Latitudinal variation in plant chemical defences drives latitudinal patterns of leaf herbivory

A long‐standing paradigm in ecology holds that herbivore pressure and thus plant defences increase towards lower latitudes. However, recent work has challenged this prediction where studies have found no relationship or opposite trends where herbivory or plant defences increase at higher latitudes. Here we tested for latitudinal variation in herbivory, chemical defences (phenolic compounds), and nutritional traits (phosphorus and nitrogen) in leaves of a long‐lived tree species, the English oak Quercus robur. We further investigated the underlying climatic and soil factors associated with such variation. Across 38 populations of Q. robur distributed along an 18° latitudinal gradient, covering almost the entire latitudinal and climatic range of this species, we observed strong but divergent latitudinal gradients in leaf herbivory and leaf chemical defences and nutrients. As expected, there was a negative relationship between latitude and leaf herbivory where oak populations from lower latitudes exhibited higher levels of leaf herbivory. However, counter to predictions there was a positive relationship between leaf chemical defences and latitude where populations at higher latitudes were better defended. Similarly, leaf phosphorus and nitrogen increased with latitude. Path analysis indicated a significant (negative) effect of plant chemical defences (condensed tannins) on leaf herbivory, suggesting that the latitudinal gradient in leaf herbivory was driven by an inverse gradient in defensive investment. Leaf nutrients had no independent influence on herbivory. Further, we found significant indirect effects of precipitation and soil porosity on leaf herbivory, which were mediated by plant chemical defences. These findings suggest that abiotic factors shape latitudinal variation in plant defences and that these defences in turn underlie latitudinal variation in leaf herbivory. Overall, this study contributes to a better understanding of latitudinal variation in plant–herbivore interactions by determining the identity and modus operandi of abiotic factors concurrently shaping plant defences and herbivory.     

Latitudinal variation in herbivory: influences of climatic drivers,herbivore identity and natural enemies

Although a number of investigations have concluded that lower latitudes are associated with increases in herbivore abundance and plant damage, the generality of this pattern is still under debate. Multiple factors may explain the lack of consistency in latitude–herbivory relationships. For instance, latitudinal variation in herbivore pressure may be shaped entirely or not by climatic variables, or vary among herbivore guilds with differing life‐history traits. Additionally, the strength of top–down effects from natural enemies on herbivores might also vary geographically and influence latitude–herbivory patterns. We carried out a field study where we investigated the effects of latitude and climate on herbivory by a seed‐eating caterpillar and leaf chewers, as well as parasitism associated to the former across 30 populations of the perennial herb Ruellia nudiflora (Acanthaceae). These populations were distributed along a 5° latitudinal gradient from northern Yucatan (Mexico) to southern Belize, representing one‐third of the species' latitudinal distribution and the entirety and one‐third of the precipitation and temperature gradient of this species' distribution (respectively). We found opposing latitudinal gradients of seed herbivory and leaf herbivory, and this difference appeared to be mediated by contrasting effects of climate on each guild. Specifically, univariate regressions showed that seed herbivory increased at higher latitudes and with colder temperatures, while leaf herbivory increased toward the equator and with wetter conditions. Multiple regressions including temperature, precipitation and latitude only found significant effects of temperature for seed herbivory and latitude for leaf herbivory. Accordingly, that latitudinal variation in seed herbivory appears to be driven predominantly by variation in temperature whereas latitudinal variation in leaf herbivory was apparently driven by other unexplored correlates of latitude. Parasitism did not exhibit variation with latitude or climatic factors. Overall, these findings underscore that the factors driving latitudinal clines in herbivory might vary even among herbivore species coexisting on the same host plant.     

Latitudinal patterns of herbivore pressure in a temperate herb support the biotic interactions hypothesis

The longstanding biotic interactions hypothesis predicts that herbivore pressure declines with latitude, but the evidence is mixed. To address gaps in previous studies, we measured herbivory and defence in the same system, quantified defence with bioassays, and considered effects of leaf age. We quantified herbivory and defence of young and mature leaves along a continental gradient in eastern North America in the native herb Phytolacca americana L. Herbivory in the field declined with latitude and was strongly correlated with lepidopteran abundance. Laboratory bioassays revealed that leaf palatability was positively correlated with latitude of origin. Young leaves were more damaged than mature leaves at lower latitudes in the field, but less palatable in bioassays. Both defence and palatability displayed non‐linear latitudinal patterns, suggesting potential mechanisms based on biological or climatic thresholds. In sum, observational and experimental studies find patterns consistent with high herbivore pressure and stronger plant defences at lower latitudes.     

