Effects of enhanced nitrogen inputs and climate warming on a forest understorey plant assessed by transplant experiments along a latitudinal gradient

Global warming and enhanced nitrogen (N) inputs are two key global-change drivers affecting temperate forest ecosystems simultaneously. Interactive effects of multiple drivers might cause species responses to differ from those in single-factor experiments; therefore, there is an urgent need for more multi-factor studies. Here, we assessed the growth and reproductive performance of multiple populations of a widespread grass of deciduous forests (Milium effusum) sampled along a latitudinal gradient and subjected to experimental manipulations of temperature and nitrogen availability. Common garden transplant experiments along the latitudinal gradient were used to manipulate temperatures and combined with experimental N addition to assess intraspecific responses of the study species to global-change drivers as well as to determine local adaptation. The total biomass, number of seeds and seedling emergence time of M. effusum increased when transplanted in the southern common garden. Apart from effects on the seed mass, the species did not respond to N addition alone. Yet, interactive effects between warming and N addition were found: N addition led to increased biomass growth but only in the northern common garden. Significant home-site advantages were apparent, most likely because of increased mycorrhizal colonization of roots of local transplants. We show that multiple global-change drivers may alter dynamics in understorey communities of temperate forests. Our study reinforces the need to increase our understanding of plant responses to future environmental changes by expanding the multi-factor research framework.     

Climate change and voltinism of Mythimna sequax: the location and choice of phenological models matter

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The 29°N latitudinal line: an important division in the Hengduan Mountains, a biodiversity hotspot in southwest China

This paper aimed to explore the division of the southern and northern Hengduan Mountains based on gradients in species similarity and richness, and to analyze species richness in each sub-region. The Hengduan Mountain region was divided into nine latitudinal belts using one degree of latitude to define the belt after which distribution of seed plants within each latitudinal belt was recorded. Latitudinal patterns of species similarity were measured using the Jaccard similarity index for each pair of adjacent latitudinal belts. Non-metric multidimentional scaling (NMDS) was also used to analyze the similarity in species composition among the nine latitudinal belts. The latitudinal pattern of species similarity and the NMDS ordination both showed a great change in species composition across the 29°N latitudinal line, essentially dividing the Hengduan Mountain region into southern and northern sub-regions. Species richness, shown by the c-value of the species–area power function, and species–area ratio along a latitudinal gradient both showed a sharp decrease across the latitudinal belt from 29°0' to 29°59'N. The southern sub-region occupied 40% of the total area of the Hengduan Mountain region, but contained more than 80% of all the seed plants in the region. The higher species richness and endemism in the southern sub-region showed it to be the core of the Hengduan biodiversity hotspot, a result not unexpected because of the greater extremes of topography and wider diversity of habitats in the southern portion.     

Arctic alpine vegetation change over 20 years

Recent arctic warming experiments have recorded significant vegetation responses, typically an increase in shrub cover and a loss of species richness. We report similar changes in vegetation along an arctic mountainside in northern Sweden over 20 years. During this time mean annual temperature increased by 2.0 °C, and growing season temperature by 2.3 °C. Growing season length increased by 28% at the bottom of our study area, in birch forest, and by 175% on the mountaintop. Neither total vegetation cover nor the cover of bare ground changed. One common dwarf shrub, Empetrum hermaphroditum , and two common forbs, Viola biflora and Geranium sylvaticum , increased in abundance over time, but no common species moved up the gradient. Species richness declined significantly over time, with an average loss of two species per 50 cm × 100 cm plot. The richness of herbaceous species at intermediate altitudes decreased significantly with increasing shrub cover. In spite of large changes in temperature, the type and magnitude of vegetation change along this mountainside were relatively modest and consistent with those from wide-spread warming experiments.     

Seasonal sea ice cover as principal driver of spatial and temporal variation in depth extension and annual production of kelp in Greenland

