Implications of global warming for the climate of African rainforests

African rainforests are likely to be vulnerable to changes in temperature and precipitation, yet there has been relatively little research to suggest how the regional climate might respond to global warming. This study presents projections of temperature and precipitation indices of relevance to African rainforests, using global climate model experiments to identify local change as a function of global temperature increase. A multi-model ensemble and two perturbed physics ensembles are used, one with over 100 members. In the east of the Congo Basin, most models (92%) show a wet signal, whereas in west equatorial Africa, the majority (73%) project an increase in dry season water deficits. This drying is amplified as global temperature increases, and in over half of coupled models by greater than 3% per °C of global warming. Analysis of atmospheric dynamics in a subset of models suggests that this could be partly because of a rearrangement of zonal circulation, with enhanced convection in the Indian Ocean and anomalous subsidence over west equatorial Africa, the Atlantic Ocean and, in some seasons, the Amazon Basin. Further research to assess the plausibility of this and other mechanisms is important, given the potential implications of drying in these rainforest regions.     

Acclimation at high temperatures increases the ability of Raphidiopsis raciborskii (Cyanobacteria) to withstand phosphate deficiency and reveals distinct strain responses

ABSTRACT

Raphidiopsis (Cylindrospermopsis) raciborskii is one of the most studied potentially harmful cyanobacteria. Single environmental factors such as increased temperature or light are reported to be promoters of R. raciborskii growth, but the interaction of two or more promoting factors is less understood. The performance of two strains of R. raciborskii (MVCC19, Uruguayan and LP1, Brazilian) were evaluated under acclimation and temperature shifts (25–32°C) in combination with two transitions from phosphorus (P) sufficiency to limiting growth conditions. When subjected to transition from high P sufficiency to a P-limiting state, strains were able to grow only at the warmer temperature if previously acclimated. The MVCC19 strain showed higher specific growth rates and a shorter growth phase than LP1. Morphological differences were also found: the MVCC19 strain produced shorter filaments, while the LP1 strain increased in length and the number of cells per filament. The results show the positive effect of high temperature on the ability of R. raciborskii to withstand P-limiting conditions, which may confer resilience of populations to periods of severe nutrient limitation in warm lakes. This finding contributes to the understanding of the success of this species in diverse environmental conditions. The effect of temperature on the tolerance of nutrient deficiency and the performance of strains under lake conditions suggests the need for a drastic reduction in nutrient loads to avoid R. raciborskii dominance in warmer lakes.     

Rapid climate driven shifts in wintering distributions of three common waterbird species

Climate change is predicted to cause changes in species distributions and several studies report margin range shifts in some species. However, the reported changes rarely concern a species' entire distribution and are not always linked to climate change. Here, we demonstrate strong north‐eastwards shifts in the centres of gravity of the entire wintering range of three common waterbird species along the North‐West Europe flyway during the past three decades. These shifts correlate with an increase of 3.8 °C in early winter temperature in the north‐eastern part of the wintering areas, where bird abundance increased exponentially, corresponding with decreases in abundance at the south‐western margin of the wintering ranges. This confirms the need to re‐evaluate conservation site safeguard networks and associated biodiversity monitoring along the flyway, as new important wintering areas are established further north and east, and highlights the general urgency of conservation planning in a changing world. Range shifts in wintering waterbirds may also affect hunting pressure, which may alter bag sizes and lead to population‐level consequences.     

Temperature‐dependent shifts in herbivore performance and interactions drive nonlinear changes in crop damages

Understanding how and to what extent the influence of temperature on physiological performance scales up to interspecific interactions and process rate patterns remains a major scientific challenge faced by ecologists. Here, we combined approaches developed by two conceptual frameworks in ecology, the stress‐gradient hypothesis (SGH), and the biodiversity–ecosystem functioning relationship (B‐EF), to test the hypothesis that interspecific difference in thermal performance modulates multiple species interactions along a thermal stress (SGH) and the subsequent richness effects on process rates (B‐EF). We designed an experiment using three species of herbivorous agricultural pests with different thermal optima for which we determined how temperature influences the direction and the strength of interaction and subsequent richness effects on crop damage (7 species interaction treatments × 6 temperature treatments × 10 replicates). We showed that both biotic interactions and species richness effects drive variations in crop damages along a thermal stress gradient, and thus have the potential to drive agro‐system responses to climate change. To help explain and generalize underlying mechanisms of richness effects on process rates, we further proposed a conceptual model that views interaction outcomes as shifting between positive and negative along a thermal stress depending on species thermal optima. Overall, our study demonstrates that nonlinear effects of temperature on process rates must be a major concern in terms of prediction and management of the consequences of global warming.     