A paradox of latitudinal leaf defense strategies in deciduous and evergreen broadleaved trees

The classical “low latitude–high defense” hypothesis is seldom supported by empirical evidence. In this context, we tested latitudinal patterns in the leaf defense traits of deciduous broadleaved (DB) and evergreen broadleaved (EGB) tree species, which are expected to affect herbivore diversity. We examined the co-occurrence of leaf defense traits (tannin and phenol content, leaf mechanical strength, leaf dry matter content, leaf mass per area, and leaf thickness) in 741 broadleaved tree species and their correlations with species geographical range in East Asian island flora. We discovered contrasting latitudinal defense strategy gradients in DB and EGB tree species. DB species employed chemical defenses (increasing tannin and phenol content) at higher latitudes and physical defenses (softer and thinner leaves) at lower latitudes, whereas EGB tree species exhibited opposite latitudinal defense patterns. The “low latitude high defense” hypothesis included a paradoxical aspect in chemical and physical defense traits across broadleaved tree species. To reconcile paradoxical defense strategies along the latitudinal gradient, we conclude that interactive correlations among leaf traits are controlled by leaf longevity, which differs between DB and EGB tree species.     

Distinct Leaf‐trait Syndromes of Evergreen and Deciduous Trees in a Seasonally Dry Tropical Forest

In seasonally dry tropical forests, tree species can be deciduous, remaining without leaves throughout the dry season, or evergreen, retaining their leaves throughout the dry season. Deciduous and evergreen trees specialize in habitats that differ in water availability (hillside and riparian forest, respectively) and in their exposure to herbivore attack (seasonal and continuous, respectively). We asked whether syndromes of leaf traits in deciduous and evergreen trees were consistent with hypothesized abiotic and biotic selective pressures in their respective habitat. We measured seven leaf traits in 19 deciduous and 11 evergreen tree species in a dry tropical forest in Western Mexico, and measured rates of herbivory on 23 of these species. We investigated the covariance of leaf traits in syndromes related to phenology and associated physiology, and to anti‐herbivory defense. We found evidence for syndromes that separated phenological strategies among four traits: toughness, water content, specific leaf area, and carbon:nitrogen (C:N) ratios. We found a trade‐off between two other traits: trichomes and latex. Overall, evergreen species exhibited lower rates of herbivory than deciduous species. Lower rates of herbivory were explained by a syndrome of higher toughness, lower water content, and higher C:N ratios, which are traits representative of evergreen trees. Phenology and trait syndromes did not exhibit significant phylogenetic signal, consistent with the hypothesis of evolutionary convergence among phenologies and associated leaf‐trait syndromes. Our results suggest that deciduous and evergreen trees could respond to differential water availability and herbivory in their respective habitats by converging on distinct leaf‐trait syndromes. Abstract in Spanish is available at http://www.blackwell‐synergy.com/loi/btp .     

The latitudinal herbivory hypothesis revisited: To be part is to be whole

As the big data accumulation in ecology picks up pace, we now have the opportunity to test several macroecological hypotheses, such as the latitudinal herbivory hypothesis (LHH) dated from the 1990s. The LHH proposes that plant–herbivore interactions decrease as latitude increases, that is, from lower latitudinal areas (i.e., the equator) to higher latitudinal areas (i.e., the poles). This hypothesis has been challenged in recent years. In this study, we used the greatest volume dataset of leaf herbivory from the study of Zhang et al. (Journal of Ecology, 104, 2016, 1089) to test the LHH at a global scale, based on a quantile regression model. We found that the mean annual temperature, mean annual precipitation, and potential net primary production were heterogeneously correlated with herbivory at different quantiles or variable intervals. Although the Northern Hemisphere (NH) and the global‐scale trends are in accordance with the expected latitudinal variation, the Southern Hemisphere (SH) was found to exhibit inverse trends. The latitude has a negative effect on plant–herbivore interactions in the NH and on a global scale; leaf herbivory decreased more at a given latitude in higher latitudinal areas, which is attributed to harsher survival conditions in these areas. The uniformity of leaf herbivory variability along the climate and latitude gradient in the NH and on a global scale motivates that the loosening of this herbivory variability in the SH is not significant enough to dismiss the prevalence of the LHH, a testable macroecology hypothesis.     

