Can amphibians take the heat? Vulnerability to climate warming in subtropical and temperate larval amphibian communities

Predicting the biodiversity impacts of global warming implies that we know where and with what magnitude these impacts will be encountered. Amphibians are currently the most threatened vertebrates, mainly due to habitat loss and to emerging infectious diseases. Global warming may further exacerbate their decline in the near future, although the impact might vary geographically. We predicted that subtropical amphibians should be relatively susceptible to warming‐induced extinctions because their upper critical thermal limits (CT_max) might be only slightly higher than maximum pond temperatures (T_max). We tested this prediction by measuring CT_max and T_max for 47 larval amphibian species from two thermally distinct subtropical communities (the warm community of the Gran Chaco and the cool community of Atlantic Forest, northern Argentina), as well as from one European temperate community. Upper thermal tolerances of tadpoles were positively correlated (controlling for phylogeny) with maximum pond temperatures, although the slope was steeper in subtropical than in temperate species. CT_max values were lowest in temperate species and highest in the subtropical warm community, which paradoxically, had very low warming tolerance (CT_max–T_max) and therefore may be prone to future local extinction from acute thermal stress if rising pond T_max soon exceeds their CT_max. Canopy‐protected subtropical cool species have larger warming tolerance and thus should be less impacted by peak temperatures. Temperate species are relatively secure to warming impacts, except for late breeders with low thermal tolerance, which may be exposed to physiological thermal stress in the coming years.     

The Michaelis–Menten kinetics of soil extracellular enzymes in response to temperature: a cross‐latitudinal study

Decomposition of soil organic matter (SOM) is mediated by microbial extracellular hydrolytic enzymes (EHEs). Thus, given the large amount of carbon (C) stored as SOM, it is imperative to understand how microbial EHEs will respond to global change (and warming in particular) to better predict the links between SOM and the global C cycle. Here, we measured the Michaelis–Menten kinetics [maximal rate of velocity (V_max) and half‐saturation constant (K_m)] of five hydrolytic enzymes involved in SOM degradation (cellobiohydrolase, β‐glucosidase, β‐xylosidase, α‐glucosidase, and N‐acetyl‐β‐d ‐glucosaminidase) in five sites spanning a boreal forest to a tropical rainforest. We tested the specific hypothesis that enzymes from higher latitudes would show greater temperature sensitivities than those from lower latitudes. We then used our data to parameterize a mathematical model to test the relative roles of V_max and K_m temperature sensitivities in SOM decomposition. We found that both V_max and K_m were temperature sensitive, with Q₁₀ values ranging from 1.53 to 2.27 for V_max and 0.90 to 1.57 for K_m. The Q₁₀ values for the K_m of the cellulose‐degrading enzyme β‐glucosidase showed a significant (P = 0.004) negative relationship with mean annual temperature, indicating that enzymes from cooler climates can indeed be more sensitive to temperature. Our model showed that K_m temperature sensitivity can offset SOM losses due to V_max temperature sensitivity, but the offset depends on the size of the SOM pool and the magnitude of V_max. Overall, our results suggest that there is a local adaptation of microbial EHE kinetics to temperature and that this should be taken into account when making predictions about the responses of C cycling to global change.     

The Fate of Organic Sources of Carbon in Moss‐rich Tundra Soil Microbial Communities: A Laboratory Experimental Study

Effects of glucose‐carbon supplementation on soil respiration and bacterial and protist biomass were investigated in laboratory studies of three soil samples from Alaskan tundra: spring tussock sample 1 (thin surface moss), spring tussock sample 2 (thick surface moss), and a summer tundra open field sample. Addition of 1% (w/v) glucose solution produced an immediate, pronounced two to three fold increase in respiration above basal rate, which declined over 4 h to baseline levels. Less than 1% (w/w) of glucose‐C supplement was respired during the respiratory spike, relative to the 89 μg/g added. A more substantial amount of the glucose‐C became incorporated in microbial biomass. The total difference in microbial carbon (μg/g) between the experimental treatments and controls without glucose after 1 wk was as follows: spring sample 1 (8), spring sample 2 (31), and summer sample (70). The percent (w/w) of glucose‐C incorporated was: spring sample 1 (5%), spring sample 2 (17%), and summer sample (39%), most attributed to biomass of bacteria and heterotrophic nanoflagellates. Although respiratory response to pulsed glucose‐C was minimal, the overall mean basal rate after 1 wk ranged between 4 and 6 nmol/min/g soil, indicating a significant assimilation and respiration of constituent soil organic carbon.     

