Relationship between heatwave‐induced forest die‐off and climatic suitability in multiple tree species

In recent decades, many forest die‐off events have been reported in relation to climate‐change‐induced episodes, such as droughts and heat waves. To understand how these extreme climatic events induce forest die‐off, it is important to find a tool to standardize the climatic conditions experienced by different populations during a specific climatic event, taking into account the historic climatic conditions of the site where these populations live (bioclimatic niche). In this study, we used estimates of climatic suitability calculated from species distribution models (SDMs) for such purpose. We studied forest die‐off across France during the 2003 heatwave that affected Western Europe, using 2,943 forest inventory plots dominated by 14 single tree species. Die‐off severity was estimated by Normalized Difference Vegetation Index (NDVI) loss using Moderate‐resolution Imaging Spectroradiometer remote sensor imagery. Climatic suitability at the local level during the historical 1979–2002 period (HCS), the episode time (2003; ECS) and suitability deviance during the historical period (HCS‐SD) were calculated for each species by means of boosted regression tree models using the CHELSA climate database and occurrences extracted from European forest inventories. Low HCS‐SD and high mean annual temperature explained the overall regional pattern of vulnerability to die‐off across different monospecific forests. The combination of high historical and low episode climatic suitability also contributed significantly to overall forest die‐off. Furthermore, we observed different species‐specific relationships between die‐off vulnerability and climatic suitability: Sub‐Mediterranean and Mediterranean species tended to be vulnerable in historically more suitable localities (high HCS), whereas Euro‐Siberian species presented greater vulnerability when the hot drought episode was more intense. We demonstrated that at regional scale, past climatic legacy plays an important role in explaining NDVI loss during the episode. Moreover, we demonstrated that SDMs‐derived indexes, such as HCS, ECS and HCS‐SD, could constitute a tool for standardizing the ways that populations and species experience climatic variability across time and space.     

Understanding and predicting frost‐induced tropical tree mortality patterns

Extreme climatic and weather events are increasing in frequency and intensity across the world causing episodes of widespread tree mortality in many forested ecosystems. However, we have a limited understanding about which local factors influence tree mortality patterns, restricting our ability to predict tree mortality, especially within topographically complex tropical landscapes with a matrix of mature and secondary forests. We investigated the effects of two major local factors, topography and forest successional type, on climate‐induced tropical tree mortality patterns using an observational and modeling approach. The northernmost Neotropical dry forest endured an unprecedented episode of frost‐induced tree mortality after the historic February 2011 cold wave hit northwestern Mexico. In a moderately hilly landscape covering mature and secondary tropical dry forests, we surveyed 454 sites for the presence or absence of frost‐induced tree mortality. In addition, across forty‐eight 1 ha plots equally split into the two forest types, we examined 6,981 woody plants to estimate a frost‐disturbance severity metric using the density of frost‐killed trees. Elevation is the main factor modulating frost effects regardless of forest type. Higher occurrence probabilities of frost‐induced tree mortality at lowland forests can be explained by the strong influence of elevation on temperature distribution since heavier cold air masses move downhill during advective frosts. Holding elevation constant, the probability of frost‐induced tree mortality in mature forests was twice that of secondary forests but severity showed the opposite pattern, suggesting a cautious use of occurrence probabilities of tree mortality to infer severity of climate‐driven disturbances. Extreme frost events, in addition to altering forest successional pathways and ecosystem services, likely maintain and could ultimately shift latitudinal and altitudinal range margins of Neotropical dry forests.     

Structural overshoot of tree growth with climate variability and the global spectrum of drought‐induced forest dieback

