Article

The role of understory phenology and productivity in the carbon dynamics of longleaf pine savannas

Authors:
  • Tall Timbers Research Station
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Abstract

Savanna ecosystems contribute ~30% of global net primary production (NPP), but they vary substantially in composition and function, specifically in the understory, which can result in complex responses to environmental fluctuations. We tested how understory phenology and its contribution to ecosystem productivity within a longleaf pine ecosystem varied at two ends of a soil moisture gradient (mesic and xeric). We used the Normalized Difference Vegetation Index (NDVI) of the understory and ecosystem productivity estimates from eddy covariance systems to understand how variation in the under-story affected overall ecosystem recovery from disturbances (drought and fire). We found that the mesic site recovered more rapidly from the disturbance of fire, compared to the xeric site, indicated by a faster increase in NDVI. During drought, understory NDVI at the xeric site decreased less compared to the mesic site, suggesting adaptation to lower soil moisture conditions. Our results also show large variation within savanna ecosystems in the contribution of the understory to ecosystem productivity and recovery, highlighting the critical need to further subcategorize global savanna ecosystems by their structural features, to accurately predict their contribution to global estimates of NPP.

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... Disturbances can shift the balance of NPP among vegetation layers directly by disproportionately removing vegetation from certain layers, and indirectly by changing resource availability and competitive interactions (Alaback, 1982;Hart & Chen, 2006;Miller et al., 2011). Although canopy productivity often dominates ecosystem productivity (Gower et al., 2001;Misson et al., 2007;Nilsson & Wardle, 2005;Tait & Schiel, 2018;Wiesner et al., 2019), intermittent or sustained disturbances to upper vegetation layers can promote understory NPP that rivals that of the canopy, with cascading community and ecosystem effects (Alaback, 1982;Hart & Chen, 2006;Lloyd et al., 2008;Miller et al., 2011;Nilsson & Wardle, 2005). For example, storms periodically reduce forest aboveground NPP by destroying trees, but create canopy gaps that increase light and boost herbaceous understory NPP, thereby altering decomposition (Kennard et al., 2020;Muscolo et al., 2014;Royo & Carson, 2006). ...
... Moreover, across a variety of ecosystems, most investigations have not achieved the long durations needed to determine how canopy and understory NPP change over the multiple cycles of disturbance and recovery that occur when disturbance regimes shift in frequency or severity (Donohue et al., 2016;Haddad et al., 2002;Knapp et al., 2012). Hence, the potential for understory vegetation to compensate for sustained losses of canopy productivity under intensified disturbance regimes is largely unknown, particularly for systems with large, complex canopies (House et al., 2003;Reich et al., 2001;Wiesner et al., 2019). ...
... By their sheer size and superior access to resources, canopy-forming species are often highly productive and exert strong control over ecosystem productivity (Gower et al., 2001;Misson et al., 2007;Nilsson & Wardle, 2005;Tait & Schiel, 2018;Wiesner et al., 2019). By extension, disturbances that disproportionately damage the canopy, such as severe winds, fire, ice storms and large waves (Dayton et al., 1992;Reich et al., 2001;Roberts, 2004), should have direct negative effects on ecosystem productivity. ...
Article
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Disturbances often disproportionately impact different vegetation layers in forests and other vertically stratified ecosystems, shaping community structure and ecosystem function. However, disturbance‐driven changes may be mediated by environmental conditions that affect habitat quality and species interactions. In a decade‐long field experiment, we tested how kelp forest net primary productivity (NPP) responds to repeated canopy loss along a gradient in grazing and substrate suitability. We discovered that habitat quality can mediate the effects of intensified disturbance on canopy and understory NPP. Experimental annual and quarterly disturbances suppressed total macroalgal NPP, but effects were strongest in high‐quality habitats that supported dense kelp canopies that were removed by disturbance. Understory macroalgae partly compensated for canopy NPP losses and this effect magnified with increasing habitat quality. Disturbance‐driven increases in understory NPP were still rising after 5–10 years of disturbance, demonstrating the value of long‐term experimentation for understanding ecosystem responses to changing disturbance regimes. In a decade‐long field experiment, we show that habitat quality can mediate the effects of intensified disturbance on canopy and understory net primary productivity (NPP) in kelp forests. Experimental annual and quarterly disturbances suppressed total macroalgal NPP; however, understory macroalgae partly compensated for canopy NPP losses and this effect magnified with increasing habitat quality. Disturbance‐driven increases in understory NPP were still rising after 5–10 years of disturbance, demonstrating the value of long‐term experimentation for understanding ecosystem responses to changing disturbance regimes.
... Due to site-specific soil properties, the xeric site, which is subject to chronic water limitation (Starr et al., 2016), has a unique structure, i.e., shorter forest canopy, sparse midstory and understory communities (Kirkman et al., 2001). Such forest structure may be more resistant to physical damage caused by hurricane-induced wind disturbance in comparison to the mesic site, which has a taller canopy and higher basal area (Wiesner et al., 2019). ...
... Our two research sites represent the ends of an edaphic gradient and are located at the Jones Center at Ichauway in southwestern Georgia, USA. The center is a 11,000 ha 2 long-leaf pine reserve (31.22°N, 84.47°W; Fig. 1), which has a subtropical humid climate (Wiesner et al., 2018(Wiesner et al., , 2019. The long-term average annual precipitation of the study area is 1310 mm. ...
... MODIS-derived LAI and GPP data were processed with the MODIS Reprojection Tool (MRT) using ArcGIS (Version 10.2; ESRI) for projection correction, image cropping, and raster calculation. Since the EC flux source area can be extended up to 600 m from the towers (Wiesner et al., 2018(Wiesner et al., , 2019, MODISderived GPP, LAI and EVI were average values within 600 m radius circles centered at each EC sites. ...
Article
Hurricanes affect the structure and function of forests by removing leaf area, reducing biomass, and causing plant mortality. Quantifying the effects of hurricanes on the phenological processes of forests can help to develop a better understanding of the responses of these systems to natural disasters. On October 10, 2018 Hurricane Michael made landfall in the northern Gulf of Mexico causing extensive damage to forests within its path. Using a phenology model, we evaluated the short-term response and recovery of phenological processes of two subtropical forests that were affected by the storm. Our results suggest that the hurricane accelerated senescence in autumn following the storm, leading to a shorter growing season. The response was dependent on the structure of the forest prior to the storm and the degree of damage; the forest with a taller canopy had greater damage, and the recovery period was prolonged compared to the forest with a shorter canopy. In the summer of the first year following the hurricane, ecosystem physiological function began to return to pre-hurricane levels which corresponded to a recovery in growing season length. The functional diversity in the understory may have aided recovery of post-hurricane spring phenology. While summer phenology was synchronized with the rate of vegetation coverage and mainly driven by increase in canopy leaf area, these forests have not completely recovered during the study. As extreme weather events and disasters induced by global climate change may become more frequent, our research can provide a reference for post-disaster forest management practices which can be adapted to local conditions and contribute to restoration efforts.
