Seasonal Precipitation Regime Research Articles

Seasonal drought treatments impact plant and microbial uptake of nitrogen in a mixed shrub grassland on the Colorado Plateau.

For many drylands, both long- and short-term drought conditions can accentuate landscape heterogeneity at both temporal (e.g., role of seasonal patterns) and spatial (e.g., patchy plant cover) scales. Furthermore, short-term drought conditions occurring over one season can exacerbate long-term, multidecadal droughts or aridification, by limiting soil water recharge, decreasing plant growth, and altering biogeochemical cycles. Here, we examine how experimentally altered seasonal precipitation regimes in a mixed shrub grassland on the Colorado Plateau impact soil moisture, vegetation, and carbon and nitrogen cycling. The experiment was conducted from 2015 to 2019, during a regional multidecadal drought event, and consisted of three precipitation treatments, which were implemented with removable drought shelters intercepting ~66% of incoming precipitation including: control (ambient precipitation conditions, no shelter), warm season drought (sheltered April-October), and cool season drought (sheltered November-March). To track changes in vegetation, we measured biomass of the dominant shrub, Ephedra viridis, and estimated perennial plant and ground cover in the spring and the fall. Soil moisture dynamics suggested that warm season experimental drought had longer and more consistent drought legacy effects (occurring two out of the four drought cycles) than either cool season drought or ambient conditions, even during the driest years. We also found that E. viridis biomass remained consistent across treatments, while bunchgrass cover declined by 25% by 2019 across all treatments, with the earliest declines noticeable in the warm season drought plots. Extractable dissolved inorganic nitrogen and microbial biomass nitrogen concentrations appeared sensitive to seasonal drought conditions, with dissolved inorganic nitrogen increasing and microbial biomass nitrogen decreasing with reduced soil volumetric water content. Carbon stocks were not sensitive to drought but were greater under E. viridis patches. Additionally, we found that under E. viridis, there was a negative relationship between dissolved inorganic nitrogen and microbial biomass nitrogen, suggesting that drought-induced increases in dissolved inorganic nitrogen may be due to declines in nitrogen uptake from microbes and plants alike. This work suggests that perennial grass plant-soil feedbacks are more vulnerable to both short-term (seasonal) and long-term (multiyear) drought events than shrubs, which can impact the future trajectory of dryland mixed shrub grassland ecosystems as drought frequency and intensity will likely continue to increase with ongoing climate change.

Effects of seasonal precipitation regimes on microbial biomass and extracellular enzyme activity during shrub foliar litter decomposition in a subtropical forest

Elucidating the mechanisms underlying microbial biomass and extracellular enzyme activity responses to the seasonal precipitation regime during foliar litter decomposition is highly important for understanding the material cycle of forest ecosystems in the context of global climate change; however, the specific underlying mechanisms remain unclear. Hence, a precipitation manipulation experiment involving a control (CK) and treatments with decreased precipitation in the dry season and extremely increased precipitation in the wet season (IE) and decreased precipitation in the dry season and proportionally increased precipitation in the wet season (IP) was conducted in a subtropical evergreen broad-leaved forest in China from October 2020 to October 2021. The moisture, microbial biomass, and extracellular enzyme activities of foliar litter from two dominant shrub species, Phyllostachys violascens and Alangium chinense, were measured at six stages during the dry and wet seasons. The results showed that (1) both IE and IP significantly decreased the microbial biomass carbon and microbial biomass nitrogen content and the activities of β-1,4-glucosidase, β-1,4-N-acetylglucosaminidase, acid phosphatase and cellulase in the dry season, while the opposite effects were observed in the wet season. (2) Compared with those of IE, the effects of IP on foliar litter microbial biomass and extracellular enzyme activity were more significant. (3) The results from the partial least squares model indicated that extracellular enzyme activity during foliar litter decomposition was strongly controlled by the foliar litter water content, microbial biomass nitrogen, the ratio of total carbon to total phosphorus, foliar litter total carbon, and foliar litter total nitrogen. These results provide an important theoretical basis for elucidating the microbial mechanisms driving litter decomposition in a subtropical forest under global climate change scenarios.

