Temperature sensitivity in aboveground net primary productivity in semi-arid grasslands

2014 Fall.Includes bibliographical references.Although climate models forecast warmer temperatures with a high degree of certainty, precipitation is the primary driver of aboveground net primary productivity (ANPP) in most grasslands. In contrast, variations in temperature seldom are related to patterns of ANPP. Thus forecasting responses to warming is a challenge, and raises the question: how sensitive will grassland ANPP be to warming? I evaluated climate and multi-year ANPP data (67 years) from eight western US grasslands arrayed along substantial mean annual temperature (MAT, ~7-14 °C) and mean annual precipitation (MAP, ~300 - 500 mm) gradients. I used regression and analysis of covariance (ANCOVA) to assess relationships between ANPP and temperature, as well as precipitation (annual and growing season) to evaluate temperature sensitivity of ANPP. I also related ANPP to the Standardized Precipitation Evaporation Index (SPEI), which combines precipitation and evapotranspiration estimates. Regression models indicated that variation in growing season temperature was negatively related to total and graminoid ANPP, but precipitation was a better predictor than temperature. Growing season temperature was also a significant parameter in more complex models, but again precipitation was consistently a stronger predictor of ANPP. Surprisingly, neither annual nor growing season SPEI was as strongly related to ANPP as was precipitation alone. I conclude that warming will affect ANPP in these grasslands, but that predicting temperature effects from natural climatic gradients is difficult. This is because unlike precipitation, warming effects are likely to be complex and site specific as well as moderated by regional shifts in the C3/C4 ratios of plant communities

Timing of growing season drought and its effects on above- and belowground production in a mesic grassland, The

2014 Fall.Includes bibliographical references.As a consequence of climate change, both the timing and amount of precipitation ecosystems receive are expected to be altered. In general, regions that are relatively dry are expected to get drier and the timing of seasonal drought - defined as a prolonged absence or marked deficiency of precipitation - is expected to change. Although drought in general has been extensively studied, particularly in grasslands, we know little about how natural ecosystems will respond to shifts in the timing of growing season drought. In this study I investigated the response of both above- and belowground net primary production (ANPP & BNPP) to reductions in precipitation in a mesic, tallgrass prairie in NE Kansas. Experimental plots were subjected to one of three drought treatments (25% reductions in the average growing season precipitation [GSP]) imposed either in late spring, early summer or mid-summer. A control treatment that received the mean GSP and a wet treatment that received 130% of the mean GSP were included to assess drought responses. In all treatments, I measured soil moisture, soil N and P content, canopy light interception and plant community composition in addition to ANPP and BNPP. I expected that ANPP would be more sensitive to drought than BNPP based on evidence from past studies that have almost always found a positive correlation between precipitation and ANPP, while trends with BNPP are less clear. I also hypothesized that early summer drought would cause the highest reduction in net primary production (ANPP + BNPP), because soil moisture would likely still be high in the late spring from late winter and early spring snow/rain, lessening the effect of reduced precipitation inputs. Moreover, because annual ANPP approaches its maximum by summer, I expected the mid-summer drought to affect NPP the least. I found that without considering timing, a 25% growing season drought reduced ANPP relative to the control by 18-26%, while ANPP in the control and wet treatment was not significantly different. Early summer and mid-summer drought resulted in significant reductions in ANPP (~25%) relative to control plots, but late spring drought did not reduce ANPP significantly despite similar reductions in soil moisture across all treatments. In contrast, neither drought nor wet treatments altered BNPP significantly. Because soil nutrients may increase during drought and plant functional type diversity may buffer productivity responses to drought, I investigated the role these played in determining responses to the treatments imposed. I found that soil nutrients were positively related to ANPP only in the wet treatment; conversely, diversity was negatively related to ANPP in the ambient and drought treatments, but not the wet treatment. I conclude that timing does play an important role in determining ecosystem response to drought with periods of no rain that occur earlier in the year having less of an impact than those that occur later. Furthermore, differences in responses between ANPP and BNPP were striking and need to be accounted for when projecting productivity responses of grasslands to climate change

Microbial and biogeochemical responses to changing precipitation patterns in grassland ecosystems

