Climate‐driven vital rates do not always mean climate‐driven population

Current climatic changes have increased the need to forecast population responses to climate variability. A common approach to address this question is through models that project current population state using the functional relationship between demographic rates and climatic variables. We argue that this approach can lead to erroneous conclusions when interpopulation dispersal is not considered. We found that immigration can release the population from climate‐driven trajectories even when local vital rates are climate dependent. We illustrated this using individual‐based data on a trans‐equatorial migratory seabird, the Scopoli's shearwater Calonectris diomedea, in which the variation of vital rates has been associated with large‐scale climatic indices. We compared the population annual growth rate λ_i, estimated using local climate‐driven parameters with ρ_i, a population growth rate directly estimated from individual information and that accounts for immigration. While λ_i varied as a function of climatic variables, reflecting the climate‐dependent parameters, ρ_i did not, indicating that dispersal decouples the relationship between population growth and climate variables from that between climatic variables and vital rates. Our results suggest caution when assessing demographic effects of climatic variability especially in open populations for very mobile organisms such as fish, marine mammals, bats, or birds. When a population model cannot be validated or it is not detailed enough, ignoring immigration might lead to misleading climate‐driven projections.     

Demographic responses of eastern bluebirds to climatic variability in northeastern Arkansas

As climate change continues to alter temperature and precipitation patterns, numerous species have declined. However, populations of some species that show responses to climate change, such as eastern bluebirds (Sialia sialis), have increased or remained stable nationwide. To understand how species are adapting to climate change, we estimated demographic parameters and their responses to climatic variability, using nesting and banding-recapture data between 2003 and 2018 in a northeastern Arkansas eastern bluebird population. Increasing variability in precipitation in the nonbreeding season negatively affected hatchability. Hatching success was negatively affected by increasing variability in maximum temperatures and the number of hot days during the breeding season, but positively affected by increasing winter snow depth. Adult survival was positively affected by increasing snow depth and variability in the number of hot days during the breeding season, but negatively affected by increasing variability in nonbreeding season temperatures. Our results demonstrate that for this study population, annual breeding parameters, though canalized against interannual environmental variation, were affected by seasonal climatic variability. Although climate change may benefit bluebird survival due to increasing variability in winter temperatures and the number of hot days, climatic variability negatively affected breeding parameters and is expected to increase. Because breeding parameters are typically the drivers of population growth rate in short-lived species, these results raise concern for the future of this population of eastern bluebirds.     

Low interannual precipitation has a greater negative effect than seedling herbivory on the population dynamics of a short‐lived shrub,Schiedea obovata

Climate projections forecast more extreme interannual climate variability over time, with an increase in the severity and duration of extreme drought and rainfall events. Based on bioclimatic envelope models, it is projected that changing precipitation patterns will drastically alter the spatial distributions and density of plants and be a primary driver of biodiversity loss. However, many other underlying mechanisms can impact plant vital rates (i.e., survival, growth, and reproduction) and population dynamics. In this study, we developed a size‐dependent integral projection model (IPM) to evaluate how interannual precipitation and mollusk herbivory influence the dynamics of a Hawaii endemic short‐lived shrub, Schiedea obovata (Caryophyllaceae). Assessing how wet season precipitation effects population dynamics it critical, as it is the timeframe when most of the foliar growth occurs, plants flower and fruit, and seedlings establish. Temporal variation in wet season precipitation had a greater effect than mollusk herbivory on S. obovata population growth rate , and the impact of interannual precipitation on vital rates shifted across plant ontogeny. Furthermore, wet season precipitation influenced multiple vital rates in contrasting ways and the effect of precipitation on the survival of larger vegetative and reproductively mature individuals contributed the most to variation in the population growth rate. Among all combination of wet season precipitation and herbivory intensities, the only scenario that led to a growing population was when high wet precipitation was associated with low herbivory. Our study highlights the importance of evaluating how abiotic factors and plant–consumer interactions influence an organism across its life cycle to fully understand the underpinning mechanisms that structure its spatial and temporal distribution and abundance. Our results also illustrate that for short‐lived species, like S. obovata, seedling herbivory can have less of an effect on the dynamics of plant populations than decreased interannual precipitation.     

