GLOBE students, teachers, and scientists demonstrate variable differences between urban and rural leaf phenology

The urban heat island effect, classically associated with high impervious surface area (ISA), low vegetation fractional cover (Fr), and high land surface temperature (LST), has been linked to changing patterns of vegetation phenology, especially spring growth. In this study, a collaboration with the Global Learning and Observations to Benefit the Environment (GLOBE) program, we investigated the effect of the urban environment on the timing of leaf budburst of native deciduous trees in seven cities: Asia (Tokyo, Japan; Bangkok and Korat, Thailand), Europe (Jyväskylä, Finland; Bishkek, Kyrgyzstan), Africa (Dakar, Senegal), and North America (Fairbanks, Alaska). The cities differed not only in population size but also in climate and vegetation type. Using Landsat satellite imagery from each city, we calculated LST, Fr, and ISA, and classified sites within each study area as rural or urban. The timing of leaf flushing, measured by students using GLOBE budburst protocols, was statistically different within all cities, with absolute differences ranging from 1 to 23 days. We assessed the classic urban phenology paradigm, which proposes higher LST, lower Fr, and earlier budburst in urban areas of temperate cities. Of the four temperate cities, Tokyo followed the classic paradigm, but no other city demonstrated consistent support. Urban budburst was advanced in three of the four temperate cities, but in only one of the three tropical cities. Results suggest that while vegetation phenology is consistently different between urban and rural areas, a uniform paradigm based on the explanatory variables in this study did not emerge. Although not testable here, it is likely that alterations to chilling requirements in temperate climates and humidity in tropical climates may also influence observed budburst differences.     

Climate and vegetational regime shifts in the late Paleozoic ice age earth

The late Paleozoic earth experienced alternation between glacial and non-glacial climates at multiple temporal scales, accompanied by atmospheric CO₂ fluctuations and global warming intervals, often attended by significant vegetational changes in equatorial latitudes of Pangaea. We assess the nature of climate–vegetation interaction during two time intervals: middle–late Pennsylvanian transition and Pennsylvanian–Permian transition, each marked by tropical warming and drying. In case study 1, there is a catastrophic intra-biomic reorganization of dominance and diversity in wetland, evergreen vegetation growing under humid climates. This represents a threshold-type change, possibly a regime shift to an alternative stable state. Case study 2 is an inter-biome dominance change in western and central Pangaea from humid wetland and seasonally dry to semi-arid vegetation. Shifts between these vegetation types had been occurring in Euramerican portions of the equatorial region throughout the late middle and late Pennsylvanian, the drier vegetation reaching persistent dominance by Early Permian. The oscillatory transition between humid and seasonally dry vegetation appears to demonstrate a threshold-like behavior but probably not repeated transitions between alternative stable states. Rather, changes in dominance in lowland equatorial regions were driven by long-term, repetitive climatic oscillations, occurring with increasing intensity, within overall shift to seasonal dryness through time. In neither case study are there clear biotic or abiotic warning signs of looming changes in vegetational composition or geographic distribution, nor is it clear that there are specific, absolute values or rates of environmental change in temperature, rainfall distribution and amount, or atmospheric composition, approach to which might indicate proximity to a terrestrial biotic-change threshold.     

Quantifying street tree regulating ecosystem services using Google Street View

The urban heat island is a particular challenge for tropical cities, which receive year-round high inputs of solar radiation. Plants can help mitigate elevated urban temperatures by providing shade and increasing evaporative cooling, although the resulting increase in humidity may negatively affect thermal comfort. Street trees offer particular potential for cooling urban microclimates, as well as providing other ecosystem services, because they can be integrated within dense urban street networks. However, we have little quantitative information about the role of street trees in providing regulating ecosystem services in tropical cities. In this study, we analysed hemispherical photographs extracted from Google Street View to quantify the proportion of green canopy coverage at 50 m intervals across more than 80% of Singapore’s road network. Canopy coverage data were then used to estimate the proportion of annual radiation that would be blocked from reaching ground level by the canopy. Across all locations, a median of 13% of the annual diffuse and direct solar radiation was shaded, and over 70% of this shading effect was due to the tree canopy. There was significant variation between different urban landuse types, with trees providing more shade in parks and low-density low-rise areas than in industrial and higher-density residential areas. Mapping the provision of street tree ecosystem services could help to prioritise areas for new planting by identifying streets or street sections with low shading. The approach developed in this article could be readily applied to quantify the proportion of canopy coverage and proportion of solar radiation shaded across other tropical cities. The method may also be applicable in temperate cities if Google Street View photographs were collected during the growing season.     

