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Dive into the research topics where Laci M. Gerhart is active.

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Featured researches published by Laci M. Gerhart.


New Phytologist | 2010

Plant responses to low [CO2] of the past

Laci M. Gerhart; Joy K. Ward

During the Last Glacial Maximum (LGM; 18,000-20,000 yr ago) and previous glacial periods, atmospheric [CO(2)] dropped to 180-190 ppm, which is among the lowest concentrations that occurred during the evolution of land plants. Modern atmospheric CO(2) concentrations ([CO(2)]) are more than twice those of the LGM and 45% higher than pre-industrial concentrations. Since CO(2) is the carbon source for photosynthesis, lower carbon availability during glacial periods likely had a major impact on plant productivity and evolution. From the studies highlighted here, it is clear that the influence of low [CO(2)] transcends several scales, ranging from physiological effects on individual plants to changes in ecosystem functioning, and may have even influenced the development of early human cultures (via the timing of agriculture). Through low-[CO(2)] studies, we have determined a baseline for plant response to minimal [CO(2)] that occurred during the evolution of land plants. Moreover, an increased understanding of plant responses to low [CO(2)] contributes to our knowledge of how natural global change factors in the past may continue to influence plant responses to future anthropogenic changes. Future work, however, should focus more on the evolutionary responses of plants to changing [CO(2)] in order to account for the potentially large effects of genetic change.


New Phytologist | 2012

Glacial trees from the La Brea tar pits show physiological constraints of low CO2

Laci M. Gerhart; John M. Harris; Jesse B. Nippert; Darren R. Sandquist; Joy K. Ward

• While studies of modern plants indicate negative responses to low [CO₂] that occurred during the last glacial period, studies with glacial plant material that incorporate evolutionary responses are rare. In this study, physiological responses to changing [CO₂] were compared between glacial (La Brea tar pits) and modern Juniperus trees from southern California. • Carbon isotopes were measured on annual rings of glacial and modern Juniperus. The intercellular:atmospheric [CO₂] ratio (c(i) /c(a) ) and intercellular [CO₂] (c(i) ) were then calculated on an annual basis and compared through geologic time. • Juniperus showed constant mean c(i) /c(a) between the last glacial period and modern times, spanning 50,000 yr. Interannual variation in physiology was greatly dampened during the last glacial period relative to the present, indicating constraints of low [CO₂] that reduced responses to other climatic factors. Furthermore, glacial Juniperus exhibited low c(i) that rarely occurs in modern trees, further suggesting limiting [CO₂] in glacial plants. • This study provides some of the first direct evidence that glacial plants remained near their lower carbon limit until the beginning of the glacial-interglacial transition. Our results also suggest that environmental factors that dominate carbon-uptake physiology vary across geologic time, resulting in major alterations in physiological response patterns through time.


Biogeochemistry | 2014

Reconstructing terrestrial nutrient cycling using stable nitrogen isotopes in wood

Laci M. Gerhart; Kendra K. McLauchlan

Although recent anthropogenic effects on the global nitrogen (N) cycle have been significant, the consequences of increased anthropogenic N on terrestrial ecosystems are unclear. Studies of the impact of increased reactive N on forest ecosystems—impacts on hydrologic and gaseous loss pathways, retention capacity, and even net primary productivity—have been particularly limited by a lack of long-term baseline biogeochemical data. Stable nitrogen isotope analysis (ratio of 15N to 14N, termed δ15N) of wood chronologies offers the potential to address changes in ecosystem N cycling on millennial timescales and across broad geographic regions. Currently, nearly 50 studies have been published utilizing wood δ15N records; however, there are significant differences in study design and data interpretation. Here, we identify four categories of wood δ15N studies, summarize the common themes and primary findings of each category, identify gaps in the spatial and temporal scope of current wood δ15N chronologies, and synthesize methodological frameworks for future research by presenting eight suggestions for common methodological approaches and enhanced integration across studies. Wood δ15N records have the potential to provide valuable information for interpreting modern biogeochemical cycling. This review serves to advance the utility of this technique for long-term biogeochemical reconstructions.