Convergence and contingency in production–precipitation relationships in North American and South African C4 grasslands

Mesic grasslands in North America and South Africa share many structural attributes, but less is known of their functional similarities. We assessed the control of a key ecosystem process, aboveground net primary production (ANPP), by interannual variation in precipitation amount and pattern via analysis of data sets (15- and 24-year periods) from long-term research programs on each continent. Both sites were dominated by C₄ grasses and had similar growing season climates; thus, we expected convergence in precipitation–ANPP relationships. Lack of convergence, however, would support an alternative hypothesis—that differences in evolutionary history and purportedly greater climatic variability in South Africa fundamentally alter the functioning of southern versus northern hemisphere grasslands. Neither mean annual precipitation nor mean ANPP differed between the South African and North American sites (838 vs. 857 mm/year, 423.5 vs. 461.4 g/m² respectively) and growing season precipitation–ANPP relationships were similar. Despite overall convergence, there were differences between sites in how the seasonal timing of precipitation affected ANPP. In particular, interannual variability in precipitation that fell during the first half of the growing season strongly affected annual ANPP in South Africa (P < 0.01), but was not related to ANPP in North America (P = 0.098). Both sites were affected similarly by late season precipitation. Divergence in the seasonal course of available soil moisture (chronically low in the winter and early spring in the South African site vs. high in the North American site) is proposed as a key contingent factor explaining differential sensitivity in ANPP to early season precipitation in these two grasslands. These long-term data sets provided no support for greater rainfall, temperature or ANPP variability in the South African versus the North American site. However, greater sensitivity of ANPP to early season precipitation in the South African grassland suggests that future patterns of productivity may be more responsive to seasonal changes in climate compared with the North American site.     

An extension of the Leslie matrix model to include a variable immature period

An extension to the Leslie matrix is presented in which the age of transformation from immature to adult has a log—normal distribution. The major effect of this is shown to be on the second largest eigenvalue. The ratio of the largest to the second largest eigenvalue |λ₁/λ₂|, which is an index of the rate of approach to the stable age distribution, is greater in the new model, even though the value of λ₁ is effectively the same. The differences in the models are most pronounced where the population is subjected to a harvesting regime.     

Interactive life‐history traits predict sensitivity of plants and animals to temporal autocorrelation

Temporal autocorrelation in demographic processes is an important aspect of population dynamics, but a comprehensive examination of its effects on different life‐history strategies is lacking. We use matrix population models from 454 plant and animal populations to simulate stochastic population growth rates (log λ_s) under different temporal autocorrelations in demographic rates , using simulated and observed covariation among rates. We then test for differences in sensitivities, or changes of log λ_s to changes in autocorrelation among two major axes of life‐history strategies, obtained from phylogenetically informed principal component analysis: the fast‐slow and reproductive‐strategy continua. Fast life histories exhibit highest sensitivities to simulated autocorrelation in demographic rates across reproductive strategies. Slow life histories are less sensitive to temporal autocorrelation, but their sensitivities increase among highly iteroparous species. We provide cross‐taxonomic evidence that changes in the autocorrelation of environmental variation may affect a wide range of species, depending on complex interactions of life‐history strategies.     

Climate change and annual survival in a temperate passerine: partitioning seasonal effects and predicting future patterns