Climate and tourism in the Black Forest during the warm season

Climate, climate change and tourism all interact. Part of the public discussion about climate change focusses on the tourism sector, with direct and indirect impacts being of equally high relevance. Climate and tourism are closely linked. Thus, climate is a very decisive factor in choices both of destination and of type of journey (active holidays, wellness, and city tours) in the tourism sector. However, whether choices about destinations or types of trip will alter with climate change is difficult to predict. Future climates can be simulated and projected, and the tendencies of climate parameters can be estimated using global and regional climate models. In this paper, the focus is on climate change in the mountainous regions of southwest Germany – the Black Forest. The Black Forest is one of the low mountain ranges where both winter and summer tourism are vulnerable to climate change due to its southern location; the strongest climatic changes are expected in areas covering the south and southwest of Germany. Moreover, as the choice of destination is highly dependent on good weather, a climatic assessment for tourism is essential. Thus, the aim of this study was to estimate climatic changes in mountainous regions during summer, especially for tourism and recreation. The assessment method was based on human-biometeorology as well as tourism-climatologic approaches. Regional climate simulations based on the regional climate model REMO were used for tourism-related climatic analyses. Emission scenarios A1B and B1 were considered for the time period 2021 to 2050, compared to the 30-year base period of 1971–2000, particularly for the warm period of the year, defined here as the months of March–November. In this study, we quantified the frequency, but not the means, of climate parameters. The study results show that global and regional warming is reflected in an increase in annual mean air temperature, especially in autumn. Changes in the spring show a slight negative trend, which is in line with the trend of a decrease in physiologically equivalent temperature as well as in thermal comfort conditions. Due to the rising air temperature, heat stress as well as sultry conditions are projected to become more frequent, affecting human health and recreation, especially at lower lying altitudes. The tops of the mountains and higher elevated areas still have the advantage of offering comfortable climatic conditions.     

Time of emergence of novel climates for North American migratory bird populations

To better understand the ecological implications of global climate change for species that display geographically and seasonally dynamic life‐history strategies, we need to determine where and when novel climates are projected to first emerge. Here, we use a multivariate approach to estimate time of emergence (ToE) of novel climates based on three climate variables (precipitation, minimum and maximum temperature) at a weekly temporal resolution within the Western Hemisphere over a 280‐yr period (2021–2300) under a high emissions scenario (RCP8.5). We intersect ToE estimates with weekly estimates of relative abundance for 77 passerine bird species that migrate between temperate breeding grounds in North America and southern tropical and subtropical wintering grounds using observations from the eBird citizen‐science database. During the non‐breeding season, migrants that winter within the tropics are projected to encounter novel climates during the second half of this century. Migrants that winter in the subtropics are projected to encounter novel climates during the first half of the next century. During the beginning of the breeding season, migrants on their temperate breeding grounds are projected to encounter novel climates during the first half of the next century. During the end of the breeding season, migrants are projected to encounter novel climates during the second half of this century. Thus, novel climates will first emerge ca 40–50 yr earlier during the second half of the breeding season. These results emphasize the large seasonal and spatial variation in the formation of novel climates, and the pronounced challenges migratory birds are likely to encounter during this century, especially on their tropical wintering grounds and during the transition from breeding to migration. When assessing the ecological implications of climate change, our findings emphasize the value of applying a full annual cycle perspective using standardized metrics that promote comparisons across space and time.     