2012 Summer.Includes bibliographical references.Global circulation models predict that precipitation patterns in grasslands will both intensify and be characterized by more severe drought in the future. In these systems, the availability of water strongly controls ecosystem function, so changes in precipitation are likely to significantly alter biological communities and biogeochemical dynamics. Since these biogeochemical changes could feed back on climate drivers by influencing regional to global scale energy and water balance, predicted changes in grassland precipitation call for a better understanding of relationships between water availability and grassland biogeochemical dynamics. My dissertation aimed to address how changing rainfall patterns affect biogeochemical cycling and soil microbial communities in grasslands. I first tested the generality of controls over soil organic matter storage in temperate grasslands by studying existing spatial gradients in soil carbon and nitrogen, as they relate to the spatial variation in average precipitation and temperature, and soil texture. I found that statistical models developed in US grasslands overestimated soil organic carbon and underestimated soil organic nitrogen in Chinese grasslands. However, when I incorporated nitrogen deposition and historical land use using a simulation model, it resulted in more accurate model estimates for this region. This work suggests that nitrogen deposition and historical land use legacies may need to be considered to accurately describe biogeochemical dynamics in Chinese grasslands and better predict the vulnerability of global carbon stocks to loss. Responses of ecosystems to changes through time are often somewhat different than relationships gleaned from large-scale spatial gradients. At the local scale, I found that an 11-year drought can significantly alter biogeochemical and ecosystem dynamics in the highly drought-resistant shortgrass steppe. Here, soil inorganic nitrogen availability increased up to 4-fold in plots receiving 25% of summer precipitation. This accumulation of nitrogen under drought may explain the higher plant tissue nitrogen and N2 flux observed under recovery. A more "open" nitrogen cycle that I observed following severe drought could affect the impact of drought on grassland ecosystems, as well as the timescale of recovery. Soil microbial community composition was also altered by this 11-year drought manipulation in the shortgrass steppe, and these differences persisted even after communities were subject to the same moisture conditions for 36 hours in the lab. In this lab experiment, I also identified specific microbial groups that grew under a certain moisture levels, presenting evidence of moisture niche partitioning in microbial communities. However, this niche differentiation wasn't realized in the field; communities that grew under dry conditions in the lab were not similar to those that emerged under long-term drought plots. Overall, this work suggests that contrary to previous assumptions, microbial communities display legacies from long-term field treatments, and that although soil moisture has the potential to drive microbial community composition through niche partitioning, this factor may not always be the primary driver of long-term community composition. Microbial communities were also sensitive to altered precipitation timing in the tallgrass prairie. In addition, communities that were subject to intensified precipitation patterns in the field respired less than control soils after laboratory rewetting events, but respiration rates of the different field treatments converged after 100 days under the same conditions. Surprisingly, species composition of these communities was more sensitive to drying and rewetting pulses in the lab than those from the control. Together, these results show that microbial communities display legacies to altered precipitation timing, in addition to drought, but community composition is not necessarily tightly linked to respiration. Overall, my dissertation work suggests that grasslands will be sensitive to extreme shifts in precipitation, and that biogeochemical and microbial responses could influence how grasslands are altered under future precipitation regimes. However, my work also shows that precipitation is not the only factor controlling biogeochemical and microbial community dynamics in grasslands, even under rainfall manipulations and across precipitation gradients. Therefore, the response of grasslands to other environmental factors - that shift with precipitation changes or are predicted to change independently - should not be overlooked

Environmental Controls on Multi-Scale Dynamics of Net Carbon Dioxide Exchange From an Alpine Peatland on the Eastern Qinghai-Tibet Plateau

Peatlands are characterized by their large carbon storage capacity and play an essential role in the global carbon cycle. However, the future of the carbon stored in peatland ecosystems under a changing climate remains unclear. In this study, based on the eddy covariance technique, we investigated the net ecosystem CO2 exchange (NEE) and its controlling factors of the Hongyuan peatland, which is a part of the Ruoergai peatland on the eastern Qinghai-Tibet Plateau (QTP). Our results show that the Hongyuan alpine peatland was a CO2 sink with an annual NEE of -226.61 and -185.35 g C m(-2) in 2014 and 2015, respectively. While, the non-growing season NEE was 53.35 and 75.08 g C m(-2) in 2014 and 2015, suggesting that non-growing seasons carbon emissions should not be neglected. Clear diurnal variation in NEE was observed during the observation period, with the maximum CO2 uptake appearing at 12:30 (Beijing time, UTC+8). The Q(10) value of the non-growing season in 2014 and 2015 was significantly higher than that in the growing season, which suggested that the CO2 flux in the non-growing season was more sensitive to warming than that in the growing season. We investigated the multi-scale temporal variations in NEE during the growing season using wavelet analysis. On daily timescales, photosynthetically active radiation was the primary driver of NEE. Seasonal variation in NEE was mainly driven by soil temperature. The amount of precipitation was more responsible for annual variation of NEE. The increasing number of precipitation event was associated with increasing annual carbon uptake. This study highlights the need for continuous eddy covariance measurements and time series analysis approaches to deepen our understanding of the temporal variability in NEE and multi-scale correlation between NEE and environmental factors