Estimation of climatic factors relating to occurrence of the maize orange leafhopper, Cicadulina bipunctata

The maize orange leafhopper, Cicadulina bipunctata is a serious pest of forage maize in East and Southeast Asia. In temperate Japan, the occurrence of this leafhopper fluctuates widely among years. Here, we examined effects of climatic factors (temperature, precipitation and sunlight) on the occurrence of C. bipunctata. Seasonal occurrence of adult C. bipunctata in a census field from July to August, when forage maize was most susceptible to the pest, could be described by a simple exponential function with two parameter: estimated density of C. bipunctata on 1 July (N ₀) and intrinsic rate of natural increase (r) for each year. Forward stepwise multiple regression analysis using seasonal occurrence data from 2004 to 2009 detected positive contributions of average temperatures in the previous December and February and a negative contribution of total precipitation during the previous winter to N ₀. The analysis also indicated that average temperature in July of the current year and N ₀ contributed positively and negatively to r, respectively. These results indicated that high temperature and little precipitation during winter and high temperature in early summer induced high occurrence of C. bipunctata in summer. A prediction model based on these factors supported the actual seasonal occurrence in 2010, suggesting that this prediction model is applicable to C. bipunctata forecasting. The prediction model indicated that further global warming in the future is likely to cause further outbreaks of C. bipunctata.     

Global assessment of experimental climate warming on tundra vegetation: heterogeneity over space and time

Understanding the sensitivity of tundra vegetation to climate warming is critical to forecasting future biodiversity and vegetation feedbacks to climate. In situ warming experiments accelerate climate change on a small scale to forecast responses of local plant communities. Limitations of this approach include the apparent site-specificity of results and uncertainty about the power of short-term studies to anticipate longer term change. We address these issues with a synthesis of 61 experimental warming studies, of up to 20 years duration, in tundra sites worldwide. The response of plant groups to warming often differed with ambient summer temperature, soil moisture and experimental duration. Shrubs increased with warming only where ambient temperature was high, whereas graminoids increased primarily in the coldest study sites. Linear increases in effect size over time were frequently observed. There was little indication of saturating or accelerating effects, as would be predicted if negative or positive vegetation feedbacks were common. These results indicate that tundra vegetation exhibits strong regional variation in response to warming, and that in vulnerable regions, cumulative effects of long-term warming on tundra vegetation - and associated ecosystem consequences - have the potential to be much greater than we have observed to date.     

The effect of altered macroclimate on N-fixation by boreal feather mosses

Plant productivity is predicted to increase in boreal forests owing to climate change, but this may depend on whether N inputs from biological N-fixation also increases. We evaluated how alteration of climatic factors affects N input from a widespread boreal N-fixer, i.e. cyanobacteria associated with the feather moss Pleurozium schreberi. In each of 10 forest stands in northern Sweden, we established climate-change plots, including a control (ambient climate) plot and three plots experiencing a +2°C temperature increase, an approximately threefold reduction in precipitation frequency, and either 0.07, 0.29 or 1.16 times normal summer precipitation. We monitored N-fixation in these plots five times between 2007 and 2009, and three times in 2010 after climate treatments ended to assess their recovery. Warmer temperatures combined with less frequent precipitation reduced feather moss moisture content and N-fixation rates regardless of total precipitation. After climate treatments ended, recovery of N-fixation rates occurred on the scale of weeks to months, suggesting resilience of N-fixation to changes in climatic conditions. These results suggest that modelling of biological N-inputs in boreal forests should emphasize precipitation frequency and evaporative water loss in conjunction with elevated temperature rather than absolute changes in mean precipitation.     

Temperature dependence of stream benthic respiration in an Alpine river network under global warming

1. Global warming has increased the mean surface temperature of the Earth by 0.6 °C in the past century, and temperature is probably to increase by an additional 3 °C by 2100. Water temperature has also increased, which in turn can affect metabolic rate in rivers. Such an increase in metabolic rate could alter the role of river networks in the global C cycle, because the fraction of allochthonous organic C that is respired may increase. 2. Laboratory‐based incubations at increasing water temperature were used to estimate the temperature dependence of benthic respiration in streams. These experiments were performed on stones taken from seven reaches with different thermal conditions (mean temperature ranging 8–19 °C) within the pre‐alpine Thur River network in Switzerland, June–October 2007. 3. The activation energy of respiration in different reaches along the river network (0.53 ± 0.12 eV, n = 94) was similar, indicating that respiration was constrained by the activation energy of the respiratory complex (E = 0.62 eV). Water temperature and the thickness of the benthic biofilm influence the temperature dependence of respiration and our results suggest that an increase of 2.5 °C will increase river respiration by an average of 20 ± 1.6%.     