Tracing the tropics across land and sea: Permian to present

The continuity through the past 300 million years of key tropical sediment types, namely coals, evaporites, reefs and carbonates, is examined. Physical controls for their geographical distributions are related to the Hadley cell circulation, and its effects on rainfall and ocean circulation. Climate modelling studies are reviewed in this context, as are biogeographical studies of key fossil groups. Low-latitude peats and coals represent everwet climates related to the Intertropical Convergence Zone near the Equator, as well as coastal diurnal rainfall systems elsewhere in the tropics and subtropics. The incidence of tropical coals and rainforests through time is variable, being least common during the interval of Pangean monsoonal climates. Evaporites represent the descending limbs of the Hadley cells and are centred at 10° to 40° north and south in latitudes that today show an excess of evaporation over precipitation. These deposits coincide with the deserts as well as seasonally rainy climates, and their latitudinal ranges seem to have been relatively constant through time. Reefs also can be related to the Hadley circulation. They thrive within the regions of clear water associated with broad areas of downwelling which are displaced toward the western portions of tropical oceans. These dynamic features are ultimately driven by the subtropical high-pressure cells which are the surface signature of the subsiding branches of the Hadley circulation. Carbonates occupy the same areas, but extend into higher latitudes in regions where terrestrial surface gradients are low and clastic runoff from the land is minimal. We argue that the palaeo-latitudinal record of all these climate-sensitive sediment types is broadly similar to their environments and latitudes of formation today, implying that dynamic effects of atmospheric and oceanic circulation control their distribution, rather than temperature gradients that would expand or contract through time.     

Methane emissions from tropical freshwater wetlands located in different climatic zones of Costa Rica

Wetlands are the largest natural source of the greenhouse gas methane to the atmosphere. Despite the fact that a large percentage of wetlands occur in tropical latitudes, methane emissions from natural tropical wetlands have not been extensively studied. The objective this research was to compare methane emissions from three natural tropical wetlands located in different climatic and ecological areas of Costa Rica. Each wetland was within a distinct ecosystem: (1) a humid flow‐through wetland slough with high mean annual temperatures (25.9 °C) and precipitation (3700 mm yr^?1); (2) a stagnant rainforest wetland with high mean annual temperatures (24.9 °C) and precipitation (4400 mm yr^?1); or (3) a seasonally wet riverine wetland with very high mean annual temperatures (28.2 °C) and lower mean annual precipitation (1800 mm yr^?1). Methane emission rates were measured from sequential gas samples using nonsteady state plastic chambers during six sampling periods over a 29‐month period from 2006 to 2009. Methane emissions were higher than most rates previously reported for tropical wetlands with means (medians) of 91 (52), 601 (79), and 719 (257) mg CH₄‐C m^?2 day^?1 for the three sites, with highest rates seen at the seasonally flooded wetland site. Methane emissions were statistically higher at the seasonally wet site than at the humid sites (P<0.001). Highest methane emissions occurred when surface water levels were between 30 and 50 cm. The interaction of soil temperature, water depth, and seasonal flooding most likely affected methanogenesis in these tropical sites. We estimate that Costa Rican wetlands produce about 0.80 Tg yr^?1 of methane, or approximately 0.6% of global tropical wetland emissions. Elevated methane emissions at the seasonally wet/warmer wetland site suggest that some current humid tropical freshwater wetlands of Central America could emit more methane if temperatures increase and precipitation becomes more seasonal with climate change.     

Geographical potential of Argentine ants (Linepithema humile Mayr) in the face of global climate change

Determining the spread and potential geographical distribution of invasive species is integral to making invasion biology a predictive science. We assembled a dataset of over 1000 occurrences of the Argentine ant (Linepithema humile), one of the world's worst invasive alien species. Native to central South America, Argentine ants are now found in many Mediterranean and subtropical climates around the world. We used this dataset to assess the species' potential geographical and ecological distribution, and to examine changes in its distributional potential associated with global climate change, using techniques for ecological niche modelling. Models developed were highly predictive of the species' overall range, including both the native distributional area and invaded areas worldwide. Despite its already widespread occurrence, L. humile has potential for further spread, with tropical coastal Africa and southeast Asia apparently vulnerable to invasion. Projecting ecological niche models onto four general circulation model scenarios of future (2050s) climates provided scenarios of the species' potential for distributional expansion with warming climates: generally, the species was predicted to retract its range in tropical regions, but to expand at higher latitude areas.     

