Publications by authors named "Laureano Gherardi"

14 Publications

  • Page 1 of 1

Global patterns and climatic controls of belowground net carbon fixation.

Proc Natl Acad Sci U S A 2020 08 3;117(33):20038-20043. Epub 2020 Aug 3.

Global Drylands Center, Arizona State University, Tempe, AZ 85287-4501.

Carbon allocated underground through belowground net primary production represents the main input to soil organic carbon. This is of significant importance, because soil organic carbon is the third-largest carbon stock after oceanic and geological pools. However, drivers and controls of belowground productivity and the fraction of total carbon fixation allocated belowground remain uncertain. Here we estimate global belowground net primary productivity as the difference between satellite-based total net primary productivity and field observations of aboveground net primary production and assess climatic controls among biomes. On average, belowground carbon productivity is estimated as 24.7 Pg y, accounting for 46% of total terrestrial carbon fixation. Across biomes, belowground productivity increases with mean annual precipitation, although the rate of increase diminishes with increasing precipitation. The fraction of total net productivity allocated belowground exceeds 50% in a large fraction of terrestrial ecosystems and decreases from arid to humid ecosystems. This work adds to our understanding of the belowground carbon productivity response to climate change and provides a comprehensive global quantification of root/belowground productivity that will aid the budgeting and modeling of the global carbon cycle.
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http://dx.doi.org/10.1073/pnas.2006715117DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7443884PMC
August 2020

Ecto- and endoparasitic nematodes respond differently across sites to changes in precipitation.

Oecologia 2020 Jul 12;193(3):761-771. Epub 2020 Jul 12.

Department of Biology and School of Global Environmental Sustainability, Colorado State University, Fort Collins, CO, 80523, USA.

Plant parasitic nematodes are among the greatest consumers of primary production in terrestrial ecosystems. Their feeding strategies can be divided into endoparasites and ectoparasites that differ substantially, not only in their damage potential to host tissue and primary production, but also in their susceptibility to environmental changes. Climate change is predicted to increase variability of precipitation in many systems, yet the effects on belowground biodiversity and associated impacts on primary productivity remain poorly understood. To examine the impact of altered precipitation on endo- and ectoparasitic soil nematodes, we conducted a 2-year precipitation manipulation study across an arid, a semiarid, and a mesic grassland. Plant parasite feeding type abundance, functional guilds, and herbivory index in response to precipitation were evaluated. Responses of endo- and ectoparasites to increased precipitation varied by grassland type. There was little response of ectoparasites to increased precipitation although their population declined at the mesic site with increased precipitation. The abundance of endoparasites remained unchanged with increasing precipitation at the arid site, increased at the semiarid, and decreased at the mesic site. The herbivory index followed closely the trends seen in the endoparasites response by stagnating at the arid site, increasing at the semiarid, and decreasing at the mesic site. Our findings suggest that altered precipitation has differing effects on plant parasite feeding strategies as well as functional guilds. This may have important implications for grassland productivity, as plant parasite pressure may exacerbate the effects of climate change on host plants.
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http://dx.doi.org/10.1007/s00442-020-04708-7DOI Listing
July 2020

Nutrient availability controls the impact of mammalian herbivores on soil carbon and nitrogen pools in grasslands.

Glob Chang Biol 2020 Feb 3. Epub 2020 Feb 3.

Department of Terrestrial Ecology, Netherlands Institute of Ecology (NIOO-KNAW), Wageningen, The Netherlands.