Seasonal precipitation changes in the western Mediterranean Basin: The case of the Spanish mainland, 1916–2015

AbstractThis study examines the spatial and temporal variations in seasonal precipitation patterns across the Spanish mainland, within the western Mediterranean Basin, spanning the years 1916–2015. Utilizing the recently developed MOPREDAS_century database at a 10 × 10 km grid resolution, our analysis reveals predominantly negative trends in precipitation across the study area over the entire period, primarily driven by declines in spring precipitation, with lesser decreases in summer and winter. Notably, these trends exhibit complexity, manifesting as two distinct pulses occurring roughly at the outset and conclusion of the 20th century. However, upon employing a moving windows analysis, we find that since the mid‐1970s, seasonal precipitation trends have largely been nonsignificant. Spatially, the precipitation regime across 1916–2015 delineates three primary zones: a winter‐maximum zone in the north, an autumn‐maximum zone along the eastern coast, and a transition from winter‐maximum to spring‐maximum towards the western and inland regions. Noteworthy shifts in this spatial distribution are evident over the four 25‐year intervals examined. Initially, during 1916–1940, winter, autumn and spring regimes occupied 44.1%, 24.2% and 31.7% of the land, respectively. Subsequently, in the periods 1941–1965 and 1966–1990, winter dominance expanded to over 50% of the grid area, while spring varied between 34.7% and 25.5%, and autumn decreased notably to 12.4% and 13.5%. Lastly, significant alterations occurred in the final period of 1991–2015, with winter coverage decreasing to 33.7%, spring to 15.8% and autumn expanding to 50.1% of the grid area. In summary, the prevalent winter and spring regimes at the dawn of the 20th century have transitioned towards an autumn‐dominated regime, particularly in much of the central and western Spanish mainland, with these shifts possibly linked to the evolving precipitation trends. These findings contribute novel insights, particularly pre‐1950, and align with published results post‐1950 across the Mediterranean, particularly in its western regions. Furthermore, they suggest a potential connection between changes in the spatial distribution of seasonal precipitation regimes and temporal variations in the primary moisture sources over the study area.

Effect of the seasonal precipitation regime on shrub litter decomposition in a subtropical forest

Changes in the seasonal precipitation regime induced by global climate change may alter moisture and the dynamics of related microbes and enzymes, thereby significantly affecting forest litter decomposition; however, the specific mechanisms remain unclear. Hence, a precipitation manipulation experiment including the treatments of control (CK), decreased precipitation in the dry season with extremely increased precipitation in the wet season (IE) and decreased precipitation in the dry season with proportionally increased precipitation in the wet season (IP) was performed from October 2020 to October 2021 in a subtropical evergreen broad-leaved forest in China. The moisture, chemical properties, microbial biomass, enzyme activity, and decomposition rates of two dominant shrub species foliar litter, Phyllostachys violascens and Alangium chinense, were measured at six stages during the dry and wet seasons. The results showed that both IE and IP significantly increased the litter decomposition rates (more than 2.0% for P. violascens and more than 4.7% for A. chinense, respectively), while the effects of IE were the most significant, which was mainly related to the precipitation availability and the frequency of dry-wet cycle. Furthermore, the IE and IP increased the wet season’s contribution to the annual total decomposition regardless of litter species, and more significant effects were also observed in IE. Additionally, the results of structural equation model indicated that the seasonal precipitation regime could regulate the decomposition process directly by changing moisture and the contents of elements and refractory substances, as well as indirectly by influencing the related microbial biomass and enzyme activity. These results deepen the understanding of the biogeochemical cycle in forest ecosystems under the global climate change scenarios.

