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Fire, hurricane and carbon dioxide: Effects on net primary production of a subtropical woodland

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Abstract

Disturbance affects most terrestrial ecosystems and has the potential to shape their responses to chronic environmental change. Scrub-oak vegetation regenerating from fire disturbance in subtropical Florida was exposed to experimentally elevated carbon dioxide (CO2 ) concentration (+350 μl l(-1) ) using open-top chambers for 11 yr, punctuated by hurricane disturbance in year 8. Here, we report the effects of elevated CO2 on aboveground and belowground net primary productivity (NPP) and nitrogen (N) cycling during this experiment. The stimulation of NPP and N uptake by elevated CO2 peaked within 2 yr after disturbance by fire and hurricane, when soil nutrient availability was high. The stimulation subsequently declined and disappeared, coincident with low soil nutrient availability and with a CO2 -induced reduction in the N concentration of oak stems. These findings show that strong growth responses to elevated CO2 can be transient, are consistent with a progressively limited response to elevated CO2 interrupted by disturbance, and illustrate the importance of biogeochemical responses to extreme events in modulating ecosystem responses to global environmental change.

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Disturbance is both a major source of temporal and spatial heterogeneity in the structure and dynamics of natural communities and an agent of natural selection in the evolution of life histories. This review emphasises the impact of disturbance on the numerical abundance of populations and on the relative abundance of species in guilds and communities. Disturbance also has an important influence on ecosystem-level processes, eg primary and secondary production, biomass accumulation, energetics, and nutrient cycling. Assemblages of sessile and mobile organisms are subject to disturbance with rather different responses. -from Author
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The immediate effect of low and high severity wildfires on the main soil properties, as well as their short‐ and medium‐term evolution under field conditions, was examined. The study was performed with three pine forest soils (two Leptosols and one Humic Cambisol, developed over granite and basic schist, respectively), located in the Atlantic humid temperate zone (Galicia, NW Spain). Samples were collected from the A‐horizon (0–5 cm depth) of the burnt and the corresponding unburnt soils, immediately and 3, 6 and 12 months after the wildfires. Most properties analysed exhibit immediate fire‐induced changes and different evolution depending on fire severity and soil type. In general, immediately after the fire pH and soil properties related to nutrients availability increased and cation exchange capacity decreased, whereas properties related to soil organic matter content (C, N, Fe and Al oxides) had a variable effect depending mainly on the soil studied; all these modifications were accentuated by fire severity. These effects were attenuated in the short term in the soil affected by a low severity wildfire, but they lasted for at least 1 year in the soils affected by high severity wildfires, particularly in the Leptosols. The results showed the importance of the fire as a disturbance agent in the dynamic of nutrients and soil organic matter that is directly related with soil quality in the Galician forest ecosystems. Copyright © 2011 John Wiley & Sons, Ltd.
Article
The structure, development, and response to fire of sand-live oak (Quercus geminata Small) and myrtle oak (Q. myrtifolia Willd.) colonies (domes) were examined in a longleaf pine-wiregrass (Pinus palustris Mill.-Aristida stricta Michaux.) community in Ocala National Forest, Florida. Dome areas ranged from 30 m2 to 1000 m2 and domes achieved heights of up to 10 m. The oak clumps excavated (<2 m tall) were clonal with roots and rhizomes concentrated in the upper 50 cm of soil. In the 3-4 year old clumps excavated (above-ground age), 2/3 of the biomass was located below-ground and 1/3 was above-ground. The proportion of above-ground stems that survived a prescribed burn increased markedly for stems >2 m tall with basal diameters >2.0 cm. Stems averaging >2 m tall in domes >200 m2 in area survived better than those in smaller domes. Oak domes <2 m tall burned back to ground level but resprouted readily from underground rhizomes. Height appeared to be the most important factor influencing oak dome persistence in pyrogenic pinelands, with domes >2 m tall having a high probability of above-ground stem survival and domes >4.5 m tall being nearly fire resistant. Height is positively correlated with dome age and therefore to the period of time needed without fire for tall dome (>4.5 m tall) development. Tall domes present within the pinelands are probably a direct result of approximately 20 years of fire suppression prior to the initiation of regular prescribed burning in the 1950's. Seedling establishment was apparently low in years when fire occurred and the year after but increased in the second, third, and fourth years following burning. Dome distribution, abundance, and height are thus dependent on fire frequency, intensity, and land-use history.