Grasslands are subject to considerable alteration due to human activities globally, including widespread changes in populations and composition of large mammalian herbivores and elevated supply of nutrients. Grassland soils remain important reservoirs of carbon (C) and nitrogen (N). Herbivores may affect both C and N pools and these changes likely interact with increases in soil nutrient availability. Given the scale of grassland soil fluxes, such changes can have striking consequences for atmospheric C concentrations and the climate. Here, we use the Nutrient Network experiment to examine the responses of soil C and N pools to mammalian herbivore exclusion across 22 grasslands, under ambient and elevated nutrient availabilities (fertilized with NPK + micronutrients). We show that the impact of herbivore exclusion on soil C and N pools depends on fertilization. Under ambient nutrient conditions, we observed no effect of herbivore exclusion, but under elevated nutrient supply, pools are smaller upon herbivore exclusion. The highest mean soil C and N pools were found in grazed and fertilized plots. The decrease in soil C and N upon herbivore exclusion in combination with fertilization correlated with a decrease in aboveground plant biomass and microbial activity, indicating a reduced storage of organic matter and microbial residues as soil C and N. The response of soil C and N pools to herbivore exclusion was contingent on temperature - herbivores likely cause losses of C and N in colder sites and increases in warmer sites. Additionally, grasslands that contain mammalian herbivores have the potential to sequester more N under increased temperature variability and nutrient enrichment than ungrazed grasslands. Our study highlights the importance of conserving mammalian herbivore populations in grasslands worldwide. We need to incorporate local-scale herbivory, and its interaction with nutrient enrichment and climate, within global-scale models to better predict land-atmosphere interactions under future climate change.
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http://dx.doi.org/10.1111/gcb.15023DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7155038PMC
February 2020

Nutrient addition increases grassland sensitivity to droughts.

Ecology 2020 05 18;101(5):e02981. Epub 2020 Feb 18.

Department of Ecology, Evolution, and Behavior, University of Minnesota, 140 Gortner Laboratory, 1479 Gortner Avenue, St. Paul, 55108, Minnesota, USA.

Grasslands worldwide are expected to experience an increase in extreme events such as drought, along with simultaneous increases in mineral nutrient inputs as a result of human industrial activities. These changes are likely to interact because elevated nutrient inputs may alter plant diversity and increase the sensitivity to droughts. Dividing a system's sensitivity to drought into resistance to change during the drought and rate of recovery after the drought generates insights into different dimensions of the system's resilience in the face of drought. Here, we examine the effects of experimental nutrient fertilization and the resulting diversity loss on the resistance to and recovery from severe regional droughts. We do this at 13 North American sites spanning gradients of aridity, five annual grasslands in California, and eight perennial grasslands in the Great Plains. We measured rate of resistance as the change in annual aboveground biomass (ANPP) per unit change in growing season precipitation as conditions declined from normal to drought. We measured recovery as the change in ANPP during the postdrought period and the return to normal precipitation. Resistance and recovery did not vary across the 400-mm range of mean growing season precipitation spanned by our sites in the Great Plains. However, chronic nutrient fertilization in the Great Plains reduced drought resistance and increased drought recovery. In the California annual grasslands, arid sites had a greater recovery postdrought than mesic sites, and nutrient addition had no consistent effects on resistance or recovery. Across all study sites, we found that predrought species richness in natural grasslands was not consistently associated with rates of resistance to or recovery from the drought, in contrast to earlier findings from experimentally assembled grassland communities. Taken together, these results suggest that human-induced eutrophication may destabilize grassland primary production, but the effects of this may vary across regions and flora, especially between perennial and annual-dominated grasslands.
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http://dx.doi.org/10.1002/ecy.2981DOI Listing
May 2020

Drought suppresses soil predators and promotes root herbivores in mesic, but not in xeric grasslands.

Proc Natl Acad Sci U S A 2019 06 11;116(26):12883-12888. Epub 2019 Jun 11.