We studied the depth distribution and production of kelp along the Greenland coast spanning Arctic to sub‐Arctic conditions from 78 ºN to 64 ºN. This covers a wide range of sea ice conditions and water temperatures, with those presently realized in the south likely to move northwards in a warmer future. Kelp forests occurred along the entire latitudinal range, and their depth extension and production increased southwards presumably in response to longer annual ice‐free periods and higher water temperature. The depth limit of 10% kelp cover was 9–14 m at the northernmost sites (77–78 ºN) with only 94–133 ice‐free days per year, but extended to depths of 21–33 m further south (73 ºN–64 ºN) where >160 days per year were ice‐free, and annual production of Saccharina longicruris and S. latissima, measured as the size of the annual blade, ranged up to sevenfold among sites. The duration of the open‐water period, which integrates light and temperature conditions on an annual basis, was the best predictor (relative to summer water temperature) of kelp production along the latitude gradient, explaining up to 92% of the variation in depth extension and 80% of the variation in kelp production. In a decadal time series from a high Arctic site (74 ºN), inter‐annual variation in sea ice cover also explained a major part (up to 47%) of the variation in kelp production. Both spatial and temporal data sets thereby support the prediction that northern kelps will play a larger role in the coastal marine ecosystem in a warmer future as the length of the open‐water period increases. As kelps increase carbon‐flow and habitat diversity, an expansion of kelp forests may exert cascading effects on the coastal Arctic ecosystem.     

Latitudinal influence on gametogenesis and host–parasite ecology in a marine bivalve model

Reproduction and parasites have significant impacts on marine animal populations globally. This study aimed to investigate the associative effects of host reproduction and a host–parasite interplay on a marine bivalve, along a geographic gradient of latitude. Cockles Cerastoderma edule were sampled from five European sites (54°N to 40°N), between April 2018 and October 2019. A histological survey provided data on trematode (metacercaria and sporocyst life stages), prevalence, and cockle stage of gametogenesis to assess the influence of a latitudinal gradient on both interplays. Sex ratios at the northernmost sites were skewed toward females, and spawning size was reduced at the lower latitudes. Trematode infection did not follow a latitudinal gradient. Localized site‐related drivers, namely seawater temperature, varied spatially, having an impact on cockle–trematode interactions. Spawning was related to elevated temperatures at all sites. Prolonged spawning occurred at southern latitudes, where seawater temperatures were warmer. Trematode prevalence and the impact of trematodes on gametogenesis were found to be spatially variable, but not latitudinally. Therefore, it is not possible to determine the likelihood of boom and bust events in cockles, based on the latitudinal location of a population. In terms of sublethal impacts, it appeared that energy was allocated to reproduction rather than somatic growth in southern populations, with less energy allocated to reproduction in the larger, northern cockles. The demonstrated spatial trend of energy allocation indicates the potential of a temporal trend of reduced cockle growth at northern sites, as a result of warming sea temperatures. This awareness of the spatially varying drivers of populations is crucial considering the potential for these drivers/inhibitors to be exacerbated in a changing marine environment.     

增温施氮对高寒草甸生产力及生物量分配的影响

在青藏高原高寒草甸区设置模拟增温和氮添加处理,研究长期增温与外源氮输入对高寒草甸群落生产及其分配的影响.结果表明:开顶箱增温装置造成小环境暖干化,即显著提高地表空气温度1.6℃,提高表层土壤温度1.4℃,降低土壤含水量4.7%.2012、2013和2014年不施氮处理下增温分别降低地上生物量61.5%、108.8%和77.1%,在高氮(40和80kg N·hm^-2·a^-1)处理下增温对群落地上生物量无显著影响,这说明增温的影响依赖于氮添加水平,且施氮补偿了增温导致的土壤氮损失.增温导致根冠比增加,2012、2013和2014年不施氮处理下增温分别增加根冠比98.6%、60.7%和97.8%.在不增温处理下,植物群落地上、地下生物量的变化率均表现出低氮(10、20 kg N·hm^-2·a^-1)促进、高氮抑制的趋势,达到饱和阈值时的氮添加剂量分别为56.0和55.5 kg N·hm^-2·a^-1;而在增温处理下,地上、地下生物量随施氮量增加呈线性增加趋势.这说明增温改变了高寒草甸生物量分配对外源氮输入的响应模式,增温导致的土壤无机氮含量变化是生物量分配模式改变的主要原因.由氮添加试验估算的高寒草甸氮饱和阈值表明,高寒草甸对氮输入的敏感性高于其他类型草地.     

Pre-winter lipid stores in young-of-year Atlantic salmon along a north–south gradient

The pre-winter lipid stores of young-of-the-year (YOY, age 0 year) Atlantic salmon Salmo salar were analysed along a north–south gradient from c. 71 to 58° N, with winter conditions ranging from >200 days of ice cover to no ice. The rivers sampled in Northern Norway represent some of the most northerly S. salar rivers. There was an increase in lipid content with increasing latitude, and mean lipid content (size adjusted to common mass) for YOY in northern rivers were almost three times higher: 0·035 g compared to 0·013 g in southern rivers. The relationship was not sensitive to variation in sampling time or variation in YOY body size. The lipid stores, however, varied markedly between rivers and also between neighbouring rivers, indicating different strategies or opportunities for pre-winter lipid storage both at latitudinal and local scales.     