Effectiveness of strict vs. multiple use protected areas in reducing tropical forest fires: a global analysis using matching methods

Protected areas (PAs) cover a quarter of the tropical forest estate. Yet there is debate over the effectiveness of PAs in reducing deforestation, especially when local people have rights to use the forest. A key analytic problem is the likely placement of PAs on marginal lands with low pressure for deforestation, biasing comparisons between protected and unprotected areas. Using matching techniques to control for this bias, this paper analyzes the global tropical forest biome using forest fires as a high resolution proxy for deforestation; disaggregates impacts by remoteness, a proxy for deforestation pressure; and compares strictly protected vs. multiple use PAs vs indigenous areas. Fire activity was overlaid on a 1 km map of tropical forest extent in 2000; land use change was inferred for any point experiencing one or more fires. Sampled points in pre-2000 PAs were matched with randomly selected never-protected points in the same country. Matching criteria included distance to road network, distance to major cities, elevation and slope, and rainfall. In Latin America and Asia, strict PAs substantially reduced fire incidence, but multi-use PAs were even more effective. In Latin America, where there is data on indigenous areas, these areas reduce forest fire incidence by 16 percentage points, over two and a half times as much as naïve (unmatched) comparison with unprotected areas would suggest. In Africa, more recently established strict PAs appear to be effective, but multi-use tropical forest protected areas yield few sample points, and their impacts are not robustly estimated. These results suggest that forest protection can contribute both to biodiversity conservation and CO2 mitigation goals, with particular relevance to the REDD agenda. Encouragingly, indigenous areas and multi-use protected areas can help to accomplish these goals, suggesting some compatibility between global environmental goals and support for local livelihoods.     

Neotropical seasonally dry forests and Quaternary vegetation changes

Seasonally dry tropical forests have been largely ignored in discussions of vegetation changes during the Quaternary. We distinguish dry forests, which are essentially tree‐dominated ecosystems, from open savannas that have a xeromorphic fire‐tolerant, grass layer and grow on dystrophic, acid soils. Seasonally dry tropical forests grow on fertile soils, usually have a closed canopy, have woody floras dominated by the Leguminosae and Bignoniaceae and a sparse ground flora with few grasses. They occur in disjunct areas throughout the Neotropics. The Chaco forests of central South America experience regular annual frosts, and are considered a subtropical extension of temperate vegetation formations. At least 104 plant species from a wide range of families are each found in two or more of the isolated areas of seasonally dry tropical forest scattered across the Neotropics, and these repeated patterns of distribution suggest a more widespread expanse of this vegetation, presumably in drier and cooler periods of the Pleistocene. We propose a new vegetation model for some areas of the Ice‐Age Amazon: a type of seasonally dry tropical forest, with rain forest and montane taxa largely confined to gallery forest. This model is consistent with the distributions of contemporary seasonally dry tropical forest species in Amazonia and existing palynological data. The hypothesis of vicariance of a wider historical area of seasonally dry tropical forests could be tested using a cladistic biogeographic approach focusing on plant genera that have species showing high levels of endemicity in the different areas of these forests.     

Similar survival of birds between wet and seasonally dry Neotropical environments

Studies in the tropics suggest a regional similarity in survival rates of adult birds; however, this literature often overlooks species in semi-arid tropical environments. Bird survival in seasonally dry environments (e.g. seasonally dry tropical forests (SDTFa)) may be lower than that in more constantly wet areas (e.g. tropical rainforests (TRFs)), especially if the birds are negatively affected by seasonal rainfall or food-limitation. However, survival could be similar across these tropical environments, as the asymmetry between young and adult mortality tends to be high in all tropical areas, and the higher risk of mortality in young animals may favour adult survival (residual reproductive investment) regardless of the local climatic conditions. To fill this knowledge gap, we tested the hypothesis that bird survival is similar between seasonally dry (SDTF) and constantly wet (TRF) Neotropical environments. We estimated the apparent survival of 27 South American bird populations from three SDTF areas and 39 populations from a TRF. Apparent survival was estimated from Cormack–Jolly–Seber (CJS) models fitted using a Bayesian structure and the resulting variation in survival rates between study areas and with body mass was explored using a Bayesian phylogenetic mixed model. Apparent annual survival of passerines did not differ between areas (geometrical mean of survival: SDTF = 0.50, 0.56, 0.64; TRF = 0.58), but body mass was positively associated with survival. The variation in bird survival was partially explained by phylogenetic relationships among species. Our results suggest that bird survival is regionally similar in Neotropical forests, despite the climatic variation. We discuss possible physiological and behavioural mechanisms adopted by birds in SDTFs to attenuate effects of environmental seasonality on survival.     

