Seasonal Climate Variability Research Articles

Seasonal predictions of energy-relevant climate variables through Euro-Atlantic Teleconnections

The goal of this analysis is the better understanding of how the large-scale atmospheric patterns affect the renewable resources over Europe and to investigate to what extent the dynamical predictions of the large-scale variability might be used to formulate empirical prediction of local climate conditions (relevant for the energy sector). The increasing integration of renewable energy into the power mix is making the electricity supply more vulnerable to climate variability, therefore increasing the need for skillful weather and climate predictions. Forecasting seasonal variations of energy relevant climate variables can help the transition to renewable energy and the entire energy industry to make better informed decision-making. At seasonal timescale climate variability can be described by recurring and persistent, large-scale patterns of atmospheric pressure and circulation anomalies that interest vast geographical areas. The main patterns of the North Atlantic region (Euro Atlantic Teleconnections, EATCs) drive variations in the surface climate over Europe. We analyze reanalysis dataset ERA5 and the multi-system seasonal forecast service provided by Copernicus Climate Change Service (C3S). We found that the observed EATC indices are strongly correlated with surface variables. However, the observed relationship between EATC patterns and surface impacts is not accurately reproduced by seasonal prediction systems. This opens the door to employ hybrid dynamical-statistical methods. The idea consists in combining the dynamical seasonal predictions of EATC indices with the observed relationship between EATCs and surface variables. We reconstructed the surface anomalies for multiple seasonal prediction systems and benchmarked these hybrid forecasts with the direct variable forecasts from the systems and also with the climatology. The analysis suggests that hybrid methodology can bring several improvements to the predictions of energy relevant Essential Climate Variables.

Effect of meteorological conditions on leisure walking: a time series analysis and the application of outdoor thermal comfort indexes.

Leisure walking is affected by meteorological conditions. However, it is still not clear what scales of meteorological conditions and thermal status affect the number of people who choose to leisure walk. Using a time series regression, this study examines the heat-leisure walking relationship by analyzing the effect of the seasons, weather, microclimate, and outdoor thermal comfort on walking count. Eight thermal indexes were selected to estimate the pedestrians' thermal comfort, and their predictive capacities in walking count were evaluated. Particular consideration was given to identifying heat thresholds of walking that determined the tolerance range of pedestrian heat stress. Four years of hourly daytime walking counts and publicly available ASOS meteorological data at Seoul-lo 7017, a pedestrian bridge in Seoul, were used for the analysis. Our findings indicate that walking count is correlated with seasonal climatic variations, with the highest number of pedestrians observed in fall and the lowest in summer. Moreover, air temperature played a vital role, showing that a 5.0°C rise in temperature was associated with a 1.34% rise in the square root of the walking count. Its impact becomes greater when combined with intense solar radiation and higher absolute humidity. The heat threshold for walking was between 23.8°C and 26.2°C. Empirical model indexes showed the highest predictive capacity in walking count at approximately 30.0%, which was followed by rational model indexes at 28.0%.

Modeling the Influence of Seasonal Climate Variability on Soybean Yield in a Temperate Environment: South Korea as a Case Study

Modeling the Influence of Seasonal Climate Variability on Soybean Yield in a Temperate Environment: South Korea as a Case Study

Insect defoliation modulates influence of climate on the growth of tree species in the boreal mixed forests of eastern Canada.

