Henry W. Loescher

ENVIRONMENTAL CONTROLS OVER NET EXCHANGES OF CARBON DIOXIDE FROM CONTRASTING FLORIDA ECOSYSTEMS

Net CO2 exchange estimated using eddy covariance and relaxed eddy accumulation indicated that evergreen pine upland and deciduous cypress wetland ecosystems in north-central Florida had similar apparent light compensation points during the growing season (125 vs. 150 μmol PPFD·m−2·s−1), but that maximum rates at 1800 μmol PPFD·m−2·s−1 at the cypress ecosystem were only 59% of those at the pine ecosystem (8.9 vs. 15.2 μmol CO2·m−2·s−1). During both the summer and winter months at the pine ecosystem, net CO2 exchange in the daytime was a curvilinear function of PPFD, with no significant seasonal differences in slope or intercept. In contrast, net CO2 exchange at the cypress ecosystem was minimal during the daytime in the winter. Net CO2 exchange during the nighttime was an exponential function of air temperature at both sites, with Q10 values of 2.0 and 1.9 for the pine and cypress ecosystems, respectively. Lower nighttime fluxes of CO2 occurred at the cypress ecosystem across the entire temperature range. Both of these relatively sparse canopies stored CO2 during stable atmospheric conditions. Mean maximum net CO2 exchange during the daytime and mean nighttime net CO2 exchange for these ecosystems were highly contrasting, and together resulted in a relatively low rate of annual carbon accumulation in the wetland when compared to the aggrading pine ecosystem. However, values reported here are within the ranges of values for other boreal, temperate, and tropical forest ecosystems.

Taking the pulse of a continent: expanding site‐based research infrastructure for regional‐ to continental‐scale ecology

Many of the most dramatic and surprising effects of global change on ecological systems will occur across large spatial extents, from regions to continents. Multiple ecosystem types will be impacted across a range of interacting spatial and temporal scales. The ability of ecologists to understand and predict these dynamics depends, in large part, on existing site-based research infrastructures developed in response to historic events. Here we review how unevenly prepared ecologists are, and more generally, ecology is as a discipline, to address regional- to continental-scale questions given these pre-existing site-based capacities, and we describe the changes that will be needed to pursue these broad-scale questions in the future. We first review the types of approaches commonly used to address questions at broad scales, and identify the research, cyber-infrastructure, and cultural challenges associated with these approaches. These challenges include developing a mechanistic understanding of the drivers and responses of ecosystem dynamics across a large, diverse geographic extent where measurements of fluxes or flows of materials, energy or information across levels of biological organization or spatial units are needed. The diversity of methods, sampling protocols, and data acquisition technologies make post-hoc comparisons of ecosystems challenging, and data collected using standardized methods across sites require coordination and teamwork. Sharing of data and analytics to create derived data products are needed for multi-site studies, but this level of collaboration is not part of the current ecological culture. We then discuss the strengths and limitations of current site-based research infrastructures in meeting these challenges, and describe a path forward for regional- to continental-scale ecological research that integrates existing infrastructures with emerging and potentially new technologies to more effectively address broad-scale questions. This new research infrastructure will be instrumental in developing an “uber network” to allow users to seamlessly identify and select, analyze, and interpret data from sites regardless of network affiliation, funding agency, or political affinity, to cover the spatial variability and extent of regional-to continental-scale questions. Ultimately, scientists must network across institutional boundaries in order to tap and expand these existing network infrastructures before these investments can address critical broad-scale research questions and needs.

Biogenic volatile organic compound emissions from a lowland tropical wet forest in Costa Rica

Atmospheric Environment 36 (2002) 3793–3802 Biogenic volatile organic compound emissions from a lowland tropical wet forest in Costa Rica Chris Geron a, *, Alex Guenther b , Jim Greenberg b , Henry W. Loescher c , Deborah Clark d , Brad Baker e a National Risk Management Research Laboratory, United States Environmental Protection Agency, Research Triangle Park, NC 27711, USA b National Center for Atmospheric Research, Boulder, CO 80303, USA c School of Forest Resources and Conservation, University of Florida, Gainesville, FL 32611, USA d Department of Biology, University of Missouri-St. Louis, St. Louis, MS 63121, USA e South Dakota School of Mines and Technology, Rapid City, SD 57701, USA Received 10 August 2001; accepted 17 April 2002 Abstract Twenty common plant species were screened for emissions of biogenic volatile organic compounds (BVOCs) at a lowland tropical wet forest site in Costa Rica. Ten of the species examined emitted substantial quantities of isoprene. These species accounted for 35–50% of the total basal area of old-growth forest on the major edaphic site types, indicating that a high proportion of the canopy leaf area is a source of isoprene. A limited number of canopy-level BVOC flux measurements were also collected by relaxed eddy accumulation (REA). These measurements verify that the forest canopy in this region is indeed a significant source of isoprene. In addition, REA fluxes of methanol and especially acetone were also significant, exceeding model estimates and warranting future investigation at this site. Leaf monoterpene emissions were non-detectable or very low from the species surveyed, and ambient concentrations and REA fluxes likewise were very low. Although the isoprene emission rates reported here are largely consistent with phylogenetic relations found in other studies (at the family, genus, and species levels), two species in the family Mimosaceae, a group previously found to consist largely of non-isoprene emitters, emitted significant quantities of isoprene. One of these, Pentaclethra macroloba (Willd.) Kuntze, is by far the most abundant canopy tree species in the forests of this area, composing 30–40% of the total basal area. The other, Zygia longifolia (Humb. & Bonpl.) Britton & Rose is a common riparian species. Our results suggest that the source strength of BVOCs is important not only to tropical atmospheric chemistry, but also may be important in determining net ecosystem carbon exchange. Published by Elsevier Science Ltd. Keywords: Isoprene; Biogenic volatile organic compounds; Relaxed eddy accumulation; Pentaclethra macroloba; Palmae; La selva biological station 1. Introduction Isoprene emission from vegetation is the world’s largest known source of non-methane volatile organic *Corresponding author. Tel.: +1-919-541-4639; fax: +1- E-mail address: geron.chris@epa.gov (C. Geron). 1352-2310/02/