Predicting climate change impacts on population size requires detailed understanding of how climate influences key demographic rates, such as survival. This knowledge is frequently unavailable, even in well‐studied taxa such as birds. In temperate regions, most research into climatic effects on annual survival in resident passerines has focussed on winter temperature. Few studies have investigated potential precipitation effects and most assume little impact of breeding season weather. We use a 19‐year capture–mark–recapture study to provide a rare empirical analysis of how variation in temperature and precipitation throughout the entire year influences adult annual survival in a temperate passerine, the long‐tailed tit Aegithalos caudatus. We use model averaging to predict longer‐term historical survival rates, and future survival until the year 2100. Our model explains 73% of the interannual variation in survival rates. In contrast to current theory, we find a strong precipitation effect and no effect of variation in winter weather on adult annual survival, which is correlated most strongly to breeding season (spring) weather. Warm springs and autumns increase annual survival, but wet springs reduce survival and alter the form of the relationship between spring temperature and annual survival. There is little evidence for density dependence across the observed variation in population size. Using our model to estimate historical survival rates indicates that recent spring warming has led to an upward trend in survival rates, which has probably contributed to the observed long‐term increase in the UK long‐tailed tit population. Future climate change is predicted to further increase survival, under a broad range of carbon emissions scenarios and probabilistic climate change outcomes, even if precipitation increases substantially. We demonstrate the importance of considering weather over the entire annual cycle, and of considering precipitation and temperature in combination, in order to develop robust predictive models of demographic responses to climate change. Synthesis Prediction of climate change impacts demands understanding of how climate influences key demographic rates. In our 19‐year mark‐recapture study of long‐tailed tits Aegithalos caudatus, weather explained 73% of the inter‐annual variation in adult survival; warm springs and autumns increased survival, wet springs reduced survival, but winter weather had little effect. Robust predictions thus require consideration of the entire annual cycle and should not focus solely on temperature. Unexpectedly, survival appeared not to be strongly density‐dependent, so we use historical climate data to infer that recent climate change has enhanced survival over the four decades in which the UK long‐tailed tit population has more than doubled. Furthermore, survival rates in this species are predicted to further increase under a wide range of future climate scenarios.     

Reduced variability in range‐edge butterfly populations over three decades of climate warming

Populations at the high latitude edge of species’ geographical ranges are thought to show larger interannual population fluctuations, with subsequent higher local extinction risk, than those within the ‘core’ climatic range. As climate envelopes shift northward under climate warming, however, we would expect populations to show dampened variability. We test this hypothesis using annual abundance indices from 19 butterfly species across 79 British monitoring sites between 1976 and 2009, a period of climatic warming. We found that populations in the latter (warmer) half of the recording period show reduced interannual population variability. Species with more southerly European distributions showed the greatest dampening in population variability over time. Our results suggest that increases in population variability occur towards climatic range boundaries. British sites, previously existing at the margins of suitable climate space, now appear to fall closer to the core climatic range for many butterfly species.     

Interannual variability in global soil respiration, 1980–94

We used a climate‐driven regression model to develop spatially resolved estimates of soil‐CO₂ emissions from the terrestrial land surface for each month from January 1980 to December 1994, to evaluate the effects of interannual variations in climate on global soil‐to‐atmosphere CO₂ fluxes. The mean annual global soil‐CO₂ flux over this 15‐y period was estimated to be 80.4 (range 79.3–81.8) Pg C. Monthly variations in global soil‐CO₂ emissions followed closely the mean temperature cycle of the Northern Hemisphere. Globally, soil‐CO₂ emissions reached their minima in February and peaked in July and August. Tropical and subtropical evergreen broad‐leaved forests contributed more soil‐derived CO₂ to the atmosphere than did any other vegetation type (～30% of the total) and exhibited a biannual cycle in their emissions. Soil‐CO₂ emissions in other biomes exhibited a single annual cycle that paralleled the seasonal temperature cycle. Interannual variability in estimated global soil‐CO₂ production is substantially less than is variability in net carbon uptake by plants (i.e., net primary productivity). Thus, soils appear to buffer atmospheric CO₂ concentrations against far more dramatic seasonal and interannual differences in plant growth. Within seasonally dry biomes (savannas, bushlands and deserts), interannual variability in soil‐CO₂ emissions correlated significantly with interannual differences in precipitation. At the global scale, however, annual soil‐CO₂ fluxes correlated with mean annual temperature, with a slope of 3.3 Pg C y^?1 per °C. Although the distribution of precipitation influences seasonal and spatial patterns of soil‐CO₂ emissions, global warming is likely to stimulate CO₂ emissions from soils.     