Respuesta ecofisiológica del espartal mediterráneo semiárido al cambio climático

[EN] Climate change is a component of the several planetary-scale shifts that are currently taking place in the Earth system, which as a whole are referred to as global change. However, climate, as a vector of energy and material fluxes, has the ability to interact with all the elements (including natural and social components) of the Earth system, also influencing their dynamics. The Mediterranean-European region has been identified as one of the most prominent climate response hot spots, and precipitation patterns are major limiting factors for human activities and natural ecosystems in this area. Climate model projections forecast an increase of 4–5°C over the course of this century in the Mediterranean-European region, and although with less confidence in specific values, they also detect a robust signal of change in precipitation patterns. These changes will consist of a decrease in the amount and frequency of precipitation, and an intensification of extreme precipitation events. Macrochloa tenacissima is a rhizomatous, C3 perennial tussock grass widespread and endemic in Western Mediterranean drylands, and is one of the few species that is usually dominant in its community. The soil in the interspaces of these tussocks is frequently covered by biological soil crust (BSC or biocrust), a community of organisms made up of cyanobacteria, green algae, heterotrophic bacteria, microfungi, lichens and bryophytes. Therefore, the major goal of this thesis is to estimate the effects of climate change on the ecophysiology of these two dominant functional types of the semiarid Western Mediterranean region[ES]El cambio climático es un componente de los diversos cambios a escala planetaria que actualmente están teniendo lugar en el Sistema tierra, los cuales en su conjunto son referidos como cambio global. Sin embargo, el clima, como vector de flujos energía y materia, tiene la habilidad de interactuar con todos los elementos (incluyendo componentes naturales y sociales) del Sistema tierra, influenciando así también en sus dinámicas. La región Mediterránea de Europa ha sido identificada como uno de los puntos calientes de respuesta al cambio climático más relevantes, y en esta área los patrones de precipitación son los principales factores limitantes para la actividad humana y los ecosistemas naturales. Las proyecciones de los modelos climáticos predicen un incremento de temperatura de 4– 5ºC durante el curso de esta centuria en la región Mediterránea de Europa, y aunque con menor fiabilidad en un valor específico, también detectan una señal de cambio robusta en los patrones de precipitación. Estos cambios consistirán en una disminución de la cantidad y frecuencia de la precipitación, así como una intensificación de los eventos de precipitación extremos. Macrochloa tenacissima es una planta C3 herbácea perenne, cespitosa y rizomatosa ampliamente distribuida y endémica de las zonas secas del Mediterráneo Occidental, y es una de las pocas especies que normalmente es dominante en su comunidad. El suelo presente entre las macollas de esta especie está frecuentemente cubierto por costra biológica del suelo (CBS o biocostra), una comunidad de organismos compuesta por cianobacterias, algas verdes, bacterias heterotróficas, microhongos, líquenes y briófitos. Por lo tanto, el principal objetivo de esta tesis es estimar los efectos del cambio climático en la ecofisiología de estos dos tipos funcionales dominantes en la región semiárida del Mediterráneo Occidental.Este trabajo ha sido posible gracias a la concesión de una beca predoctoral en el marco del Programa “Junta de Ampliación de Estudios” (JAE) para ser desarrollado en la Estación Experimental de Zonas Áridas, instituto perteneciente al Consejo Superior de Investigaciones Científicas (EEZA-CSIC). El trabajo se ha financiado mayoritariamente por el proyecto PREVEA (CGL2007-63258/BOS) concedido por el Plan Estatal I+D+I del Ministerio de Economía y Competitividad. También ha sido parcialmente financiado por los proyectos CARBORAD (CGL2011-27493) y Bacarcos (CGL2011-29429) del Ministerio de Economía y Competitividad; MesoTopos (RNM 04023), COSTRAS (RNM-3614) y GEOCARBO (P08-RNM-3721) de la Consejería de Innovación, Ciencia y Empresa de la Junta de Andalucía; BIOCOM (ERC 242658) del Consejo Europeo de Investigación bajo el VII Programa Marco de la Comunidad Europea (FP7/2007-2013); SCIN-Soil Crust InterNational (PRI-PIMBDV-2011-0874), proyecto Europeo BIODIVERSA.Peer reviewe

Soil Ecosystem Responses to Climate Change and Land-Use Simulations and Estimation of Carbon Stocks in Steppe and Forest Ecosystems in Northern Mongolia