Ongoing climate change poses significant threats to plant function and distribution. Increased temperatures and altered precipitation regimes amplify drought frequency and intensity, elevating plant stress and mortality. Large‐scale forest mortality events will have far‐reaching impacts on carbon and hydrological cycling, biodiversity, and ecosystem services. However, biogeographical theory and global vegetation models poorly represent recent forest die‐off patterns. Furthermore, as trees are sessile and long‐lived, their responses to climate extremes are substantially dependent on historical factors. We show that periods of favourable climatic and management conditions that facilitate abundant tree growth can lead to structural overshoot of aboveground tree biomass due to a subsequent temporal mismatch between water demand and availability. When environmental favourability declines, increases in water and temperature stress that are protracted, rapid, or both, drive a gradient of tree structural responses that can modify forest self‐thinning relationships. Responses ranging from premature leaf senescence and partial canopy dieback to whole‐tree mortality reduce canopy leaf area during the stress period and for a lagged recovery window thereafter. Such temporal mismatches of water requirements from availability can occur at local to regional scales throughout a species geographical range. As climate change projections predict large future fluctuations in both wet and dry conditions, we expect forests to become increasingly structurally mismatched to water availability and thus overbuilt during more stressful episodes. By accounting for the historical context of biomass development, our approach can explain previously problematic aspects of large‐scale forest mortality, such as why it can occur throughout the range of a species and yet still be locally highly variable, and why some events seem readily attributable to an ongoing drought while others do not. This refined understanding can facilitate better projections of structural overshoot responses, enabling improved prediction of changes in forest distribution and function from regional to global scales.     

Measuring canopy loss and climatic thresholds from an extreme drought along a fivefold precipitation gradient across Texas

Globally, trees are increasingly dying from extreme drought, a trend that is expected to increase with climate change. Loss of trees has significant ecological, biophysical, and biogeochemical consequences. In 2011, a record drought caused widespread tree mortality in Texas. Using remotely sensed imagery, we quantified canopy loss during and after the drought across the state at 30‐m spatial resolution, from the eastern pine/hardwood forests to the western shrublands, a region that includes the boundaries of many species ranges. Canopy loss observations in ~200 multitemporal fine‐scale orthophotos (1‐m) were used to train coarser Landsat imagery (30‐m) to create 30‐m binary statewide canopy loss maps. We found that canopy loss occurred across all major ecoregions of Texas, with an average loss of 9.5%. The drought had the highest impact in post oak woodlands, pinyon‐juniper shrublands and Ashe juniper woodlands. Focusing on a 100‐km by ~1,000‐km transect spanning the State's fivefold east–west precipitation gradient (~1,500 to ~300 mm), we compared spatially explicit 2011 climatic anomalies to our canopy loss maps. Much of the canopy loss occurred in areas that passed specific climatic thresholds: warm season anomalies in mean temperature (+1.6°C) and vapor pressure deficit (VPD, +0.66 kPa), annual percent deviation in precipitation (?38%), and 2011 difference between precipitation and potential evapotranspiration (?1,206 mm). Although similarly low precipitation occurred during the landmark 1950s drought, the VPD and temperature anomalies observed in 2011 were even greater. Furthermore, future climate data under the representative concentration pathway 8.5 trajectory project that average values will surpass the 2011 VPD anomaly during the 2070–2099 period and the temperature anomaly during the 2040–2099 period. Identifying vulnerable ecological systems to drought stress and climate thresholds associated with canopy loss will aid in predicting how forests will respond to a changing climate and how ecological landscapes will change in the near term.     

A climatic threshold triggers the die‐off of peat mosses during an extreme heat wave

Heat waves, which are projected to be more frequent and intense in a warmer climate, could become a serious threat to plants that rely on water surplus availability, such as bryophytes. Here, I take the advantage of the European summer 2003 climate anomaly to assess the impact of an extreme heat wave on peat mosses of the genus Sphagnum, a group of bryophytes forming the bulk of living and dead biomass in peatlands. With this aim, 20 selected bogs in the Italian Alps were checked for Sphagnum survival in the years following the heat wave. Over the study area, the period May–September 2003 was characterized by higher mean monthly air temperature (13.5 °C) and lower mean monthly precipitation (87 mm) compared with normal climatic conditions (11.5 °C and 117 mm, respectively) so that the heat wave coincided with a drought spell. As a consequence of the unusual water stress, I documented an increased mortality of peat mosses forming high hummocks. In particular, at habitat scale, the distribution of desiccated peat mosses was restricted to the hummock face receiving the greatest amount of solar irradiation. However, at regional scale, the present study identified a climatic threshold, simply defined by the ratio of precipitation to temperature (P : T), which triggered an irreversible desiccation of peat mosses when mean monthly P : T dropped below 6.5 (mm : °C) during May–September 2003. The absence of any sign of recovery after 4 years since the drought must be seen as a harbinger of the deleterious effects of extreme heat waves on organisms not adapted to cope with abrupt climate anomaly.     