Wood anatomical spectrum of co-occurring species in early and late-successional tropical dry forest communities

Key messageAnalysis of wood anatomical traits revealed that drought tolerance predominates in early-successional communities, and vessel variability is relevant for species’ ecological success in seasonally dry tropical environments.Successional tropical dry forests harbor a diverse array of species subjected to a strong seasonal precipitation regime. Considering that wood encompasses diverse functional properties related to stem hydraulics and mechanical support, in this study, we asked which quantitative anatomical traits are exhibited by co-occurring species in early (EC)- and late (LC)-successional communities. We used generalized linear mixed-effects models to estimate and compare means and standard deviations of wood traits (vessel area, vessel density, vessel grouping, and fiber dimensions) between eight EC and between 13 LC co-occurring species. Wood traits were highly heterogeneous. High vessel redundancy, a property associated with water safety involved in the prevention of hydraulic failure, prevails among species coexisting in EC but is maintained through succession by the same species in LC. Highly variable fiber dimensions indicate that species coexisting in LC possess different characteristics associated with mechanical resistance. Low wood density species typical of LC, which are absent from EC, displayed wood anatomical features associated with drought-evasion mechanisms. This result implies that deforestation not only reduces taxonomic but also functional diversity. Future research should adopt a standard deviation analysis approach, as this will allow confirming the high variability in vessel diameter, even in diffuse porous species, as a key feature for the ecological success of plants facing succession-driven changes in seasonally dry environments.

Linking upwelling intensity and orbital-scale climate variability in South Africa's winter rainfall zone: Insights from a ∼70,000-year hyrax midden record

The climate of Africa's southwestern Cape is characterised by a strongly seasonal winter precipitation regime, with late Quaternary climate variability generally considered to have been driven by the position of the southern westerlies. This paper presents a unique ∼70,000 year-long palaeoclimatic record from a rock hyrax midden from South Africa's winter rainfall zone, enabling the analysis of regional climate systems since the beginning of marine isotope stage 4. The data suggest that the last glacial period was relatively humid compared to the Holocene, likely due to cooler temperatures, more extensive Antarctic sea-ice extent and an equatorward displacement of the westerly storm track. However, orbital-scale climate variability associated with the 23 kyr precessional cycle primarily correlates with changes in upwelling intensity in the Benguela system, implying an important role for the blocking of tropical easterly flow in driving long-term climatic variability. These factors combined during glacial periods to significantly amplify rainfall seasonality in the southwestern Cape, bringing more winter rainfall via mid-latitude frontal systems, while reducing the proportion of summer rainfall, particularly during the glacial periods of the late Quaternary. The results therefore highlight the need to consider a complex suite of circulation systems and dynamics when inferring drivers of long-term environmental change in the region.

Implications of submonthly oxygen and carbon isotope variations in late Pleistocene Melanopsis shells for regional and local hydroclimate in the upper Jordan River valley

AbstractMany water-stressed regions of the globe have a highly seasonal precipitation regime. However, seasonality in the past and under changing climates is little studied. Submonthly records of sclerochronological δ18O and δ13C values of Melanopsis shells from the Jordan River Dureijat archaeological site (JRD) in the upper Jordan River valley presented here document the hydrology of paleo-Lake Hula. These records were assessed for changes in seasonal hydrology in the lake and compared with modern shells collected from present-day waterbodies in northern Israel and with models of δ18Oshell. Results from shells in sediments dating from the last glacial maximum (LGM) to the Bølling-Allerød imply changes in waterbody size that qualitatively parallel changes in the late Pleistocene Lake Lisan levels; Hula Lake was well buffered when Lake Lisan stood at a high stand and poorly buffered when water levels were lower. Furthermore, data from shells dated to the LGM suggest inflowing water with lower δ18O values than local rainfall, providing evidence for a greater proportion of snow in the catchment than today. Reconstruction of water δ18O and mixing-model calculations suggest that snowmelt contribution to spring water during the LGM may have been more than twice the amount in the modern-day catchment.