Delayed phenological timing of dragonfly emergence in Japan over five decades

Recent increases in air temperature have affected species phenology, resulting in the earlier onset of spring life-cycle events. Trends in the first appearance of adult dragonflies across Japan were analysed using a dataset consisting of observations from 1953 to 2005. Dynamic factor analysis was used to evaluate underlying common trends in a set of 48 time series. The appearance of the first adult dragonfly has significantly shifted to later in the spring in the past five decades. Generalized linear mixing models suggested that this is probably the result of increased air temperatures. Increased summer and autumn temperatures may provide longer bivoltine periods and a faster growth rate; thus, the second generation, which previously hatched in summer, can emerge in the autumn causing the size of the population of dragonflies that emerge in spring to decrease. It is also possible that reduced dragonfly populations along with human development are responsible for a delay in the first observed dragonflies in the spring. However, human population density did not appear to strongly affect the appearance date. This study provides the first evidence of a delay in insect phenological events over recent decades.     

Effect of seawater temperature on the productivity of <Emphasis Type="Italic">Laminaria japonica</Emphasis> in the Uwa Sea,southern Japan

Recent studies on global climate change report that increase in seawater temperature leads to coastal ecosystem change, including coral bleaching in the tropic. In order to assess the effect of increased seawater temperature on a temperate coastal ecosystem, we studied the inter-annual variation in productivity of Laminaria japonica using long-term oceanographic observations for the Uwa Sea, southern Japan. The annual productivity estimates for L. japonica were 2.7 ± 2.5 (mean ± SD) kg wet wt. m⁻¹ (length of rope) (2003/2004), 1.0 ± 0.6 kg wet wt. m⁻¹ (2004/2005) and 12.1 ± 12.5 kg wet wt. m⁻¹ (2005/2006). Our previous study using the same methodology at the same locality reported that the productivity was estimated for the 2001/2002 (33.3 ± 15.2 kg wet wt. m⁻¹) and 2002/2003 (34.0 ± 8.7 kg wet wt. m⁻¹) seasons. Productivity in 2003/2004 and 2004/2005 was significantly lower than in years 2001/2002, 2002/2003 and 2005/2006. A comparison of oceanographic conditions among the 5 years revealed the presence of threshold seawater temperature effects. When the average seawater temperature during the first 45 days of each experiment exceeded 15.5°C, productivity was reduced to about 10 % of that in cooler years. Moreover the analysis of growth and erosion rates indicates that when the seawater temperature was over 17.5°C, erosion rate exceeded growth rate. Thus, an increase of seawater temperature of just 1°C during winter drastically reduces the productivity of L. japonica in the Uwa Sea.     

Long-term changes in tree-ring–climate relationships at Mt. Patscherkofel (Tyrol,Austria) since the mid-1980s

Although growth limitation of trees at Alpine and high-latitude timberlines by prevailing summer temperature is well established, the loss of thermal response of radial tree growth during last decades has repeatedly been addressed. We examined long-term variability of climate–growth relationships in ring width chronologies of Stone pine (Pinus cembra L.) by means of moving response functions (MRF). The study area is situated in the timberline ecotone (ca. 2,000–2,200 m a.s.l.) on Mt. Patscherkofel (Tyrol, Austria). Five site chronologies were developed within the ecotone with constant sample depth (≥19 trees) throughout most of the time period analysed. MRF calculated for the period 1866–1999 and 1901–1999 for ca. 200- and ca. 100-year-old stands, respectively, revealed that mean July temperature is the major and long-term stable driving force of Pinus cembra radial growth within the timberline ecotone. However, since the mid-1980s, radial growth in timberline and tree line chronologies strikingly diverges from the July temperature trend. This is probably a result of extreme climate events (e.g. low winter precipitation, late frost) and/or increasing drought stress on cambial activity. The latter assumption is supported by a <10% increase in annual increments of ca. 50-year-old trees at the timberline and at the tree line in 2003 compared with 2002, when extraordinary hot and dry conditions prevailed during summer. Furthermore, especially during the second half of the twentieth century, influence of climate variables on radial growth show abrupt fluctuations, which might also be a consequence of climate warming on tree physiology.