Article
Forest fires are known to severely pertubate the nutrient budgets of forested ecosystems by altering the distribution, element species composition and availability of organic matter and associated nutrients. Although several studies have reported on the biogeochemical effects of fires on soil properties and element pools, relatively few have measured the water-flux driven release of nutrients from the forest floor into the mineral soil by lysimeters following low-intensity duff fires. This study was designed to test the effect of a low-intensity fire on the short-term release of nutrients from two different forest floor types developed under a mixed spruce/pine and a beech forest representing the most prevalent forest types in Germany. We found that even low-intensity fires remarkably promoted the amount of leachable nutrients under beech. Compared to the control, the fire treatment at the beech site significantly enhanced the fluxes of DOC by 75%, of DN, NO3–N, NH4–N and DON by, 233, 123, 380 and 158%, respectively, and by 158% for PO4–P. However, at the coniferous site, flux increases were less pronounced exhibiting enhancement rates for DOC of 38%, for DN, NO3–N, NH4–N and DON of 56, 41, 64 and 57%, respectively, and of 19% for PO4–P. Reasons for the different fire effects on the amount and composition of released elements at the beech and conifer site, might be due to the thicker Oa-layer of the coniferous site likely buffering the element pulses into the mineral soil more efficiently, or to the nutrient up-take by the re-establishing ground vegetation. In essence, the study clearly demonstrated that information on the potential range of short-term variability of matter fluxes induced by ecosystem disturbances is necessary, to understand differing filter and trigger mechanisms caused by varying environmental conditions within and between ecosystems.
Article
Vegetation fires influence the properties and turnover of soil organic matter in numerous ecosystems. Thermal alteration tends to increase the stability of soil organic matter. Fire induces the destruction of above‐ground biomass, which is associated with a large production of new partially-charred litter. Plant regeneration is directly affected by the fire itself, and by the ensuing modifications to the C, N and nutrient cycles associated with soil organic matter transformations. The objectives of the present study were to determine 1) changes in soil organic matter composition induced by fire, and 2) the dynamics of soil organic matter recovery in the first two decades following a fire event.. In a Florida scrub oak ecosystem, a chronosequence of soils protected from vegetation fire for 1 to 20 years was studied. The bulk organic matter and oxidation resistant elemental carbon were quantified in depths 0–5 cm, 5–15 cm, and 15–25 cm. The soil organic matter was characterized by solid-state 13C NMR spectroscopy and Curie point pyrolysis.The combination of these techniques allowed us to identify three steps in soil organic matter evolution: first the un-charred litter brought by the fire degraded between 1 and 4 years after fire; second the contribution of aryl carbon, and most probably the pyrogenic carbon, significantly decreased between 4 and 11 years after fire; Thirdly, after 11 years, the soil organic matter quality appeared driven again by the fresh litter input from regenerating plant ecosystem. In this study, the soil organic matter did not appear strongly modified by the fire. The pyrogenic carbon did not dominate the soil organic matter composition and underwent significant degradation at the decadal timescale. Our results also highlight a potential underestimated effect of dead root input to soil organic matter after the fire.