Department of Biology & School of Global Environmental Sustainability, Colorado State University, Fort Collins, CO 80523

Precipitation changes among years and locations along gradients of mean annual precipitation (MAP). The way those changes interact and affect populations of soil organisms from arid to moist environments remains unknown. Temporal and spatial changes in precipitation could lead to shifts in functional composition of soil communities that are involved in key aspects of ecosystem functioning such as ecosystem primary production and carbon cycling. We experimentally reduced and increased growing-season precipitation for 2 y in field plots at arid, semiarid, and mesic grasslands to investigate temporal and spatial precipitation controls on the abundance and community functional composition of soil nematodes, a hyper-abundant and functionally diverse metazoan in terrestrial ecosystems. We found that total nematode abundance decreased with greater growing-season precipitation following increases in the abundance of predaceous nematodes that consumed and limited the abundance of nematodes lower in the trophic structure, including root feeders. The magnitude of these nematode responses to temporal changes in precipitation increased along the spatial gradient of long-term MAP, and significant effects only occurred at the mesic site. Contrary to the temporal pattern, nematode abundance increased with greater long-term MAP along the spatial gradient from arid to mesic grasslands. The projected increase in the frequency of extreme dry years in mesic grasslands will therefore weaken predation pressure belowground and increase populations of root-feeding nematodes, potentially leading to higher levels of plant infestation and plant damage that would exacerbate the negative effect of drought on ecosystem primary production and C cycling.
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http://dx.doi.org/10.1073/pnas.1900572116DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6600938PMC
June 2019

Effect of interannual precipitation variability on dryland productivity: A global synthesis.

Glob Chang Biol 2019 01 22;25(1):269-276. Epub 2018 Nov 22.

Global Drylands Center, Arizona State University, Tempe, Arizona.

Climate-change assessments project increasing precipitation variability through increased frequency of extreme events. However, the effects of interannual precipitation variance per se on ecosystem functioning have been largely understudied. Here, we report on the effects of interannual precipitation variability on the primary production of global drylands, which include deserts, steppes, shrublands, grasslands, and prairies and cover about 40% of the terrestrial earth surface. We used a global database that has 43 datasets, which are uniformly distributed in parameter space and each has at least 10 years of data. We found (a) that at the global scale, precipitation variability has a negative effect on aboveground net primary production. (b) Expected increases in interannual precipitation variability for the year 2,100 may result in a decrease of up to 12% of the global terrestrial carbon sink. (c) The effect of precipitation interannual variability on dryland productivity changes from positive to negative along a precipitation gradient. Arid sites with mean precipitation under 300 mm/year responded positively to increases in precipitation variability, whereas sites with mean precipitation over 300 mm/year responded negatively. We propose three complementary mechanisms to explain this result: (a) concave-up and concave-down precipitation-production relationships in arid vs. humid systems, (b) shift in the distribution of water in the soil profile, and (c) altered frequency of positive and negative legacies. Our results demonstrated that enhanced precipitation variability will have direct impacts on global drylands that can potentially affect the future terrestrial carbon sink.
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http://dx.doi.org/10.1111/gcb.14480DOI Listing
January 2019

Regional grassland productivity responses to precipitation during multiyear above- and below-average rainfall periods.

Glob Chang Biol 2018 05 27;24(5):1935-1951. Epub 2018 Feb 27.

United States Department of Agriculture - Agricultural Research Service, Grazinglands Research Laboratory, El Reno, OK, USA.

There is considerable uncertainty in the magnitude and direction of changes in precipitation associated with climate change, and ecosystem responses are also uncertain. Multiyear periods of above- and below-average rainfall may foretell consequences of changes in rainfall regime. We compiled long-term aboveground net primary productivity (ANPP) and precipitation (PPT) data for eight North American grasslands, and quantified relationships between ANPP and PPT at each site, and in 1-3 year periods of above- and below-average rainfall for mesic, semiarid cool, and semiarid warm grassland types. Our objective was to improve understanding of ANPP dynamics associated with changing climatic conditions by contrasting PPT-ANPP relationships in above- and below-average PPT years to those that occurred during sequences of multiple above- and below-average years. We found differences in PPT-ANPP relationships in above- and below-average years compared to long-term site averages, and variation in ANPP not explained by PPT totals that likely are attributed to legacy effects. The correlation between ANPP and current- and prior-year conditions changed from year to year throughout multiyear periods, with some legacy effects declining, and new responses emerging. Thus, ANPP in a given year was influenced by sequences of conditions that varied across grassland types and climates. Most importantly, the influence of prior-year ANPP often increased with the length of multiyear periods, whereas the influence of the amount of current-year PPT declined. Although the mechanisms by which a directional change in the frequency of above- and below-average years imposes a persistent change in grassland ANPP require further investigation, our results emphasize the importance of legacy effects on productivity for sequences of above- vs. below-average years, and illustrate the utility of long-term data to examine these patterns.
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http://dx.doi.org/10.1111/gcb.14024DOI Listing
May 2018