... The xeric site lies on well-drained deep sandy soils with no argillic horizon [39]. All sites are situated within 10 km of each other ( [40]). Water holding capacity: cm water per cm soil, measured in upper 3 m of soil. ...
... Previous studies have reported the long-term precipitation dynamics of these sites during the study period (2009-2017) [40,42], and showed short-term seasonal annual drought and water stress from June to August [38]. This caused significant day-to-day fluctuations in summer ecosystem productivity (GPP decline and recovery) and changes in summer phenological processes across sites. ...
... Prescribed fire and weather events (meteorological drought and flood) affected ecosystem-scale phenological processes and productivity [40,49], especially summer phenology (Table 2; Appendix B, Figure A2) because of the variability of water availability. To develop a more thorough understanding of the phenological patterns of the study sites, we investigated SOS, EOS and LOS patterns and characteristics at an inter-annual scale ( Figure 6; Appendix B, Table A1 and Figure A2). ...
Article
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Understanding plant phenological change is of great concern in the context of global climate change. Phenological models can aid in understanding and predicting growing season changes and can be parameterized with gross primary production (GPP) estimated using the eddy covariance (EC) technique. This study used nine years of EC-derived GPP data from three mature subtropical longleaf pine forests in the southeastern United States with differing soil water holding capacity in combination with site-specific micrometeorological data to parameterize a photosynthesis-based phenological model. We evaluated how weather conditions and prescribed fire led to variation in the ecosystem phenological processes. The results suggest that soil water availability had an effect on phenology, and greater soil water availability was associated with a longer growing season (LOS). We also observed that prescribed fire, a common forest management activity in the region, had a limited impact on phenological processes. Dormant season fire had no significant effect on phenological processes by site, but we observed differences in the start of the growing season (SOS) between fire and non-fire years. Fire delayed SOS by 10 d ± 5 d (SE), and this effect was greater with higher soil water availability, extending SOS by 18 d on average. Fire was also associated with increased sensitivity of spring phenology to radiation and air temperature. We found that interannual climate change and periodic weather anomalies (flood, short-term drought, and long-term drought), controlled annual ecosystem phenological processes more than prescribed fire. When water availability increased following short-term summer drought, the growing season was extended. With future climate change, subtropical areas of the Southeastern US are expected to experience more frequent short-term droughts, which could shorten the region’s growing season and lead to a reduction in the longleaf pine ecosystem’s carbon sequestration capacity.
... The xeric site lies on well-drained deep sandy soils with no argillic horizon [39]. All sites are situated within 10 km of each other ( [40]). Water holding capacity: cm water per cm soil, measured in upper 3 m of soil. ...
... Previous studies have reported the long-term precipitation dynamics of these sites during the study period (2009-2017) [40,42], and showed short-term seasonal annual drought and water stress from June to August [38]. This caused significant day-to-day fluctuations in summer ecosystem productivity (GPP decline and recovery) and changes in summer phenological processes across sites. ...
... Prescribed fire and weather events (meteorological drought and flood) affected ecosystem-scale phenological processes and productivity [40,49], especially summer phenology (Table 2; Appendix B, Figure A2) because of the variability of water availability. To develop a more thorough understanding of the phenological patterns of the study sites, we investigated SOS, EOS and LOS patterns and characteristics at an inter-annual scale ( Figure 6; Appendix B, Table A1 and Figure A2). ...
Article
Understanding plant phenological change is of great concern in the context of global climate change. Phenological models can aid in understanding and predicting growing season changes and can be parameterized with gross primary production (GPP) estimated using the eddy covariance (EC) technique. This study used nine years of EC-derived GPP data from three mature subtropical longleaf pine forests in the southeastern United States with differing soil water holding capacity in combination with site-specific micrometeorological data to parameterize a photosynthesis-based phenological model. We evaluated how weather conditions and prescribed fire led to variation in the ecosystem phenological processes. The results suggest that soil water availability had an effect on phenology, and greater soil water availability was associated with a longer growing season (LOS). We also observed that prescribed fire, a common forest management activity in the region, had a limited impact on phenological processes. Dormant season fire had no significant effect on phenological processes by site, but we observed differences in the start of the growing season (SOS) between fire and non-fire years. Fire delayed SOS by 10 d � 5 d (SE), and this effect was greater with higher soil water availability, extending SOS by 18 d on average. Fire was also associated with increased sensitivity of spring phenology to radiation and air temperature. We found that interannual climate change and periodic weather anomalies (flood, short-term drought, and longterm drought), controlled annual ecosystem phenological processes more than prescribed fire. When water availability increased following short-term summer drought, the growing season was extended. With future climate change, subtropical areas of the Southeastern US are expected to experience more frequent short-term droughts, which could shorten the region’s growing season and lead to a reduction in the longleaf pine ecosystem’s carbon sequestration capacity.
... Their proximity also suggests these populations are not reproductively isolated and therefore not genetically separate. Differences in these particular populations have been studied for >20 years and the soil drainage differences generate consistent differences in productivity [47][48][49][50][51][52][53][54][55][56][57][58]. We measured sugar and starch concentrations from lateral root, main stem, mid canopy branch, and upper canopy branch tissues for a year, and we compared the temporal variation of these concentrations to infer differences in carbon allocation. ...
... Its soils are primarily classified as Typic Quartzipsammants with inclusions of Arenic or Grossarenic Kandiudults [51]. Other than soil hydrology, both sites experience similar light, temperature, and vapor pressure environments ( Figure 1 in [54]; Figure 2 in [57]). Xeric site annual precipitation was 94% of mesic site annual precipitation for 2008-2015 (Table 1 in [55]). ...
... Xeric site annual precipitation was 94% of mesic site annual precipitation for 2008-2015 (Table 1 in [55]). Soil volumetric water content was 1%-3% higher at the mesic site during the study period and the preceding five years (Figure 1 in [53]; Figure 2 in [57]). The hydrological differences resulted in stark differences in net ecosystem exchange (−208.2 ...