Abundance and diversity of birch‐feeding leafminers along latitudinal gradients in northern Europe

Latitudinal patterns in biotic interactions, including herbivory, have been widely debated during the past years. In particular, recent meta‐analysis questioned the hypothesis that herbivory increases from the poles towards the equator. Our study was designed to verify this hypothesis by exploring latitudinal patterns in abundance and diversity of birch‐feeding insect herbivores belonging to the leafminer guild in northern Europe, from 59° to 69°N. We collected branches from five mature trees of two birch species (Betula pendula and B. pubescens) at each study site (ten sites for each of five latitudinal gradients) twice per season (in early and late summer of 2008–2011) and attributed all mines found on leaves of these branches to a certain taxon of insects. Latitudinal patterns were quantified by calculating Spearman rank correlation coefficients between both abundance and diversity of leafmining taxa and latitudes of sampling sites. In general, both abundance and diversity of leafminers significantly decreased with latitude. However, we discovered pronounced variation in patterns of latitudinal changes among study years and leafminer taxa. Variation among study years was best explained by mean temperatures in July at the northern ends of our gradients. During cold years, abundance of leafminers significantly decreased with latitude, while during warm years the abundance was either independent of latitude or even increased towards the pole. In the northern boreal forests (66° to 69°N), herbivores demonstrated larger changes in densities in response to temperature variations than in the boreo‐nemoral forests (59° to 62°N). Our data suggest that climate warming will result in a stronger increase in herbivory at higher latitudes than at lower latitudes.     

Incidence of leaf mining in different vegetation types across rainfall,canopy cover and latitudinal gradients

Abstract Leaf miners are insects whose larval stages live between layers of leaf epidermis, feeding on mesophyll and lower epidermis to create mine‐like cavities. Little is known about the ecology or distribution of leaf miners in Australia. We investigated the incidence of leaf miners in relation to aridity, vegetation types, host plant taxonomy, leaf traits, canopy cover and latitude. We surveyed leaf miners at 15 sites in NSW, eastern Australia, situated along a rainfall gradient from 300 to 1700 mm per annum and a latitudinal gradient of 28°S to 33°S, within four vegetation types (mallee, heath, woodland and rainforest). Leaf mining was recorded from 36 plant species, 89% of which had no previous record of mining. The proportion of mined plant species at each site varied, but there was no significant difference between vegetation types. Leaf mining presence was positively correlated with both total leaf length and leaf thickness. No significant correlations were found between the proportion of mined species at a site and rainfall, latitude or foliar projected cover. We conclude that leaf mining is a widespread type of insect herbivory whose distribution patterns are more likely to be influenced by biotic than abiotic factors.     

Geographic signatures in species turnover: decoupling colonization and extinction across a latitudinal gradient

High latitude communities have low species richness and are rapidly warming with climate change. Thus, temporal changes in community composition are expected to be greatest at high latitudes. However, at the same time traits such as body size can also change with latitude, potentially offsetting or increasing changes to community composition over time. We tested how zooplankton communities (copepods and cladocerans) have changed over a 25–75 year time span by assessing colonization and extinction rates from lakes across an 1800 km latitudinal gradient, and further tested whether species traits predict rates of community change over time. Lake‐level dissimilarity, measured with Sorenson distance, decreased at higher latitudes. This decrease was due to higher colonization rates of cladocerans in lower latitude lakes and consistent extinction rates across the latitudinal gradient. At the species level, colonization increased with regional occupancy, and tended to be higher for smaller bodied, locally abundant, species. Local extinction rates were negatively correlated with local abundance and regional occupancy, but were not influenced by body size. None of these species‐specific characteristics changed predictably with latitude. Contrary to our expectations, low‐latitude zooplankton communities changed more rapidly than high‐latitude communities by becoming more species rich, not by losing species that were historically present. Moreover, colonization and extinction trends suggest that lakes have become increasingly dominated by species with smaller body sizes and that are already common locally and regionally. Together, these findings indicate that rates of species turnover in freshwater lakes across a latitudinal gradient are not predicted by rates of temperature change, but that turnover is nonetheless resulting in trait‐shifts that favour small, generalist species.     