Article
Post-fire resprouting is an important process in the Mediterranean climate regions of the world and involves considerable rearrangement of biomass allocation. We have investigated the morphological changes occurring in the fine root portion of Quercus pubescens seedlings growing in controlled conditions in which fire disturbance is superimposed on drought-stressed plants. We measured the absolute (length, number of apices) and relative (specific root length and root tissue density) morphometric traits of fine roots, and the biomass and water content of the main plant compartments (leaves, shoot, taproot and lateral fine roots). Initially, soil drying significantly increased the fine root standing mass and decreased the specific fine root length irrespective of the fire, but fine root biomass declined after a critical length of time. Fire significantly decreased the above-ground biomass and its water content notwithstanding the drought stress interruption. On the contrary, time, water supply and fire disturbance factors showed significant interaction effects for the plastic morphological traits, namely, length and number of apices. In fact, fire reduced and postponed the peak of root growth in terms of the thinnest fine root fraction (0.0–0.5mm diameter) and number of apices. These findings indicate the advantages of shedding over maintaining the roots under a condition of severe drought. Indeed, shedding makes the overall reduction of the root system more functional, and induces a partial increase in water particularly in the thicker fraction of the fine roots (0.5–2.0mm). Shoot removal by fire seems to lessen and prolong the acclimation process to drought, but the decrease in non structural carbohydrate reserves appears to impede the recovery process at least after persistent drought.
Article
Alpine plant species have been shown to exhibit a more pronounced increase in leaf photosynthesis under elevated CO2 than lowland plants. In order to test whether this higher carbon fixation efficiency will translate into increased biomass production under CO2 enrichment we exposed plots of narrow alpine grassland (Swiss Central Alps, 2470 m) to ambient (355 µl l-1) and elevated (680 µl l-1) CO2 concentration using open top chambers. Part of the plost received moderate mineral nutrient additions (40 kg ha-1 year-1 of nitrogen in a complete fertilizer mix). Under natural nutrient supply CO2 enrichment had no effect on biomass production per unit land area during any of the three seasons studied so far. Correspondingly, the dominant species Carex curvula and Leontodon helveticus as well as Trifolium alpinum did not show a growth response either at the population level or at the shoot level. However, the subdominant generalistic species Poa alpina strongly increased shoot growth (+47%). Annual root production (in ingrowth cores) was significantly enhanced in C. curvula in the 2nd and 3rd year of investigation (+43%) but was not altered in the bulk samples for all species. Fertilizer addition generally stimulated above-ground (+48%) and below-ground (+26%) biomass production right from the beginning. Annual variations in weather conditions during summer also strongly influenced above-ground biomass production (19–27% more biomass in warm seasons compared to cool seasons). However, neither nutrient availability nor climate had a significant effect on the CO2 response of the plants. Our results do not support the hypothesis that alpine plants, due to their higher carbon uptake efficiency, will increase biomass production under future atmospheric CO2 enrichment, at least not in such late successional communities. However, as indicated by the response of P. alpina, species-specific responses occur which may lead to altered community structure and perhaps ecosystem functioning in the long-term. Our findings further suggest that possible climatic changes are likely to have a greater impact on plant growth in alpine environments than the direct stimulation of photosynthesis by CO2. Counter-intuitively, our results suggest that even under moderate climate warming or enhanced atmospheric nitrogen deposition positive biomass responses to CO2 enrichment of the currently dominating species are unlikely.
Article
The Atlantic Ocean has spawned three hurricanes in three years that have made direct hits on the U.S. east coast. The unusual activity has sparked a question of whether East Coast landfalling hurricanes are increasing in frequency and intensity. Results of data studied from 1900-98 indicate that there is a slight increase in hurricane frequency and a fairly constant intensity level for East Coast landfalling hurricanes. The recent landfall of Hurricane Bonnie in North Carolina, Hurricane George in Alabama/Mississippi, and simultaneous formation of four hurricanes in the mid-Atlantic during the 1998 season have stimulated the question of whether hurricanes are becoming more frequent and intense along the East Coast heading into the twenty-first century.