Historical and event‐based bioclimatic suitability predicts regional forest vulnerability to compound effects of severe drought and bark beetle infestation

Vulnerability to climate change, and particularly to climate extreme events, is expected to vary across species ranges. Thus, we need tools to standardize the variability in regional climatic legacy and extreme climate across populations and species. Extreme climate events (e.g., droughts) can erode populations close to the limits of species' climatic tolerance. Populations in climatic‐core locations may also become vulnerable because they have developed a greater demand for resources (i.e., water) that cannot be enough satisfied during the periods of scarcity. These mechanisms can become exacerbated in tree populations when combined with antagonistic biotic interactions, such as insect infestation. We used climatic suitability indices derived from Species Distribution Models (SDMs) to standardize the climatic conditions experienced across Pinus edulis populations in southwestern North America, during a historical period (1972–2000) and during an extreme event (2001–2007), when the compound effect of hot drought and bark beetle infestation caused widespread die‐off and mortality. Pinus edulis climatic suitability diminished dramatically during the die‐off period, with remarkable variation between years. P. edulis die‐off occurred mainly not just in sites that experienced lower climatic suitability during the drought but also where climatic suitability was higher during the historical period. The combined effect of historically high climatic suitability and a marked decrease in the climatic suitability during the drought best explained the range‐wide mortality. Lagged effects of climatic suitability loss in previous years and co‐occurrence of Juniperus monosperma also explained P. edulis die‐off in particular years. Overall, the study shows that past climatic legacy, likely determining acclimation, together with competitive interactions plays a major role in responses to extreme drought. It also provides a new approach to standardize the magnitude of climatic variability across populations using SDMs, improving our capacity to predict population's or species' vulnerability to climatic change.     

Relationships between individual‐tree mortality and water‐balance variables indicate positive trends in water stress‐induced tree mortality across North America

Accounting for water stress‐induced tree mortality in forest productivity models remains a challenge due to uncertainty in stress tolerance of tree populations. In this study, logistic regression models were developed to assess species‐specific relationships between probability of mortality (P_m) and drought, drawing on 8.1 million observations of change in vital status (m) of individual trees across North America. Drought was defined by standardized (relative) values of soil water content (W_s,z) and reference evapotranspiration (ET_r,z) at each field plot. The models additionally tested for interactions between the water‐balance variables, aridity class of the site (AC), and estimated tree height (h). Considering drought improved model performance in 95 (80) per cent of the 64 tested species during calibration (cross‐validation). On average, sensitivity to relative drought increased with site AC (i.e. aridity). Interaction between water‐balance variables and estimated tree height indicated that drought sensitivity commonly decreased during early height development and increased during late height development, which may reflect expansion of the root system and decreasing whole‐plant, leaf‐specific hydraulic conductance, respectively. Across North America, predictions suggested that changes in the water balance caused mortality to increase from 1.1% yr^?1 in 1951 to 2.0% yr^?1 in 2014 (a net change of 0.9 ± 0.3% yr^?1). Interannual variation in mortality also increased, driven by increasingly severe droughts in 1988, 1998, 2006, 2007 and 2012. With strong confidence, this study indicates that water stress is a common cause of tree mortality. With weak‐to‐moderate confidence, this study strengthens previous claims attributing positive trends in mortality to increasing levels of water stress. This ‘learn‐as‐we‐go’ approach – defined by sampling rare drought events as they continue to intensify – will help to constrain the hydraulic limits of dominant tree species and the viability of boreal and temperate forest biomes under continued climate change.     

Changes in structure of over‐ and midstory tree species in a Mediterranean‐type forest after an extreme drought‐associated heatwave

Worldwide, extreme climatic events such as drought and heatwaves are associated with forest mortality. However, the precise drivers of tree mortality at individual and stand levels vary considerably, with substantial gaps in knowledge across studies in biomes and continents. In 2010–2011, a drought‐associated heatwave occurred in south‐western Australia and drove sudden and rapid forest canopy collapse. Working in the Northern Jarrah (Eucalyptus marginata) Forest, we quantified the response of key overstory (E. marginata, Corymbia calophylla) and midstory (Banksia grandis, Allocasuarina fraseriana) tree species to the extreme climate event. Using transects spanning a gradient of drought impacts (minimal (50–100 m), transitional (100–150 m) and severe (30–60 m)), tree species mortality in relation to stand characteristics (stand basal area and stem density) and edaphic factors (soil depth) was determined. We show differential mortality between the two overstory species and the two midstory species corresponding to the drought‐associated heatwave. The dominant overstory species, E. marginata, had significantly higher mortality (~19%) than C. calophylla (~7%) in the severe zone. The midstory species, B. grandis, demonstrated substantially higher mortality (~59%) than A. fraseriana (~4%) in the transitional zone. Banksia grandis exhibited a substantial shift in structure in response to the drought‐associated heatwave in relation to tree size, basal area and soil depth. This study illustrates the role of climate extremes in driving ecosystem change and highlights the critical need to identify and quantify the resulting impact to help predict future forest die‐off events and to underpin forest management and conservation.     