A spatially explicit model for density that accounts for availability: a case study withMojavedesert tortoises

AbstractEstimating population density and identifying those areas where density is changing through time are central to prioritizing conservation and management strategies. Obtaining reliable estimates of density and trends can be challenging, however, especially for long‐lived species that are rare, have broad geographic distributions, and are difficult to detect reliably during field surveys. We developed a hierarchical model for distance‐sampling data that characterizes spatial variation in density at two scales and simultaneously estimates regional trends while accounting for variation in detection probability and availability across surveys. We applied the model to data collected over a 20‐year period (2001–2020) in an area that encompassed most of the geographic range of the Mojave desert tortoise (Gopherus agassizii). Density of adult tortoises varied with multiple biotic and abiotic features, including topography, aspect, geology, and seasonal precipitation and temperature regimes. Across the entire period and study area, the density of adult tortoises decreased by an average of 1.8% per year (95% CI = −3.5% to −0.2%). Trends varied geographically, however, with the steepest declines in the western part of the range (−4.1%, −6.9% to −1.3%). Accounting for habitat loss across our study area, the abundance of this threatened species declined by an estimated 129,000 adults (36%) between 2001 and 2020. Our modeling approach extends traditional distance‐sampling frameworks by accounting for ecological and observational processes that could mask spatiotemporal variation in density and, at the same time, provides spatially explicit estimates to guide conservation and management strategies for tortoises and other rare species.

Seasonal temperature and precipitation regimes drive variation in the wood of oak species (Quercus) along a climatic gradient in western Mexico

Summary To achieve optimal balance among wood’s support, storage, and transport functions, tree species present adaptive variation in wood anatomical traits. Oaks (Quercus) are woody plants that are dominant in many ecosystems, possess wide variation in wood traits, and can tolerate water stress in diverse habitats. Mountain regions contain steep climatic gradients that exert strong environmental pressures on wood traits. The aims of this study were: (1) to quantify the anatomical variation in 21 oak species distributed along an environmental gradient and (2) to analyze the interspecific variation among the studied species. Fibres and parenchyma fractions were the most abundant cell types. We found a trade-off between total fibre fraction and total parenchyma fraction modulated by the precipitation during the warmest quarter in which oak species in drier areas invested more in support cells. The 21 oak species studied showed vasicentric tracheids, which could be essential to compensate for the transport system in drier areas. Wood density was negatively correlated with the axial parenchyma fraction and fibre lumen width. Oak species have high interspecific variation in the total vessel fraction and vessel diameter. Anatomical variation in oak species along a climatic gradient showed a broad continuum of fibre characteristics from wide to narrow wall and lumen, abundant to scarce parenchyma, and wide to narrow vessels. These variations imply a diversification of ecological strategies within the genus that includes the vascular design of the stem to define the tolerance and competitiveness of the oak species.

Regional differences in the response of California’s rangeland production to climate and future projection

Rangelands support many important ecosystem services and are highly sensitive to climate change. Understanding temporal dynamics in rangeland gross primary production (GPP) and how it may change under projected future climate, including more frequent and severe droughts, is critical for ranching communities to cope with future changes. Herein, we examined how climate regulates the interannual variability of GPP in California’s diverse annual rangeland, based on the contemporary records of satellite derived GPP at 500 m resolution since 2001. We built Gradient Boosted Regression Tree models for 23 ecoregion subsections, relating annual GPP with 30 climatic variables, to disentangle the partial dependence of GPP on each climate variable. The machine learning results showed that GPP was most sensitive to growing season (GS) precipitation, with a reduction in GPP up to 200 g cm−2 yr−1 when GS precipitation decreased from 400 to 100 mm yr−1 in one of the driest subsections. We also found that years with more evenly distributed GS precipitation had higher GPP. Warmer winter minimum air temperature enhanced GPP in approximately two-thirds of the subsections. In contrast, average GS air temperatures showed a negative relationship with annual GPP. When the pre-trained models were forced by downscaled future climate projections, changes in the predicted rangeland productivity by mid- and end of century were more remarkable at the ecoregion subsection scale than at the state level. Our machine learning-based analysis highlights key regional differences in GPP vulnerability to climate and provides insights on the intertwining and potentially counteracting effects of seasonal temperature and precipitation regimes. This work demonstrates the potential of using remote sensing to enhance field-based rangeland monitoring and, combined with machine learning, to inform adaptive management and conservation within the context of weather extremes and climate change.