Article
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Research linking soil moisture availability to nonstructural carbohydrate (NSC) storage suggests greater NSC reserves promote survival under acute water stress, but little is known about how NSC allocation responds to long-term differences in water availabilty. We hypothesized populations experiencing chronic or frequent water stress shift carbon allocation to build greater NSC reserves for increased survival probability during drought relative to populations rarely experiencing water stress. Over a year, we measured soluble sugar and starch concentrations from branches, stems, and coarse roots of mature Pinus palustris trees at two sites differing in long-term soil moisture availability. Xeric and mesic populations exhibited a cycle of summer depletion-winter accumulation in root starch. Xeric populations reached a maximum root starch concentration approximately 1–2 months later than mesic populations, indicating delayed summer depletion. Xeric and mesic populations reached the same minimum root starch at similar times, suggesting extended winter accumulation for xeric populations. These results suggest seasonal mobilization from root starch is compressed into a shorter interval for xeric populations instead of consistently greater reserves as hypothesized. Seasonal trends differed little between xeric and mesic populations for starch and sugars, suggesting the importance of roots in seasonal carbon dynamics and the primacy of starch for storage. If roots are the primary organ for longterm storage, then our results suggest that whole-plant mobilization and allocation respond to chronic differences in water availability.
... To date, this metric has primarily been used to study vegetation recovery after infrequent, stand-replacing fires where yearly satellite images are sufficient to detect changes over time [9][10][11][12][13]. In burned savannas, where frequent imagery is needed to detect vegetation recovery, researchers often employ ground-based remote sensing [14], because differences in canopy density and phenology may skew NDVI [15,16]. Additionally, satellite imagery typically does not have the spatial and temporal resolution needed to accurately detect rapid changes in understory vegetation [17]. ...
... In savannas, remote sensing has been widely used at the landscape level to detect overall differences in stand structure resulting from management practices [30,31]. In contrast, data needed at small temporal and spatial scales are often collected using time-intensive measurements of shrub and tree height or expensive ground-based imagery, because vegetation recovery after fire is swift in these ecosystems [14,32,33]. Our study shows that NDVI values derived from satellite imagery could be used to gain ecological information in pine savannas where a high temporal resolution is needed to accurately detect differences, although there are still some challenges to overcome. ...
... This time period also corresponded with the most rapid vegetation growth when the time-to-recovery should have been the shortest, and our need for imagery was the greatest. Despite this drawback, our results were similar to those of another study in xeric, pine savannas that found that NDVI increased quickly after spring, growing season fires [14]. ...
Article
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Research Highlights: Fire-frequented savannas are dominated by plant species that regrow quickly following fires that mainly burn through the understory. To detect post-fire vegetation recovery in these ecosystems, particularly during warm, rainy seasons, data are needed on a small, temporal scale. In the past, the measurement of vegetation regrowth in fire-frequented systems has been labor-intensive, but with the availability of daily satellite imagery, it should be possible to easily determine vegetation recovery on a small timescale using Normalized Difference Vegetation Index (NDVI) in ecosystems with a sparse overstory. Background and Objectives: We explore whether it is possible to use NDVI calculated from satellite imagery to detect time-to-vegetation recovery. Additionally, we determine the time-to-vegetation recovery after fires in different seasons. This represents one of very few studies that have used satellite imagery to examine vegetation recovery after fire in southeastern U.S.A. pine savannas. We test the efficacy of using this method by examining whether there are detectable differences between time-to-vegetation recovery in subtropical savannas burned during different seasons. Materials and Methods: NDVI was calculated from satellite imagery approximately monthly over two years in a subtropical savanna with units burned during dry, dormant and wet, growing seasons. Results: Despite the availability of daily satellite images, we were unable to precisely determine when vegetation recovered, because clouds frequently obscured our range of interest. We found that, in general, vegetation recovered in less time after fire during the wet, growing, as compared to dry, dormant, season, albeit there were some discrepancies in our results. Although these general patterns were clear, variation in fire heterogeneity and canopy type and cover skewed NDVI in some units. Conclusions: Although there are some challenges to using satellite-derived NDVI, the availability of satellite imagery continues to improve on both temporal and spatial scales, which should allow us to continue finding new and efficient ways to monitor and model forests in the future.
... MODIS-derived LSP data were processed with the MODIS Reprojection Tool using ArcGIS (Version 10.2; ESRI) for projection correction, image cropping, and raster calculation. Since the EC flux source area extends 500 m from the towers (Wiesner et al., 2018(Wiesner et al., , 2019, we used the average of the MODIS-derived phenology dates obtained within a 500-m radius circle centered at each EC site. However, we also calculated the minimum and maximum value within the footprint to allow evaluation across the possible range of values obtained from MODIS EVI and further assess relationships between MODIS dates and those of the nine-parameter function. ...
... During summer, variation in Re may be caused by site-level water availability (Starr et al., 2016;Wiesner et al., 2018Wiesner et al., , 2019, which also adds uncertainty in the timing of SOP and EOP. These fluctuations cause the growth rate of Re to appear as multiple peaks in summer (Appendix S1: Figures S1-S2), and the GR and TD method may not give a biologically reasonable LOP (Appendix S1: Figure S6). ...
Article
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The seasonal dynamics of plant communities are important indicators for assessment of long‐term vegetation patterns and provide valuable information to predict ecosystem responses to climate change. However, increased frequency of extreme weather events can force ecosystems into unstable states, which leads to greater uncertainty in determining phenological metrics (e.g., growing season length). To better understand these uncertainties, we utilized 9 years of eddy covariance and remote sensing data to parameterize models of seasonal ecosystem respiration (Re) for two subtropical longleaf pine forests (mesic and xeric), with similar vegetation but different water holding capacity. We compared two commonly used algorithms to extract phenology metrics, the growth rate (GR) and third derivative (TD) methods, which are usually used without justification. We determined the impact of algorithm selection on estimating key biological dates related to plant community carbon dynamics (e.g., start, end, and length of physiologically active season, specifically Re), characterized the model's response to extreme weather events, and compared estimates to those derived via remotely sensed greenness from the enhanced vegetation index (EVI). We observed that periods of winter warming increased duration of physiological activity in terms of Re, and summer water limitation caused multi‐peaked, asymmetric behavior, creating significant uncertainties. We found that choice of phenology metric extraction algorithm significantly impacted biological event dates; the GR method estimated longer phenophases than the TD in both sites, as well as earlier starting and later ending dates for phenophases. Because the TD method was unable to give estimates during the buffer period of phenophase transition under certain weather conditions, the GR method may be more suitable for studies in subtropical forests. Dates derived from EVI greenness rarely matched those of plant community seasonal dynamics models, especially in spring and summer. The estimated length of Re from the model was significantly longer than that derived from EVI, indicating that the use of EVI could result in shorter growing season estimates and greater uncertainty. Our results provide direction for optimization of future approaches to extract phenological metrics and better scientific understanding of forest land surface phenology, as weather anomalies become more common with climate change.