An ecoclimatic framework for evaluating the resilience of vegetation to water deficit

The surge in global efforts to understand the causes and consequences of drought on forest ecosystems has tended to focus on specific impacts such as mortality. We propose an ecoclimatic framework that takes a broader view of the ecological relevance of water deficits, linking elements of exposure and resilience to cumulative impacts on a range of ecosystem processes. This ecoclimatic framework is underpinned by two hypotheses: (i) exposure to water deficit can be represented probabilistically and used to estimate exposure thresholds across different vegetation types or ecosystems; and (ii) the cumulative impact of a series of water deficit events is defined by attributes governing the resistance and recovery of the affected processes. We present case studies comprising Pinus edulis and Eucalyptus globulus, tree species with contrasting ecological strategies, which demonstrate how links between exposure and resilience can be examined within our proposed framework. These examples reveal how climatic thresholds can be defined along a continuum of vegetation functional responses to water deficit regimes. The strength of this framework lies in identifying climatic thresholds on vegetation function in the absence of more complete mechanistic understanding, thereby guiding the formulation, application and benchmarking of more detailed modelling.     

Drought survival is a threshold function of habitat size and population density in a fish metapopulation

Because smaller habitats dry more frequently and severely during droughts, habitat size is likely a key driver of survival in populations during climate change and associated increased extreme drought frequency. Here, we show that survival in populations during droughts is a threshold function of habitat size driven by an interaction with population density in metapopulations of the forest pool dwelling fish, Neochanna apoda. A mark–recapture study involving 830 N. apoda individuals during a one‐in‐seventy‐year extreme drought revealed that survival during droughts was high for populations occupying pools deeper than 139 mm, but declined steeply in shallower pools. This threshold was caused by an interaction between increasing population density and drought magnitude associated with decreasing habitat size, which acted synergistically to increase physiological stress and mortality. This confirmed two long‐held hypotheses, firstly concerning the interactive role of population density and physiological stress, herein driven by habitat size, and secondly, the occurrence of drought survival thresholds. Our results demonstrate how survival in populations during droughts will depend strongly on habitat size and highlight that minimum habitat size thresholds will likely be required to maximize survival as the frequency and intensity of droughts are projected to increase as a result of global climate change.     

Changes in soil bacterial community triggered by drought‐induced gap succession preceded changes in soil C stocks and quality

The aim of this study was to understand how drought‐induced tree mortality and subsequent secondary succession would affect soil bacterial taxonomic composition as well as soil organic matter (SOM) quantity and quality in a mixed Mediterranean forest where the Scots pine (Pinus sylvestris) population, affected by climatic drought‐induced die‐off, is being replaced by Holm‐oaks (HO; Quercus ilex). We apply a high throughput DNA pyrosequencing technique and ¹³C solid‐state Nuclear Magnetic Resonance (CP‐MAS ¹³C NMR) to soils within areas of influence (defined as an surface with 2‐m radius around the trunk) of different trees: healthy and affected (defoliated) pines, pines that died a decade ago and healthy HOs. Soil respiration was also measured in the same spots during a spring campaign using a static close‐chamber method (soda lime). A decade after death, and before aerial colonization by the more competitive HOs have even taken place, we could not find changes in soil C pools (quantity and/or quality) associated with tree mortality and secondary succession. Unlike C pools, bacterial diversity and community structure were strongly determined by tree mortality. Convergence between the most abundant taxa of soil bacterial communities under dead pines and colonizer trees (HOs) further suggests that physical gap colonization was occurring below‐ground before above‐ground colonization was taken place. Significantly higher soil respiration rates under dead trees, together with higher bacterial diversity and anomalously high representation of bacteria commonly associated with copiotrophic environments (r‐strategic bacteria) further gives indications of how drought‐induced tree mortality and secondary succession were influencing the structure of microbial communities and the metabolic activity of soils.     