Conservation implications of native and introduced ungulates in a changing climate

In many grasslands, grazing by large native or introduced ungulates drives ecosystem structure and function. The behavior of these animals is important as it directs the spatial effects of grazing. To the degree that temperature drives spatial components of foraging, understanding how changes in climate alter grazing behavior will provide guidance for the conservation of ecosystem goods and services. We determined the behavioral response of native bison (Bison bison) and introduced cattle (Bos taurus) to temperature in tallgrass prairie within the Great Plains, USA. We described the thermal environment by measuring operative temperature (the temperature perceived by animals) through space and time. Site selection preferences of ungulates were quantified using resource selection functions. Woody vegetation in tallgrass prairie provided a cooler thermal environment for large ungulates, decreasing operative temperature up to 16 °C in the heat of the summer. Cattle began to seek thermal refugia at lower air temperatures (24 °C) by selecting areas closer to woody vegetation and water sources. Bison, however, sought refugia within wooded areas at higher air temperatures (36 °C), which occurred much less frequently. Both species became more attracted to riparian areas as air temperature increased, with preferences increasing tenfold during the hottest periods. As predicted warming occurs across the Great Plains and other grasslands, grazing behavior and subsequent grazing effects will be altered. Riparian areas, particularly those with both water and woody vegetation, will receive greater utilization and selection by large ungulates. The use of native grazers for conservation or livestock production may mitigate negative effects caused by increased temperatures.     

Local genetic adaptation generates latitude‐specific effects of warming on predator–prey interactions

Temperature effects on predator–prey interactions are fundamental to better understand the effects of global warming. Previous studies never considered local adaptation of both predators and prey at different latitudes, and ignored the novel population combinations of the same predator–prey species system that may arise because of northward dispersal. We set up a common garden warming experiment to study predator–prey interactions between Ischnura elegans damselfly predators and Daphnia magna zooplankton prey from three source latitudes spanning >1500 km. Damselfly foraging rates showed thermal plasticity and strong latitudinal differences consistent with adaptation to local time constraints. Relative survival was higher at 24 °C than at 20 °C in southern Daphnia and higher at 20 °C than at 24 °C, in northern Daphnia indicating local thermal adaptation of the Daphnia prey. Yet, this thermal advantage disappeared when they were confronted with the damselfly predators of the same latitude, reflecting also a signal of local thermal adaptation in the damselfly predators. Our results further suggest the invasion success of northward moving predators as well as prey to be latitude‐specific. We advocate the novel common garden experimental approach using predators and prey obtained from natural temperature gradients spanning the predicted temperature increase in the northern populations as a powerful approach to gain mechanistic insights into how community modules will be affected by global warming. It can be used as a space‐for‐time substitution to inform how predator–prey interaction may gradually evolve to long‐term warming.     

Ocean acidification and warming scenarios increase microbioerosion of coral skeletons

Biological mediation of carbonate dissolution represents a fundamental component of the destructive forces acting on coral reef ecosystems. Whereas ocean acidification can increase dissolution of carbonate substrates, the combined impact of ocean acidification and warming on the microbioerosion of coral skeletons remains unknown. Here, we exposed skeletons of the reef‐building corals, Porites cylindrica and Isopora cuneata, to present‐day (Control: 400 μatm – 24 °C) and future pCO₂–temperature scenarios projected for the end of the century (Medium: +230 μatm – +2 °C; High: +610 μatm – +4 °C). Skeletons were also subjected to permanent darkness with initial sodium hypochlorite incubation, and natural light without sodium hypochlorite incubation to isolate the environmental effect of acidic seawater (i.e., Ω_aragonite <1) from the biological effect of photosynthetic microborers. Our results indicated that skeletal dissolution is predominantly driven by photosynthetic microborers, as samples held in the dark did not decalcify. In contrast, dissolution of skeletons exposed to light increased under elevated pCO₂–temperature scenarios, with P. cylindrica experiencing higher dissolution rates per month (89%) than I. cuneata (46%) in the high treatment relative to control. The effects of future pCO₂–temperature scenarios on the structure of endolithic communities were only identified in P. cylindrica and were mostly associated with a higher abundance of the green algae Ostreobium spp. Enhanced skeletal dissolution was also associated with increased endolithic biomass and respiration under elevated pCO₂–temperature scenarios. Our results suggest that future projections of ocean acidification and warming will lead to increased rates of microbioerosion. However, the magnitude of bioerosion responses may depend on the structural properties of coral skeletons, with a range of implications for reef carbonate losses under warmer and more acidic oceans.     