Increasing air temperatures and changing precipitation patterns due to climate change can affect tree growth in boreal forests. Periodic insect outbreaks affect the growth trajectory of trees, making it difficult to quantify the climate signal in growth dynamics at scales longer than a year. We studied climate‐driven growth trends and the influence of spruce budworm (Choristoneura fumiferana Clem.) outbreaks on these trends by analyzing the basal area increment (BAI) of 2058 trees of Abies balsamea (L.) Mill., Picea glauca (Moench) Voss, Thuja occidentalis L., Populus tremuloides Michx., and Betula papyrifera Marsh, which co‐occurs in the boreal mixedwood forests of western Quebec. We used a generalized additive mixed model (GAMM) to analyze species‐specific trends in BAI dynamics from 1967 to 1991. The model relied on tree size, cambial age, degree of spruce budworm defoliation, and seasonal climatic variables. Overall, we observed a decreasing growth rate of the spruce budworm host species, A. balsamea and P. glauca between 1967 and 1991, and an increasing growth rate for the non‐host, P. tremuloides, B. papyrifera, and T. occidentalis. Our results suggest that insect outbreaks may offset growth increases resulting from a warmer climate. The observation warrants the inclusion of the spruce budworm defoliation into models predicting future forest productivity.

Changes in rumen microbial community composition in yak in response to seasonal variations.

Yak is a dominant ruminant, well adapted to grazing on pasture year around in the harsh climate of the 3000-meter-high Qinghai-Tibetan Plateau. The complex microbial community that resides within the yak rumen is responsible for fermentation and contributes to its climatic adaptation. This study aimed to characterize the rumen microbiota responses to wide seasonal variations, especially those necessary for survival in the cold seasons. In the present study, we performed 16s rRNA gene sequencing to investigate the seasonal variations in microbiota composition, diversity and associated volatile fatty acids (VFAs) in yak rumen. The results showed that rumen microbiota were dominated by Bacteroides (72.13%-78.54%) and Firmicutes; the relative abundance of Firmicutes was higher in summer (17.44%) than in winter (10.67%; p<0.05). The distribution of taxa differed among spring, summer and winter rumen communities (PERMANOVA, p=0.001), whereas other taxa (e.g., Fibrobacter, Verrucomicrobia, Anaerostipes and Paludibacter), which could potentially help overcome harsh climate conditions were observed in higher abundance during the cold spring and winter seasons. The highest total VFA concentration in the yak rumen was obtained in summer (p<0.05), followed by spring and winter, and both positive and negative correlations between VFAs and specific genera were revealed. Microbiota in yak rumen appear to be highly responsive to seasonal variations. Considering environmental factors, we suggest that seasonal adaptation by microbial communities in rumen enables their hosts to survive seasonal scarcity and cold stress in the spring and winter. The present study furthers our understanding of how microbial adaptation to seasonal variations in nutrient availability and climate may function in high plateau ruminants, providing insights into the tripartite relationship between the environment, host and microbiota.

Not Only Severe Events: Moderate Dry Periods Impact the Hydraulic Functioning and Survival of Planted Ponderosa Pine Seedlings

Juvenile tree survival will increasingly shape the persistence of ponderosa pine forests in the western United States. In contrast to severe pulse disturbances that induce widespread adult and juvenile tree mortality, moderate periods of low rainfall and warm temperatures may reduce forest persistence by killing juvenile trees at the seedling stage. Intensification of these periods in a changing climate could therefore increasingly restrict both natural regeneration and artificial regeneration of planted seedlings. We conducted a controlled field experiment at a single site in the Front Range of Colorado, USA, to determine the responses and survival of 3 Colorado subpopulations of &lt;1 year old potted ponderosa pines to moderately dry conditions, variation in small rainfall events based on observed patterns, and shaded and unshaded microsite environments. Near surface soil moisture increased slightly following small rainfall events, but declined over the 45-day experimental period. Seedling transpiration and associated canopy cooling declined after ∼13 days, and further declines in transpiration and canopy cooling suggest that the majority of trees in lower rainfall treatments experienced hydraulic dysfunction between days ∼20–30. After 45 days, mortality across all subpopulations and treatments, inferred by relative water loss, exceeded 90–95%. Despite some uncertainty pertaining to the stress tolerance of nursery grown versus naturally germinated conifers, our results show that planted ponderosa pine seedlings &lt;1 year old are unlikely to survive moderate dry periods of 20+ days relying on small rainfall events. Although microsite conditions and soil moisture availability shaped tree hydraulic functioning early in the experiment (days 1–13), later functioning was shaped predominately by the legacy of rainfall treatments. Our results illustrate the importance of moderate dry events that occur consistently as part of seasonal variation in climate, and show how their intensification may constitute a sustained press that limits opportunities for natural and artificial regeneration.