Annual Variation of Soil Respiration and Precipitation in a Temperate Forest (<Emphasis Type="Italic">Quercus serrata</Emphasis> and <Emphasis Type="Italic">Carpinus laxiflora</Emphasis>) Under East Asian Monsoon Climate

The consequence of changing pattern of precipitation on soil CO₂ emission is poorly understood in montane forest ecosystems under monsoon climate in Asia. In this paper, the results of 3-year field measurements are reported on the annual soil respiration (R _s) from a temperate deciduous broad-leaved forest (Quercus serrata and Carpinus laxiflora) in Korea, and its interannual variations are examined associated with changing precipitation. Based on biweekly chamber measurements from 2001 to 2004, the annual soil CO₂ emission averaged to be 7.8 t C ha⁻¹ with an annual variability of ∼20%. The soil temperature explained 22–97% of seasonal variations of R _s each year whereas the water-filled porosity (WFP) and precipitation pattern had a major effect on the observed interannual variation. The optimum values of WFP during the main growing season depended not only on the amount but also on the intensity and frequency of precipitation. These results indicate that the changes in catchment hydrology can significantly alter the carbon sink/source strength of forest ecosystems in monsoon Asia.     

Carbon dioxide exchange over multiple temporal scales in an arid shrub ecosystem near La Paz,Baja California Sur,Mexico

Arid environments represent 30% of the global terrestrial surface, but are largely under‐represented in studies of ecosystem carbon flux. Less than 2% of all FLUXNET eddy covariance sites exist in a hot desert climate. Long‐term datasets of these regions are vital for capturing the seasonal and interannual variability that occur due to episodic precipitation events and climate change, which drive fluctuations in soil moisture and temperature patterns. The objectives of this study were to determine the meteorological variables that drive carbon flux on diel, seasonal, and annual scales and to determine how precipitation events control annual net ecosystem exchange (NEE). Patterns of NEE from 2002 to 2008 were investigated, providing a record with multiple replicates of seasons and conditions. Precipitation was extremely variable (55–339 mm) during the study period, and reduced precipitation in later years (2004–2008) appears to have resulted in annual moderate to large carbon sources (62–258 g C m^?2 yr^?1) in contrast to the previously reported sink (2002–2003). Variations in photosynthetically active radiation were found to principally drive variations in carbon uptake during the wet growing season while increased soil temperatures at a 5 cm depth stimulated carbon loss during the dry dormant season. Monthly NEE was primarily driven by soil moisture at a 5 cm depth, and years with a higher magnitude of precipitation events showed a longer growing season with annual net carbon uptake, whereas years with lower magnitude had drier soils and displayed short growing seasons with annual net carbon loss. Increased precipitation frequency was associated with increased annual NEE, which may be a function of increased microbial respiration to more small precipitation events. Annual precipitation frequency and magnitude were found to have effects on the interannual variability of NEE for up to 2 years.     

Estimating annual N2O emissions from agricultural soils in temperate climates

The Kyoto protocol requires countries to provide national inventories for a list of greenhouse gases including N₂O. A standard methodology proposed by the Intergovernmental Panel on Climate Change (IPCC) estimates direct N₂O emissions from soils as a constant fraction (1.25%) of the nitrogen input. This approach is insensitive to environmental variability. A more dynamic approach is needed to establish reliable N₂O emission inventories and to propose efficient mitigation strategies. The objective of this paper is to develop a model that allows the spatial and temporal variation in environmental conditions to be taken into account in national inventories of direct N₂O emissions. Observed annual N₂O emission rates are used to establish statistical relationships between N₂O emissions, seasonal climate and nitrogen‐fertilization rate. Two empirical models, MCROPS and MGRASS, were developed for croplands and grasslands. Validated with an independent data set, MCROPS shows that spring temperature and summer precipitation explain 35% of the variance in annual N₂O emissions from croplands. In MGRASS, nitrogen‐fertilization rate and winter temperature explain 48% of the variance in annual N₂O emissions from grasslands. Using long‐term climate observations (1900–2000), the sensitivity of the models with climate variability is estimated by comparing the year‐to‐year prediction of the model to the precision obtained during the validation process. MCROPS is able to capture interannual variability of N₂O emissions from croplands. However, grassland emissions show very small interannual variations, which are too small to be detectable by MGRASS. MCROPS and MGRASS improve the statistical reliability of direct N₂O emissions compared with the IPCC default methodology. Furthermore, the models can be used to estimate the effects of interannual variation in climate, climate change on direct N₂O emissions from soils at the regional scale.     