Contrasting growth forecasts across the geographical range of Scots pine due to altitudinal and latitudinal differences in climatic sensitivity

Ongoing changes in global climate are altering ecological conditions for many species. The consequences of such changes are typically most evident at the edge of a species’ geographical distribution, where differences in growth or population dynamics may result in range expansions or contractions. Understanding population responses to different climatic drivers along wide latitudinal and altitudinal gradients is necessary in order to gain a better understanding of plant responses to ongoing increases in global temperature and drought severity. We selected Scots pine (Pinus sylvestris L.) as a model species to explore growth responses to climatic variability (seasonal temperature and precipitation) over the last century through dendrochronological methods. We developed linear models based on age, climate and previous growth to forecast growth trends up to year 2100 using climatic predictions. Populations were located at the treeline across a latitudinal gradient covering the northern, central and southernmost populations and across an altitudinal gradient at the southern edge of the distribution (treeline, medium and lower elevations). Radial growth was maximal at medium altitude and treeline of the southernmost populations. Temperature was the main factor controlling growth variability along the gradients, although the timing and strength of climatic variables affecting growth shifted with latitude and altitude. Predictive models forecast a general increase in Scots pine growth at treeline across the latitudinal distribution, with southern populations increasing growth up to year 2050, when it stabilizes. The highest responsiveness appeared at central latitude, and moderate growth increase is projected at the northern limit. Contrastingly, the model forecasted growth declines at lowland‐southern populations, suggesting an upslope range displacement over the coming decades. Our results give insight into the geographical responses of tree species to climate change and demonstrate the importance of incorporating biogeographical variability into predictive models for an accurate prediction of species dynamics as climate changes.     

Impact of Future Climate on Radial Growth of Four Major Boreal Tree Species in the Eastern Canadian Boreal Forest

Immediate phenotypic variation and the lagged effect of evolutionary adaptation to climate change appear to be two key processes in tree responses to climate warming. This study examines these components in two types of growth models for predicting the 2010–2099 diameter growth change of four major boreal species Betula papyrifera, Pinus banksiana, Picea mariana, and Populus tremuloides along a broad latitudinal gradient in eastern Canada under future climate projections. Climate-growth response models for 34 stands over nine latitudes were calibrated and cross-validated. An adaptive response model (A-model), in which the climate-growth relationship varies over time, and a fixed response model (F-model), in which the relationship is constant over time, were constructed to predict future growth. For the former, we examined how future growth of stands in northern latitudes could be forecasted using growth-climate equations derived from stands currently growing in southern latitudes assuming that current climate in southern locations provide an analogue for future conditions in the north. For the latter, we tested if future growth of stands would be maximally predicted using the growth-climate equation obtained from the given local stand assuming a lagged response to climate due to genetic constraints. Both models predicted a large growth increase in northern stands due to more benign temperatures, whereas there was a minimal growth change in southern stands due to potentially warm-temperature induced drought-stress. The A-model demonstrates a changing environment whereas the F-model highlights a constant growth response to future warming. As time elapses we can predict a gradual transition between a response to climate associated with the current conditions (F-model) to a more adapted response to future climate (A-model). Our modeling approach provides a template to predict tree growth response to climate warming at mid-high latitudes of the Northern Hemisphere.     

Discovery of latitudinal gradient of triidothyronine concentrations in ectotherms as revealed from a cyprinid fish,the common roach Rutilus rutilus

Important life history traits in groups of ectotherms have been shown to vary along a latitudinal axis. Despite sustained interest to this phenomenon, the underlying physiological mechanisms of latitudinal adaptation remain poorly understood. Thyroid hormones (THs) are key regulators of metabolism, development, and growth, and are involved in shaping adult phenotypes in lower vertebrates, fishes and amphibians. We tested the hypothesis that concentrations of triiodothyronine (T₃), the most active form of THs, correlate with latitudinal gradient in ectotherms using a cyprinid fish, the common roach Rutilus rutilus as an example. Fish from seven locations between 46°45′ and 58°04′ N were studied for T₃ concentrations. Our results show a strong positive correlation between latitude and T₃ concentrations. There was a three-fold difference between the means of the extreme southern and northern samples. This is a first finding of latitudinal gradient of thyroid hormones in ectotherms. Photoperiodism and temperature were tested as main environmental factors influencing TH levels. In our results, the increase in T₃ concentrations along the south–north axis in ectotherms is, seemingly, associated with adaptation to differing thermal environments.     