Article
The effects of elevated CO2 (ambient, +175, and +350 μl l−1) and nitrogen fertilization (0, 100, and 200 kg N ha−1 yr−1 as ammonium sulfate) on C and N accumulations in biomass and soils planted with ponderosa pine (Pinus ponderosa Laws) over a 6-year study period are reported. Both nitrogen fertilization and elevated CO2 caused increases in C and N contents of vegetation over the study period. The pattern of responses varied over time. Responses to CO2 decreased in the +175 μl l−1 and increased in the +350 μl l−1 after the first year, whereas responses to N decreased after the first year and became non-significant by year six. Foliar N concentrations were lower and tree C:N ratios were higher with elevated CO2 in the early years, but this was offset by the increases in biomass, resulting in substantial increases in N uptake with elevated CO2. Nitrogen budget estimates showed that the major source of the N for unfertilized trees, with or without elevated CO2, was likely the soil organic N pool. There were no effects of elevated CO2 on soil C, but a significant decrease in soil N and an increase in soil C:N ratio in year six. Nitrogen fertilization had no significant effect on tree C:N ratios, foliar N concentrations, soil C content, soil N content, or soil C:N ratios. There were no significant interactions between CO2 and N treatments, indicating that N fertilization had no effect on responses to CO2 and that CO2 treatments had no effect on responses to N fertilization. These results illustrate the importance of long-term studies involving more than one level of treatment to assess the effects of elevated CO2.
Article
Elevated atmospheric CO2 concentrations ([CO2 ]) generally increase primary production of terrestrial ecosystems. Production responses to elevated [CO2 ] may be particularly large in deserts but information on their long-term response is unknown. We evaluated the cumulative effects of elevated [CO2 ] on primary production at the Nevada Desert FACE Facility. Aboveground and belowground perennial plant biomass was harvested in an intact Mojave Desert ecosystem at the end of a 10-year elevated [CO2 ] experiment. We measured community standing biomass, biomass allocation, canopy cover, leaf area index (LAI), carbon and nitrogen content, and isotopic composition of plant tissues for five to eight dominant species. We provide the first long-term results of elevated [CO2 ] on biomass components of a desert ecosystem and offer information on understudied Mojave Desert species. In contrast to initial expectations, ten years of elevated [CO2 ] had no significant effect on standing biomass, biomass allocation, canopy cover, and C:N ratios of above- and belowground components. However, elevated [CO2 ] increased short-term responses, including leaf water-use efficiency as measured by carbon isotope discrimination and increased plot-level LAI. Standing biomass, biomass allocation, canopy cover, and C:N ratios of above- and belowground pools significantly differed among dominant species, but responses to elevated [CO2 ] did not vary among species, photosynthetic pathway (C3 vs. C4 ), or growth form (drought-deciduous shrub vs. evergreen shrub vs. grass). Thus, even though previous and current results occasionally show increased leaf-level photosynthetic rates, water-use efficiency, LAI, and plant growth under elevated [CO2 ] during the 10-year experiment, most responses were in wet years and did not lead to sustained increases in community biomass. We presume that the lack of sustained biomass responses to elevated [CO2 ] is explained by inter-annual differences in water availability. Therefore, the high frequency of low precipitation years may constrain cumulative biomass responses to elevated [CO2 ] in desert environments. © 2013 Blackwell Publishing Ltd.
Article
Potential nitrogen mineralization and nitrification were examined in an age sequence of clearcut and control hardwood forests. Results of laboratory incubations indicated that nitrification was always greater in clearcut soils than in control forest soils, while mineralization was significantly greater only in a 4-year-old clearcut. Increasing moisture increased rates of mineralization and nitrification in both clearcut and control forest soils. In situ incubations indicated that warmer soil temperatures increased rates of mineralization and nitrification in the youngest clearcut. Regardless of the immediate effects of temperature and moisture, all clearcut soils produced more nitrate than control soils under identical conditions. The results of a microlysimeter experiment suggested that these differences are due to the initial population size of nitrifying bacteria. Forest Sci. 27:781-791.