Notes on the life history ofCenturio senex(Chiroptera: Phyllostomidae) from northern Central America

AbstractThe wrinkled-faced bat (Centurio senex) is an enigmatic species widely distributed in Mesoamerica. Besides its wide distribution it is considered rare in almost the entire range and little is known about its life history. We provide notes and discuss on the natural history of the species and performed a distribution model and an elevation profile in order to understand the distribution patterns in Northern Central America. We found thatC. senexinhabits ecosystems from lowlands to 1943 m, with higher probability of occurrence in the Pacific region and the mountains of Guatemala in which seasonal precipitation regimes predominates.

Delayed wet season increases soil net N mineralization in a seasonally dry tropical forest

Seasonal precipitation regime plays a vital role in regulating nutrient dynamics in seasonally dry tropical forests. Present evidence suggests that not only wet season precipitation is increasing in the tropics of South China, but also that the wet season is occurring later. However, it is unclear how nutrient dynamics will respond to the projected precipitation regime changes. We assessed the impacts of altered seasonal precipitation on soil net N mineralization in a secondary tropical forest. Since 2013, by reducing throughfall and/or irrigating experimental plots, we delayed the wet season by two months from April–September to June–November (DW treatment) or increased annual precipitation by 25% in July and August (WW treatment). We measured soil net N mineralization rates and assessed soil microbial communities in January, April, August and November in 2015 and 2017. We found that a wetter wet season did not significantly affect soil microbes or net N mineralization rates, even in the mid-wet season (August) when soil water content in the WW treatment increased significantly. By contrast, a delayed wet season enhanced soil microbial biomass and altered microbial community structure, resulting in a two-fold increase in net N mineralization rates relative to controls in the early dry season (November). Structural equation modeling showed that the changes in net N mineralization during the early dry season were associated with altered soil microbial communities, dissolved organic N, and litterfall, which were all affected by enhanced soil water content. Our findings suggest that a delayed wet season could have a greater impact on N dynamics than increased precipitation during the wet season. Changes in the seasonal timing of rainfall might therefore influence the functioning of seasonally dry tropical forests.

Current and future projections of flood risk dynamics under seasonal precipitation regimes in the Hyrcanian Forest region

The main aim of this research is to predict the impact of seasonal precipitation regimes on flood hazard applying machine learning models. For this purpose, twelve static variables and eight rainfall dynamics variables for 2050s (RCP 2.6 and 8.5) were used as conditioning factors. Four machine learning algorithms including K-Nearest Neighbour (KNN), Extremely Randomize Trees (ERT), Random Forest (RF) and Oblique-Random Forest (ORF) were used to model flood risk. Considering the area under curve (AUC) and other indices, the ORF was the most optimal model. The AUC of the KNN, ERT, RF and ORF for the validation datasets were 0.85, 0.90, 0.89 and 0.92 respectively. The results showed that under two RCPs, spatial distribution of high flood risk areas will change in the future and the trends will be different from the current. These results could provide valuable insights in simulating, predicting and reducing future flood risk.

Paleoproductivity Modes in Central Mediterranean During MIS 20—MIS 18: Calcareous Plankton and Alkenone Variability

AbstractPaleoproductivity is reconstructed across a Mediterranean benchmark record, the Early/Middle Pleistocene Montalbano Jonico section, cropping out in southern Italy. High‐resolution coccolithophore and alkenone data (C37 and C37:2/C38:2 ratio) were collected in order to extend the data set on Mediterranean paleoproductivity pattern and forcing mechanisms. The multi‐proxy record indicates low productivity during glacial and stadial phases and enhanced productivity during interglacial and interstadials. Increased surface water turbidity, cold‐water temperature and polar‐subpolar low salinity water incursion appear as the dominant controls for low productivity during Marine Isotope Stage (MIS) 20. Enhanced productivity during MIS 19c was sustained by warmer surface waters, coupled with a seasonal precipitation regime, providing higher nutrient availability. Productivity increases during interstadials with respect to stadials, in relation with enhanced land‐derived nutrient input through river discharge during wetter winters. The productivity scenario we propose is similar to those reconstructed from deep‐sea records in the central and western Mediterranean during Dansgaard‐Oeschger oscillations over the last 70 ka. This indicates that similar forcing mechanisms acted on productivity dynamics on a regional scale over different times. We suggest that migration of the westerly wind system over the Mediterranean and the polar water inflow influenced productivity on a regional scale. The acquired data set provides new evidences on the environmental significance of the C37:2/C38:2 ratio and on its relation with surface water productivity.