Rising temperature and development in dragonfly populations at different latitudes

1. For modelling the future ecological responses to climate change, data on individual species and on variation within and between populations from different latitudes are required. 2. We examined life cycle regulation and growth responses to temperature in Mediterranean and temperate populations of a widespread European odonate, Orthetrum cancellatum. In an experiment, offspring from individual females from different parts of the range were kept separately to elucidate differences between families. 3. The experiment was run outdoors at 52°N at a natural photoperiod for almost a year. We used four temperature regimes, ambient (i.e. following local air temperature) and ambient temperature increased by 2, 4 and 6 °C, to mimic future temperature rise. A mathematical model was used to categorise the type of seasonal regulation and estimate parameters of the temperature response curve. 4. Growth rate varied significantly with temperature sum, survival and geographic origin, as well as with family. Offspring of all females from the temperate part of the range had a life cycle with a 12 h day‐length threshold necessary to induce diapause (i.e. diapause was induced once day length fell below 12 h). By contrast, Mediterranean families had a 10 h threshold or had an unregulated life cycle allowing winter growth. The temperature response did not significantly differ between populations, but varied between families with a greater variation in the optimum temperature for growth in the Mediterranean population. 5. The variation in seasonal regulation leads to a diversity in voltinism patterns within species, ranging from bivoltine to semivoltine along a latitudinal gradient. Given that the type of seasonal regulation is genetically fixed, rising temperatures will not allow faster than univoltine development in temperate populations. We discuss the consequences of our results in the light of rising temperature in central Europe.     

Seasonal adaptations to day length in ecotypes of Diorhabda spp. (Coleoptera: Chrysomelidae) inform selection of agents against saltcedars (Tamarix spp.)

Seasonal adaptations to daylength often limit the effective range of insects used in biological control of weeds. The leaf beetle Diorhabda carinulata (Desbrochers) was introduced into North America from Fukang, China (latitude 44° N) to control saltcedars (Tamarix spp.), but failed to establish south of 38° N latitude because of a mismatched critical daylength response for diapause induction. The daylength response caused beetles to enter diapause too early in the season to survive the duration of winter at southern latitudes. Using climate chambers, we characterized the critical daylength response for diapause induction (CDL) in three ecotypes of Diorhabda beetles originating from 36, 38, and 43° N latitudes in Eurasia. In a field experiment, the timing of reproductive diapause and voltinism were compared among ecotypes by rearing the insects on plants in the field. CDL declined with latitude of origin among Diorhabda ecotypes. Moreover, CDL in southern (<39° N latitude) ecotypes was shortened by more than an hour when the insects were reared under a fluctuating 35-15°C thermoperiod than at a constant 25°C. In the northern (>42° N latitude) ecotypes, however, CDL was relatively insensitive to temperature. The southern ecotypes produced up to four generations when reared on plants in the field at sites south of 38° N, whereas northern ecotypes produced only one or two generations. The study reveals latitudinal variation in how Diorhabda ecotypes respond to daylength for diapause induction and how these responses affect insect voltinism across the introduced range.     

Mollusk species diversity in the Southeastern Pacific: why are there more species towards the pole?

The most ubiquitous and well recognized diversity pattern at large spatial scales is the latitudinal increase in species richness near the equator and decline towards the poles. Although several exceptions to this pattern have been documented, shallow water mollusks, the most specious group of marine invertebrates, are the epitome of the monotonic decline in species diversity toward higher latitudes along the Pacific and Atlantic coasts of North America. Here we analyze the geographic diversity of 629 mollusk species along the Pacific South American shelf. Our analyses are based on the most complete database of invertebrates assembled for this region of the world, consisting of latitudinal ranges of over 95% of all described mollusks between 10° and 55°S. Along this coast, mollusk diversity did not follow the typical latitudinal trend. The number of species remained constant and relatively low at intermediate latitudes and sharply increased toward higher latitudes, south of 42°S. This trend was explained by changes in shelf area, but not by sea surface temperature, unlike the pattern documented for Northern Hemisphere mollusks. Direct sampling of soft bottom communities along the gradient suggests that regional trends in species richness are produced by increased alpha diversity, and not only by artifacts produced by the increase in sampling area. We hypothesize that increased shelf area south of 42°S, geographic isolation produced by divergence of major oceanic currents, and the existence of refugia during glaciations, enabled species diversification. Radiation could have been limited by narrow continental shelves between 10°–42°. Asymmetries in latitudinal diversity trends between hemispheres show that there is not a single general factor determining large-scale diversity patterns.     