Effects of climate variability on the demography of wild geladas

Nonhuman primates are an essential part of tropical biodiversity and play key roles in many ecosystem functions, processes, and services. However, the impact of climate variability on nonhuman primates, whether anthropogenic or otherwise, remains poorly understood. In this study, we utilized age‐structured matrix population models to assess the population viability and demographic variability of a population of geladas (Theropithecus gelada) in the Simien Mountains, Ethiopia with the aim of revealing any underlying climatic influences. Using data from 2008 to 2019 we calculated annual, time‐averaged, and stochastic population growth rates (λ) and investigated relationships between vital rate variability and monthly cumulative rainfall and mean temperature. Our results showed that under the prevailing environmental conditions, the population will increase (λ _s = 1.021). Significant effects from rainfall and/or temperature variability were widely detected across vital rates; only the first year of infant survival and the individual years of juvenile survival were definitively unaffected. Generally, the higher temperature in the hot‐dry season led to lower survival and higher fecundity, while higher rainfall in the hot‐dry season led to increased survival and fecundity. Overall, these results provide evidence of greater effects of climate variability across a wider range of vital rates than those found in previous primate demography studies. This highlights that although primates have often shown substantial resilience to the direct effects of climate change, their vulnerability may vary with habitat type and across populations.     

Deriving a new phenological indicator of interannual net carbon exchange in contrasting boreal deciduous and evergreen forests

Phenology is an important variable affecting the annual net ecosystem production (NEP) of terrestrial ecosystems. A new phenological indicator was proposed based on the ratio of respiration season length and growing season length (respiration–growth length ratio, RGR). Validation of this new phenological indicator was conducted using continuous flux measurements at contrasting boreal deciduous and evergreen forests in Canada. Analyses based on yearly anomalies of both annual NEP and phenological indicators indicated that the RGR can explain more proportion of interannual NEP variability compared to existing phenological metrics, including the carbon uptake period and the autumn lag. A multivariate regression model was used to predict the respiration–growth length ratio anomaly using anomalies of spring air temperature, autumn radiation and soil water content (SWC), which serves as a prerequisite for this indicator being scaled up for regional applications where flux data were unavailable. By normalization growing season length, interannual NEP showed comparable sensitivity to RGR variations of different plant functional types, which is a great advantage over other phenological indicators. The high potential of RGR in explaining interannual NEP variability may highlight the importance of respiration process in controlling annual NEP, which has probably been overlooked or underestimated in existing phenological studies. The comparable sensitivity of RGR to annual NEP observed at different plant functional types would favor its application in tracking interannual variability of NEP regionally and complementary to existing indices to promote our understanding of carbon sequestration with future climate change.     

气候年际变率对全球植被平均分布的影响

采用改进后的通用陆面模式的动态植被模式(CLM-DGVM)研究当前气候条件下气候年际变率对全球潜在植被平均分布的影响。设计两组区域数值实验,一组使用基于NCEP再分析资料衍生的1960-1999年多年气象数据循环驱动,对照实验使用这40a的气候平均态或单年气象资料驱动(即没有气候年际变率),分别考察有无气候年际变化对热带、温带和寒带的潜在植被分布平衡态的影响。在此基础上以1950-1999年上述数据及对应的气候平均态为驱动做两组全球实验。结果表明气候年际变率导致全球植被总覆盖度下降,其中树和灌木减少而草增加;全球平均覆盖度的变化按常绿树、草、灌木、落叶树顺序递减,而相对变化(即格点覆盖度差异的绝对值的全球平均值与气候平均态下植物覆盖度的比值)按灌木、草、落叶树、常绿树顺序递减。在温度、降水、风速、比湿、光照、气压等6种气候因子中降水年际变率对于植被平均分布影响最显著。受降水影响,当年降水小于1200mm时植被总覆盖度的差异随其变率增加而下降,其它时候影响不明显。年降水小于1500mm时树减少,幅度随其年际变率变大而增加。常绿树无论降水多寡均减少,而落叶树在年降水大于1500mm时随其变率变大而增加。草在年降水小于1500mm、变率为中等时差异最大,降水较大时其年际变化对草的影响不大。温度年际变率对落叶树分布影响不大而使常绿树减少,尤其是在寒带,其幅度大致随变率增加而变大。草主要在温度高于-10℃增加而灌木在温度低于0℃增加。植被总体覆盖度在温度高于0℃时受影响普遍降低,降低的区域对应于温度年际变率较大的区域。以上结果说明用气候模式或生物地理模式预测未来植物分布时要同时考虑气候平均态和气候变率两方面的变化。     