Plant Physiology | 2016

Examining Plant Physiological Responses to Climate Change through an Evolutionary Lens

Katie M. Becklin; Jill T. Anderson; Laci M. Gerhart; Susana M. Wadgymar; Carolyn A. Wessinger; Joy K. Ward

Integrating knowledge from physiological ecology, evolutionary biology, phylogenetics, and paleobiology provides novel insights into factors driving plant physiological responses to both past and future climate change.


Scientific Reports | 2017

Centennial-scale reductions in nitrogen availability in temperate forests of the United States

Kendra K. McLauchlan; Laci M. Gerhart; John J. Battles; Joseph M. Craine; Andrew J. Elmore; Phil E. Higuera; Michelle M Mack; Brendan E. McNeil; David M. Nelson; Neil Pederson; Steven S. Perakis

Forests cover 30% of the terrestrial Earth surface and are a major component of the global carbon (C) cycle. Humans have doubled the amount of global reactive nitrogen (N), increasing deposition of N onto forests worldwide. However, other global changes—especially climate change and elevated atmospheric carbon dioxide concentrations—are increasing demand for N, the element limiting primary productivity in temperate forests, which could be reducing N availability. To determine the long-term, integrated effects of global changes on forest N cycling, we measured stable N isotopes in wood, a proxy for N supply relative to demand, on large spatial and temporal scales across the continental U.S.A. Here, we show that forest N availability has generally declined across much of the U.S. since at least 1850 C.E. with cool, wet forests demonstrating the greatest declines. Across sites, recent trajectories of N availability were independent of recent atmospheric N deposition rates, implying a minor role for modern N deposition on the trajectory of N status of North American forests. Our results demonstrate that current trends of global changes are likely to be consistent with forest oligotrophication into the foreseeable future, further constraining forest C fixation and potentially storage.


Oecologia | 2017

Warmest extreme year in U.S. history alters thermal requirements for tree phenology

Jacob M. Carter; Maria E. Orive; Laci M. Gerhart; Jennifer H. Stern; Renée M. Marchin; Joane Nagel; Joy K. Ward

The frequency of extreme warm years is increasing across the majority of the planet. Shifts in plant phenology in response to extreme years can influence plant survival, productivity, and synchrony with pollinators/herbivores. Despite extensive work on plant phenological responses to climate change, little is known about responses to extreme warm years, particularly at the intraspecific level. Here we investigate 43 populations of white ash trees (Fraxinus americana) from throughout the species range that were all grown in a common garden. We compared the timing of leaf emergence during the warmest year in U.S. history (2012) with relatively non-extreme years. We show that (a) leaf emergence among white ash populations was accelerated by 21 days on average during the extreme warm year of 2012 relative to non-extreme years; (b) rank order for the timing of leaf emergence was maintained among populations across extreme and non-extreme years, with southern populations emerging earlier than northern populations; (c) greater amounts of warming units accumulated prior to leaf emergence during the extreme warm year relative to non-extreme years, and this constrained the potential for even earlier leaf emergence by an average of 9 days among populations; and (d) the extreme warm year reduced the reliability of a relevant phenological model for white ash by producing a consistent bias toward earlier predicted leaf emergence relative to observations. These results demonstrate a critical need to better understand how extreme warm years will impact tree phenology, particularly at the intraspecific level.