How interactions between microbial resource demands,soil organic matter stoichiometry,and substrate reactivity determine the direction and magnitude of soil respiratory responses to warming

Recent empirical and theoretical advances inform us about multiple drivers of soil organic matter (SOM) decomposition and microbial responses to warming. Absent from our conceptual framework of how soil respiration will respond to warming are adequate links between microbial resource demands, kinetic theory, and substrate stoichiometry. Here, we describe two important concepts either insufficiently explored in current investigations of SOM responses to temperature, or not yet addressed. First, we describe the complete range of responses for how warming may change microbial resource demands, physiology, community structure, and total biomass. Second, we describe how any relationship between SOM activation energy of decay and carbon (C) and nitrogen (N) stoichiometry can alter the relative availability of C and N as temperature changes. Changing availabilities of C and N liberated from their organic precursors can feedback to microbial resource demands, which in turn influence the aggregated respiratory response to temperature we observe. An unsuspecting biogeochemist focused primarily on temperature sensitivity of substrate decay thus cannot make accurate projections of heterotrophic CO₂ losses from diverse organic matter reservoirs in a warming world. We establish the linkages between enzyme kinetics, SOM characteristics, and potential for microbial adaptation critical for making such projections. By examining how changing microbial needs interact with inherent SOM structure and composition, and thus reactivity, we demonstrate the means by which increasing temperature could result in increasing, unchanging, or even decreasing respiration rates observed in soils. We use this exercise to highlight ideas for future research that will develop our abilities to predict SOM feedbacks to climate.     

Variable temperature effects of Open Top Chambers at polar and alpine sites explained by irradiance and snow depth

Environmental manipulation studies are integral to determining biological consequences of climate warming. Open Top Chambers (OTCs) have been widely used to assess summer warming effects on terrestrial biota, with their effects during other seasons normally being given less attention even though chambers are often deployed year‐round. In addition, their effects on temperature extremes and freeze‐thaw events are poorly documented. To provide robust documentation of the microclimatic influences of OTCs throughout the year, we analysed temperature data from 20 studies distributed across polar and alpine regions. The effects of OTCs on mean temperature showed a large range (?0.9 to 2.1 °C) throughout the year, but did not differ significantly between studies. Increases in mean monthly and diurnal temperature were strongly related (R² = 0.70) with irradiance, indicating that PAR can be used to predict the mean warming effect of OTCs. Deeper snow trapped in OTCs also induced higher temperatures at soil/vegetation level. OTC‐induced changes in the frequency of freeze‐thaw events included an increase in autumn and decreases in spring and summer. Frequency of high‐temperature events in OTCs increased in spring, summer and autumn compared with non‐manipulated control plots. Frequency of low‐temperature events was reduced by deeper snow accumulation and higher mean temperatures. The strong interactions identified between aspects of ambient environmental conditions and effects of OTCs suggest that a detailed knowledge of snow depth, temperature and irradiance levels enables us to predict how OTCs will modify the microclimate at a particular site and season. Such predictive power allows a better mechanistic understanding of observed biotic response to experimental warming studies and for more informed design of future experiments. However, a need remains to quantify OTC effects on water availability and wind speed (affecting, for example, drying rates and water stress) in combination with microclimate measurements at organism level.     

The effect of heat waves,elevated [CO2] and low soil water availability on northern red oak (Quercus rubra L.) seedlings

The frequency and intensity of heat waves are predicted to increase. This study investigates whether heat waves would have the same impact as a constant increase in temperature with the same heat sum, and whether there would be any interactive effects of elevated [CO₂] and soil moisture content. We grew Quercus rubra seedlings in treatment chambers maintained at either ambient or elevated [CO₂] (380 or 700 μmol CO₂ mol^?1) with temperature treatments of ambient, ambient +3 °C, moderate heat wave (+6 °C every other week) or severe heat wave (+12 °C every fourth week) temperatures. Averaged over a 4‐week period, and the entire growing season, the three elevated temperature treatments had the same average temperature and heat sum. Half the seedlings were watered to a soil water content near field capacity, half to about 50% of this value. Foliar gas exchange measurements were performed morning and afternoon (9:00 and 15:00 hours) before, during and after an applied heat wave in August 2010. Biomass accumulation was measured after five heat wave cycles. Under ambient [CO₂] and well‐watered conditions, biomass accumulation was highest in the +3 °C treatment, intermediate in the +6 °C heat wave and lowest in the +12 °C heat wave treatment. This response was mitigated by elevated [CO₂]. Low soil moisture significantly decreased net photosynthesis (A_net) and biomass in all [CO₂] and temperature treatments. The +12 °C heat wave reduced afternoon A_net by 23% in ambient [CO₂]. Although this reduction was relatively greater under elevated [CO₂], A_net values during this heat wave were still 34% higher than under ambient [CO₂]. We concluded that heat waves affected biomass growth differently than the same amount of heat applied uniformly over the growing season, and that the plant response to heat waves also depends on [CO₂] and soil moisture conditions.