Long-term population dynamics of a managed burrowing owl colony

We analyzed the population dynamics of a burrowing owl (Athene cunicularia) colony at Mineta San Jose International Airport in San Jose, California, USA from 1990–2007. This colony was managed by using artificial burrows to reduce the occurrence of nesting owls along runways and within major airport improvement projects during the study period. We estimated annual reproduction in natural and artificial burrows and age-specific survival rates with mark–recapture techniques, and we estimated the relative contribution of these vital rates to population dynamics using a life table response experiment. The breeding colony showed 2 distinct periods of change: high population growth from 7 nesting pairs in 1991 to 40 pairs in 2002 and population decline to 17 pairs in 2007. Reproduction was highly variable: annual nesting success (pairs that raised ≥1 young) averaged 79% and ranged from 36% to 100%, whereas fecundity averaged 3.36 juveniles/pair and ranged from 1.43 juveniles/pair to 4.54 juveniles/pair. We estimated annual adult survival at 0.710 during the period of colony increase from 1996 to 2001 and 0.465 during decline from 2002 to 2007, but there was no change in annual survival of juveniles between the 2 time periods. Long-term population growth rate (λ) estimated from average vital rates was λ_a = 1.072 with λ_i = 1.288 during colony increase and λ_d = 0.921 (Δλ = 0.368) during decline. A life table response experiment showed that change in adult survival rate during increasing and declining phases explained more than twice the variation in growth rate than other vital rates. Our findings suggest that management and conservation of declining burrowing owl populations should address factors that influence adult survival. © 2011 The Wildlife Society.     

非生长季降水对青藏高原高寒草甸优势种生物量稳定性的影响

受全球气候变化的影响,青藏高原在过去的几十年间整体上呈现暖湿化的趋势,相比于年际之间温度和降水的变化外,生长季和非生长季气候变化模式的差异可能会对生态系统产生更重要的影响,但相关的研究尚不充分。以青藏高原东部的高寒草甸为研究对象,基于2001年至2017年17年的野外观测数据,包括优势植物紫花针茅的高度、多度以及生物量、次优势物种洽草的生物量,结合生长季和非生长季平均温度和降水量的变化,通过线性回归以及结构方程模型,探究生长季/非生长季不对称气候变化对于青藏高原高寒草甸优势物种生物量稳定性的影响。研究结果表明：1)青藏高原东部年均温和年降水在过去的17年间显著增加,呈现暖湿化的趋势,但是非生长的降水却变化不明显;2)紫花针茅的高度、多度以及生物量在过去17年没有显著的趋势,但是洽草的生物量稳定性显著减少;3)非生长降水结合紫花针茅的高度、多度以及洽草的生物量稳定性促进了紫花针茅的生物量稳定性。研究结果可以为青藏高原高寒草甸在未来气候变化的背景下合理保护与利用提供科学依据。     

Region-Specific Sensitivity of Anemophilous Pollen Deposition to Temperature and Precipitation