Spatial variation of throughfall volume in an old-growth tropical wet forest, Costa Rica

- see front matter Published by Elsevier Science Ltd. PII: S 1 3 5 2 - 2 3 1 0 ( 0 2 ) 0 0 3 0 1 - 1 compounds (NMVOCs). Its importance is further amplified by its high reactivity with the hydroxyl radical compared to other abundant atmospheric NMVOCs. It is estimated that over 90% of isoprene emission is from vegetation (Guenther et al., 1995). Modeling (Guenther et al., 1995) and limited measurement (Guenther et al., 1999, 1996) studies have suggested that at least 50% of the global annual isoprene flux is from tropical

Controls on carbon dynamics by ecosystem structure and climate for southeastern U.S. slash pine plantations

Throughfall volume and interception of bulk precipitation events were measured during individual rain events of differing magnitudes in a primary wet tropical forest at La Selva Biological Station, Costa Rica. The relationship between canopy structure and throughfall were examined to identify key sources of spatial variation. Geostatistical analyses were also used to examine the spatial variation in throughfall, spatial autocorrelation and to determine minimum distances for independence of collectors. Throughfall volume was collected from 56 ground-based (funnel-style) collectors. Throughfall was collected for 26 separate precipitation events during July and August 1998. Per cent cover, distance to nearest tree, distance to nearest leaf were also estimated for each collection point. A weak relationship was found with per cent cover (r 2 = 0.11). No relationship was found between throughfall and distance to the nearest leaf above the collector. Estimated interception was 1.88 mm (r 2 = 0.94) with increased variance as bulk precipitation increased. A range distance of 45 m was estimated from variograms, strongly suggesting that large tree canopies and gaps are the source of much of the spatial variance in throughfall volume. Interception was reduced by 19% if only spatially independent collectors were used.

Design of the AmeriFlux Portable Eddy Covariance System and Uncertainty Analysis of Carbon Measurements

Planted pine forests (plantations) in the southeastern United States are an important component of the continents carbon balance. Forest carbon dynamics are affected by a range of factors including climatic variability. Multiyear droughts have affected the region in the past, and an increase in the frequency of drought events has been predicted. How this increased climatic variability will affect the capacity of the regions pine plantations to sequester carbon is not known. We used eddy covariance and biometric approaches to measure carbon dynamics over nine years in two slash pine plantations (Pinus elliottii var elliottii Englm) in north Florida, consisting of a newly planted and a mid-rotation stand. During this time, the region experienced two multiyear droughts (1999-2002 and 2006-2008), separated by a three-year wet period. Net ecosystem carbon accumulation measured using both approaches showed the same trends and magnitudes during plantation development. The newly planted site released 15.6 Mg C/ha during the first three years after planting, before becoming a carbon sink in year 4. Increases in carbon uptake during the early stages of stand development were driven by the aggrading leaf area index (LAI). After canopy closure, both sites were continuous carbon sinks with net carbon uptake (NEE) fluctuating between 4 and ; 8M g Cha � 1 � yr � 1 , depending on environmental conditions. Drought reduced NEE by ;25% through its negative effects on both LAI and radiation-use efficiency, which resulted in a larger impact on gross ecosystem carbon exchange than on ecosystem respiration. While results indicate that responses to drought involved complex interactions among water availability, LAI, and radiation-use efficiency, these ecosystems remain carbon sinks under current management strategies and climatic variability. Variation within locations is primarily due to major disturbances, such as logging in the current study and, to a much lesser extent, local environmental fluctuations. When data from this study are compared to flux data from a broad range of forests worldwide, these ecosystems fill a data gap in the warm-temperate zone and support a broad maximum NEE (for closed-canopy forests) between 88C and 208C mean annual temperature.