Herbivore damage along a latitudinal gradient: relative impacts of different feeding guilds

We present the first broad-scale test for a latitudinal gradient in herbivory made with consistent methods, in similar habitat type, over the entire lifespan of leaves (phyllodes). We assessed the degree of chewing, sap-sucking and mining herbivory on Acacia falcata along its entire coastal latitudinal range (1150 km) in Australia. We found no significant differences in the rate of herbivory among latitudes. Mature phyllodes had a higher rate of herbivory compared to young phyllodes, and mining was higher on mature phyllodes from the most tropical latitude. We found significant differences in phyllode toughness and specific leaf (phyllode) area among latitudes, but no significant differences among latitudes in carbon: nitrogen. This study provides a useful model for further testing of the generalisation that herbivory is more intense in tropical versus temperate regions.     

Contrasting responses to water deficits of Nothofagus species from tropical New Guinea and high‐latitude temperate forests: can rainfall regimes constrain latitudinal range?

Aim Comparative responses of Nothofagus species to water deficits were studied to determine whether rainfall regimes could limit the latitudinal ranges of tropical and temperate forest species. Location The study species are native to New Guinea, New Caledonia, Australia, New Zealand, Chile and Argentina. Methods Seedlings of Nothofagus species from a broad latitudinal range were grown in a common environment. Changes in conductance, relative water content and water potential were measured in detached shoots, and together with measurements of tissue injury and biomass allocation, were compared between tropical and temperate species. Results Differences in responses to water deficits between tropical and temperate species appear to reflect differences in climate regimes. In particular, species native to ever‐wet rainfall regimes in New Guinea, where water deficits are generally likely to be short‐lived, were effective at conserving water by reduced stomatal conductance. In contrast, high‐latitude evergreen species on average showed greater development of traits that should enhance water uptake. This was particularly evident in Nothofagus cunninghamii from southern Australia, which developed low water potentials at moderate levels of tissue water deficit and higher root:leaf biomass than tropical species, potentially allowing carbon assimilation to be maximized during warmer, but drier, summer months. However, water relations varied among high‐latitude species. In particular, deciduous species on average showed higher rates of conductance, even during moderate levels of tissue water deficit, than evergreen species. Main conclusions The tropical species appear to conserve water during periods of water deficit (relative to temperate species), which is unlikely to have substantial opportunity costs for growth in ever‐wet climates. However, spread of tropical species to higher latitudes may be limited by water conservation strategies that limit carbon gain in climates in which temperature seasonality is often paired with drier summers. Evergreen species from high latitudes, such as N. cunninghamii, commonly showed traits that should increase water uptake. However, this strategy, while probably maximizing growth in temperate climates with cool winters and drier summers, must limit competitiveness at lower latitudes in summer‐wet climates. We conclude that responses to water regimes may make a significant contribution to the latitudinal limits of some evergreen rain forest species.     

Biotic interactions mediate the expansion of black mangrove (Avicennia germinans) into salt marshes under climate change

Many species are expanding their distributions to higher latitudes due to global warming. Understanding the mechanisms underlying these distribution shifts is critical for better understanding the impacts of climate changes. The climate envelope approach is widely used to model and predict species distribution shifts with changing climates. Biotic interactions between species, however, may also influence species distributions, and a better understanding of biotic interactions could improve predictions based solely on climate envelope models. Along the northern Gulf of Mexico coast, USA, subtropical black mangrove (Avicennia germinans) at the northern limit of its distribution grows sympatrically with temperate salt marsh plants in Florida, Louisiana, and Texas. In recent decades, freeze‐free winters have led to an expansion of black mangrove into salt marshes. We examined how biotic interactions between black mangrove and salt marsh vegetation along the Texas coast varied across (i) a latitudinal gradient (associated with a winter‐temperature gradient); (ii) the elevational gradient within each marsh (which creates different marsh habitats); and (iii) different life history stages of black mangroves (seedlings vs. juvenile trees). Each of these variables affected the strength or nature of biotic interactions between black mangrove and salt marsh vegetation: (i) Salt marsh vegetation facilitated black mangrove seedlings at their high‐latitude distribution limit, but inhibited black mangrove seedlings at lower latitudes; (ii) mangroves performed well at intermediate elevations, but grew and survived poorly in high‐ and low‐marsh habitats; and (iii) the effect of salt marsh vegetation on black mangroves switched from negative to neutral as black mangroves grew from seedlings into juvenile trees. These results indicate that the expansion of black mangroves is mediated by complex biotic interactions. A better understanding of the impacts of climate change on ecological communities requires incorporating context‐dependent biotic interactions into species range models.     