Sensitivity to thermal extremes in Australian Drosophila implies similar impacts of climate change on the distribution of widespread and tropical species

Climatic factors influence the distribution of ectotherms, raising the possibility that distributions of many species will shift rapidly under climate change and/or that species will become locally extinct. Recent studies have compared performance curves of species from different climate zones and suggested that tropical species may be more susceptible to climate change than those from temperate environments. However, in other comparisons involving responses to thermal extremes it has been suggested that mid‐latitude populations are more susceptible. Using a group of 10 closely related Drosophila species with known tropical or widespread distribution, we undertake a detailed investigation of their growth performance curves and their tolerance to thermal extremes. Thermal sensitivity of life history traits (fecundity, developmental success, and developmental time) and adult heat resistance were similar in tropical and widespread species groups, while widespread species had higher adult cold tolerance under all acclimation regimes. Laboratory measurements of either population growth capacity or acute tolerance to heat and cold extremes were compared to daily air temperature under current (2002–2007) and future (2100) conditions to investigate if these traits could explain current distributions and, therefore, also forecast future effects of climate change. Life history traits examining the thermal sensitivity of population growth proved to be a poor predictor of current species distributions. In contrast, we validate that adult tolerance to thermal extremes provides a good correlate of current distributions. Thus, in their current distribution range, most of the examined species experience heat exposure close to, but rarely above, the functional heat resistance limit. Similarly, adult functional cold resistance proved a good predictor of species distribution in cooler climates. When using the species’ functional tolerance limits under a global warming scenario, we find that both tropical and widespread Drosophila species will face a similar proportional reduction in distribution range under future warming.     

Climate-growth relations of congeneric tree species vary across a tropical vegetation gradient in Brazil

Seasonally dry tropical forests are an important global climatic regulator, a main driver of the global carbon sink dynamics and are predicted to suffer future reductions in their productivity due to climate change. Yet, little is known about how interannual climate variability affects tree growth and how climate-growth responses vary across rainfall gradients in these forests. Here we evaluate changes in climate sensitivity of tree growth along an environmental gradient of seasonally dry tropical vegetation types (evergreen forest – savannah – dry forest) in Northeastern Brazil, using congeneric species of two common neotropical genera: Aspidosperma and Handroanthus. We built tree-ring width chronologies for each species × forest type combinations and explored how growth variability correlated with local (precipitation, temperature) and global (the El Niño Southern Oscillation - ENSO) climatic factors. We also assessed how growth sensitivity to climate and the presence of growth deviations varied along the gradient. Precipitation stimulates tree growth and was the main growth-influencing factor across vegetation types. Trees in the dry forest site showed highest growth sensitivity to interannual variation in precipitation. Temperature and ENSO phenomena correlated negatively with growth and sensitivity to both climatic factors were similar across sites. Negative growth deviations were present and found mostly in the dry-forest species. Our results reveal a dominant effect of precipitation on tree growth in seasonally dry tropical forests and suggest that along the gradient, dry forests are the most sensitivity to drought. These forests may therefore be the most vulnerable to the deleterious effects of future climatic changes. These results highlight the importance of understanding the climatic sensitivity of different tropical forests. This understanding is key to predict the carbon dynamics in tropical regions, and sensitivity differences should be considered when prioritizing conservation measures of seasonally dry topical forests.     

The impact of thermal seasonality on terrestrial endotherm food web dynamics: a revision of the Exploitation Ecosystem Hypothesis

Many terrestrial endotherm food webs constitute three trophic level cascades. Others have two trophic level dynamics (food limited herbivores; plants adapted to tackle intense herbivory) or one trophic level dynamic (herbivorous endotherms absent, thus plants compete for the few places where they can survive and grow). According to the Exploitation Ecosystems Hypothesis (EEH), these contrasting dynamics are consequences of differences in primary productivity. The productivity thresholds for changing food web dynamics were assumed to be global constants. We challenged this assumption and found that several model parameters are sensitive to the contrast between persistently warm and seasonally cold climates. In persistently warm environments, three trophic level dynamics can be expected to prevail almost everywhere, save the most extreme deserts. We revised EEH accordingly and tested it by compiling direct evidence of three and two trophic level dynamics and by studying the global distribution of felids. In seasonally cold environments, we found evidence for three trophic level dynamics only in productive ecosystems, while evidence for two trophic level dynamics appeared in ecosystems with low primary productivity. In persistently warm environments, we found evidence for three trophic level dynamics in all types of ecosystems. The distribution of felids corroborated these results. The empirical evidence thus indicates that two trophic level dynamics, as defined by EEH, are restricted to seasonally cold biomes with low primary productivity, such as the artic–alpine tundra and the temperate steppe.     