... Frequent fire has maintained the structure of these forests, with low intensity prescribed burns taking place during the past ∼75 years (Mitchell et al., 1999). Both sites have been burned in odd-numbered years since 2009 (Wiesner et al., 2019). The composition and abundance of other overstory and understory species is site dependent . ...
... To estimate the time needed to sequester carbon loss due to salvage logging from our site, we used long-term annual estimates of NEE, 10.1029/2021JG006452 5 of 17 assuming future productivity would equal the long-term average. To describe uncertainties, we also calculated a range of carbon sequestration capacity from least productive year and most productive year for each site (Whelan et al., 2013;Wiesner et al., 2019). ...
Article
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Tropical cyclones can physically alter ecosystems, causing immediate and potentially long‐lasting effects on carbon dynamics. In 2018, Hurricane Michael hit the southeastern United States with category 5 winds at landfall and category 2 winds reaching over 100 miles inland, resulting in extensive damage. Longleaf pine woodlands in the path of the hurricane were damaged, but severity varied based on the storm track. We used a combination of eddy covariance measurements, airborne LiDAR, and forest inventory data to determine whether hurricane affects structure, function, and recovery of two longleaf pine woodlands at the ends of an edaphic gradient. We found that the carbon sink potentials in both sites were diminished following the storm, with reductions in net ecosystem exchange (NEE) primarily due to lower rates of photosynthesis, as respiration only increased marginally. The xeric site carbon losses and physiological reductions were smaller following the disturbance, which led to the recovery of ecosystem physiological activity to prestorm rates before that of the mesic site, as indicated by maximum ecosystem CO2 uptake rates. Two years following the hurricane both stands continued to have reduced NEE, which signaled altered function. We expect both locations to recover their lost carbon stocks in ∼10–35 years; however, long‐term studies are needed to examine how longleaf woodlands respond to compounding disturbances, such as drought, fire, or other wind storms, which vary significantly across the ecosystem's range. Additionally, hurricanes are intensifying due to climate change, potentially amplifying the degree to which they will alter this ecosystem in the future.
... LAI expansion increases shaded areas and interception, causing a decline in radiation and water for the understory vegetation Kwon et al., 2018;Vickers et al., 2012). At the study site, the understory was mostly composed of grasses, which have high transpiration rates when water and energy are available, but perish when environmental conditions become unfavorable due to higher temperatures and water stress (Pereira et al., 2007;Wiesner et al., 2019). In contrast, trees generally have more capacity to adjust transpiration, through the regulation of their stomatal conductance, which makes trees more hydrologically conservative (Jones, 2013;O'Grady et al., 1999;Pita et al., 2013). ...
Article
The establishment and expansion of commercial plantations for timber production and carbon sequestration raises concerns because of their large water use. Eucalyptus globulus (blue gum) is one of the most planted species globally, as it grows rapidly and is adaptable to a range of climatic conditions. The dearth of experimental observations on water use and growth in blue gum plantations in their early years after establishment makes it difficult to develop management practices. This study quantified the trade-offs between water use and carbon assimilation in a blue gum plantation in the first 4 years after establishment. The study site is located in southwest Victoria, Australia, where energy, water and CO2 fluxes were continuously measured above the tree canopy for 4 years after the trees were planted. During the first year after establishment, understory vegetation and ecosystem respiration had a major impact on the net ecosystem exchange (NEE), with the plantation being a net carbon source. Subsequently, the trees started dominating the contributions to NEE, and after approximately 2 years the plantation became a consistent carbon sink. These shifts in NEE were accompanied by smaller increases in annual evapotranspiration rates, which was 70% of the annual precipitation in the first year and 74% in the 3rd year of measurements. As a result, yearly averages of water use efficiency increased from 2.86 gCkg−1H2O in 2018 to 4.3 gCkg−1H2O in 2020, following tree development. This shows a remarkable increase in productivity at the expense of a small amount of water.
... Prescribed fire objectives necessitate a detailed and nuanced understanding of the fine-scale micrometeorological ) and phenological conditions (Wiesner et al. 2019) that influence fuel moisture dynamics of both live and dead fuels (Jolly et al. 2014;Kreye et al. 2018). Changes over time represent a "fourth dimension" of fuels characterization. ...
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Abstract Understanding plant–plant facilitation is critical for predicting how plant community function will respond to changing disturbance and climate. In longleaf pine (Pinus palustris Mill.) ecosystems of the southeastern United States, understanding processes that affect pine reproduction is imperative for conservation efforts that aim to maintain ecosystem resilience across its wide geographic range and edaphic gradients. Variation in wildland fire and plant–plant interactions may be overlooked in “coarse filter” restoration management, where actions are often prescribed over a variety of ecological conditions with an assumed outcome. For example, hardwood reduction techniques are commonly deemed necessary for ecological restoration of longleaf pine ecosystems, as hardwoods are presumed competitors with longleaf pine seedlings. Natural regeneration dynamics are difficult to test experimentally given the infrequent and irregular mast seed events of the longleaf pine. Using a long‐term, large‐scale restoration experiment and a long‐term monitoring data site at Eglin Air Force Base, Florida (USA), this study explores the influence of native fire‐intolerant oaks on longleaf regeneration. We test for historical observations of hardwood facilitation against the null hypothesis of competitive exclusion. Our results provide evidence of hardwood facilitation on newly germinated longleaf pine seedlings (
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Least-squares means are predictions from a linear model, or averages thereof. They are useful in the analysis of experimental data for summarizing the effects of factors, and for testing linear contrasts among predictions. The lsmeans package (Lenth 2016) provides a simple way of obtaining least-squares means and contrasts thereof. It supports many models fitted by R (R Core Team 2015) core packages (as well as a few key contributed ones) that fit linear or mixed models, and provides a simple way of extending it to cover more model classes.