Asymmetric responses of primary productivity to precipitation extremes: A synthesis of grassland precipitation manipulation experiments.

Glob Chang Biol 2017 10 9;23(10):4376-4385. Epub 2017 May 9.

Department of Microbiology and Plant Biology, University of Oklahoma, Norman, OK, USA.

Climatic changes are altering Earth's hydrological cycle, resulting in altered precipitation amounts, increased interannual variability of precipitation, and more frequent extreme precipitation events. These trends will likely continue into the future, having substantial impacts on net primary productivity (NPP) and associated ecosystem services such as food production and carbon sequestration. Frequently, experimental manipulations of precipitation have linked altered precipitation regimes to changes in NPP. Yet, findings have been diverse and substantial uncertainty still surrounds generalities describing patterns of ecosystem sensitivity to altered precipitation. Additionally, we do not know whether previously observed correlations between NPP and precipitation remain accurate when precipitation changes become extreme. We synthesized results from 83 case studies of experimental precipitation manipulations in grasslands worldwide. We used meta-analytical techniques to search for generalities and asymmetries of aboveground NPP (ANPP) and belowground NPP (BNPP) responses to both the direction and magnitude of precipitation change. Sensitivity (i.e., productivity response standardized by the amount of precipitation change) of BNPP was similar under precipitation additions and reductions, but ANPP was more sensitive to precipitation additions than reductions; this was especially evident in drier ecosystems. Additionally, overall relationships between the magnitude of productivity responses and the magnitude of precipitation change were saturating in form. The saturating form of this relationship was likely driven by ANPP responses to very extreme precipitation increases, although there were limited studies imposing extreme precipitation change, and there was considerable variation among experiments. This highlights the importance of incorporating gradients of manipulations, ranging from extreme drought to extreme precipitation increases into future climate change experiments. Additionally, policy and land management decisions related to global change scenarios should consider how ANPP and BNPP responses may differ, and that ecosystem responses to extreme events might not be predicted from relationships found under moderate environmental changes.
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http://dx.doi.org/10.1111/gcb.13706DOI Listing
October 2017

Enhanced interannual precipitation variability increases plant functional diversity that in turn ameliorates negative impact on productivity.

Ecol Lett 2015 Dec 6;18(12):1293-300. Epub 2015 Oct 6.

School of Life Sciences, Arizona State University, Tempe, AZ, 85287-4501, USA.

Although precipitation interannual variability is projected to increase due to climate change, effects of changes in precipitation variance have received considerable less attention than effects of changes in the mean state of climate. Interannual precipitation variability effects on functional diversity and its consequences for ecosystem functioning are assessed here using a 6-year rainfall manipulation experiment. Five precipitation treatments were switched annually resulting in increased levels of precipitation variability while maintaining average precipitation constant. Functional diversity showed a positive response to increased variability due to increased evenness. Dominant grasses decreased and rare plant functional types increased in abundance because grasses showed a hump-shaped response to precipitation with a maximum around modal precipitation, whereas rare species peaked at high precipitation values. Increased functional diversity ameliorated negative effects of precipitation variability on primary production. Rare species buffered the effect of precipitation variability on the variability in total productivity because their variance decreases with increasing precipitation variance.
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http://dx.doi.org/10.1111/ele.12523DOI Listing
December 2015

Enhanced precipitation variability decreases grass- and increases shrub-productivity.

