Joseph O'Brien | The University of Georgia (original) (raw)

Papers by Joseph O'Brien

Applied Vegetation Science, 2013

Aim: Non-destructive methods for quantifying above-ground plant biomass are important tools in ma... more Aim: Non-destructive methods for quantifying above-ground plant biomass are important tools in many ecological studies and management endeavours, but estimation methods can be labour intensive and particularly difficult in structurally diverse vegetation types. We aimed to develop a low-cost, but reasonably accurate, estimation technique within early-successional Bahamian broadleaf shrub vegetation. Location: Eleuthera, The Bahamas. Methods: Our biomass estimation technique was based on a visual obstruction method originally developed for use in grassland vegetation, but modified to suit our mixed vegetation structure. Visual obstruction measures were recorded for a total of 90 0.25-m 2 quadrats from which all standing vegetation was subsequently collected, dried and weighed. Regression models were then developed to predict vegetation dry weight (biomass) from visual obstruction. Results: The field time and equipment costs required to obtain our visual obstruction measures were low, and the accuracy of our above-ground plant biomass predictions was on par with that reported for other estimation techniques. Should increased accuracy or additional structural information be desired, our basic technique can be enhanced by adding other easily obtained measures (e.g. woody stem circumference). Conclusions: Our low-cost technique yields reasonably accurate estimates of above-ground plant biomass and should be transferable to any shrubland vegetation type where the maximum height of vegetation is typically below 2 m.

Biogeochemistry, 2003

Our objectives were to quantify and compare soil CO2 efflux of two dominant soil types in an old-... more Our objectives were to quantify and compare soil CO2 efflux of two dominant soil types in an old-growth neotropical rain forest in the Atlantic zone of Costa Rica, and to evaluate the control of environmental factors on CO2 release. We measured soil CO2 efflux from eight permanent soil chambers on six Oxisol sites. Three sites were developed on old river

The Wilson Journal of Ornithology, 2006

Cuban Parrots (Amazona leucocephala) on the island of Great Abaco in the Bahamas forage and nest ... more Cuban Parrots (Amazona leucocephala) on the island of Great Abaco in the Bahamas forage and nest in native pine forests. The population is unique in that the birds nest in limestone solution holes on the forest floor. Bahamian pine forests are fire-dependent with a frequent surface fire regime. The effects of fire on the parrots, especially while nesting, are not well known. We measured ambient conditions inside a cavity characteristic of the Cuban Parrot's Abaconian population as a prescribed fire passed over it. Cavity conditions were relatively benign; although temperatures immediately outside the cavity rose to Ͼ800Њ C, inside temperatures increased only 5Њ C at 30 cm inside the entrance and 0.4Њ C at the cavity floor (cavity depth was ϳ120 cm). CO 2 levels briefly rose to 2,092 ppm as the flames passed, but dropped to nearly ambient levels approximately 15 min later. Smoke levels also were elevated only briefly, with 0.603 mg of total suspended particulates filtered from 0.1 m 3 of air. Smokey conditions lasted approximately 20 min.

Soil Science Society of America Journal, 2000

tent. Recently, a FDR sensor was developed for continuous measurement of soil water content. This... more tent. Recently, a FDR sensor was developed for continuous measurement of soil water content. This system Recently, a frequency domain reflectometry (FDR) was developed uses the dielectric properties of water but in a different for measuring soil water content. It has a multivibrator that sends electromagnetic waves along its probes, and it measures the frequency approach than TDR (Bilskie, 1997). Time domain reof the reflected wave, which varies with water content. This FDR flectometry measures the apparent dielectric permittivsensor has several advantages over time domain reflectometry (TDR); ity of soil (K a) using a cable tester. As the dielectric it is less expensive, has a lower power consumption, and continuous permittivity of water is an order of magnitude greater monitoring of soil moisture at several remote locations is easily autothan other soil constituents, changes in K a can be attribmated using dataloggers. Our goal was to derive a calibration function uted to changes in soil water content in nonexpanding for the FDR sensor with the following criteria: it should be applicable soils. The FDR sensor, on the other hand, sends an to soils with high clay and organic matter contents and with bulk electromagnetic wave along its probes and measures the densities between 0.7 and 1.1 g cm Ϫ3. We used undisturbed soil samples frequency of the reflected wave, which varies with water to account for the natural heterogeneity in soils. Our results show content. In contrast to TDR, where K a is obtained, the that the calibration functions derived from a three-phase mixing model performed better than the manufacturer's empirically derived function output frequency of the FDR sensor has no direct physifor the soil volumetric content () range of 0.45 to 0.70 m 3 m Ϫ3 .