Ommastrephid squid paralarvae potential nursery habitat in the tropical-subtropical convergence off Mexico

The Gulf of California and adjacent Pacific have passed through a prolonged warming period that affects the Humboldt squid D. gigas life cycle, which was the third largest fishery in Mexico until its total collapse after 2010. The aims of the present study were (i) to characterize the nursery habitat of paralarvae of the complex Sthenoteuthis oualaniensis–Dosidicus gigas (SD complex) on the basis of 1112 paralarvae collected from zooplankton trawls carried out during ten oceanographic cruises off northwestern Mexico between 2010 and 2017; and (ii) to model the seasonal and interannual variability of the nursery habitat, applying a generalized additive model (GAM) using a regional long-term (1922–2017) hydrographic database. The GAM showed significant relationships of paralarvae abundance with the seasonal average of thermocline depth, absolute salinity, and conservative temperature. The potential nursery habitat of the SD complex paralarvae showed its maximum expected abundance in the entrance of the Gulf of California during spring, when the thermocline depth was shallowest (<30 m). During summer the nursery habitat moved northward along the Pacific west coast of the peninsula. Autumn and winter were less suitable seasons for paralarvae nursery because the thermocline was deep throughout the area of study. Potential nursery habitat did not change interannually in location during La Niña conditions, but did increase the expected abundance in its southern extension. El Niño events contracted the nursery habitat near Cabo San Lucas, and caused the habitat to move northwards over both coasts of the Baja California peninsula. Despite seasonal and interannual climatic variability, two regions remained suitable habitats almost throughout the year. One was associated with coastal upwelling regions (Cabo Corrientes) and the second comprised the oceanic region of high mesoscale kinetic energy located south of Cabo San Lucas. We conclude that the potential nursery habitat of the SD complex is more coastal than oceanic, and is associated with a shallow thermocline in the tropical-subtropical convergence off Mexico. The recent anomalous warming period in this region could drive the changes in the morphology of D. gigas in the Gulf of California, given the effects of environmental variability over early stages of development throughout the adult ontogenesis.

Seasonal variation of leaf thickness: An overlooked component of functional trait variability.

The dry and wet seasons in the Neotropics have strong effects on soil water and nutrient availability, as well as on forest dynamics. Despite these major effects on forest ecology, little is known on how leaf traits vary throughout the seasons in tropical rainforest trees. Here, we investigated the influence of seasonal variations in climate and soil characteristics on leaf trait variation in two tropical tree species. We measured two leaf traits, thickness and water mass per area, in 401 individuals of two species of Symphonia (Clusiaceae) in the Paracou research station in French Guiana tropical lowland rainforest. We found a significant effect of seasonal variation on these two leaf traits. Soil relative extractable water was a strong environmental predictor of leaf trait variation in response to seasonal variation. Reduced soil water availability during the dry season was associated with increased leaf thickness and water mass per area, possibly as a result of stomatal closure. Our findings advocate the need to account for environmental seasonality when studying leaf traits in seasonal ecosystems such as tropical forests.