Global change‐type drought‐induced tree mortality: vapor pressure deficit is more important than temperature per se in causing decline in tree health

Drought‐induced tree mortality is occurring across all forested continents and is expected to increase worldwide during the coming century. Regional‐scale forest die‐off influences terrestrial albedo, carbon and water budgets, and land‐surface energy partitioning. Although increased temperatures during drought are widely identified as a critical contributor to exacerbated tree mortality associated with “global‐change‐type drought”, corresponding changes in vapor pressure deficit (D) have rarely been considered explicitly and have not been disaggregated from that of temperature per se. Here, we apply a detailed mechanistic soil–plant–atmosphere model to examine the impacts of drought, increased air temperature (+2°C or +5°C), and increased vapor pressure deficit (D; +1 kPa or +2.5 kPa), singly and in combination, on net primary productivity (NPP) and transpiration and forest responses, especially soil moisture content, leaf water potential, and stomatal conductance. We show that increased D exerts a larger detrimental effect on transpiration and NPP, than increased temperature alone, with or without the imposition of a 3‐month drought. Combined with drought, the effect of increased D on NPP was substantially larger than that of drought plus increased temperature. Thus, the number of days when NPP was zero across the 2‐year simulation was 13 or 14 days in the control and increased temperature scenarios, but increased to approximately 200 days when D was increased. Drought alone increased the number of days of zero NPP to 88, but drought plus increased temperature did not increase the number of days. In contrast, drought and increased D resulted in the number of days when NPP = 0 increasing to 235 (+1 kPa) or 304 days (+2.5 kPa). We conclude that correct identification of the causes of global change‐type mortality events requires explicit consideration of the influence of D as well as its interaction with drought and temperature.     

Rapid warming accelerates tree growth decline in semi‐arid forests of Inner Asia

Forests around the world are subject to risk of high rates of tree growth decline and increased tree mortality from combinations of climate warming and drought, notably in semi‐arid settings. Here, we assess how climate warming has affected tree growth in one of the world's most extensive zones of semi‐arid forests, in Inner Asia, a region where lack of data limits our understanding of how climate change may impact forests. We show that pervasive tree growth declines since 1994 in Inner Asia have been confined to semi‐arid forests, where growing season water stress has been rising due to warming‐induced increases in atmospheric moisture demand. A causal link between increasing drought and declining growth at semi‐arid sites is corroborated by correlation analyses comparing annual climate data to records of tree‐ring widths. These ring‐width records tend to be substantially more sensitive to drought variability at semi‐arid sites than at semi‐humid sites. Fire occurrence and insect/pathogen attacks have increased in tandem with the most recent (2007–2009) documented episode of tree mortality. If warming in Inner Asia continues, further increases in forest stress and tree mortality could be expected, potentially driving the eventual regional loss of current semi‐arid forests.     

Satellite sun‐induced chlorophyll fluorescence detects early response of winter wheat to heat stress in the Indian Indo‐Gangetic Plains

Extremely high temperatures represent one of the most severe abiotic stresses limiting crop productivity. However, understanding crop responses to heat stress is still limited considering the increases in both the frequency and severity of heat wave events under climate change. This limited understanding is partly due to the lack of studies or tools for the timely and accurate monitoring of crop responses to extreme heat over broad spatial scales. In this work, we use novel spaceborne data of sun‐induced chlorophyll fluorescence (SIF), which is a new proxy for photosynthetic activity, along with traditional vegetation indices (Normalized Difference Vegetation Index NDVI and Enhanced Vegetation Index EVI) to investigate the impacts of heat stress on winter wheat in northwestern India, one of the world's major wheat production areas. In 2010, an abrupt rise in temperature that began in March adversely affected the productivity of wheat and caused yield losses of 6% compared to previous year. The yield predicted by satellite observations of SIF decreased by approximately 13.9%, compared to the 1.2% and 0.4% changes in NDVI and EVI, respectively. During early stage of this heat wave event in early March 2010, the SIF observations showed a significant reduction and earlier response, while NDVI and EVI showed no changes and could not capture the heat stress until late March. The spatial patterns of SIF anomalies closely tracked the temporal evolution of the heat stress over the study area. Furthermore, our results show that SIF can provide large‐scale, physiology‐related wheat stress response as indicated by the larger reduction in fluorescence yield (SIF_yield) than fraction of photosynthetically active radiation during the grain‐filling phase, which may have eventually led to the reduction in wheat yield in 2010. This study implies that satellite observations of SIF have great potential to detect heat stress conditions in wheat in a timely manner and assess their impacts on wheat yields at large scales.     