Ecosystems | 2016

A Framework to Assess Biogeochemical Response to Ecosystem Disturbance Using Nutrient Partitioning Ratios

J. Marty Kranabetter; Kendra K. McLauchlan; Sara K. Enders; Jennifer M. Fraterrigo; Philip E. Higuera; Jesse L. Morris; Edward B. Rastetter; Rebecca T. Barnes; Brian Buma; Daniel G. Gavin; Laci M. Gerhart; Lindsey Gillson; Peter Hietz; Michelle C. Mack; Brenden E. McNeil; Steven S. Perakis

Disturbances affect almost all terrestrial ecosystems, but it has been difficult to identify general principles regarding these influences. To improve our understanding of the long-term consequences of disturbance on terrestrial ecosystems, we present a conceptual framework that analyzes disturbances by their biogeochemical impacts. We posit that the ratio of soil and plant nutrient stocks in mature ecosystems represents a characteristic site property. Focusing on nitrogen (N), we hypothesize that this partitioning ratio (soil N: plant N) will undergo a predictable trajectory after disturbance. We investigate the nature of this partitioning ratio with three approaches: (1) nutrient stock data from forested ecosystems in North America, (2) a process-based ecosystem model, and (3) conceptual shifts in site nutrient availability with altered disturbance frequency. Partitioning ratios could be applied to a variety of ecosystems and successional states, allowing for improved temporal scaling of disturbance events. The generally short-term empirical evidence for recovery trajectories of nutrient stocks and partitioning ratios suggests two areas for future research. First, we need to recognize and quantify how disturbance effects can be accreting or depleting, depending on whether their net effect is to increase or decrease ecosystem nutrient stocks. Second, we need to test how altered disturbance frequencies from the present state may be constructive or destructive in their effects on biogeochemical cycling and nutrient availability. Long-term studies, with repeated sampling of soils and vegetation, will be essential in further developing this framework of biogeochemical response to disturbance.


Scientific Reports | 2017

Changes in biomass allocation buffer low CO2 effects on tree growth during the last glaciation

Guangqi Li; Laci M. Gerhart; Sandy P. Harrison; Joy K. Ward; John M. Harris; I. Colin Prentice

Isotopic measurements on junipers growing in southern California during the last glacial, when the ambient atmospheric [CO2] (ca) was ~180 ppm, show the leaf-internal [CO2] (ci) was approaching the modern CO2 compensation point for C3 plants. Despite this, stem growth rates were similar to today. Using a coupled light-use efficiency and tree growth model, we show that it is possible to maintain a stable ci/ca ratio because both vapour pressure deficit and temperature were decreased under glacial conditions at La Brea, and these have compensating effects on the ci/ca ratio. Reduced photorespiration at lower temperatures would partly mitigate the effect of low ci on gross primary production, but maintenance of present-day radial growth also requires a ~27% reduction in the ratio of fine root mass to leaf area. Such a shift was possible due to reduced drought stress under glacial conditions at La Brea. The necessity for changes in allocation in response to changes in [CO2] is consistent with increased below-ground allocation, and the apparent homoeostasis of radial growth, as ca increases today.


Quaternary Geochronology | 2014

Ultrafiltration for asphalt removal from bone collagen for radiocarbon dating and isotopic analysis of Pleistocene fauna at the tar pits of Rancho La Brea, Los Angeles, California

Benjamin T. Fuller; Simon M. Fahrni; John M. Harris; Aisling B. Farrell; Joan Brenner Coltrain; Laci M. Gerhart; Joy K. Ward; R.E. Taylor; John Southon


Revista Mexicana De Biodiversidad | 2012

Climate-change and mass mortality events in overwintering monarch butterflies

Narayani Barve; Alvin J. Bonilla; Julia Brandes; J. Christopher Brown; Nathaniel A. Brunsell; Ferdouz V. Cochran; Rebecca J. Crosthwait; Jodi Gentry; Laci M. Gerhart; Trish Jackson; Anna J. Kern; Karen S. Oberhauser; Hannah L. Owens; A. Townsend Peterson; Alexis S. Reed; Jorge Soberón; Adam D. Sundberg; Linda M. Williams

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John M. Harris

Natural History Museum of Los Angeles County

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Hannah L. Owens

Florida Museum of Natural History

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