Developing a national indicator of functional connectivity

Habitat loss is a significant driver of biodiversity loss, causing fragmentation into small, isolated patches of suitable land cover. This reduces the permeability of landscapes to the movement of individuals and reduces the likelihood of metapopulation persistence. Quantifying functional connectivity, the ability of a focal species to move between resource patches, is therefore essential for conservation management. There is substantial evidence supporting a technique based on ‘population synchrony’- the degree of correlation in time-series of annual population growth rates between different long-term monitoring sites, to provide a measure of functional connectivity. However, synchronised population dynamics are not only driven by the movement of individuals between sites, but also shared environmental conditions which must be accounted for. Here, we use species survey data from over four decades to investigate average levels and temporal trends in population synchrony for 58 British bird and butterfly species. We first show that population synchrony is significantly associated with synchrony in some seasonal climatic variables. Once we accounted for spatiotemporal climatic patterns, we found that synchrony in butterflies declined over time by 71% between 1985 and 2000 but increased by 64% in recent years. Synchrony in birds showed some decline between 1999 and 2005, after which there appears to being recovery, however most species (74%) show no significant overall change in synchrony. Our proposed indicator provides a ‘species-eye-view’ of functional connectivity using widely available abundance data. Developing such indicators of functional connectivity, which can be updated annually, is crucial to improve the effectiveness of land management strategies for conservation under increasing environmental change.

Eddy-induced meridional transport variability at ocean western boundary

Meridional transport variability induced by westward-propagating eddies impinging on the western boundary is investigated both analytically and numerically. A simple theory is first developed in the framework of the reduced-gravity model which relates eddy-induced meridional transport to eddy thickness anomalies propagated into the western boundary by long Rossby waves, and this is followed by a suite of numerical model experiments. It is found that eddies impinging on the western boundary excite boundary waves that propagate equatorward along the western boundary, which leads to coherent meridional overturning circulation (MOC) anomalies equatorward of the incident eddy field. The magnitude and duration of eddy-induced MOC anomalies are variable and irregular, ranging from less than 1 Sv to over 5 Sv and from less than 10 days to over 100 days. Importantly, these eddy-induced MOC anomalies lead to considerable meridional heat transport variability across the latitudes, with implications for seasonal and interannual climate variability and prediction.

Effect of the Seasonal Climatic Variations on the Accumulation of Fruit Volatiles in Four Grape Varieties Under the Double Cropping System.

The double cropping system has been widely applied in many subtropical viticultural regions. In the 2-year study of 2014–2015, four grape varieties were selected to analyze their fruit volatile compounds in four consecutive seasons in the Guangxi region of South China, which had a typical subtropical humid monsoon climate. Results showed that berries of winter seasons had higher concentrations of terpenes, norisoprenoids, and C6/C9 compounds in “Riesling,” “Victoria,” and “Muscat Hamburg” grapes in both of the two vintages. However, in the “Cabernet Sauvignon” grapes, only the berries of the 2014 winter season had higher terpene concentrations, but lower norisoprenoid concentrations than those of the corresponding summer season. The Pearson correlation analysis showed the high temperature was the main climate factor that affected volatile compounds between the summer and winter seasons. Hexanal, γ-terpinene, terpinen-4-ol, cis-furan linalool oxide, and trans-pyran linalool oxide were all negatively correlated with the high-temperature hours in all of the four varieties. Transcriptome analysis showed that the upregulated VviDXSs, VviPSYs, and VviCCDs expressions might contribute to the accumulations of terpenes or norisoprenoids in the winter berries of these varieties. Our results provided insights into how climate parameters affected grape volatiles under the double cropping system, which might improve the understanding of the grape berries in response to the climate changes accompanied by extreme weather conditions in the future.

Retracted] HANA: A Performance‐Based Machine Learning and Neural Network Approach for Climate Resilient Agriculture

The importance of agricultural crops in India has been understated in terms of production during the last two decades, owing to global warming and other factors. Policymakers and farmers would benefit from imagining crop yields well before harvest to help them make appropriate marketing and storage decisions. Crop yields will benefit from such estimates as well. Several systems for predicting and modelling agricultural yields have been developed in the past, with varying degrees of effectiveness, due to the fact that they do not objectively account for meteorological aspects and seasonal climate variations. The importance of agricultural crops in India has been understated in terms of production during the last two decades, owing to global warming and other factors. Policymakers and farmers would benefit from imagining crop yields well before harvest to help them make appropriate marketing and storage decisions. Crop yields will benefit from such estimates as well. Several systems for predicting and modelling agricultural yields have been developed in the past, with varying degrees of effectiveness, due to the fact that they do not objectively account for meteorological aspects and seasonal climate variations. The method makes use of climate data from Thanjavur, India’s soils. To choose the optimal crop for a particular set of input and climate conditions, the system leverages real‐time input of location‐specific soil attributes. The model is been tested with various machine learning techniques such as NB, KNN, SVM, and decision tree. Among all these methods, the SVM gives the good results. The accuracy of the model was determined using LSTM, SVM, and Tabu search optimization. TSO with SVM has an 89% accuracy value.