Plant, Cell and Environment, 2004

In order to quantify and characterize the variance in rainforest tree physiology, whole tree sap ... more In order to quantify and characterize the variance in rainforest tree physiology, whole tree sap flow responses to local environmental conditions were investigated in 10 species of trees with diverse traits at La Selva Biological Station, Costa Rica. A simple model was developed to predict tree sap flow responses to a synthetic environmental variable generated by a principle components analysis. The best fit was obtained with a sigmoid function which explained between 74 and 93% of the variation in sap flux of individual trees. Sap flow reached an asymptote where higher light and evaporative demand did not cause sap flux to increase further. Soil moisture had little influence on sap flux. The morphological characteristics of the trees significantly affected sap flow; taller trees responded to changes in environmental variables sooner than shorter trees and high liana cover buffered tree sap flow responses to weather. The effect of species-specific differences on the model was small; the mean effectiveness of the model was reduced by 6% when parameters were estimated from a single pool of measurements taken from all individuals. The results indicate that sap flow response could be effectively estimated using a simple general model and composite environmental index for these 10 diverse tree species.

Oecologia, 1992

Predawn water potential of representative plant species, together with stable isotope composition... more Predawn water potential of representative plant species, together with stable isotope composition of stem water and potential water sources were investigated in four low-elevation tropical hardwood hammocks in the Lower Florida Keys, during a one year period. Hammock species had the lowest water potentials when soil water content was low and/or soil salinity was high, but differences in groundwater salinity had no effect on the water potential. Comparison of D/H ratio of plant stem water with soil and ground water corroborates the conclusion that they are primarily utilizing soil water and not groundwater. Thus, tropical hardwood hammocks are buffered from saline groundwater, and are able to thrive in areas where groundwater salinity is as high as 25%~ The effect of sea level rise on these forests may depend more on changes in the frequency of tidal inundation of the soil surface than on changes in groundwater salinity.

Oecologia, 1991

Salinity and isotope ratios were determined in water from several wells in the Florida Keys, and ... more Salinity and isotope ratios were determined in water from several wells in the Florida Keys, and tidal inlets. Both D/H and lsO/160 ratios of water from wells and tidal inlets were highly correlated to their salinity. Water from standing pools was enriched in deuterium and oxygen-18 relative to their salinity because of evaporation processes, l sO/160 and D/H ratios of stem water from plants of several different communities at Sugar Loaf Key, ranging from hardwood hammocks to mangroves, were highly correlated to their predawn water potential. The correlation was consistent with the presence of high salinity in waters with high 180 and D content. Most individuals from each community were either utilizing water with isotopic characteristics typical of freshwater or of ocean water, while only a few individuals had stem water with isotopic ratios intermediate to these two water sources.

Fire Ecology, 2005

A variety of techniques that estimate temperature and/or heat output during fires are available. ... more A variety of techniques that estimate temperature and/or heat output during fires are available. We assessed the predictive ability of metal and tile pyrometers, calorimeters of different sizes, and fuel consumption to time-temperature metrics derived from thick and thin thermocouples at 140 points distributed over 9 management-scale burns in a longleaf pine forest in the southeastern US. While all of these devices underestimate maximum flame temperatures, we found several to be useful for characterizing other metrics of fire behavior. While the degree to which thermocouples underestimated maximum temperatures was based on thickness, metrics derived from thermocouple data that integrated time and temperature minimized this discrepancy between thin and thick thermocouples. Thermocouples also provided the most detailed spatial and temporal data of the devices tested. Pyrometers underestimated maximum temperatures relative to thermocouples, but due to their low cost, can be useful for examining spatial variation in temperature during fires. Use of calorimeters is disadvantageous given their lack of precision and high labor cost. Simple fire behavior observations taken during burns and indicators of fire severity observed post-burn were inexpensive to estimate and revealed useful differences among fires. Due to the wide variation among these techniques in cost, labor, accuracy, and level of detail of results, their suitability for a particular project will vary according to research objectives and available resources. Researchers should ensure that the fire behavior parameter measured has a logical relationship to the effect of interest, is measured at an appropriate level of detail, and is reported with attention to the limitations of the measuring devices used.