Climatic triggers and phenological responses in Isoetes cangae (Isoetaceae), an endemic quillwort from Amazon Iron Rocky Outcrops, Brazil

South America is the center of diversity of the genus Isoetes and several new species have been described in the last decade, especially in Brazil. Isoetes cangae J.B.S. Pereira, Salino & Stützel was first recognized in 2016 as an endemic species of a single lake in Serra dos Carajás, Brazil, in the southeastern Amazon region. The climate of Amazon is warm with a seasonal precipitation regime, which affects aquatic ecosystem properties and regulates plant phenology. Understanding how climatic and hydrological drivers affect I. cangae is fundamental for its conservation because the species is quite vulnerable due to its restricted distribution. In this study, we evaluated the effects of seasonal climatic variations and water level on the phenology of I. cangae over two years. The maximum leaf length, the number of sporophylls (fertile leaves), and the total number of leaves were assessed. Sporophylls were classified into mature and immature, megasporophylls and microsporophylls. The number of leaves, leaf length, and the number of sporophylls changed drastically among the hydrological seasons, but not the ratio between sporophylls and leaves, suggesting that reproductive effort is distributed accordingly over the years. All these factors also varied between years, indicating that inter-annual climatic variations affect the morphology and reproduction of I. cangae. Sporophylls are produced throughout the year, but their maturation and release increase in the rainy and early dry seasons, respectively. Megasporophylls predominate over microsporophylls throughout the year, and the peak of sporophyll maturation occurs during the rainy season, but lasts longer (till early dry season) for megasporophylls. The reproductive traits were associated with lake water level, suggesting that development is stimulated during the driest and hottest periods. Therefore, our results show that environmental factors play an important role in I. cangae phenology and must be considered in the management and conservation efforts to preserve this ancient Amazonian plant.

The environmental implications of upper Paleozoic plant-fossil assemblages with mixtures of wetland and drought-tolerant taxa in tropical Pangea

We evaluate the influences of elevation and climate on the spatio-temporal distribution of wetland and dryland biomes during the Pennsylvanian and early Permian in tropical Pangea. The longstanding “upland model” places drought-tolerant vegetation in elevated habitats, where slope and drainage created moisture-limited substrates under a humid climate that simultaneously promoted peat accumulation in contemporaneous lowlands. Upland plants were periodically transported to, and buried in, lowlands. Rare preservation of dryland vegetation thus reflects its general absence in basins, and taphonomic vagaries of long-distance transport. The alternative “climate model” proposes that drought-tolerant plants dominated tropical habitats when climate was seasonally dry, with wetland vegetation reduced to scattered refugia. Environmental changes attending glacial-interglacial cycles caused alternating wetter-drier conditions, and the relative abundance of wetland versus dryland biomes in basinal lowlands thus varied with climatic oscillations. The paucity of drought-tolerant plants reflects a preservational megabias against habitats with seasonal moisture deficits.The environmental signal of “mixed” plant-fossil assemblages, comprising taxa characteristic of both wetland and dryland biomes, may help resolve these debates. We review key Pennsylvanian and lower Permian mixed assemblages from tropical Euramerican Pangea, and interpret their original habitats and climatic contexts based on multidisciplinary lines of evidence, including sedimentology, taphonomy, physiology, and paleoecology. Evaluations also consider patterns of vegetational distribution and taphonomy in modern tropical environments. We suggest that even a cursory view of current tropical plant distribution exposes flaws in the upland model. Where tropical climate is sufficiently humid to support peat swamps, slopes and elevated habitats do not host drought-tolerant vegetation, but are occupied by plants similar to those in lowland settings. This occurs because equable, high precipitation strongly dampens water-table variation across entire landscapes. Furthermore, taphonomic studies indicate that most plant-fossil assemblages record vegetation living near the burial site. Fossil floras thus reflect environmental conditions near their growth site, excluding an upland origin for most occurrences of drought-tolerant taxa. Conversely, the climate model is consistent with modern tropical vegetational distribution and soundly explains late Paleozoic floristic patterns. When Pangean tropical lowlands experienced seasonally dry conditions, plants tolerant of moisture deficits dominated most habitats, whereas wetland vegetation was restricted to wetter sites with greater preservation potential. This occurred because topographic variations are magnified under seasonal precipitation regimes, creating a complex habitat mosaic with wetland patches in a landscape subject to seasonal drought. Accordingly, we propose that a macrofloral assemblage with even rare drought-tolerant taxa indicates seasonality in the broader landscape.At larger spatio-temporal scales, disagreement also persists about whether tectonic uplift or long-term climatic drying was the primary driver of changes in late Paleozoic floristic patterns and areal extent of tropical peat swamps. We argue that tectonic activity alone cannot explain the drastic reduction in peat swamps or coincident changes in dominance-diversity of wetland vegetation. Rates of plant dispersal and evolution far outpace that of mountain building, and peat-forming wetlands persisted in elevated habitats well into the Late Pennsylvanian. Therefore, progressive late Paleozoic aridification was the most probable driver of changing floral patterns and the distribution of wetland and dryland biomes in tropical Pangea.