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in ecosystem phenology play an important role in the definition of inter-annual variability of net ecosystem carbon uptake. A good estimate at the global scale of ecosystem phenology, mainly that of photosynthesis or gross primary productivity (GPP), may be provided by vegetation indices derived from MODIS satellite image data. However, the relationship between the start date of a growing (or greening) season (SGS) when derived from different vegetation indices (VI's), and the starting day of carbon uptake is not well elucidated. Additionally, the validation of existing phenology data with in-situ measurements is largely missing. We have investigated the possibility to use different VI's to predict the starting day of the growing season for 28 FLUXNET sites as well as MODIS data. This analysis included main plant functional types (PFT's). Of all VI's taken into account in this paper, the NDVI (Normalised Difference Vegetation Index) shows the highest correlation coefficient for the relationship between the starting day of the growing season as observed with MODIS and in-situ observations. However, MODIS observations elicit a 20-21 days earlier SGS date compared to in-situ observations. The prediction for the NEE start of the growing season diverges when using different VI's, and seems to depend on the amplitude for carbon and VI and on PFT. The optimal VI for estimation of a SGS date was PFT-specific - for example the WRDVI for cropland, but the MODIS NDVI performed best when applied as an estimator for Net Ecosystem Exchange and when considering all PFT's pooled.
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Growth is a major component of fitness in all organisms, an important mediator of competitive interactions in plant communities, and a central determinant of yield in crops. Understanding what limits plant growth is therefore of fundamental importance to plant evolution, ecology, and crop science, but each discipline views the process from a different perspective. This review highlights the importance of source–sink interactions as determinants of growth. The evidence for source- and sink-limitation of growth, and the ways in which regulatory molecular feedback systems act to maintain an appropriate source:sink balance, are first discussed. Evidence clearly shows that future increases in crop productivity depend crucially on a quantitative understanding of the extent to which sources or sinks limit growth, and how this changes during development. To identify bottlenecks limiting growth and yield, a holistic view of growth is required at the whole-plant scale, incorporating mechanistic interactions between physiology, resource allocation, and plant development. Such a holistic perspective on source–sink interactions will allow the development of a more integrated, whole-system level understanding of growth, with benefits across multiple disciplines.
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Common oak trees display endogenous rhythmic growth with alternating shoot and root flushes. To explore the mechanisms involved, microcuttings of the Quercus robur L. clone DF159 were used for (13)C/(15)N labelling in combination with RNA sequencing (RNASeq) transcript profiling of shoots and roots. The effect of plant internal resource availability on the rhythmic growth of the cuttings was tested through inoculation with the ectomycorrhizal fungus Piloderma croceum. Shoot and root flushes were related to parallel shifts in above- and below-ground C and, to a lesser extent, N allocation. Increased plant internal resource availability by P. croceum inoculation with enhanced plant growth affected neither the rhythmic growth nor the associated resource allocation patterns. Two shifts in transcript abundance were identified during root and shoot growth cessation, and most concerned genes were down-regulated. Inoculation with P. croceum suppressed these transcript shifts in roots, but not in shoots. To identify core processes governing the rhythmic growth, functions [Gene Ontology (GO) terms] of the genes differentially expressed during the growth cessation in both leaves and roots of non-inoculated plants and leaves of P. croceum-inoculated plants were examined. Besides genes related to resource acquisition and cell development, which might reflect rather than trigger rhythmic growth, genes involved in signalling and/or regulated by the circadian clock were identified. The results indicate that rhythmic growth involves dramatic oscillations in plant metabolism and gene regulation between below- and above-ground parts. Ectomycorrhizal symbiosis may play a previously unsuspected role in smoothing these oscillations without modifying the rhythmic growth pattern. © The Author 2015. Published by Oxford University Press on behalf of the Society for Experimental Biology.
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In order to understand how carbon storage and allocation patterns vary among plantation types, we estimated carbon allocation between above- and below-ground compartments in four subtropical plantations and a naturally recovered shrubland (as a control). Results indicated that the carbon storage and allocation pattern varied greatly among forest types and was highly dependent on specific traits of trees and understory vegetation. The fast-growing species, such as Eucalyptus urophylla, accumulated more carbon in plant biomass. The biomass carbon was about 1.9- and 2.2-times greater than the 10-species mixed plantation and Castanopsis hystrix plantations, respectively. Meanwhile, the plantations sequestered 1.5- to 3-times more carbon in biomass than naturally recovered shrubland. The carbon allocation pattern between above- and below-ground compartments also varied with plantation type and stand age. The ratio of tree root carbon to tree aboveground carbon decreased with stand age for Eucalyptus urophylla and the 10-species mixed plantation. In contrast, the ratio increased for Acacia crassicarpa. Our data suggested that planting the fast-growing species in the degraded land of subtropical China was an effective choice in terms of carbon sequestration. The information about carbon allocation patterns was also valuable for decision making in sustainable forest management and climate change mitigation.
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Reforested plantations have substantial effects on terrestrial carbon cycling due to their large coverage area. Although understory plants are important components of reforested plantations, their effects on ecosystem carbon dynamics remain unclear. This study was designed to investigate the effects of vegetation removal/understory removal and tree girdling on soil respiration and ecosystem carbon dynamics in Eucalyptus plantations of South China with contrasting ages (2 and 24 years old). We conducted a field manipulation experiment from 2008 to 2009. Understory removal reduced soil respiration in both plantations, whereas tree girdling decreased soil respiration only in the 2-year-old plantations. The net ecosystem production was approximately three times greater in the 2-year-old plantations (13.4 t C ha(-1) yr(-1)) than in the 24-year-old plantations (4.2 t C h(-1) yr(-1)). The biomass increase of understory plants was 12.6 t ha(-1) yr(-1) in the 2-year-old plantations and 2.9 t ha(-1) yr(-1) in the 24-year-old plantations, accounting for 33.9% and 14.1% of the net primary production, respectively. Our findings confirm the ecological importance of understory plants in subtropical plantations based on the 2 years of data. These results also indicate that Eucalyptus plantations in China may be an important carbon sink due to the large plantation area.
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Vegetation phenology plays an important role in regulating processes of terrestrial ecosystems. Dynamic ecosystem models (DEMs) require representation of phenology to simulate the exchange of matter and energy between the land and atmosphere. Location-specific parameterization with phenological observations can potentially improve the performance of phenological models embedded in DEMs. As ground-based phenological observations are limited, phenology derived from remote sensing can be used as an alternative to parameterize phenological models. It is important to evaluate to what extent remotely sensed phenological metrics are capturing the phenology observed on the ground. We evaluated six methods based on two vegetation indices (VIs) (i.e., Normalized Difference Vegetation Index and Enhanced Vegetation Index) for retrieving the phenology of temperate forest in the Agro-IBIS model. First, we compared the remotely sensed phenological metrics with observations at Harvard Forest and found that most of the methods have large biases regardless of the VI used. Only two methods for the leaf onset and one method for the leaf offset showed a moderate performance. When remotely sensed phenological metrics were used to parameterize phenological models, the bias is maintained, and errors propagate to predictions of gross primary productivity and net ecosystem production. Our results show that Agro-IBIS has different sensitivities to leaf onset and offset in terms of carbon assimilation, suggesting it might be better to examine the respective impact of leaf onset and offset rather than the overall impact of the growing season length.