Climatic Change, 2011

Low-elevation islands face threats from sea level rise (SLR) and increased storm intensity. Evide... more Low-elevation islands face threats from sea level rise (SLR) and increased storm intensity. Evidence of endangered species' population declines and shifts in vegetation communities are already underway in the Florida Keys. SLR predictions indicate large areas of these habitats may be eliminated in the next century. Using the Florida Keys as a model system, we present a process for evaluating conservation options for rare and endemic taxa. Considering species characteristics and habitat, we

Canadian Journal of Forest Research, 2000

This study presents the results of a comparison of digital and film hemispherical photography as ... more This study presents the results of a comparison of digital and film hemispherical photography as means of characterizing forest light environments and canopy openness. We also compared hemispherical photography to spherical densiometry. Our results showed that differences in digital image quality due to the loss of resolution that occurred when images were processed for computer analysis did not affect estimates of unweighted openness. Weighted openness and total site factor estimates were significantly higher in digital images compared with film photos. The differences between the two techniques might be a result of underexposure of the film images or differences in lens optical quality and field of view. We found densiometer measurements significantly increased in consistency with user practice and were correlated with total site factor and weighted-openness estimates derived from hemispherical photography. Digital photography was effective and more convenient and inexpensive than...

Biotropica, 2007

The relationship between phenology and tree stem diameter increment is largely unexplored in trop... more The relationship between phenology and tree stem diameter increment is largely unexplored in tropical species, especially in wet tropical forests. To explore links between these phenomena, we measured stem diameter increment and phenology of ten canopy tree species from a range of functional types in the Atlantic lowlands of Costa Rica to test for seasonal and interannual patterns. We measured stem diameter increment using band dendrometers and visually assessed leaf and reproductive phenology monthly from 1997 to 2000. We categorized the species into groups based on patterns of leaf exchange and reproduction. Species were either deciduous with synchronous or asynchronous leaf drop, or evergreen with continuous or seasonal leaf flushing. Flowering occurred supra-annually, annually, or continuously. Of the ten species studied, four species, Cecropia insignis, Dipteryx panamensis, Lecythis ampla, and Simarouba amara, had consistent seasonal stem diameter increment patterns in both years. Dipteryx panamensis and L. ampla were deciduous with synchronized leaf drop. Cecropia insignis was evergreen and produced new leaves continuously. Simarouba amara, also evergreen, exchanged leaves over a brief period once a year. We tested whether stem diameter increment was correlated to phenology using logistic regression. Leaflessness significantly explained patterns in stem diameter increment but reproductive phenology did not. Deciduous trees were 2.6-9.3 times more likely to grow less than average the month following leaffall than in months when trees had full crowns.

Journal of Forestry, 2009

Fire is a dominant disturbance within many forested ecosystems worldwide. Understanding the compl... more Fire is a dominant disturbance within many forested ecosystems worldwide. Understanding the complex feedbacks among vegetation as a fuel for fire, the effects of fuels on fire behavior, and the impact of fire behavior on future vegetation are critical for sustaining biodiversity in fire-dependent forests. Nonetheless, understanding in fire ecology has been limited in part by the difficulties in establishing the connections between fire behavior and vegetation response. To address this issue, we present the concept of the ecology of fuels, which emphasizes the critical role that fuels play in conceptually linking feedbacks between fire and vegetation. This article explores the ecology of the fuels concept for longleaf pine woodlands and illustrates its utility by evaluating the principles of ecological forestry (incorporating legacies of disturbances, understanding intermediate stand development processes, and allowing for recovery periods) in this chronically disturbed ecosystem. We...