Proc Natl Acad Sci U S A 2015 Oct 28;112(41):12735-40. Epub 2015 Sep 28.

School of Life Sciences, Arizona State University, Tempe, AZ 85287-4501; School of Sustainability, Arizona State University, Tempe, AZ, 85287-4501.

Although projections of precipitation change indicate increases in variability, most studies of impacts of climate change on ecosystems focused on effects of changes in amount of precipitation, overlooking precipitation variability effects, especially at the interannual scale. Here, we present results from a 6-y field experiment, where we applied sequences of wet and dry years, increasing interannual precipitation coefficient of variation while maintaining a precipitation amount constant. Increased precipitation variability significantly reduced ecosystem primary production. Dominant plant-functional types showed opposite responses: perennial-grass productivity decreased by 81%, whereas shrub productivity increased by 67%. This pattern was explained by different nonlinear responses to precipitation. Grass productivity presented a saturating response to precipitation where dry years had a larger negative effect than the positive effects of wet years. In contrast, shrubs showed an increasing response to precipitation that resulted in an increase in average productivity with increasing precipitation variability. In addition, the effects of precipitation variation increased through time. We argue that the differential responses of grasses and shrubs to precipitation variability and the amplification of this phenomenon through time result from contrasting root distributions of grasses and shrubs and competitive interactions among plant types, confirmed by structural equation analysis. Under drought conditions, grasses reduce their abundance and their ability to absorb water that then is transferred to deep soil layers that are exclusively explored by shrubs. Our work addresses an understudied dimension of climate change that might lead to widespread shrub encroachment reducing the provisioning of ecosystem services to society.
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http://dx.doi.org/10.1073/pnas.1506433112DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4611653PMC
October 2015

Biotic mechanisms of community stability shift along a precipitation gradient.

Ecology 2014 Jun;95(6):1693-700

Understanding how biotic mechanisms confer stability in variable environments is a fundamental quest in ecology, and one that is becoming increasingly urgent with global change. Several mechanisms, notably a portfolio effect associated with species richness, compensatory dynamics generated by negative species covariance and selection for stable dominant species populations can increase the stability of the overall community. While the importance of these mechanisms is debated, few studies have contrasted their importance in an environmental context. We analyzed nine long-term data sets of grassland species composition to investigate how two key environmental factors, precipitation amount and variability, may directly influence community stability and how they may indirectly influence stability via biotic mechanisms. We found that the importance of stability mechanisms varied along the environmental gradient: strong negative species covariance occurred in sites characterized by high precipitation variability, whereas portfolio effects increased in sites with high mean annual precipitation. Instead of questioning whether compensatory dynamics are important in nature, our findings suggest that debate should widen to include several stability mechanisms and how these mechanisms vary in importance across environmental gradients.
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http://dx.doi.org/10.1890/13-0895.1DOI Listing
June 2014

Sensitivity of grassland plant community composition to spatial vs. temporal variation in precipitation.

Ecology 2013 Aug;94(8):1687-96

Ecology, Behavior, and Evolution Section, University of California-San Diego, La Jolla, California 92093, USA.

Climate gradients shape spatial variation in the richness and composition of plant communities. Given future predicted changes in climate means and variability, and likely regional variation in the magnitudes of these changes, it is important to determine how temporal variation in climate influences temporal variation in plant community structure. Here, we evaluated how species richness, turnover, and composition of grassland plant communities responded to interannual variation in precipitation by synthesizing long-term data from grasslands across the United States. We found that mean annual precipitation,(MAP) was a positive predictor of species richness across sites, but a positive temporal relationship between annual precipitation and richness was only evident within two sites with low MAP. We also found higher average rates of species turnover in dry sites that in turn had a high proportion of annual species, although interannual rates of species turnover were surprisingly high across all locations. Annual species were less abundant than perennial species at nearly all sites, and our analysis showed that the probability of a species being lost or gained from one year to the next increased with decreasing species abundance. Bray-Curtis dissimilarity from one year to the next, a measure of species composition change that is influenced mainly by abundant species, was insensitive to precipitation at all sites. These results suggest that the richness and turnover patterns we observed were driven primarily by rare species, which comprise the majority of the local species pools at these grassland sites. These findings are consistent with the idea that short-lived and less abundant species are more sensitive to interannual climate variability than longer-lived and more abundant species. We conclude that, among grassland ecosystems, xeric grasslands are likely to exhibit the greatest responsiveness of community composition (richness and turnover) to predicted future increases in interannual precipitation variability. Over the long-term, species composition may shift to reflect spatial patterns of mean precipitation; however, perennial-dominated systems will be buffered against rising interannual variation, while systems that have a large number of rare, annual species will show the greatest temporal variability in species composition in response to rising interannual variability in precipitation.
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http://dx.doi.org/10.1890/12-1006.1DOI Listing
August 2013