Article
Seasonal patterns of leaf photosynthetic capacity and conductance were determined for deciduous hardwood tree species in natural habitats in northern lower Michigan. Leaves of bigtooth aspen and red oak at the top of the canopy had higher maximum CO2 Exchange Rate (CER) (10–15 μmol m ² s ¹) than leaves of sugar maple, red maple, red oak, and beech growing in the understory (4–5 μmol m ² s ¹). In all leaves, CER measured at light-saturation increased to a maximum near the completion of leaf expansion in early June, was constant until mid-September, and then rapidly declined until leaf death. A similar pattern was seen for CER measured in low light (1.5% full sun). Respiration rate in the dark was highest in young leaves and decreased during leaf expansion; a relatively constant rate was then maintained for the rest of leaf lifespan. The seasonal pattern of the initial slope of the light response of CER paralleled the pattern of light-saturated CER. The initial slope in midsummer ranged from values of 37 to 44 μmol/mol for species in the understory to 51 and 56 μmol/mol for red oak and bigtooth aspen, respectively, at the top of the canopy. Leaf conductance was constant throughout most of leaf lifespan, with some decline occurring in autumn. Leaves at the top of the canopy had higher conductances for water vapor (2–5 mm/s) than leaves in the understory (1–2 mm/s). All species maintained leaf intercellular CO, mole fractions (c,) near 200 uML/L until autumn, when c, increased during leaf senescence.
Article
Tight coupling between below-ground autotrophic respiration and the availability of recently assimilated carbon (C) has become a paradigm in the ecophysiological literature. Here, we show that stored carbohydrates can decouple respiration from assimilation for prolonged periods by mobilizing reserves from transport roots to absorptive roots. We permanently disrupted the below-ground transfer of recently assimilated C using stem girdling and root trenching and measured soil CO2 efflux for over 1 yr in longleaf pine (Pinus palustris), a species that has large reserves of stored carbohydrates in roots. Soil CO2 efflux was not influenced by girdling or trenching through the 14-month observation period. Stored carbohydrate concentrations in absorptive roots were not affected by the disrupted supply of current photosynthate for over 1 yr; however, carbohydrate concentrations in transport roots decreased. Our results indicate that root respiration can be decoupled from recent canopy assimilation and that stored carbohydrates can be mobilized from transport roots to absorptive roots to maintain respiration for over 1 yr. This refines the current paradigm that canopy assimilation and below-ground respiration are tightly coupled and provides evidence of the mechanism and dynamics responsible for decoupling the above- and below-ground processes.
Article
This study examines the complex feedback mechanisms that regulate a positive relationship between species richness and productivity in a longleaf pine-wiregrass woodland. Across a natural soil moisture gradient spanning wet-mesic to xeric conditions, two large scale manipulations over a 10-yr period were used to determine how limiting resources and fire regulate plant species diversity and productivity at multiple scales. A fully factorial experiment was used to examine productivity and species richness responses to N and water additions. A separate experiment examined standing crop and richness responses to N addition in the presence and absence of fire. Specifically, these manipulations addressed the following questions: (1) How do N and water addition influence annual aboveground net primary productivity of the midstory/overstory and ground cover? (2) How do species richness responses to resource manipulations vary with scale and among functional groups of ground cover species? (3) How does standing crop (including overstory, understory/midstory, and ground cover components) differ between frequently burned and fire excluded plots after a decade without fire? (4) What is the role of fire in regulating species richness responses to N addition? This long- Term study across a soil moisture gradient provides empirical evidence that species richness and productivity in longleaf pine woodlands are strongly regulated by soil moisture. After a decade of treatment, there was an overall species richness decline with N addition, an increase in richness of some functional groups with irrigation, and a substantial decline in species richness with fire exclusion. Changes in species richness in response to treatments were scale-dependent, occurring primarily at small scales (≤10 m²). Further, with fire exclusion, standing crop of ground cover decreased with N addition and non-pine understory/midstory increased in wet-mesic sites. Non-pine understory/midstory standing crop increased in xeric sites with fire exclusion, but there was no influence of N addition. This study highlights the complexity of interactions among multiple limiting resources, frequent fire, and characteristics of dominant functional groups that link species richness and productivity.
Article
Wildland fire radiant energy emission is one of the only measurements of combustion that can be made at wide spatial extents and high temporal and spatial resolutions. Furthermore, spatially and temporally explicit measurements are critical for making inferences about fire effects and useful for examining patterns of fire spread. In this study we describe our methods for capturing and analysing spatially and temporally explicit long-wave infrared (LWIR) imagery from the RxCADRE (Prescribed Fire Combustion and Atmospheric Dynamics Research Experiment) project and examine the usefulness of these data in investigating fire behaviour and effects. We compare LWIR imagery captured at fine and moderate spatial and temporal resolutions (from 1 cm(2) to 1 m(2); and from 0.12 to 1 Hz) using both nadir and oblique measurements. We analyse fine-scale spatial heterogeneity of fire radiant power and energy released in several experimental burns. There was concurrence between the measurements, although the oblique view estimates of fire radiative power were consistently higher than the nadir view estimates. The nadir measurements illustrate the significance of fuel characteristics, particularly type and connectivity, in driving spatial variability at fine scales. The nadir and oblique measurements illustrate the usefulness of the data for describing the location and movement of the fire front at discrete moments in time at these fine and moderate resolutions. Spatially and temporally resolved data from these techniques show promise to effectively link the combustion environment with post-fire processes, remote sensing at larger scales and wildland fire modelling efforts.