Understanding relations between climate and pollen production is important for several societal and ecological challenges, importantly pollen forecasting for pollinosis treatment, forensic studies, global change biology, and high-resolution palaeoecological studies of past vegetation and climate fluctuations. For these purposes, we investigate the role of climate variables on annual-scale variations in pollen influx, test the regional consistency of observed patterns, and evaluate the potential to reconstruct high-frequency signals from sediment archives. A 43-year pollen-trap record from the Netherlands is used to investigate relations between annual pollen influx, climate variables (monthly and seasonal temperature and precipitation values), and the North Atlantic Oscillation climate index. Spearman rank correlation analysis shows that specifically in Alnus, Betula, Corylus, Fraxinus, Quercus and Plantago both temperature in the year prior to (T_-1), as well as in the growing season (T), are highly significant factors (T_April r_s between 0.30 [P<0.05[ and 0.58 [P<0.0001]; T_Juli-1 rs between 0.32 [P<0.05[ and 0.56 [P<0.0001]) in the annual pollen influx of wind-pollinated plants. Total annual pollen prediction models based on multiple climate variables yield R² between 0.38 and 0.62 (P<0.0001). The effect of precipitation is minimal. A second trapping station in the SE Netherlands, shows consistent trends and annual variability, suggesting the climate factors are regionally relevant. Summer temperature is thought to influence the formation of reproductive structures, while temperature during the flowering season influences pollen release. This study provides a first predictive model for seasonal pollen forecasting, and also aides forensic studies. Furthermore, variations in pollen accumulation rates from a sub-fossil peat deposit are comparable with the pollen trap data. This suggests that high frequency variability pollen records from natural archives reflect annual past climate variability, and can be used in palaeoecological and -climatological studies to bridge between population- and species-scale responses to climate forcing.     

The sensitivity of annual grassland carbon cycling to the quantity and timing of rainfall

Global climate models predict significant changes to the rainfall regimes of the grassland biome, where C cycling is particularly sensitive to the amount and timing of precipitation. We explored the effects of both natural interannual rainfall variability and experimental rainfall additions on net C storage and loss in annual grasslands. Soil respiration and net primary productivity (NPP) were measured in treatment and control plots over four growing seasons (water years, or WYs) that varied in wet‐season length and the quantity of rainfall. In treatment plots, we increased total rainfall by 50% above ambient levels and simulated one early‐ and one late‐season storm. The early‐ and late‐season rain events significantly increased soil respiration for 2–4 weeks after wetting, while augmentation of wet‐season rainfall had no significant effect. Interannual variability in precipitation had large and significant effects on C cycling. We observed a significant positive relationship between annual rainfall and aboveground NPP across the study (P=0.01, r²=0.69). Changes in the seasonal timing of rainfall significantly affected soil respiration. Abundant rainfall late in the wet season in WY 2004, a year with average total rainfall, led to greater net ecosystem C losses due to a ～50% increase in soil respiration relative to other years. Our results suggest that C cycling in annual grasslands will be less sensitive to changes in rainfall quantity and more affected by altered seasonal timing of rainfall, with a longer or later wet season resulting in significant C losses from annual grasslands.     

Interannual variability of pollen productivity and transport in mid-North America from 1997 to 2009

Understanding the causes of interannual variability in atmospheric pollen concentration is an important but elusive goal for public health and environmental change. We analyzed long-term daily records of pollen counts from urban Kansas City, Missouri, USA collected from 1997 to 2009 for three pollen groups: Ambrosia, Poaceae, and a third group which is mostly composed of arboreal pollen types. The annual pollen index varied from 8,368 to 80,822 over the thirteen-year period. Although Ambrosia pollen is often thought to be associated with droughts and disturbance, years with high Ambrosia pollen were associated with high summer precipitation to the south of Kansas City. Years with high Poaceae pollen were associated with high spring precipitation to the south of the city. In support of the southern influence to Kansas City pollen, Ambrosia and Poaceae pollen mostly arrived on southern winds. In contrast to the other two pollen groups, the arboreal pollen was most associated with growing season precipitation to the east of Kansas City, although it was still highest on days with southern winds. Based on the correlations with climate, the severity of an upcoming allergy season may be predicted with early-season precipitation data, while short-term severity can be forecast from local weather patterns.