Weather and fireblight in England

Agriculture in tropical countries produces many of the world's most valuable crops, but productivity is limited by a multiplicity of different diseases, many of which have been inadequately studied. Certain characteristics of tropical climates have a fundamental influence on the incidence and severity of many of these diseases. The main seasonal variations which control crop growth and disease epidemiology depend on differences in moisture availability rather than daylength and temperature differences. Marked dry seasons hinder the survival of many pathogens outside hosts unless they can produce drought-resistant spores. In many tropical areas continuous cropping throughout the year is possible, enabling disease epidemics to continue without interruption over long periods. Much agriculture in the tropics is at fairly high altitudes enabling temperate crops to be grown, but low temperatures may present special disease hazards. Violent storms are a feature of many tropical climates and have been implicated in the spread of many bacterial diseases. Drought is a factor which influences the severity of many root diseases. The various effects which moisture, as rain or dew, temperature and cropping cycle can have are illustrated by discussion of sigatoka disease of bananas (Mycosphaerella musicola Leach), South American leaf blight of rubber (Microcyclus ulei (R. Henn.) Arx) and coffee berry disease (Colletotrichum coffeanum Noack).     

Thermal tolerance ranges and climate variability: A comparison between bivalves from differing climates

The climate variability hypothesis proposes that in variable temperate climates poikilothermic animals have wide thermal tolerance windows, whereas in constant tropical climates they have small thermal tolerance windows. In this study we quantified and compared the upper and lower lethal thermal tolerance limits of numerous bivalve species from a tropical (Roebuck Bay, north western Australia) and a temperate (Wadden Sea, north western Europe) tidal flat. Species from tropical Roebuck Bay had higher upper and lower lethal thermal limits than species from the temperate Wadden Sea, and Wadden Sea species showed an ability to survive freezing temperatures. The increased freezing resistance of the Wadden Sea species resulted in thermal tolerance windows that were on average 7 °C greater than the Roebuck Bay species. Furthermore, at a local-scale, the upper lethal thermal limits of the Wadden Sea species were positively related to submersion time and thus to encountered temperature variation, but this was not the case for the Roebuck Bay species. A review of previous studies, at a global scale, showed that upper lethal thermal limits of tropical species are closer to maximum habitat temperatures than the upper lethal thermal limits of temperate species, suggesting that temperate species are better adapted to temperature variation. In this study, we show for the first time, at both local and global scales, that the lethal thermal limits of bivalves support the climate variability effect in the marine environment.     

Phenology of temperate trees in tropical climates

Several North American broad-leaved tree species range from the northern United States at 47°N to moist tropical montane forests in Mexico and Central America at 15–20°N. Along this gradient the average minimum temperatures of the coldest month (T _Jan), which characterize annual variation in temperature, increase from –10 to 12°C and tree phenology changes from deciduous to leaf-exchanging or evergreen in the southern range with a year-long growing season. Between 30 and 45°N, the time of bud break is highly correlated with T _Jan and bud break can be reliably predicted for the week in which mean minimum temperature rises to 7°C. Temperature-dependent deciduous phenology—and hence the validity of temperature-driven phenology models—terminates in southern North America near 30°N, where T _Jan>7°C enables growth of tropical trees and cultivation of frost-sensitive citrus fruits. In tropical climates most temperate broad-leaved species exchange old for new leaves within a few weeks in January-February, i.e., their phenology becomes similar to that of tropical leaf-exchanging species. Leaf buds of the southern ecotypes of these temperate species are therefore not winter-dormant and have no chilling requirement. As in many tropical trees, bud break of Celtis, Quercus and Fagus growing in warm climates is induced in early spring by increasing daylength. In tropical climates vegetative phenology is determined mainly by leaf longevity, seasonal variation in water stress and day length. As water stress during the dry season varies widely with soil water storage, climate-driven models cannot predict tree phenology in the tropics and tropical tree phenology does not constitute a useful indicator of global warming.     