The Physical Climate at Global Warming Thresholds as Seen in the U.K. Earth System Model

AbstractA key goal of the 2015 Paris Climate Agreement is to keep global mean temperature change at 2°C and if possible under 1.5°C by the end of the century. To investigate the likelihood of achieving this target, we calculate the year of exceedance of a given global warming threshold (GWT) temperature across 32 CMIP6 models for Shared Socioeconomic Pathway (SSP) and radiative forcing combinations included in the Tier 1 ScenarioMIP simulations. Threshold exceedance year calculations reveal that a majority of CMIP6 models project warming beyond 2°C by the end of the century under every scenario or pathway apart from the lowest emission scenarios considered, SSP1–1.9 and SSP1–2.6, which is largely a function of the ScenarioMIP experiment design. The U.K. Earth System Model (UKESM1) ScenarioMIP projections are analyzed in detail to assess the regional and seasonal variations in climate at different warming levels. The warming signal emerging by midcentury is identified as significant and distinct from internal climate variability in all scenarios considered and includes warming summers in the Mediterranean, drying in the Amazon, and heavier Indian monsoons. Arctic sea ice depletion results in prominent amplification of warming and tropical warming patterns emerge that are distinct from interannual variability. Climate changes projected for a 2°C warmer world are in almost all cases exacerbated with further global warming (e.g., to a 4°C warmer world).

Seasonal Distribution of the Broad-Tailed Hummingbird (Selasphorus platycercus): A Climatic Approach.

The seasonal movements of birds are a phenomenon that has historically been of interest in ecology and biogeography. Despite this, information on how environmental conditions influence migratory behavior and its regulation is still scarce. In this work, we study the Broad-Tailed hummingbird Selasphorus platycercus from an analysis of its populations through longitudinal and latitudinal gradients. We use the frequencies of monthly presence records throughout the annual cycle to identify the breeding areas (corresponding to the summer months), of winter presence (corresponding to the winter months), and annual residence (presence records throughout the year). Subsequently, we use ecological niche models to reconstruct the potential distribution of the summer and winter niches by correlating the climates of each season with the corresponding records. We evaluate the species' climatic preferences between the breeding and winter seasons by transferring the niches from each season to the opposite and by their capacity to inter-predict records between seasons. In addition, we quantify the overlap between the summer and winter niches using a niche similarity analysis. Geographically, we see a clear seasonal turnover pattern along a north-south gradient and records throughout the year (resident populations) in the south-central region of its distribution. We observed a low inter-prediction of records between seasons. Together with the similarity analysis, we suggest that the species is niche-switching (i.e., has different seasonal niches). We identified three seasonal migration patterns among the species' populations: long-distance migratory, short-distance summer migrant, and resident. Our findings suggest that the different migration patterns in this species' populations all over its distribution can be explained through seasonal climatic variations throughout the year.

Effect of the Seasonal Climatic Variations on the Flavonoid Accumulation in Vitis vinifera cvs. 'Muscat Hamburg' and 'Victoria' Grapes under the Double Cropping System.