Floral,climatic and soil pH controls on leaf ash content in China's terrestrial plants

Aim To investigate broad‐scale patterns of plant leaf ash content and their possible causes in China. Location Mainland China and Hainan island, with the geographic ranges for the data used from 18.7° N to 49.2° N and 76.0° E to 128.3° E. Methods By analysing a data set of 2022 leaf samples, involving 704 species of terrestrial plants. Results Leaf ash content increases with increasing latitude at an average rate of 2.7 mg ash g^?1 dry weight per degree latitude from south to north of China. Plant functional group shows a more powerful influence on the spatial variation in leaf ash than soil pH and climate. Fast‐growing species or those with leaves with a short life span have higher leaf ash than slow‐growing species or those with a long leaf life span. Plants from alkaline soils have higher leaf ash than those from acid soils (39.5 mg g^?1 increase in leaf ash content per unit increase of pH). Increasing precipitation significantly reduces leaf ash (with a mean rate of 4.8 mg g^?1 for every 100 mm rainfall), whereas the effect of temperature appears to be nonlinear. Main conclusions This study shows a significant latitudinal trend in leaf ash content in China. This geographic pattern is possibly shaped by the floral, edaphic and climatic factors that control the biogeochemical cycling of plant minerals. The results suggest that leaf ash content is a useful biogeographic indicator that can be used to explore the complex interactions between plants and the environment.     

Leaf life span as a simple predictor of evergreen forest zonation in China

Aim Our aim was to investigate how the average life span of canopy leaves might be used to predict the geographical distribution of natural forests at large geographical scales, and to explore the link between leaf characteristics and ecosystem functioning. We examine whether there is a general relationship between canopy mean leaf life span and climate (i.e. temperature and precipitation) that can be used to predict evergreen forest zonation in China. Location Forest areas in China. Methods During July and August of 2002–2004, we conducted a latitudinal forest transect spanning about 30° of latitude in eastern China. The canopy mean leaf life span was calculated to include all tree species (groups) in each forest plot through weighted averages scaled up from branch‐level measurements. Data from our previous work conducted in the Tibetan Alpine Vegetation Transects (TAVT) and from other investigators were compiled to supplement our results. Based on regression equations developed on the pooled data, and using gridded temperature and precipitation datasets, we simulated the distribution of canopy mean leaf life span for forests in China. The predicted leaf life span zonation was compared with a map of Chinese forest vegetation divisions published in 1980. Results Canopy mean leaf life span across 10 evergreen forest plots in eastern China showed a decreasing trend as mean annual temperature increased, following a common logistic pattern consistent with the data from the TAVT and other investigators. In pooled data for 40 evergreen forest plots across tropical and boreal regions, canopy mean leaf life span generally showed a negative relationship with mean annual temperature (r² = 0.72, P < 0.001), and a positive correlation with mean annual precipitation where mean annual temperature was > 8°C (r² = 0.45, P < 0.01). The climate‐based simulations of leaf life span zonation compared well with the previously published boundaries of forest vegetation divisions in eastern China. Main conclusions Our results reveal that mean leaf life span in evergreen forests follows a common logistic pattern associated with mean annual temperature and precipitation, which can in turn be used to predict evergreen forest zonation in eastern China.     