Applications of a universal thermal index: physiological equivalent temperature

The physiological equivalent temperature, PET, is a thermal index derived from the human energy balance. It is well suited to the evaluation of the thermal component of different climates. As well as having a detailed physiological basis, PET is preferable to other thermal indexes like the predicted mean vote because of its unit (°C), which makes results more comprehensible to urban or regional planners, for example, who are not so familiar with modern human-biometeorological terminology. PET results can be presented graphically or as bioclimatic maps. Graphs mostly display the temporal behaviour of PET, whereas spatial distribution is specified in bioclimatic maps. In this article, some applications of PET are discussed. They relate to the evaluation of the urban heat island in cities in both temperate climates and warm climates at high altitude. The thermal component of the microclimate in the trunk space of a deciduous forest is also evaluated by PET. As an example of the spatial distribution of PET, a bioclimatic map for Greece in July (Mediterranean climate) is presented. Received: 7 April 1999 / Accepted: 26 May 1999     

Photosynthetic responses to vapour pressure deficit in temperate and tropical evergreen rainforest trees of Australia

Rainforests occur in high precipitation areas of eastern Australia, along a gradient in seasonality of precipitation, ranging from a summer dry season in the temperate south to a winter dry season in the tropical north. The response of net photosynthesis to increasing vapour pressure deficit (VPD) was measured in a range of Australian rainforest trees from different latitudes to investigate possible differences in their response to atmospheric drought. Plants were grown in glasshouses under ambient or low VPD to determine the effect of growth VPD on the photosynthetic response. Temperate species, which experience low summer precipitation, were found to maintain maximum net photosynthesis over the measurement range of VPD (0.5–1.9 kPa). In contrast, the tropical species from climates with high summer precipitation showed large reductions in net photosynthesis with increasing VPD. Temperate species showed higher intrinsic water-use efficiencies under low VPD than the tropical species, whereas their efficiencies were similar under high VPD. Growing plants under a low VPD had little effect on either the photosynthetic response to VPD or the intrinsic water-use efficiency of the species. These different responses of gas exchange to VPD shown by the tropical and temperate rainforest species may reflect different strategies to maximise productivity in their respective climates.     

Multiregional comparison of the ecological and phylogenetic structure of butterfly species richness gradients

Aim To examine butterfly species richness gradients in seven regions/countries and to quantify geographic mean root distance (MRD) patterns. My primary goal is to determine the extent to which an explanation for butterfly richness patterns based on tropical niche conservatism and the evolution of cold tolerance, proposed for the fauna of Canada and the USA, applies to other parts of the world. Location USA/Canada, Mexico, Europe/NW Africa, Transbaikal Siberia, Chile, South Africa and Australia. Methods Digitized range maps for butterfly species in each region were used to map richness patterns in summer (for all areas) and winter (for USA/Canada, Europe/NW Africa and Australia). A phylogeny resolved to subfamily was used to map the geographic MRD patterns. Regression trees and general linear models examined climatic and vegetation correlates of species richness and MRD within and among regions. Results Various combinations of climate and vegetation were strong predictors of species richness gradients within regions, but unresolved ‘regional’ factors contributed to the multiregional pattern. Regionally based differences in phylogenetic structure also exist, but MRD is negatively correlated with temperature both within and across areas. MRD patterns consistent with tropical niche conservatism occur in most areas. With a possible partial exception of Mexico, faunas in cold climates and in mountains are more derived than faunas in lowlands and tropical/subtropical climates. In USA/Canada, Europe and Australia, winter faunas are more derived than summer faunas. Main conclusions The phylogenetic pattern previously found in the USA and Canada is widespread in both the Northern and Southern Hemispheres, and niche conservatism and the evolution of cold tolerance is the likely explanation for the development of the global butterfly species richness gradient over evolutionary time. Contemporary climate also influences species richness patterns but is unlikely to be a complete explanation globally. The importance of climate is also manifested in the seasonal loss of more basal butterfly elements outside the tropics in winter.