Jordan G. Barr

Ecosystem carbon dioxide fluxes after disturbance in forests of North America

Disturbances are important for renewal of North American forests. Here we summarize more than 180 site years of eddy covariance measurements of carbon dioxide flux made at forest chronosequences in North America. The disturbances included stand-replacing fire (Alaska, Arizona, Manitoba, and Saskatchewan) and harvest (British Columbia, Florida, New Brunswick, Oregon, Quebec, Saskatchewan, and Wisconsin) events, insect infestations (gypsy moth, forest tent caterpillar, and mountain pine beetle), Hurricane Wilma, and silvicultural thinning (Arizona, California, and New Brunswick). Net ecosystem production (NEP) showed a carbon loss from all ecosystems following a stand-replacing disturbance, becoming a carbon sink by 20 years for all ecosystems and by 10 years for most. Maximum carbon losses following disturbance (g C m−2y−1) ranged from 1270 in Florida to 200 in boreal ecosystems. Similarly, for forests less than 100 years old, maximum uptake (g C m−2y−1) was 1180 in Florida mangroves and 210 in boreal ecosystems. More temperate forests had intermediate fluxes. Boreal ecosystems were relatively time invariant after 20 years, whereas western ecosystems tended to increase in carbon gain over time. This was driven mostly by gross photosynthetic production (GPP) because total ecosystem respiration (ER) and heterotrophic respiration were relatively invariant with age. GPP/ER was as low as 0.2 immediately following stand-replacing disturbance reaching a constant value of 1.2 after 20 years. NEP following insect defoliations and silvicultural thinning showed lesser changes than stand-replacing events, with decreases in the year of disturbance followed by rapid recovery. NEP decreased in a mangrove ecosystem following Hurricane Wilma because of a decrease in GPP and an increase in ER.

Controls on mangrove forest‐atmosphere carbon dioxide exchanges in western Everglades National Park

August 2005. Maximum daytime NEE ranged from −20 to −25 mmol (CO2 )m −2 s −1 between March and May. Respiration (Rd) was highly variable (2.81 ± 2.41 mmol (CO2) m −2 s −1 ), reaching peak values during the summer wet season. During the winter dry season, forest CO2 assimilation increased with the proportion of diffuse solar irradiance in response to greater radiative transfer in the forest canopy. Surface water salinity and tidal activity were also important controls on NEE. Daily light use efficiency was reduced at high (>34 parts per thousand (ppt)) compared to low (<17 ppt) salinity by 46%. Tidal inundation lowered daytime Rd by ∼0.9 mmol (CO2 )m −2 s −1 and nighttime Rd by ∼0.5 mmol (CO2 )m −2 s −1 . The forest was a sink for atmospheric CO2, with an annual NEP of 1170 ± 127 g C m −2 during 2004. This unusually high NEP was attributed to year‐round productivity and low ecosystem respiration which reached a maximum of only 3 g C m −2 d −1 . Tidal export of dissolved inorganic carbon derived from belowground respiration likely lowered the estimates of mangrove forest respiration. These results suggest that carbon balance in mangrove coastal systems will change in response to variable salinity and inundation patterns, possibly resulting from secular sea level rise and climate change.

Seasonal evapotranspiration patterns in mangrove forests

Diurnal and seasonal controls on water vapor fluxes were investigated in a subtropical mangrove forest in Everglades National Park, Florida. Energy partitioning between sensible and latent heat fluxes was highly variable during the 2004–2005 study period. During the dry season, the mangrove forest behaved akin to a semiarid ecosystem as most of the available energy was partitioned into sensible heat, which gave Bowen ratio values exceeding 1.0 and minimum latent heat fluxes of 5 MJ d−1. In contrast, during the wet season the mangrove forest acted as a well-watered, broadleaved deciduous forest, with Bowen ratio values of 0.25 and latent heat fluxes reaching 18 MJ d−1. During the dry season, high salinity levels (> 30 parts per thousand, ppt) caused evapotranspiration to decline and correspondingly resulted in reduced canopy conductance. From multiple linear regression, daily average canopy conductance to water vapor declined with increasing salinity, vapor pressure deficit, and daily sums of solar irradiance but increased with air temperature and friction velocity. Using these relationships, appropriately modified Penman-Monteith and Priestley-Taylor models reliably reproduced seasonal trends in daily evapotranspiration. Such numerical models, using site-specific parameters, are crucial for constructing seasonal water budgets, constraining hydrological models, and driving regional climate models over mangrove forests.

Inferring the source strength of isoprene from ambient concentrations

This paper reports on the application of an inverse Lagrangian technique that uses localized near-field (LNF) theory to calculate the source strength profile of isoprene from deciduous forest canopies. The basic tenet considered in this study is that the prevailing ambient isoprene concentrations observed over forests represent the source strength of the underlying surface as the scalar is transported from the sites of biosynthesis to the measurement point above forest canopies. Using information on the distribution of active isoprene biomass and the plant canopy environment, a two-storey canopy model was developed and applied to estimate isoprene emission rate profiles for a monoculture aspen forest whose isoprene source is homogeneously distributed throughout the landscape. Modelled results show that isoprene emission rates strongly vary with canopy depth, with maximum values coinciding with canopy layers with largest amount of active biomass. The model also captures the strong diurnal patterns of isoprene emissions from the forest canopy. We conclude that the present modelling system provides a practical method for estimating isoprene emission rate profiles based on the knowledge of atmospheric turbulence and ambient concentrations.