Article
Factors controlling savanna woody vegetation structure vary at multiple spatial and temporal scales, and as a consequence, unraveling their combined effects has proven to be a classic challenge in savanna ecology. We used airborne LiDAR (light detection and ranging) to map three-dimensional woody vegetation structure throughout four savanna watersheds, each contrasting in geologic substrate and climate, in Kruger National Park, South Africa. By comparison of the four watersheds, we found that geologic substrate had a stronger effect than climate in determining watershed-scale differences in vegetation structural properties, including cover, height and crown density. Generalized Linear Models were used to assess the spatial distribution of woody vegetation structural properties, including cover, height and crown density, in relation to mapped hydrologic, topographic and fire history traits. For each substrate and climate combination, models incorporating topography, hydrology and fire history explained up to 30% of the remaining variation in woody canopy structure, but inclusion of a spatial autocovariate term further improved model performance. Both crown density and the cover of shorter woody canopies were determined more by unknown factors likely to be changing on smaller spatial scales, such as soil texture, herbivore abundance or fire behavior, than by our mapped regional-scale changes in topography and hydrology. We also detected patterns in spatial covariance at distances up to 50-450 m, depending on watershed and structural metric. Our results suggest that large-scale environmental factors play a smaller role than is often attributed to them in determining woody vegetation structure in southern African savannas. This highlights the need for more spatially-explicit, wide-area analyses using high resolution remote sensing techniques.
Article
Ongoing shifts in the species composition of Eastern US forests necessitate the development of frameworks to explore how species-specific water-use strategies influence ecosystem-scale carbon (C) cycling during drought. Here, we develop a diagnostic framework to classify plant drought-response strategies along a continuum of isohydric to anisohydric regulation of leaf water potential (Ψ L). The framework is applied to a 3-year record of weekly leaf-level gas exchange and Ψ measurements collected in the Morgan-Monroe State Forest (Indiana, USA), where continuous observations of the net ecosystem exchange of CO2 (NEE) have been ongoing since 1999. A severe drought that occurred in the middle of the study period reduced the absolute magnitude of NEE by 55 %, though species-specific responses to drought conditions varied. Oak species were characterized by anisohydric regulation of Ψ L that promoted static gas exchange throughout the study period. In contrast, Ψ L of the other canopy dominant species was more isohydric, which limited gas exchange during the drought. Ecosystem-scale estimates of NEE and gross ecosystem productivity derived by upscaling the leaf-level data agreed well with tower-based observations, and highlight how the fraction of isohydric and anisohydric species in forests can mediate net ecosystem C balance.
Article
Plant growth is the balance of photosynthetic gains and respiratory losses, and it is therefore essential to consider respiration in analyses of plant productivity. The partitioning of dark respiratory losses into two functional components, a growth component and a maintenance component, has proved useful. The growth loss is that associated with synthesis of new biomass while the maintenance loss is that associated with maintenance of existing biomass. Experimental evidence indicates that the respiratory cost of maintenance in herbaceous plants is about equal to the cost of growth over a growing season, with daily maintenance expenditures less important in the small, rapidly growing plant but increasing in significance as plant size increases and the relative growth rate decreases. Because it is such a large fraction of the total carbon budget of a plant, any variations in maintenance requirements may result in significant alterations in productivity. In the present work the theoretical and empirical bases of maintenance respiration are described; magnitudes of maintenance expenditures are summarized; and applications to models of plant growth and productivity are discussed. It is concluded that the costs of maintenance should be included in analyses of plant growth.
Article
Frequency and intensity of fire determines the structure and regulates the function of savanna ecosystems worldwide, yet our understanding of prescribed fire impacts on carbon in these systems is rudimentary. We combined eddy covariance (EC) techniques and fuel consumption plots to examine the short-term response of longleaf pine forest carbon dynamics to one prescribed fire at the ends of an edaphic gradient (mesic and xeric sites). We also introduce novel (to the EC research community) statistical time-series approaches to quantify the drivers of carbon dynamics in these systems. We determined that our mesic site was a moderate sink of carbon (−157.7 ± 25.1 g C m−2 year−1), while the xeric site was carbon neutral (5.9 ± 32.8 g C m−2 year−1) during the study. The fire released 408 and 153 g C m−2 year−1 for the mesic and xeric sites, respectively. When loss associated with fire was combined with net ecosystem exchange rates, both sites became moderate carbon sources for the year. Analyses of assimilation and respiration parameters (e.g., maximum photosynthesis, quantum efficiency, and daytime ecosystem respiration) showed a positive trend over time pre-fire and a negative trend over time post-fire for maximum ecosystem CO2 uptake rates, and the opposite relationship for daytime ecosystem respiration rates. Within 30 days following fire, ecosystem physiological activity was statistically similar to pre-fire and appeared to be driven by the pine canopy. Our results suggest that prescribed fire (low intensity, high frequency) maintains the existing structure and function (in this case, carbon flux rates) because longleaf pine ecosystems have evolved with fire. This study, 1 year in length, provides a foundational understanding of the complex interaction between fire and carbon dynamics for longleaf pine ecosystems. Moreover, it provides a case study for applying time series analysis methods to EC data where there are complex relationships between ecosystem physiological activity and environmental drivers. However, to elicit a broader understanding of the complex interaction occurring between fire and carbon dynamics long- term studies are needed.
Article
In order to understand how carbon storage and allocation patterns vary among plantation types, we estimated carbon allocation between above- and below-ground compartments in four subtropical plantations and a naturally recovered shrubland (as a control). Results indicated that the carbon storage and allocation pattern varied greatly among forest types and was highly dependent on specific traits of trees and understory vegetation. The fast-growing species, such as Eucalyptus urophylla, accumulated more carbon in plant biomass. The biomass carbon was about 1.9- and 2.2-times greater than the 10-species mixed plantation and Castanopsis hystrix plantations, respectively. Meanwhile, the plantations sequestered 1.5- to 3-times more carbon in biomass than naturally recovered shrubland. The carbon allocation pattern between above- and below-ground compartments also varied with plantation type and stand age. The ratio of tree root carbon to tree aboveground carbon decreased with stand age for Eucalyptus urophylla and the 10-species mixed plantation. In contrast, the ratio increased for Acacia crassicarpa. Our data suggested that planting the fast-growing species in the degraded land of subtropical China was an effective choice in terms of carbon sequestration. The information about carbon allocation patterns was also valuable for decision making in sustainable forest management and climate change mitigation.
Article
Forests are experiencing simultaneous changes in climate, disturbance regimes, and management, all of which affect ecosystem function. Climate change is shifting ranges and altering forest productivity. Disturbance regimes are changing with the potential for novel interactions among disturbance types. In some areas, forest management practices are intensifying, whereas in other areas, lower-impact ecological methods are being used. Interactions among these changing factors are likely to alter ecosystem structure and function at regional to continental scales. A macrosystems approach is essential to assessing the broadscale impacts of these changes and quantify cross-scale interactions, emergent patterns, and feedbacks. A promising line of analysis is the assimilation of data with ecosystem models to scale processes to the macrosystem and generate projections based on alternative scenarios. Analyses of these projections can characterize the range of future variability in forest function and provide information to guide policy, industry, and science in a changing world.