Microsatellite markers show decreasing diversity but unchanged level of clonality in Dryas octopetala (Rosaceae) with increasing latitude

? Premise of the study: Average arctic temperatures have increased at almost twice the global average in the past 100 years. Most studies on biodiversity along latitudinal gradients have focused on species richness or genetic diversity at lower latitudes, and only a few studies have inferred genetic diversity within a species along a latitudinal gradient at higher latitudes, even though these areas might be most affected by recent climate changes. Here, intraspecific genetic diversity of the arctic-alpine Dryas octopetala (Rosaceae) is studied along a latitudinal gradient to test the hypotheses that genetic diversity decreases and vegetative clonal growth increases with latitude. ? Methods: Ten microsatellite markers have been developed for D. octopetala and analyzed with population genetic methods in five populations along a latitudinal transect spanning from 59.0°N to 79.9°N. ? Key results: The nine microsatellites that were used in the final analyses resulted in a resolution high enough to distinguish between ramets while providing useful information at a larger geographical scale. Three genetic clusters were indicated, a southern Norway group, a northern Norway group, and a Svalbard group, with corresponding decreasing genetic diversity. No trend was found with regard to clonality along the gradient. ? Conclusions: The newly developed microsatellite markers provide a useful tool for further genetic studies of D. octopetala and its close relatives, addressing population structure as well as phylogeographic patterns. The results of this study support the hypothesis of decreasing genetic diversity with increasing latitude, which may have implications for future adaptability to climate change.     

Latitudinal Variation in Leaf and Tree Traits of the Mangrove Avicennia germinans (Avicenniaceae) in the Central Region of the Gulf of Mexico

The variation in leaf mass per area, leaf nutrients (% carbon, nitrogen and phosphorus), and the allometric relation between tree height and diameter of the black mangrove, Avicennia germinans, were explored in nine mangrove forests in similar environments along a 5° latitudinal gradient in the central region of the Gulf of Mexico, as indicated by a southward increase in temperature and precipitation. There was no correlation between leaf nitrogen or phosphorus content and latitude. Leaf mass per area and leaf carbon content were positively correlated with latitude and negatively correlated with temperature and annual rainfall, whereas asymptotic tree height and maximum diameter showed the opposite trend. Such patterns suggest a trade-off between leaf traits and tree size which may be constrained by the same environmental factors along a dry-cold to humid-warm latitudinal gradient.     

The response of forest plant regeneration to temperature variation along a latitudinal gradient

Background and Aims

The response of forest herb regeneration from seed to temperature variations across latitudes was experimentally assessed in order to forecast the likely response of understorey community dynamics to climate warming.

Methods

Seeds of two characteristic forest plants (Anemone nemorosa and Milium effusum) were collected in natural populations along a latitudinal gradient from northern France to northern Sweden and exposed to three temperature regimes in growth chambers (first experiment). To test the importance of local adaptation, reciprocal transplants were also made of adult individuals that originated from the same populations in three common gardens located in southern, central and northern sites along the same gradient, and the resulting seeds were germinated (second experiment). Seedling establishment was quantified by measuring the timing and percentage of seedling emergence, and seedling biomass in both experiments.

Key Results

Spring warming increased emergence rates and seedling growth in the early-flowering forb A. nemorosa. Seedlings of the summer-flowering grass M. effusum originating from northern populations responded more strongly in terms of biomass growth to temperature than southern populations. The above-ground biomass of the seedlings of both species decreased with increasing latitude of origin, irrespective of whether seeds were collected from natural populations or from the common gardens. The emergence percentage decreased with increasing home-away distance in seeds from the transplant experiment, suggesting that the maternal plants were locally adapted.

Conclusions

Decreasing seedling emergence and growth were found from the centre to the northern edge of the distribution range for both species. Stronger responses to temperature variation in seedling growth of the grass M. effusum in the north may offer a way to cope with environmental change. The results further suggest that climate warming might differentially affect seedling establishment of understorey plants across their distribution range and thus alter future understorey plant dynamics.     