RED MANGROVES EMIT HYDROCARBONS

Abstract The objective of this study is to investigate hydrocarbon species and amounts released by red mangrove foliage and determine if these quantities warrant future research on atmospheric chemical processing of these compounds. The field investigation took place during July 2001 at Key Largo, Florida Bay, Florida. Foliage still attached to plants was enclosed in cuvettes while air of known flow rates circulated around leaves to study hydrocarbon emissions. Cuvette air samples underwent gas chromatographic analyses to determine species and amounts of hydrocarbons released by mangrove foliage. Red mangrove foliage emits isoprene and trace amounts of the monoterpenes of α-pinene, β-pinene, camphene, and d-limonene. The mangrove flowers released these latter compounds in amounts ranging from 0.5 to 10 mg (monoterpene) per gram of dry biomass per hour. These fluxes are normalized to the foliage temperature of 30 °C. When normalized to the foliage temperature of 30 °C and light levels of 1000 µmol m−2 s−1, isoprene emission rates as high as 0.092 ± 0.109 µg (isoprene) per gram of dry biomass per hour were measured. Compared to terrestrial forest ecosystems, red mangroves are low isoprene emitters. During peak flowering periods in the summertime, however, red mangroves may emit sufficient amounts of monoterpenes to alter ground-level ozone concentrations and contribute to biogenic aerosol formation.

Radiative forcing of phytogenic aerosols

gave rise to maximum phytogenic aerosol concentrations of circa 5000 particles per cm 3 . The amount of diffuse and attenuated solar irradiance resulting from the interaction of aerosols with incoming irradiance was quantified using a one-dimensional spectral radiative transfer model and measured aerosol sized distributions and concentrations. Results show that aerosols in the atmospheric boundary layer contributed to only moderate levels of diffuse irradiance but generated substantial attenuation of the incoming solar irradiance stream. For the irradiance levels measured in eastern Canada during cloudless days in July and with aerosol concentrations in the range of 2000 to 5000 particles per cm 3 , average daytime solar irradiance attenuation amounted to 0.04 W m � 2 with a diffuse component of 0.01 W m � 2 . The maximum solar irradiance extinction reached 0.2 W m � 2 . Assuming a uniform spatial aerosol distribution, this negative radiative influence could offset substantial fractions of the regional thermal forcing resulting from increased levels of greenhouse gases such as carbon dioxide. It is concluded that greater radiative influences (cooling) could be present over regions dominated by hydrocarbon productive forest ecosystems. INDEX TERMS: 0315 Atmospheric Composition and Structure: Biosphere/atmosphere interactions; 0322 Atmospheric Composition and Structure: Constituent sources and sinks; 0330 Atmospheric Composition and Structure: Geochemical cycles; 3307 Meteorology and Atmospheric Dynamics: Boundary layer processes; KEYWORDS: biogenic hydrocarbons, biogenic aerosols, radiative forcing, energy balance, Mie scattering, monoterpenes

Differing temporal patterns of Chara hornemannii cover correlate to alternate regimes of phytoplankton and submerged aquatic-vegetation dominance

Cover of the alga Chara hornemannii Wallman and water-quality parameters were measured over a 3-year period in adjacent mangrove subestuaries in Florida Bay, so as to describe temporal variability and infer relationships betweenCharacoverandwaterqualitythatwillassistresourcemanagerstorestoreCharaabundancetohistoricallyhigher levels. A seasonal pattern of Chara cover was observed in the Alligator Creek subestuary that coincided with seasonal changes in water transparency in a relatively high-nutrient and phytoplankton environment. In contrast, higher Chara cover in the relatively low-nutrient and phytoplankton-abundance McCormick Creek subestuary did not exhibit a repeatable seasonal pattern, but was temporally negatively correlated with salinity and water depth. These observations suggest that water transparency may determine the importance of the salinity driver in these Chara communities. The present study demonstrates the differential importance of water quality and environmental drivers in estuaries distinguished by alternate regimes of phytoplankton and submerged aquatic-vegetation (SAV) dominance, and explains how differences in local estuarine geography may filter the response of SAV communities to environmental stressors.

AmeriFlux US-Skr Shark River Slough (Tower SRS-6) Everglades

This is the AmeriFlux version of the carbon flux data for the site US-Skr Shark River Slough (Tower SRS-6) Everglades. Site Description - The Florida Everglades Shark River Slough Mangrove Forest site is located along the Shark River in the western region of Everglades National Park. Also referred to as site SRS6 of the Florida Coastal Everglades LTER program, freshwater in the mangrove riverine floods the forest floor under a meter of water twice per day. Transgressive discharge of freshwater from the Shark river follows annual rainfall distributions between the wet and dry seasons. Hurricane Wilma struck the site in October of 2005 causing significant damage. The tower was offline until the following October in order to continue temporally consistent measurements. In post-hurricane conditions, ecosystem respiration rates and solar irradiance transfer increased. 2007- 2008 measurements indicate that these factors led to an decline in both annual -NEE and daily NEE from pre-hurricane conditions in 2004-2005.

Understanding Coastal Carbon Cycling by Linking Top- Down and Bottom-Up Approaches

The coastal zone, despite occupying a small fraction of the Earths surface area, is an important component of the global carbon (C) cycle. Coastal wetlands, including mangrove forests, tidal marshes, and seagrass meadows, compose a domain of large reservoirs of biomass and soil C [Fourqurean et al., 2012; Donato et al., 2011; Pendleton et al., 2012; Regnier et al., 2013; Bauer et al., 2013]. These wetlands and their associated C reservoirs (2 to 25 petagrams C; best estimate of 7 petagrams C [Pendleton et al., 2012]) provide numerous ecosystem services and serve as key links between land and ocean.