Article
Increased atmospheric CO2 concentration (Ca) produces a short-term stimulation of photosynthesis and plant growth across terrestrial ecosystems. However, the long-term response remains uncertain and is thought to depend on environmental constraints. In the longest experiment on natural ecosystem response to elevated Ca, we measured the shoot-density, biomass and net CO2 exchange (NEE) responses to elevated Ca from 1987 to 2003 in a Scirpus olneyi wetland sedge community of the Chesapeake Bay, MD, USA. Measurements were conducted in five replicated open-top chambers per CO2 treatment (ambient and elevated). In addition, unchambered control plots were monitored for shoot density. Responses of daytime NEE, Scirpus plant biomass and shoot density to elevated Ca were positive for any single year of the 17-year period of study. Daytime NEE stimulation by elevated Ca rapidly dropped from 80% at the onset of the experiment to a long-term stimulation average of about 35%. Shoot-density stimulation by elevated Ca increased linearly with duration of exposure (r2=0.89), exceeding 120% after 17 years. Although of lesser magnitude, the shoot biomass response to elevated Ca was similar to that of the shoot density. Daytime NEE response to elevated Ca was not explained by the duration of exposure, but negatively correlated with salinity of the marsh, indicating that this elevated-Ca response was decreased by water-related stress. By contrast, circumstantial evidence suggested that salinity stress increased the stimulation of shoot density by elevated Ca, which highlights the complexity of the interaction between water-related stresses and plant community responses to elevated Ca. Notwithstanding the effects of salinity stress, we believe that the most important finding of the present research is that a species response to elevated Ca can continually increase when this species is under stress and declining in its natural environment. This is particularly important because climate changes associated with elevated Ca are likely to increase environmental stresses on numerous species and modify their present distribution. Our results point to an increased resilience to change under elevated Ca when plants are exposed to adverse environmental conditions.
Article
The fate of immobilized N in soils is one of the great uncertainties in predicting C sequestration at increased CO2 and N deposition. In a dual isotope tracer experiment (13C, 15N) within a 4-year CO2 enrichment (+200 ppmv) study with forest model ecosystems, we (i) quantified the effects of elevated CO2 on the partitioning of N; (ii) traced immobilized N into physically separated pools of soil organic matter (SOM) with turnover rates known from their 13C signals; and (iii) estimated the remobilization and thus, the bio-availability of newly sequestered C and N. (1) CO2 enrichment significantly decreased NO3− concentrations in soil waters and export from 1.5 m deep lysimeters by 30–80%. Consequently, elevated CO2 increased the overall retention of N in the model ecosystems. (2) About 60–80% of added 15NH415NO3 were retained in soils. The clay fraction was the greatest sink for the immobilized 15N sequestering 50–60% of the total new soil N. SOM associated with clay contained only 25% of the total new soil C pool and had small C/N ratios (<13), indicating that it consists of humified organic matter with a relatively slow turn over rate. This implies that added 15N was mainly immobilized in stable mineral-bound SOM pools. (3) Incubation of soils for 1 year showed that the remobilization of newly sequestered N was three to nine times smaller than that of newly sequestered C. Thus, inorganic inputs of N were stabilized more effectively in soils than C. Significantly less newly sequestered N was remobilized from soils previously exposed to elevated CO2. In summary, our results show firstly that a large fraction of inorganic N inputs becomes effectively immobilized in relative stable SOM pools and secondly that elevated CO2 can increase N retention in soils and hence it may tighten N cycling and diminish the risk of nitrate leaching to groundwater.
Article
Ecology Letters (2011) 14: 349–357AbstractThe earth’s future climate state is highly dependent upon changes in terrestrial C storage in response to rising concentrations of atmospheric CO2. Here we show that consistently enhanced rates of net primary production (NPP) are sustained by a C-cascade through the root-microbe-soil system; increases in the flux of C belowground under elevated CO2 stimulated microbial activity, accelerated the rate of soil organic matter decomposition and stimulated tree uptake of N bound to this SOM. This process set into motion a positive feedback maintaining greater C gain under elevated CO2 as a result of increases in canopy N content and higher photosynthetic N-use efficiency. The ecosystem-level consequence of the enhanced requirement for N and the exchange of plant C for N belowground is the dominance of C storage in tree biomass but the preclusion of a large C sink in the soil.