Genetic and environmental influences on leaf phenology and cold hardiness of native and introduced riparian trees

To explore the roles of plasticity and genetic variation in the response to spatial and temporal climate variation, we established a common garden consisting of paired collections of native and introduced riparian trees sampled along a latitudinal gradient. The garden in Fort Collins, Colorado (latitude 40.6°N), included 681 native plains cottonwood (Populus deltoides subsp. monilifera) and introduced saltcedar (Tamarix ramosissima, T. chinensis and hybrids) collected from 15 sites at 29.2–47.6°N in the central United States. In the common garden both species showed latitudinal variation in fall, but not spring, leaf phenology, suggesting that the latitudinal gradient in fall phenology observed in the field results at least in part from inherited variation in the critical photoperiod, while the latitudinal gradient in spring phenology observed in the field is largely a plastic response to the temperature gradient. Populations from higher latitudes exhibited earlier bud set and leaf senescence. Cold hardiness varied latitudinally in both fall and spring for both species. For cottonwood, cold hardiness began earlier and ended later in northern than in southern populations. For saltcedar northern populations were hardier throughout the cold season than southern populations. Although cottonwood was hardier than saltcedar in midwinter, the reverse was true in late fall and early spring. The latitudinal variation in fall phenology and cold hardiness of saltcedar appears to have developed as a result of multiple introductions of genetically distinct populations, hybridization and natural selection in the 150 years since introduction.     

A comparative ecological study of Norwegian mountain plants in relation to possible future climatic change

Mountain plants constitute an important part of the Norwegian flora. They are also believed to be the plant group in Norway most threatened by the expected climatic warming due to an enhanced greenhouse effect in the near future. In this study the distributions of 107 mountain Norwegian vascular plants were modelled in relation to present-day climate using Gaussian logit regression. Most species are found to have a surprisingly broad amplitude to mean July and January temperatures, suggesting that a 2°C increase in summer temperature and 4°C increase in winter temperature (as expected with a 2×CO₂ increase) may not have a dramatic direct effect on most of the species investigated. A comparative study between estimated July and January temperature optima and tolerances and other ecological attributes such as habitat characteristics, dispersal mechanisms, range sizes and other climatic optima and tolerances was done using multivariate analysis. The results suggest that species most vulnerable to climatic warming, namely the species with narrow July and January temperature tolerances, are characterized by small range sizes and small population sizes, i.e. they are nationally rare species. Furthermore, these vulnerable species are found in all habitats along the major moisture gradient in alpine vegetation. A classification of the species into Rabinowitz's seven forms of rarity confirms that the species most vulnerable to climatic warming are characterized by being habitat specialists with a small geographic range size.     

Soil respiration response to climate change in Pacific Northwest prairies is mediated by a regional Mediterranean climate gradient

Soil respiration is expected to increase with rising global temperatures but the degree of response may depend on soil moisture and other local factors. Experimental climate change studies from single sites cannot discern whether an observed response is site‐dependent or generalizable. To deconvolve site‐specific vs. regional climatic controls, we examined soil respiration for 18 months along a 520 km climate gradient in three Pacific Northwest, USA prairies that represents increasingly severe Mediterranean conditions from north to south. At each site we implemented a fully factorial combination of 2.5–3 °C warming and 20% added precipitation intensity. The response of soil respiration to warming was driven primarily by the latitudinal climate gradient and not site‐specific factors. Warming increased respiration at all sites during months when soil moisture was not limiting. However, these gains were offset by reductions in respiration during seasonal transitions and summer drought due to lengthened periods of soil moisture limitation. The degree of this offset varied along the north–south climate gradient such that in 2011 warming increased cumulative annual soil respiration 28.6% in the northern site, 13.5% in the central site, and not at all in the southern site. Precipitation also stimulated soil respiration more frequently in the south, consistent with an increased duration of moisture limitation. The best predictors of soil respiration in nonlinear models were the Normalized Difference Vegetation Index (NDVI), antecedent soil moisture, and temperature but these models provided biased results at high and low soil respiration. NDVI was an effective integrator of climate and site differences in plant productivity in terms of their combined effects on soil respiration. Our results suggest that soil moisture limitation can offset the effect of warming on soil respiration, and that greater growing‐season moisture limitation would constrain cumulative annual responses to warming.