Firebrands are an important agent of wildfire spread and structure fire ignitions at the wildland... more Firebrands are an important agent of wildfire spread and structure fire ignitions at the wildland urban interface. Bark flake morphology has been highlighted as an important, yet poorly characterized factor in firebrand generation, transport, deposition, and ignition of unburned material. Using pine species where bark flakes are the documented source of embers, we conducted experiments to investigate how bark structure changes in response to diurnal drying. Over a 3-day period in a longleaf pine (Pinus palustris Mill.) stand in Florida, we recorded changes in temperature, moisture content and structure of bark across different facing aspects of mature pine trees to examine the effects of varying solar exposure on bark moisture. We further compared results to bark drying in a pitch pine (Pinus rigida Mill.) plantation in New Jersey. Under all conditions, bark peeled and lifted away from the tree trunk over the study periods. Tree bole aspect and the time of day interacted to signific...

International Journal of Wildland Fire, 2020

The relationship between wildland fire spread rate and wind has been a topic of study for over a ... more The relationship between wildland fire spread rate and wind has been a topic of study for over a century, but few laboratory studies report measurements in controlled winds exceeding 5ms−1. In this study, measurements of fire rate of spread, flame residence time and energy release are reported for fires burning under controlled atmospheric conditions in shallow beds of pine needles subject to winds ranging from 0 to 27ms−1 (measured 5m above ground level). The data suggested that under constant flow conditions when winds are less than 10ms−1, fire rate of spread increases linearly at a rate of ~3% of the wind speed, which generally agrees with other laboratory-based models. When wind speed exceeds 10ms−1, the fire rate of spread response to wind remains linear but with a much stronger dependence, spreading at a rate of ~13% of the wind speed. Radiative and convective heating correlated directly to wind speed, with radiant heating increasing approximately three-fold as much as convec...

Forests, 2018

Carbohydrate reserves provide advantages for mature trees experiencing frequent disturbances; how... more Carbohydrate reserves provide advantages for mature trees experiencing frequent disturbances; however, it is unclear if selective pressures operate on this characteristic at the seedling or mature life history stage. We hypothesized that natural selection has favored carbohydrate reserves in species that have an evolutionary history of frequent disturbance and tested this using three southern pine species that have evolved across a continuum of fire frequencies. Longleaf pine (Pinus palustris) roots exhibited higher maximum starch concentrations than slash (P. elliottii) and loblolly (P. taeda), which were similar. Longleaf also relied on starch reserves in roots more than slash or loblolly, depleting 64, 41, and 23 mg g−1 of starch, respectively, between seasonal maximum and minimum, which represented 52%, 45%, and 26% of reserves, respectively. Starch reserves in stems did not differ among species or exhibit temporal dynamics. Our results suggest that an evolutionary history of di...

International Journal of Wildland Fire, 2019

The FireFlux II experiment was conducted in a tall grass prairie located in south-east Texas on 3... more The FireFlux II experiment was conducted in a tall grass prairie located in south-east Texas on 30 January 2013 under a regional burn ban and high fire danger conditions. The goal of the experiment was to better understand micrometeorological aspects of fire spread. The experimental design was guided by the use of a coupled fire–atmosphere model that predicted the fire spread in advance. Preliminary results show that after ignition, a surface pressure perturbation formed and strengthened as the fire front and plume developed, causing an increase in wind velocity at the fire front. The fire-induced winds advected hot combustion gases forward and downwind of the fire front that resulted in acceleration of air through the flame front. Overall, the experiment collected a large set of micrometeorological, air chemistry and fire behaviour data that may provide a comprehensive dataset for evaluating and testing coupled fire–atmosphere model systems.

