Laureano Gherardi Arbizu

Arizona State University - School of Life Sciences
Mail code 4601
Postdoctoral Research Assoc.
Post Doctoral Scholars
TEMPE Campus
Mailcode
4501

Research Interests

From water to nutrients and even to carbon dioxide, availability of resources is one of the major controls of ecosystem functioning. In this regard, multiple studies built theories and hypotheses on how availability of single or combined resources affected ecosystem structure and processes (1-3). However, most efforts have focused on the first moment of resource availability, the mean. My dissertation pursued the study of the second moment of precipitation, the effect of precipitation variance independent from changes in the mean on grassland aboveground net primary productivity and functional diversity. Climate change predictions of increased frequency of extreme events with uncertain changes in precipitation mean, support the importance of this largely overlooked threat of climate change on ecosystems.

I developed a rainfall manipulation system able to both reduce and increase precipitation on experimental plots that was of easy implementation, low cost and needed no connection to the power grid. I designed such system using rainout shelters for the rainfall interception component, and a solar powered automated irrigation system that would divert water collected by rainout shelters to irrigated plots (4).  Such innovation allowed me to carry out unprecedented PhD work that consisted of a six-year experiment where I manipulated incoming precipitation on 50 plots switching rainfall exclusion and irrigation treatments every year in order to increase interannual precipitation variation while maintaining average precipitation constant.

My first important finding was that interannual precipitation variance had a significant negative effect on productivity independent from precipitation amount (5). The impact was considerably large, productivity in high precipitation-variability treatments decreased 49% compared to the ambient precipitation treatment. Overall ecosystem response resulted from the aggregated response of individual plant functional types. Perennial grasses showed a negative response while shrubs increased productivity with precipitation variability. Opposite responses of different plant types suggested a potential plant-community composition change with unforeseen consequences.

I was also able to provide strong support for mechanisms behind observed responses to enhanced precipitation variability (5). The effect of precipitation variability on primary production resulted from the shape of the yearly-productivity response to annual precipitation. Total and perennial grass productivity responded in a saturating fashion peaking at precipitation levels similar to the long-term median with no further productivity increase at higher rainfall. Therefore, dry years resulted in relatively large productivity decrease while wet years did not produce proportionally large productivity increases explaining the negative effect of increased precipitation variability. Shrubs responded in a concave up fashion showing stable production during dry years that was similar to that of average years and increased productivity during wet years. Therefore, wet years led to ANPP increases that overcompensated small decreases caused by dry years explaining the positive effect of enhanced precipitation variability on shrubs. I concluded that interannual precipitation coefficient of variation has an effect itself on the functioning of this ecosystem. Such effect is mediated by differential responses of plant functional types to water availability. These findings have important ecological implications highlighting the importance of studying effects of resource availability variance itself, not only precipitation but all resources, on ecosystem functioning.

Since plant-functional-type responses played such a critical role explaining the effect of precipitation variance on productivity, I decided to study the effect of precipitation variability on functional diversity and its consequences for ecosystem functioning (6). I found that interannual precipitation variance had a positive effect on plant diversity. This was a transformative finding since most studies concentrate on the consequences of decreases in biodiversity for ecosystem stability.

Increases in functional diversity were due to a decrease in the relative abundance of perennial grasses and an increase in the proportion of rare plant-functional types. Enhanced functional diversity had a positive effect on ecosystem productivity that ameliorated, but did not fully compensate, the productivity reduction caused by direct effects of precipitation variability. On the other hand, increased functional diversity enhanced ecosystem stability. Rare-species-productivity variance decreased with increasing precipitation variance reducing ecosystem productivity variance over time (6). These results show that ecosystem response to climate change would include reduction in process rates but increases in diversity that buffer the overall impact.

The following step was to assess large-scale impacts of precipitation variability on primary production. In order to do this, I explored the effect of precipitation variability through abiotic mechanisms. By the means of a very interesting collaboration, we used a process-based ecosystem model to simulate water losses and soil-water availability at 35 grassland locations in the central US (7). We studied the abiotic effect of temporal scale and intensity of precipitation variability change on ecosystem water flows. We found that the scale of temporal variability had a larger effect on soil-water availability than the magnitude of variability, and that inter- and multi-annual variability had much larger effects than intra-annual variability. Further, the effect of precipitation variability was modulated by mean annual precipitation. Arid-semiarid locations showed increases in water availability as a result of enhanced precipitation variability while mesic locations showed a decrease in soil water implying that ANPP should increase in arid sites and decrease in mesic sites with precipitation variability.

