional groups. Community structure of bacterial functional groups were correlated with soil POC but not MAOC.

Linking resource- and disturbance-based models to explain tree-grass coexistence in savannas.

Savannas cover a significant fraction of the Earth's land surface. In these ecosystems, C3 trees and C4 grasses coexist persistently, but the mechanisms explaining coexistence remain subject to debate. Different quantitative models have been proposed to explain coexistence, but these models make widely contrasting assumptions about which mechanisms are responsible for savanna persistence. Here, we show that no single existing model fully captures all key elements required to explain tree-grass coexistence across savanna rainfall gradients, but many models make important contributions. We show that recent empirical work allows us to combine many existing elements with new ideas to arrive at a synthesis that combines elements of two dominant frameworks: Walter's two-layer model and demographic bottlenecks. We propose that functional rooting separation is necessary for coexistence and is the crux of the coexistence problem. It is both well-supported empirically and necessary for tree persistence, given the comprehensive grass superiority for soil moisture acquisition. We argue that eventual tree dominance through shading is precluded by ecohydrological constraints in dry savannas and by fire and herbivores in wet savannas. Strong asymmetric grass-tree competition for soil moisture limits tree growth, exposing trees to persistent demographic bottlenecks.

Manganese Oxidation States in Volcanic Soils across Annual Rainfall Gradients.

Manganese (Mn) exists as Mn(II), Mn(III), or Mn(IV) in soils, and the Mn oxidation state controls the roles of Mn in numerous environmental processes. However, the variations of Mn oxidation states with climate remain unknown. We determined the Mn oxidation states in highly weathered bulk volcanic soils (primary minerals free) across two rainfall gradients covering mean annual precipitation (MAP) of 0.25-5 m in the Hawaiian Islands. With increasing MAP, the soil redox conditions generally shifted from oxic to suboxic and to anoxic despite fluctuating at each site; concurrently, the proportions of Mn(IV) and Mn(II) decreased and increased, respectively. Mn(III) was low at both low and high MAP, but accumulated substantially, up to 80% of total Mn, in soils with prevalent suboxic conditions at intermediate MAP. Mn(III) was likely hosted in Mn(III,IV) and iron(III) oxides or complexed with organic matter, and its distribution among these hosts varied with soil redox potentials and soil pH. Soil redox conditions and rainfall-driven leaching jointly controlled exchangeable Mn(II) in soils, with its concentration peaking at intermediate MAP. The Mn redox chemistry was at disequilibrium, with the oxidation states correlating with long-term average soil redox potentials better than with soil pH. The soil redox conditions likely fluctuated between oxic and anoxic conditions more frequently at intermediate than at low and high MAP, creating biogeochemical hot spots where Mn, Fe, and other redox-sensitive elements may be actively cycled.

Edaphic controls of soil organic carbon in tropical agricultural landscapes

Predicting soil organic carbon (SOC) is problematic in tropical soils because mechanisms of SOC (de)stabilization are not resolved. We aimed to identify such storage mechanisms in a tropical soil landscape constrained by 100 years of similar soil inputs and agricultural disturbance under the production of sugarcane, a C4 grass and bioenergy feedstock. We measured soil physicochemical parameters, SOC concentration, and SOC dynamics by soil horizon to one meter to identify soil parameters that can predict SOC outcomes. Applying correlative analyses, linear mixed model (LMM) regression, model selection by AICc, and hierarchical clustering we found that slow moving SOC was related to many soil parameters, while the fastest moving SOC was only related to soil surface charge. Our models explained 78–79%, 51–57%, 7–8% of variance in SOC concentration, slow pool decay, and fast pool decay, respectively. Top SOC predictors were roots, the ratio of organo-complexed iron (Fe) to aluminum (Al), water stable aggregates (WSagg), and cation exchange capacity (CEC). Using hierarchical clustering we also assessed SOC predictors across gradients of depth and rainfall with strong reductions in Roots, SOC, and slow pool decay associated with increasing depth, while increased rainfall was associated with increased Clay and WSagg and reduced CEC in surface soils. Increased negative surface charge, water stable aggregation, organo-Fe complexation, and root inputs were key SOC protection mechanisms despite high soil disturbance. Further development of these relationships is expected to improve understanding of SOC storage mechanisms and outcomes in similar tropical agricultural soils globally.