Xylem growth rings and leaf phenological patterns in tree species of a subtropical seasonal forest

ABSTRACT Growth rings have been reported for several tropical species under seasonal precipitation regimes and have often been related to leaf phenology. We investigated growth ring distinctiveness, wood markers, and leaf shedding and flushing patterns of 16 tree species from a subtropical seasonal deciduous forest under abundant and well-distributed rainfall regime in southern Brazil. Distinct growth rings were found in 13 species, 10 of which presented clear anatomical boundaries. Seven species were deciduous, five semideciduous and four perennial. Leaf shedding peaked during winter and spring months for the deciduous species, while it peaked during spring and summer months for the perennial and semideciduous species. Leaf flushing peaked in spring and summer for all species. All the deciduous species exhibited growth rings with clear boundaries. Marginal parenchyma, associated or not with other anatomical markers, was present in deciduous species but was not present in species with other leaf shedding patterns. Growth rings in Allophylus edulis, Erythrina falcata, Jacaranda micrantha and Luehea divaricata were described for the first time. The presence of seasonal leaf phenological patterns and growth rings in most of the species suggests that seasonality of the photoperiod and/or temperature influence the development of trees in moist subtropical seasonal deciduous forests.

Estimating Surface Water Presence and Infiltration in Ephemeral to Intermittent Streams in the Southwestern US

Streamflow in arid and semi-arid regions is predominantly temporary, an integral part of mountain block hydrology and of significant importance for groundwater recharge and biogeochemical processes. However, temporary streamflow regimes, especially ephemeral flow, remain poorly quantified. We use electrical resistance sensors and USGS stream gauge data in 15 southern Arizona streams spanning a climate gradient (mean annual precipitation from 160 to 750 mm) to quantify temporary streamflow as streamflow presence and water presence, which includes streamflow, ponding and soil moisture. We use stream channel sediment data to estimate saturated hydraulic conductivity and potential annual infiltration. Annual streamflow ranged 0.6–82.4% or 2–301 days; while water presence ranged from 2.6 to 82.4% or 10 to over 301 days, or 4–33 times longer than streamflow. We identified 5 statistically distinct flow regimes based on the annual percent streamflow and water presence: (1) dry-ephemeral, (2) wet-ephemeral, (3) dry-intermittent, (4) wet-intermittent, and (5) seasonally-intermittent. In contrast to our expectations, stream channel density was a better predictor of annual streamflow and water presence than annual rainfall alone. Whereas, the dry-ephemeral and wet-ephemeral flow regimes varied with seasonal precipitation, the dry-intermittent, wet intermittent and seasonally-intermittent flow regimes did not. These results coupled with the potential infiltration estimates indicate that streamflow at the driest sites occurs in response to rainfall and overland flow while groundwater discharge and vadose zone contributions enhance streamflow at the wetter sites. We suggest that on a short temporal scale, and with respect to water presence, wetter sites might be buffered better against shifts in the timing and distribution of precipitation in response to climate change. Flow regime classifications that include both stream flow and water presence, rather than on stream flow alone, may be important for predicting thresholds in ecological functions and refugia in these dryland systems.