Northern Mongolia currently sequesters 31 Tg C yr-1 but it may become a carbon source if respiration rates increase due to climate change and overgrazing, or if projected boundary shifts between forest and steppe cause a change in the carbon storage of ecosystems. The objectives of the thesis are to study soil ecosystem response to simulated climate change and grazing, and to assess C stocks in the steppe and forest. Open-top chambers (OTCs) have been frequently used for simulating climate change. However, the pattern of temperature increase by OTCs contradicted the IPCC predictions. An alternative method, open-sided chambers (OSCs), was evaluated based on its effects on abiotic and biotic factors. The results indicated that OSCs manipulated air temperature in a pattern that was predicted by IPCC models, but the overall effect was too small, hence it is not an optimal device. In the subsequent study, OTCs were used to study soil respiration response to experimental warming in three ecosystems. Temperature increase by OTCs had no effect on soil respiration in the steppe but increased soil respiration in the forest (by 0.20 g CO2 m-2 h-1), demonstrating the importance of ecosystem setting. Although warming increased soil respiration, it decreased its temperature sensitivity as well (Q10 = 5.82 in control versus 2.22 in OTC). In addition to OTCs, watering and grazing effects on CO2 effluxes (ecosystem and soil respiration) were studied across the topographical gradients in the steppe. Our results show a robust, positive effect of soil moisture on CO2 effluxes across topography, and the contrasting effects of grazing on CO2 effluxes. Interactive effects of the treatments were minimal. Soil carbon of the forest was the same (8.3 kg C m-2) as the steppe (8.1 kg C m-2) but aboveground carbon in the forest (2.9 kg C m-2) was 3-7 times greater than that in the steppe. In summary, the results show that warming will slightly increase soil respiration in the forest, but in steppe precipitation will have stronger effect on CO2 flux than temperature change. The results also indicated that overgrazing and deforestation could trigger a greater loss of carbo

Interannual variation in seasonal drivers of soil respiration in a semi-arid Rocky Mountain meadow

pre-printSemi-arid ecosystems with annual moisture inputs dominated by snowmelt cover much of the western United States, and a better understanding of their seasonal drivers of soil respiration is needed to predict consequences of climatic change on soil CO2 efflux. We assessed the relative importance of temperature, moisture, and plant phenology on soil respiration during seasonal shifts between cold, wet winters and hot, dry summers in a Rocky Mountain meadow over 3.5 separate growing seasons. We found a consistent, unique pattern of seasonal hysteresis in the annual relationship between soil respiration and temperature, likely representative for this ecosystem type, and driven by (1) continued increase in soil T after summer senescence of vegetation, and (2) reduced soil respiration during cold, wet periods at the beginning versus end of the growing season. The timing of meadow senescence varied between years with amount of cold season precipitation, but on average occurred days before soil temperature peaked in late-summer. Autumn soil respiration was greatest when substantial autumn precipitation events occurred early. Surface CO2 efflux was temporarily decoupled from respiratory production during winter 2006/2007, due to effects of winter surface snow and ice on mediating the diffusion of CO2 from deep soil horizons to the atmosphere. Upon melt of a capping surface ice layer, release of soil-stored CO2 was determined to be 65 g C, or *10 % of the total growing season soil respiration for that year. The shift between soil respiration sources arising from moisture-limited spring plant growth and autumn decomposition indicates that annual mineralization of soil carbon will be less dependent on projected changes in temperature than on future variations in amount and timing of precipitation for this site and similar semiarid ecosystems

The impact of altered precipitation patterns on plant productivity, soil respiration and plant water-use efficiency in a northern Great Plains grassland

Precipitation patterns are expected to shift towards larger but fewer rain events, with longer intermittent dry periods, associated with climate change. The larger rain events may compensate for and help to mitigate climate change effects on key ecosystem functions in semi-arid grasslands. I experimentally manipulated the amount and frequency of simulated precipitation added to treatment plots that were covered by rain shelters, and measured the response in plant productivity, soil respiration and water-use efficiency in a native, grassland near Lethbridge, Alberta. The observed responses were compared to the predictions of a conceptual ecosystem response model developed by Knapp et al. 2008. Two experiments were conducted during 14 weeks of the growing season from May-August. The first experiment applied total growing season precipitation of 180 mm (climate normal), and the second experiment applied total precipitation of 90 mm (reduced amount). In both experiments, precipitation was applied at two frequencies, 1 rain event every week (normal frequency) and 1 rain event every two weeks (reduced frequency).Plant productivity decreased in response to larger but fewer rain events in the first experiment, but was not significantly different in the second experiment. Soil respiration rate was significantly higher for the larger but fewer rain events in the second experiment, as well as for the normal (NN+NR) compared to the reduced (RN+RR) amount treatments. Stable carbon isotope composition of plant tissue was largely insensitive to precipitation alterations, but showed significantly lower δ13C values for the normal compared to the reduced amount treatments. The results of this study have implications for understanding the mechanisms underlying ecosystem responses to anticipated precipitation change in the Great Plains.NSERC Discovery Gran