Under the double cropping system, berries usually showed significant quality variations in the summer and winter seasons. In the two-year/four-consecutive-season study, two table grapes of ‘Muscat Hamburg’ and ‘Victoria’ were investigated to determine the phenolic compounds in their berries. Different from those of the summer season, the berries in the winter season suffered no high-temperature stress since veraison to harvest in 2014 and 2015. The variations in the season temperatures led to a higher anthocyanin concentration in the winter season berries of ‘Muscat Hamburg’ grapes than that in the summer berries, while the summer season berries had higher proportions of acylated and methylated anthocyanins than those in the winter season berries. Similar to the anthocyanins, the winter season berries also had a higher flavonol concentration in both varieties. Transcriptome analysis showed that the upregulated genes involved in the flavonoid pathway in the winter season berries were agreed with the changes found in the metabolites. However, the influence of the growing seasons on the flavanols was not consistent in the two varieties, and the variations in VviLARs between the grapes of ‘Muscat Hamburg’ and ‘Victoria’ might be the cause. This research helped us better understand the double cropping system and how the climate factors affected the phenolic compounds in the double cropping system.

Inter‐comparison of historical simulation and future projections of rainfall and temperature by CMIP5 and CMIP6 GCMs over Egypt

AbstractThe global climate models (GCMs) performances of the recently released Coupled Model Intercomparison Project phase 6 (CMIP6) compared to its predecessor, CMIP5, are evaluated to anticipate the expected changes in climate over Egypt. Thirteen GCMs and their multi‐model ensemble (MME) of both CMIPs were used for this purpose. The future projections were compared for two radiative concentration pathways (RCP 4.5 and 8.5) and two shared socio‐economic pathways (SSP 2–4.5 and 5–8.5). The results revealed improvement in most CMIP6 models in replicating historical rainfall, maximum temperature (Tmax), and minimum temperature (Tmin) climatology over Egypt. The MME of the CMIPs revealed that both could reproduce Egypt's spatial distribution and seasonal climate variability. However, the bias in CMIP5 was higher than that for CMIP6. The uncertainties in simulating seasonal variability of rainfall and temperatures were lower for CMIP6 compared to CMIP5. The future projection of rainfall using CMIP6 MME revealed a higher reduction of precipitation (10–26 mm) in the economically crucial northern region than that estimated using CMIP5 (0–17 mm), compared to 133.5 mm rainfall in the base period. CMIP6 also projected a 0.74–1.63°C more rise in Tmax and Tmin compared to CMIP5 by the end of the century. The study indicates more aggravated scenarios of climate changes in Egypt than anticipated earlier, using the CMIP5 models. Therefore, Egypt needs to streamline the existing adaptation and mitigation measures to account for climate projections.

The role of climatic variability on Eucalyptus regeneration in southeastern Australia

In the forested landscapes of southeastern Australia, bushfires and timber harvesting are the primary catalysts for regeneration in Eucalyptus regnans , E. delegatensis , and high elevation mixed species (HEMS) forests. Quantifying the role of climate, topography and edaphic conditions on plant regeneration is important for understanding current and future risks of climate change. In this study, we investigated the post-disturbance regeneration dynamics in these forests. We sought to determine the direct impact of climate variability on regeneration and the interactive effects of climate, topography, and edaphic factors on the regeneration success of Eucalyptus . Data was collated from regeneration surveys conducted in 881 coupes, totaling ~55,000 ha of forest harvested and regenerated between 2004 and 2012. The time-period analyzed included the Millennium Drought, an intense and prolonged drought that lasted from 1996 to 2010. To test the influence of climatic, topographic, and edaphic variables on the occurrence and abundance of Eucalyptus regeneration, we used machine learning models. We found that regeneration success was closely tied to the timing of the Millennium Drought. Between 2004 and 2005 Eucalyptus seedlings occurred in less than 50% of survey plots across all three forest types. Declines in regeneration during the period of drought were greater in E. regnans and E. delegatensis than HEMS forests, suggesting that regeneration in the HEMS forests is more resistant to drought. We found that seasonal precipitation and temperature had the greatest influence on regeneration success of Eucalyptus . Responses varied by species, however autumn precipitation had the strongest influence on abundance of regeneration of E. regnans and E. delegatensis, while summer temperature was more important for HEMS species. Our findings highlight the importance of seasonal and annual climate variability on Eucalyptus regeneration and portend potential declines in regeneration success in a warmer and drier future for E. regnans and E. delegatensis .