Preference for different inorganic nitrogen forms among plant functional types and species of the Patagonian steppe.

Oecologia 2013 Nov 29;173(3):1075-81. Epub 2013 Jun 29.

School of Life Sciences, Arizona State University, 427 E Tyler Mall, Tempe, AZ, 85287, USA,

We have explored species-specific preferences for nitrate (NO3(-)) and ammonium (NH4(+)) as an alternative niche separation in ecosystems where nitrogen (N) is present mostly in inorganic forms. The Patagonian steppe is dominated by shrubs and grasses. Shrubs absorb water and nutrients from deep soil layers, which are poor in N, while grasses have the opposite pattern, absorbing most of their water and nutrients from the upper layers of the soil. We hypothesized that the preferences of shrub and grass for inorganic N forms are different and that the rate of potential N uptake is greater in shrubs than in grasses. To test this hypothesis, we grew individuals of six dominant species in solutions of different NH4(+):NO3(-) concentration ratios. Nitrate uptake was found to be higher in shrubs, while ammonium uptake was similar between plant functional types. The NH4(+):NO3(-) uptake ratio was significantly lower for shrubs than grasses. Shrubs, which under field conditions have deeper rooting systems than grasses, showed a higher N absorption capacity than grasses and a preference for the more mobile N form, nitrate. Grasses, which had lower N uptake rates than shrubs, preferred ammonium over nitrate. These complementary patterns between grasses and shrubs suggest a more thorough exploitation of resources by diverse ecosystems than those dominated by just one functional type. The loss of one plant functional group or a significant change in its abundance would therefore represent a reduction in resource use efficiency and ecosystem functioning.
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http://dx.doi.org/10.1007/s00442-013-2687-7DOI Listing
November 2013

Legacies of precipitation fluctuations on primary production: theory and data synthesis.

Philos Trans R Soc Lond B Biol Sci 2012 Nov;367(1606):3135-44

School of Sustainability, Arizona State University, Tempe, AZ, USA.

Variability of above-ground net primary production (ANPP) of arid to sub-humid ecosystems displays a closer association with precipitation when considered across space (based on multiyear averages for different locations) than through time (based on year-to-year change at single locations). Here, we propose a theory of controls of ANPP based on four hypotheses about legacies of wet and dry years that explains space versus time differences in ANPP-precipitation relationships. We tested the hypotheses using 16 long-term series of ANPP. We found that legacies revealed by the association of current- versus previous-year conditions through the temporal series occur across all ecosystem types from deserts to mesic grasslands. Therefore, previous-year precipitation and ANPP control a significant fraction of current-year production. We developed unified models for the controls of ANPP through space and time. The relative importance of current-versus previous-year precipitation changes along a gradient of mean annual precipitation with the importance of current-year PPT decreasing, whereas the importance of previous-year PPT remains constant as mean annual precipitation increases. Finally, our results suggest that ANPP will respond to climate-change-driven alterations in water availability and, more importantly, that the magnitude of the response will increase with time.
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http://dx.doi.org/10.1098/rstb.2011.0347DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3479688PMC
November 2012