My work used complementary tools from field experimentation to simulation modelling to address ecosystem response to enhanced precipitation variability. Field experiments provided an in-depth understanding of ecological mechanisms whereas, simulation modelling allowed me to extrapolate results from a single location to 35 locations at continental scale. Our novel understanding informs the management of grasslands that under enhanced precipitation variability will experience a decrease in productivity and increased diversity. Opposite responses of grasses and shrubs suggests that large areas of the grassland biome may shift to shrubland in the future, with large impacts on the supply of ecosystem services. Grasslands provide more and better forage for livestock production than shrublands. Livestock production is the major ecosystem service derived from grasslands, which cover 40% of the terrestrial earth surface. While enhanced climate variability may decrease one type of ecosystem service, livestock production; it may increase other such as conservation resulting from the positive effect of precipitation variability on biodiversity.

References

1.            O. E. Sala, L. A. Gherardi, L. G. Reichmann, E. Jobbagy, D. P. C. Peters, Legacies of precipitation fluctuations on primary production: Theory and data synthesis. Phil. Trans. R. Soc. B, 3135 (2012).

2.            A. K. Knapp, M. D. Smith, Variation among biomes in temporal dynamics of aboveground primary production. Science 291, 481 (2001).

3.            D. A. Wedin, D. Tilman, Influence of nitrogen loading and species composition on the carbon balance of grasslands. Science 274, 1720 (1996).

4.            L. A. Gherardi, O. E. Sala, Automated rainfall manipulation system: a reliable and inexpensive tool for ecologists. Ecosphere 4, art18 (2013/02/01, 2013).

5.            L. A. Gherardi, O. E. Sala, Enhanced precipitation variability decreases grass- and increases shrub-productivity. Proceedings of the National Academy of Sciences 112, 12735 (October 13, 2015, 2015).

6.            L. A. Gherardi, O. E. Sala, Enhanced interannual precipitation variability increases plant functional diversity that in turn ameliorates negative impact on productivity. Ecol. Lett. 18, 1293 (2015).

7.            O. Sala, L. Gherardi, D. C. Peters, Enhanced precipitation variability effects on water losses and ecosystem functioning: differential response of arid and mesic regions. Clim. Change 131, 213 (2015/04/03, 2015).

Courses

Spring 2018
Course Number Course Title
SOS 429 Human Impacts: Ecosys Function
BIO 429 Human Impacts: Ecosys Function
SOS 528 Human Impacts: Ecosys Function
BIO 528 Human Impacts: Ecosys Function
Spring 2017
Course Number Course Title
SOS 429 Human Impacts: Ecosys Function
BIO 429 Human Impacts: Ecosys Function
SOS 528 Human Impacts: Ecosys Function
BIO 528 Human Impacts: Ecosys Function
Spring 2016
Course Number Course Title
SOS 429 Human Impacts: Ecosys Function
BIO 429 Human Impacts: Ecosys Function
SOS 528 Human Impacts: Ecosys Function
BIO 528 Human Impacts: Ecosys Function
Spring 2015
Course Number Course Title
SOS 429 Human Impacts: Ecosys Function
BIO 429 Human Impacts: Ecosys Function
SOS 528 Human Impacts: Ecosys Function
BIO 528 Human Impacts: Ecosys Function
Fall 2013
Course Number Course Title
BIO 182 General Biology II

Presentations

Invited Lectures

Spring 2014 Ecosystem Ecology. Professor: Dr. Sharon Hall, guest lecturer on Effects of temporal variability on ecosystem response. Arizona State University, School of Life Sciences.

Spring 2013 Ecosystem Ecology. Professor: Dr. Osvaldo E. Sala, guest lecturer on Nitrogen use by plants. Arizona State University, School of Life Sciences.

Honors/Awards

2014 Completion RA Award. ASU, School of Life Sciences. PI: $11,084.50

Editorships

2016- present Subject Matter Editor, Ecosphere. Ecological Society of America. Editor in Chief: Dr. Debra Peters

Professional Associations

Ecological Society of America & Asociaci�n Argentina de Ecolog�a

Service

2013- present International Network of Next Generation Ecologist. Working group participant and node for South America

2013-present Asombro Institute for Science Education. Contributor on the development of K-12 activities based on current experimental data

2012-2014 Jornada Basin Long Term Ecological Research. Graduate student representative

2011-2014 Graduate and Professional Student Association. Research and travel grant proposal reviewer