АНАЛИЗ СИСТЕМЫ «ОСАДКИ-ПОЧВА-ДРЕНАЖ» В УСЛОВИЯХ ИЗМЕНЕНИЯ КЛИМАТА НА СЕВЕРО-ЗАПАДЕ РОССИИ

Purpose: to analyze the soil-climatic and anthropogenic reasons of decreasing drainage efficiency of closed pottery tubular drainage on the reclaimed lands of the North-West of Russia under the observed local weather conditions against the background of global climate changes. Materials and methods. The objects of research are seasonal precipitation regimes, reclaimed lands and drainage systems of closed tubular drainage. The methods of applied mathematical statistics, the ensemble method of processing and generalization of climate forecasts of the Intergovernmental Panel on Climate Change (IPCC), modernized by the authors were used in research. Monitoring data on changes in the physical properties of soils of reclaimed lands during their long-term operation were obtained using laboratory agrophysical methods for studying samples taken in the field. Results. It is shown that under the conditions of the observed climatic changes, the frequency, intensity and extremeness of atmospheric precipitation are increased. Long-term exploitation of lands with a leaching drainage regime is accompanied by changes in the hydrophysical properties of soils, for example, the coefficient of heterogeneity of the subsurface horizons of soils texture of automorphic genesis decreased from 26 to 6. The system “precipitation – soil – drainage” in climatically abnormal weather conditions exceeding the initial calculated precipitation level mode by 10–20 %, is unable to ensure the removal of excess moisture from the root layer. Conclusions: when creating and reconstructing such reclamation drainage systems it is recommended to take into account the tendencies of changes in the spatial-temporal statistical structure of precipitation and to design technologies for regulating the water regime with systems for intercepting and diverting surface runoff on such reclaimed lands.

Impact of temperature on the growth of a Neotropical tree species (Hymenaea courbaril, Fabaceae) at its southern distribution limit.

Widely distributed tree species usually face different growth conditions across gradients of climate variables. Hymenaea courbaril inhabits most of Neotropical lowlands, where its growth is limited by low precipitation under seasonal precipitation regimes. However, it is still unclear what are the drivers of growth variability at its distribution limits, where populations are most vulnerable to climate change. We evaluated the role of precipitation and temperature variability on the growth rate of two populations of H. courbaril at the southern limits of its occurrence. Sampling sites comprise two semi-deciduous forest fragments with weathered and chemically poor soils, similar temperature conditions, only differing in size and in precipitation regime. To achieve that goal, we built two tree-ring chronologies using standard dendrochronological methods, one with 21 trees (37 radii) and the other one with 13 trees (24 radii). First, we evaluated if site conditions would affect average growth patterns, and then, we tested the climate-growth relationships and the teleconnections with the Equatorial Pacific sea surface temperature (SST). The results show that trees display similar average growth rates throughout life without evidence of influence from differing fragment sizes. Nonetheless, precipitation positively influences annual growth in the drier site, while it has a negative effect on growth in the wetter site. In contrast to previous studies, temperature has a stronger influence than precipitation on the growth of these trees. Monthly, seasonal, and annual mean temperatures showed a negative influence on trees growth. The variability of the regional temperature and, consequently, of the growth rate of the trees is partially dependent on the SST of the Equatorial Pacific. In conclusion, this study shows that temperature is a key limiting growth factor for this species at its southern distribution limits and periods with warmer temperature will likely reduce annual growth rate.