Interannual variation of carbon fluxes from three contrasting evergreen forests: the role of forest dynamics and climate

The AmeriFlux network continues to improve the understanding of carbon, water, and energy fluxes across temporal and spatial scales. The network includes 120 research sites that contribute to the understanding of processes within and among ecosystems. To improve the networks ability and confidence to synthesize data across multiple sites, the AmeriFlux quality assurance and quality control laboratory was established to reduce the within- and among-site uncertainties. This paper outlines the design of the portable eddy covariance system (PECS) and subsequent data processing procedures used for site comparisons. Because the PECS makes precision measurements of atmospheric CO2, the authors also present the results of uncertainty analyses in determining the polynomials for an infrared gas analyzer, estimating the CO2 in secondary standards, and estimating ambient CO2 in field measurements. Under field conditions, drift in the measurement of CO2 increased the uncertainty in flux measurements across 7 days by 5% and was not dependent on the magnitude or direction of the flux. The maximum relative flux measurement error for unstable conditions was 10.03 mol CO2 m 2 s 1 .

Spatial Variation in Soil Properties among North American Ecosystems and Guidelines for Sampling Designs

Interannual variation of carbon fluxes can be attributed to a number of biotic and abiotic controls that operate at different spatial and temporal scales. Type and frequency of disturbance, forest dynamics, and climate regimes are important sources of variability. Assessing the variability of carbon fluxes from these specific sources can enhance the interpretation of past and current observations. Being able to separate the variability caused by forest dynamics from that induced by climate will also give us the ability to determine if the current observed carbon fluxes are within an expected range or whether the ecosystem is undergoing unexpected change. Sources of interannual variation in ecosystem carbon fluxes from three evergreen ecosystems, a tropical, a temperate coniferous, and a boreal forest, were explored using the simulation model STANDCARB. We identified key processes that introduced variation in annual fluxes, but their relative importance differed among the ecosystems studied. In the tropical site, intrinsic forest dynamics contributed approximately 30% of the total variation in annual carbon fluxes. In the temperate and boreal sites, where many forest processes occur over longer temporal scales than those at the tropical site, climate controlled more of the variation among annual fluxes. These results suggest that climate-related variability affects the rates of carbon exchange differently among sites. Simulations in which temperature, precipitation, and radiation varied from year to year (based on historical records of climate variation) had less net carbon stores than simulations in which these variables were held constant (based on historical records of monthly average climate), a result caused by the functional relationship between temperature and respiration. This suggests that, under a more variable temperature regime, large respiratory pulses may become more frequent and high enough to cause a reduction in ecosystem carbon stores. Our results also show that the variation of annual carbon fluxes poses an important challenge in our ability to determine whether an ecosystem is a source, a sink, or is neutral in regard to CO2 at longer timescales. In simulations where climate change negatively affected ecosystem carbon stores, there was a 20% chance of committing Type II error, even with 20 years of sequential data.

Enhancing Water Cycle Measurements for Future Hydrologic Research

Soils are highly variable at many spatial scales, which makes designing studies to accurately estimate the mean value of soil properties across space challenging. The spatial correlation structure is critical to develop robust sampling strategies (e.g., sample size and sample spacing). Current guidelines for designing studies recommend conducting preliminary investigation(s) to characterize this structure, but are rarely followed and sampling designs are often defined by logistics rather than quantitative considerations. The spatial variability of soils was assessed across ∼1 ha at 60 sites. Sites were chosen to represent key US ecosystems as part of a scaling strategy deployed by the National Ecological Observatory Network. We measured soil temperature (Ts) and water content (SWC) because these properties mediate biological/biogeochemical processes below- and above-ground, and quantified spatial variability using semivariograms to estimate spatial correlation. We developed quantitative guidelines to inform sample size and sample spacing for future soil studies, e.g., 20 samples were sufficient to measure Ts to within 10% of the mean with 90% confidence at every temperate and sub-tropical site during the growing season, whereas an order of magnitude more samples were needed to meet this accuracy at some high-latitude sites. SWC was significantly more variable than Ts at most sites, resulting in at least 10× more SWC samples needed to meet the same accuracy requirement. Previous studies investigated the relationship between the mean and variability (i.e., sill) of SWC across space at individual sites across time and have often (but not always) observed the variance or standard deviation peaking at intermediate values of SWC and decreasing at low and high SWC. Finally, we quantified how far apart samples must be spaced to be statistically independent. Semivariance structures from 10 of the 12-dominant soil orders across the US were estimated, advancing our continental-scale understanding of soil behavior.

Seasonal patterns in energy partitioning of two freshwater marsh ecosystems in the Florida Everglades

The Consortium of Universities for the Advancement of Hydrologic Sciences, Inc., established the Hydrologic Measurement Facility to transform watershed-scale hydrologic research by facilitating access to advanced instrumentation and expertise that would not otherwise be available to individual investigators. We outline a committee-based process that determined which suites of instrumentation best fit the needs of the hydrological science community and a proposed mechanism for the governance and distribution of these sensors. Here, we also focus on how these proposed suites of instrumentation can be used to address key scientific challenges, including scaling water cycle science in time and space, broadening the scope of individual subdisciplines of water cycle science, and developing mechanistic linkages among these subdisciplines and spatiotemporal scales.