Species‐specific stomatal response of trees to drought – a link to vegetation dynamics?

Question: Is stomatal regulation specific for climate and tree species, and does it reveal species‐specific responses to drought? Is there a link to vegetation dynamics? Location: Dry inner alpine valley, Switzerland Methods: Stomatal aperture (θ_E) of Pinus sylvestris, Quercus pubescens, Juniperus communis and Picea abies were continuously estimated by the ratio of measured branch sap flow rates to potential transpiration rates (adapted Penman‐Monteith single leaf approach) at 10‐min intervals over four seasons. Results: θ_E proved to be specific for climate and species and revealed distinctly different drought responses: Pinus stomata close disproportionately more than neighbouring species under dry conditions, but has a higher θ_E than the other species when weather was relatively wet and cool. Quercus keeps stomata more open under drought stress but has a lower θ_E under humid conditions. Juniperus was most drought‐tolerant, whereas Picea stomata close almost completely during summer. Conclusions: The distinct microclimatic preferences of the four tree species in terms of θ_E strongly suggest that climate (change) is altering tree physiological performances and thus species‐specific competitiveness. Picea and Pinus currently live at the physiological limit of their ability to withstand increasing temperature and drought intensities at the sites investigated, whereas Quercus and Juniperus perform distinctly better. This corresponds, at least partially, with regional vegetation dynamics: Pinus has strongly declined, whereas Quercus has significantly increased in abundance in the past 30 years. We conclude that θ_E provides an indication of a species' ability to cope with current and predicted climate.     

Insect‐induced tree mortality of boreal forests in eastern Canada under a changing climate

Forest insects are major disturbances that induce tree mortality in eastern coniferous (or fir-spruce) forests in eastern North America. The spruce budworm (SBW) (Choristoneura fumiferana [Clemens]) is the most devastating insect causing tree mortality. However, the relative importance of insect-caused mortality versus tree mortality caused by other agents and how this relationship will change with climate change is not known. Based on permanent sample plots across eastern Canada, we combined a logistic model with a negative model to estimate tree mortality. The results showed that tree mortality increased mainly due to forest insects. The mean difference in annual tree mortality between plots disturbed by insects and those without insect disturbance was 0.0680 per year (P < 0.0001, T-test), and the carbon sink loss was about 2.87t C ha⁻¹ year⁻¹ larger than in natural forests. We also found that annual tree mortality increased significantly with the annual climate moisture index (CMI) and decreased significantly with annual minimum temperature (T_min), annual mean temperature (T_mean) and the number of degree days below 0°C (DD0), which was inconsistent with previous studies (Adams et al. 2009; van Mantgem et al. 2009; Allen et al. 2010). Furthermore, the results for the trends in the magnitude of forest insect outbreaks were consistent with those of climate factors for annual tree mortality. Our results demonstrate that forest insects are the dominant cause of the tree mortality in eastern Canada but that tree mortality induced by insect outbreaks will decrease in eastern Canada under warming climate.     

The form of a trade‐off determines the response to competition

Understanding how populations and communities respond to competition is a central concern of ecology. A seminal theoretical solution first formalised by Levins (and re‐derived in multiple fields) showed that, in theory, the form of a trade‐off should determine the outcome of competition. While this has become a central postulate in ecology it has evaded experimental verification, not least because of substantial technical obstacles. We here solve the experimental problems by employing synthetic ecology. We engineer strains of Escherichia coli with fixed resource allocations enabling accurate measurement of trade‐off shapes between bacterial survival and multiplication in multiple environments. A mathematical chemostat model predicts different, and experimentally verified, trajectories of gene frequency changes as a function of condition‐specific trade‐offs. The results support Levins' postulate and demonstrates that otherwise paradoxical alternative outcomes witnessed in subtly different conditions are predictable.