Background/Question/Methods Earthworms function as ecosystem engineers by changing and constructi... more Background/Question/Methods Earthworms function as ecosystem engineers by changing and constructing environments, and by influencing the community structure of other soil organisms. Therefore, invasion of earthworms into ecosystems can cause significant alterations. Amynthas agrestis is known to have invaded the USA from Asia, and has expanded its distribution range to include relatively undisturbed forest ecosystems. First, to clarify the life history of A. agrestis, we reared individuals under 7 different conditions of food provision (only litter, only Oa/Oe, only soil, litter and Oa/Oe, litter and soil, Oa/Oe and soil, all materials), and compared the weights of worms, survival rates, and cocoon production rates. We also collected A. agrestis in the field, and analyzed their stable isotope ratios to investigate their food sources. Second, we examined whether prescribed fire (a common forest management practice) can be used to manage invasive earthworms by direct mortality to eart...

Forest Ecology and Management, 2015

Two conservation goals of the early 20th century, extensive reforestation and reduced wildfire th... more Two conservation goals of the early 20th century, extensive reforestation and reduced wildfire through fire exclusion, may have contributed to declining pollinator abundance as forests became denser and shrub covered. To examine how forest structure affects bees we selected 5 stands in each of 7 forest types including: cleared forest; dense young pines; thinned young pines; mature open pine with extensive shrub/sapling cover; mature open pine with extensive herbaceous plant cover and little shrub cover; mature upland hardwood forest; and mature riparian hardwood forest. We sampled bees during the 2008 growing season using pan traps and measured overstory tree density, understory herbaceous plant and shrub diversity and cover, light penetration, and leaf area index. Numbers of bees and numbers of species per plot were highest in cleared forest and in mature pine stands with an herbaceous plant understory. Estimates of asymptotic species richness were highest in mature riparian hardwood forests, cleared forests and open pine forests with an herbaceous plant understory. Bee communities in the cleared forests and in the mature pine with an herbaceous plant understory were grouped together in ordination space which was consistent with perMANOVA results. The best predictor variable for bee species density was total tree basal area which was negatively correlated (r 2 = 0.58), while the best model for predicting bee abundance (r 2 = 0.62) included canopy openness, plant species density (both positively correlated) and shrub cover (negatively correlated). Our results combined with many others show that thinning forests combined with shrub control provides good bee habitat, is compatible with habitat restoration and management for other species, and the resulting forests will be healthier and less susceptible to old (e.g., southern pine beetle, Dendroctonus frontalis) and new (European woodwasp, Sirex noctilio) threats.

Journal of Plant Ecology, 2015

Aims The fruits of Erithalis fruticosa l. and Lantana involucrata l. are important in the diet of... more Aims The fruits of Erithalis fruticosa l. and Lantana involucrata l. are important in the diet of us federally endangered Kirtland's Warblers (Setophaga kirtlandii) wintering in the bahamas archipelago. These two shrubs occur in tropical and subtropical dry forests, including forests that have been subjected to recent disturbance. Despite their importance to the endangered warbler, the disturbance ecology of these shrubs is poorly understood. We sought to determine, based on functional characteristics of the plants, whether their presence is favored by a particular type or regime of disturbance. Methods We used data from field experiments (seed broadcasting and shrub cutting) conducted on the island of Eleuthera, The bahamas to determine mechanisms of and conditions favoring establishment and persistence ('vital attributes') of E. fruticosa and L. involucrata, which enabled categorization according to the plant functional types defined by Noble and slatyer (1980). We then compared hypothesized distributions of these plant functional types among different anthropogenic disturbance regimes to observed distributions of E. fruticosa and L. involucrata in order to identify disturbance regimes most likely to produce habitat used by Kirtland's Warblers. Important Findings E. fruticosa and L. involucrata were functionally categorized as widely dispersed but largely shade intolerant species capable of establishing or regenerating individuals after disturbance via both seeds and vegetative mechanisms. both hypothesized and observed distribution patterns indicated the shrubs were favored by a regime of frequent disturbance producing open canopy and ground layers. among the anthropogenic disturbances we examined, areas of large-scale land clearing combined with subsequent goat grazing most often supported E. fruticosa and L. involucrata, while the shrubs were relatively rare in burned areas. utilizing the plant functional type framework in combination with field data to evaluate predictions of species occurrence among different disturbances regimes provides a strong theoretical basis for conservation strategies. understanding which disturbance types favor a habitat of concern and the mechanisms by which they do so can aid the prioritization of areas for protection or the design of habitat management protocols.