Assessing seasonal climate variability impact on the malaria patient’s cases in the north of Côte d'Ivoire

In recent years, the impact of climate change on human health has attracted considerable attention including their effects on malaria, which have been of particular interest due to its disease burden and transmission sensitivity to climatic conditions. The main objective of the study is to examine the relationship between intra-annual climate variability and malaria transmission in the north of Côte d’Ivoire. The monthly incidence of malaria in Korhogo 1 health district was treated as a dependent variable, and meteorological variables were independent variables. Our results highlighted that malaria is influenced by all three climatic variables (rainfall, temperature and relative humidity). The highest effect of temperature, relative humidity, and rainfall on malaria cases is observed in the months of June, July, August, September, October and November respectively. The monthly rainfall and the monthly mean relative humidity were highly correlated to monthly incidence of malaria in population under-five years old. Meanwhile, the monthly mean temperature was highly correlated to malaria incidence of pregnant women over 15 years old and to monthly incidence of malaria in under-five. Moreover, combining monthly rainfall and monthly mean temperature, monthly rainfall and monthly mean relative humidity and monthly mean temperature and relative humidity were highly correlated to malaria incidence in patients influenced by the rainy season exclusively. The combination between monthly rainfall and monthly mean relative humidity was closely correlated to malaria incidence in malaria patient influencedby rainy and/or dry season.

Responsiveness of Dichrostachys cinerea to seasonal variations in temperature and rainfall in central Namibia

Woody plants provide natural archives of climatic variation which can be investigated by applying dendroclimatological methods. Such studies are limited in Southern Africa but have great potential of improving our understanding of past climates and plant functional adaptations in the region. This study therefore investigated the responsiveness of Dichrostachys cinerea to seasonal variations in temperature and rainfall at two sites in central Namibia, Waterberg and Kuzikus. Dichrostachys cinerea is one of the encroacher species thriving well in Namibia. A moving correlation and response function analysis were used to test its responsiveness to seasonal climatic variations over time. Dichrostachys cinerea growth rings showed relationships to late summer warming, lasting up to half of the rainy season. The results also revealed that past temperatures had been fluctuating and their influence on growth rings had been intensifying over the years, but to varying extents between the two sites. Temperature was a more important determinant of ring growth at the drier site (Kuzikus), while rainfall was more important at the wetter site (Waterberg). Growth ring responsiveness to rainfall was not immediate but showed a rather lagged pattern. We conclude that D. cinerea differentially responds to variations in rainfall and temperature across short climatic gradients. This study showed that the species, due to its somewhat wide ecological amplitude, has great potential for dendroclimatological studies in tropical regions.

Seasonal Performance Evaluation of Pavement Base Using Recycled Materials

Using recycled pavement materials to construct new pavement base is currently an important construction strategy bringing improved sustainability. This study investigates the long-term performance of pavement bases constructed with recycled concrete aggregate (RCA), reclaimed asphalt pavement (RAP), and blends with natural aggregates in a seasonal frost region. The stabilization effect of fly ash on RAP was studied as well. In situ falling weight deflectometer (FWD) tests were routinely conducted to provide seasonal deflection data, which were used to back-calculate the layer modulus. Seasonal changes in the base layer modulus along with the pavement ride quality were monitored. One of the two lanes at the test sections was consistently subjected to traffic loading, whereas the other one was not. Findings from this field research indicated that after undergoing over 8 years of naturally seasonal freeze-thaw conditions, 100% RCA, 50% RCA, plus 50% natural aggregates, and 100% RAP, presented improved performance over 100% natural aggregates. However, 50% RAP blended with 50% natural aggregates performed comparably to natural aggregates only, and fly ash did not provide considerable improvement on the long-term performance of 50% RAP plus 50% natural aggregate base. Seasonal climatic variations turned out to affect pavement performance more critically than traffic loading.