Different mechanisms explain feeding type-specific patterns of latitudinal variation in herbivore damage among diverse feeding types of herbivorous insects

To determine whether latitudinal variation in herbivore impact exists, we examined three major herbivorous insect feeding types (chewers, gallers, and miners) on/in leaves of Japanese beech. Herbivores were collected with litter traps deployed in forests across a latitudinal gradient of 10°. Leaf litter analyses demonstrated that chewing herbivory increased with increasing latitude of collection site. However, the densities of miners and gallers decreased with latitude. To test whether latitudinal variation in herbivore damage occurs in the absence of geographically differentiated environmental cueing (e.g., physical stresses or herbivore damage), we measured both genetically determined constitutive leaf traits and herbivore damage in a common-garden experiment. In this experiment, miner density decreased with latitude, but chewing herbivory did not vary latitudinally. Galler density was higher on trees from native provenances than on trees from unrelated provenances likely because of local adaptations. Leaf mass per unit area (LMA), tannin, and phenolics all decreased with latitude of provenance. The latitudinal variation in one constitutive leaf trait (LMA) best explained latitudinal variation in chewing herbivory. Thus, different mechanisms account for feeding type-specific patterns of latitudinal variation in herbivore damage among different herbivore feeding types.     

Opposed latitudinal patterns of network‐derived and dietary specialization in avian plant–frugivore interaction systems

Latitudinal patterns of biodiversity have been studied for centuries, but it is only during the last decades that species interaction networks have been used to examine the proposed latitudinal gradient of biotic specialization. These studies have given idiosyncratic results, which may either be because of genuine biological differences between systems, different concepts and scales used to quantify biotic specialization or because the methodological approaches used to compare interaction networks were inappropriate. Here we carefully examine the latitudinal specialization gradient using a global dataset of avian plant–frugivore assemblages and interaction networks. In particular, we test whether network‐derived specialization patterns differ from patterns based on assemblage‐level information on avian dietary preferences on specific food types. We found that network‐derived measures of specialization (complementary specialization H₂′ and < d’>, modularity Q) increased with latitude, i.e. frugivorous birds divide the niche of fruiting plants most finely at high latitudes where they also formed more modular interaction networks than at tropical latitudes. However, the strength and significance of the relationship between specialization metrics and latitude was influenced by the methodological approach. On the other hand, assemblage‐level information on avian specialization on fruit diet (i.e. the proportion of obligate frugivorous bird species feeding primarily on fruit) revealed an opposed latitudinal pattern as more bird species were specialized on fruit diet in tropical than in temperate assemblages. This difference in the latitudinal specialization gradient reflects that obligate frugivores require a high diversity of fruit plants, as observed in tropical systems, and fulfil more generalized roles in plant–frugivore networks than bird species feeding on different food types. Future research should focus on revealing the underlying ecological, historical and evolutionary mechanisms shaping these patterns. Our results highlight the necessity of comparing different scales of biotic specialization for a better understanding of geographical patterns of specialization in resource–consumer interactions.     

Urbanization drives unique latitudinal patterns of insect herbivory and tree condition

Urban landscapes are characterized by high proportions of impervious surface resulting in higher temperatures than adjacent natural landscapes. In some cities, like those at cooler latitudes, trees may benefit from warmer urban temperatures, but trees in many cities are beset with problems like drought stress and increased herbivory. What drives patterns of urban tree health across urbanization and latitudinal temperature gradients? In natural systems, latitude–herbivory relationships are well‐studied, and recent temperate studies have shown that herbivory generally increases with decreasing latitudes (warmer temperatures). However, the applicability of this latitude–herbivory theory in already‐warmed urban systems is unknown. In this study, we investigated how the interaction of urbanization, latitudinal warming and scale insect abundance affected urban tree health. We predicted that trees in warmer, lower latitude cities would be in poorer health at lower levels of urbanization than trees at cooler, higher latitudes due to the interaction of urbanization, latitudinal temperature and herbivory. To evaluate our predictions, we surveyed the abundance of scale insect herbivores on a single, common tree species Acer rubrum in eight US cities spanning 10° of latitude. We estimated urbanization at two extents, a local one that accounted for the direct effects on an individual tree, and a larger one that captured the surrounding urban landscape. We found that urban tree health did not vary with latitudinal temperature but was best predicted by local urbanization and herbivore abundance. We did not observe increased herbivore abundance in warmer, lower latitudes cities, but instead herbivore abundance peaked in the mid latitudes of our study. This study demonstrates that urban landscapes may deviate from classical theory developed in natural systems and reinforces the need for research reconciling ecological patterns in urban landscapes.     