Article
Compost addition to soil can increase nutrient availability, but if added to sandy soils, nutrients can be rapidly leached. Clay added to compost could increase nutrient retention and reduce nutrient leaching due to binding to the clay. An incubation experiment was conducted to assess the effect of addition of a fine-textured soil (34% clay) to garden waste compost on nutrient availability and leaching in a sandy soil. The sandy soil was non-amended or amended with compost only, at a rate 27.3 g kg(-1), or with a mixture of compost and 5% or 20% (w/w) of fine-textured soil. Two additional treatments included sandy soil amended with only the fine-textured soil at rates similar to those added with compost. Soil, compost, and fine-textured soil were mixed and packed to a bulk density of 1.22 g cm(-3). Soil respiration was measured over 23 days. On days 1, 5, and 23, the soils were leached with 50 mL reverse-osmosis water, and the following parameters were measured in the leachate: water-soluble organic carbon (OC), inorganic nitrogen (N), and phosphorus (P); water-soluble OC and available N and P were measured in the soil after leaching. Compost increased nutrient availability and leaching compared with the non-amended control. Addition of the fine-textured soil to compost reduced cumulative respiration and N and P leaching, with the effect more pronounced at 20% (w/w). Addition of the fine-textured soil alone had no effect on nutrient availability and leaching because of the low nutrient concentration in this soil. This study showed that addition of fine-textured soil to compost can reduce N and P leaching, which could enhance and prolong the positive effects of compost on soil fertility.
Conference Paper
Background/Question/Methods Longleaf pine (Pinus palustris) ecosystems once dominated 60-90 million acres and it is suggested that some remnant stands continue to support one of the most diverse floras in North America. It is well-known that longleaf pine ecosystems must burn frequently to maintain natural structure and function. This vegetation type ranks as one of the most fire-dependent ones in the country and must burn frequently (multiple times a decade) for natural structure and function to be maintained. Frequent fires maintain relatively low fuel loads and so many burns do not directly affect adult longleaf trees. However, other species are immediately affected by each fire that burns through a stand. Because many resident species are perennials that re-sprout after fires, it likely takes multiple burns to change the plant assemblage of the ground layer. There is a need is for better insight into fire effects on small woody stems in the ground layer. How and when fire is applied should influence the likelihood as to whether hardwood stems will overtake the ground layer, cast increasing shade over herbaceous species long before the woody species become understory shrubs and trees. Results/Conclusions For decades, most prescribed fires in longleaf pine ecosystems were conducted during the winter months, usually on a 3-year cycle. In 1974, the US Forest Service established field trials on the Escambia Experimental Forest (EEF) in south-central Alabama to study the impact of biennial prescribed burns in the spring, summer, and winter, as well as a no burn treatment on longleaf pine as well as the woody and herbaceous layer. As a result of the negative impacts to longleaf pine growth, a study was established in 1984 to follow up the above study. The summer fire treatment was removed, and the frequency between fires was expanded to include 3-year and 5-year return intervals. The hardwood species composition from each of the season of burn and fire frequency treatments will be discussed. Winter burning has not removed what are considered to be fire intolerant species such as water oak (Quercus nigra), and sweetgum (Liquidambar styraciflua), from the landscape. These species will make future fires more difficult to make and eventually make it difficult to regenerate longleaf pine. We need to do a better job with our burning or longleaf pine will be lost from the landscape.
Article
We assessed an existing method of remote sensing of wildland fire burn severity for its applicability in south-eastern USA vegetation types. This method uses Landsat satellite imagery to calculate the Normalised Burn Ratio (NBR) of reflectance bands sensitive to fire effects, and the change in NBR from pre- to post fire (dNBR) to estimate burn severity. To ground-truth ranges of NBR and dNBR that correspond to levels of burn severity, we measured severity using the Composite Burn Index at 731 locations stratified by plant community type, season of measurement, and time since fire. Best-fit curves relating Composite Burn Index to NBR or dNBR were used to determine reflectance value breakpoints that delimit levels of burn severity. Remotely estimated levels of burn severity within 3 months following fire had an average of 78% agreement with ground measurements using NBR and 75% agreement using dNBR. However, percentage agreement varied among habitat types and season of measurement, with either NBR or dNBR being advantageous under specific combinations of conditions. The results suggest this method will be useful for monitoring burned area and burn severity in south-eastern USA vegetation types if the provided recommendations and limitations are considered.
Article
Fire regulates the structure and function of longleaf pine ecosystems, including potential nutrient controls on productivity, forest floor and groundcover nutrient pools, and nutrient availability. Little is known about comparative influences of seasonality of fire, litter types, and mass on N and P balance and soil processes in longleaf pine ecosystems. This study primarily addresses the hypothesis that nutrient volatilization during growing season burning, due to combustion of live biomass, exceeds losses from winter burning of standing dead plant litter. Summer and winter burns were conducted experimentally in different groundcover types with ambient, double-ambient and no litter loadings to contrast 2-3 years of litter accumulation with very low and high fuels. As a comparison, the seasonal burns were repeated with fuel and temperature measurements on sites that had actual fuel accumulations ranging from 1 to 3 years following the last fire. Peak fire temperatures and duration of burning were similar, but with high variation across groundcover types and seasons due to variation in fuel moisture content. The highest pine litter loadings produced maximum mineral soil/litter interface temperatures that never exceeded 700°C. Groundcovers without pine litter burned incompletely and with low temperatures. Biomass and N content were greater in summer groundcover than winter groundcover, and were greater in wiregrass than old-field groundcover. More N was lost from growing season burning as biomass had higher N in green foliage at that time. With ambient litter loadings, mass losses were 88-94% of total litter and groundcover. Percentages of N lost were comparable (80-90% across all groundcovers and seasons), but amounts of N lost were below that estimated to be replenished by legume N fixation and regional atmospheric deposition over a dormant season prescribed fire cycle. Net N balances with growing season fire were generally negative only if growing season burning was projected exclusively over the long-term. P content was not significantly different among groundcovers, but summer standing stocks were higher than winter. No P losses were detected with any experimental treatments and, following burning, all P was returned to soil pools, attributable to soil surface temperatures remaining largely below 700°C. We conclude that frequent, dormant season, or even variable season burning should not seriously deplete long-term nitrogen balance of longleaf pine ecosystems.