Leaf life spans in wild perennial herbaceous plants: a survey and attempts at a functional interpretation

Summary Leaf longevity in 29 herbaceous plant species of Central Europe was studied by inspecting tagged leaves at weekly intervals. About half of the species are elements of the lowland meadow flora, the other half comprises a representative sample of species from the highest sites where vascular plants grow in the Alps. Shaded and water-stressed sites were avoided. Overall mean leaf longevity did not differ significantly between sites and amounted to 71±5 days at low and 68±4 days at high altitude. Leaf life spans ranged (with no clear altitudinal trend) from 41 to 95 days. Low-altitude forbs and grasses produced several leaf cohorts during their growth period, while most alpine species produced only one. Correlations were found between leaf duration and percent nitrogen content and carbon-cost/carbon-gain ratios, but not with leaf dry mass per unit leaf area and photosynthetic capacity alone. As leaf life spans increase, more C tends to be invested per unit CO₂ uptake and less N is invested per unit invested C. Thus, mass relationships rather than area relationships seem to be linked to leaf life span in these species, suggesting that leaf duration is associated with properties other than the efficiency of light utilization (e.g. mechanical strength, herbivory or pathogen resistance). It seems that the explanations of leaf duration that have been developed for evergreen/deciduous plants and for plants along steep light gradients do not apply to the variable life spans in leaves of perennial herbaceous plants of open habitats.     

Weak latitudinal gradients in insect herbivory for dominant rangeland grasses of North America

Patterns of insect herbivory may follow predictable geographical gradients, with greater herbivory at low latitudes. However, biogeographic studies of insect herbivory often do not account for multiple abiotic factors (e.g., precipitation and soil nutrients) that could underlie gradients. We tested for latitudinal clines in insect herbivory as well as climatic, edaphic, and trait‐based drivers of herbivory. We quantified herbivory on five dominant grass species over 23 sites across the Great Plains, USA. We examined the importance of climate, edaphic factors, and traits as correlates of herbivory. Herbivory increased at low latitudes when all grass species were analyzed together and for two grass species individually, while two other grasses trended in this direction. Higher precipitation was related to more herbivory for two species but less herbivory for a different species, while higher specific root length was related to more herbivory for one species and less herbivory for a different species. Taken together, results highlight that climate and trait‐based correlates of herbivory can be highly contextual and species‐specific. Patterns of insect herbivory on dominant grasses support the hypothesis that herbivory increases toward lower latitudes, though weakly, and indicates that climate change may have species‐specific effects on plant–herbivore interactions.     

Latitudinal variation in sexual dimorphism in life‐history traits of a freshwater fish

Sexual dimorphism is common across the animal kingdom, but the contribution of environmental factors shaping differences between the sexes remains controversial. In ectotherms, life‐history traits are known to correlate with latitude, but sex‐specific responses are not well understood. We analyzed life‐history trait variation between the sexes of European perch (Perca fluviatilis L.), a common freshwater fish displaying larger female size, by employing a wide latitudinal gradient. We expected to find sex‐dependent latitudinal variation in life‐history variables: length at age, length increment, and size at maturity, with females showing consistently higher values than males at all latitudes. We further anticipated that this gender difference would progressively decrease with the increasingly harsh environmental conditions toward higher latitude. We hypothesized that growth and length increment would decrease and size/age at maturity would increase at higher latitudes. Our results confirmed female‐biased sexual size dimorphism at all latitudes and the magnitude of sexual dimorphism diminished with increase in latitude. Growth of both sexes decreased with increase in latitude, and the female latitudinal clines were steeper than those of males. Hence, we challenge two predominant ecological rules (Rensch's and Bergmann's rules) that describe common large‐scale patterns of body size variation. Our data demonstrate that these two rules are not universally applicable in ectotherms or female‐biased species. Our study highlights the importance of sex‐specific differences in life‐history traits along a latitudinal gradient, with evident implications for a wide range of studies from individual to ecosystems level.