Article
Fine root dynamics play an important role in the cycling of carbon belowground. Previous studies have indicated that CO2 enrichment results in increased root productivity, mortality and relative turnover; however, our understanding of the duration and long-term trends of this effect are limited. Non-destructive minirhizotron observation tubes were used to measure effects of elevated CO2 on root dynamics and survivorship in a fire dominated scrub-oak ecosystem. Open-top chambers were exposed to elevated atmospheric CO2 for 10 years at Kennedy Space Center, Florida. In this study, initial fine root dynamics from an earlier published study from this experiment (Dilustro et al., 2002) were compared to our findings 5 years later. Significant increases in root productivity, mortality, and turnover due to CO2 enrichment were no longer present after 9 years of treatment. However, the vertical variation in these parameters suggests the upper 50 cm of the soil are the most dynamic. A greater proportion of the fine roots were deeper in the soil profile later in the study, but no CO2 effect was observed. Survivorship analysis suggested the smallest fine roots (i.e. <0.1 mm in diameter and <0.25 mm in length) were most susceptible to mortality. In addition, increased root persistence was correlated with greater soil depth, suggesting that a nutrient and water limited scrub-oak ecosystem at root closure or carrying capacity produces larger, longer-lived fine roots at greater depths. Mean root diameter increased in the upper and lower portions of the soil profile. Seasonal cohort analysis implied that roots appearing in the spring and summer typically had the highest risk of mortality in the fall, although environmental factors influencing this relationship are not clear. The results from this study indicated that CO2 enrichment is no longer driving changes in fine root dynamics, but rather root closure in the upper portions of the soil profile seem to be the strongest influence. Fine roots comprise nearly 25% of the total plant biomass in the scrub-oak ecosystem and their turnover and persistence is an important pathway for carbon inputs into the soil. In order to develop accurate predictive models of the impacts of increasing anthropogenic CO2 on carbon cycling, it is imperative to examine long-term fine root dynamics rather than just shorter observations that could result in misleading conclusions regarding ecosystem responses.
Article
Plant growth is stimulated by elevated atmospheric pCO(2), and hence demand for nutrients increases. In this context, nitrogen is a very prominent element; it can either be supplied from the limited available soil N or through biological (e.g. symbiotic) nitrogen fixation. In this study, the effect of elevated pCO(2) (60 Pa) on symbiotic N-2 fixation (N-15-isotope dilution method) was investigated using Free-Air-CO2-Enrichment (FACE) technology over a period of two growing seasons. Trifolium repens L, was cultivated either alone or in mixed swards together with Lolium perenne L. (non-fixing reference crop). In T. repens, percentage of plant N derived from symbiotic N-2 fixation (%Nsym) increased from 59 to 66% under elevated pCO(2). The major part of the additionally assimilated N was derived from symbiotic N-2 fixation. In the mixed swards, increased N yield was entirely due to increased symbiotic N-2 fixation. It is suggested that increased N-2 fixation is an important factor in the satisfaction of increased N demand in both clover and the associated grass under elevated pCO(2).
Article
A scrub-oak woodland has maintained higher aboveground biomass accumulation after 11 years of atmospheric CO2 enrichment (ambient +350 μmol CO2 mol−1), despite the expectation of strong nitrogen (N) limitation at the site. We hypothesized that changes in plant available N and exploitation of deep sources of inorganic N in soils have sustained greater growth at elevated CO2. We employed a suite of assays performed in the sixth and 11th year of a CO2 enrichment experiment designed to assess soil N dynamics and N availability in the entire soil profile. In the 11th year, we found no differences in gross N flux, but significantly greater microbial respiration (P≤0.01) at elevated CO2. Elevated CO2 lowered extractable inorganic N concentrations (P=0.096) considering the whole soil profile (0–190 cm). Conversely, potential net N mineralization, although not significant in considering the entire profile (P=0.460), tended to be greater at elevated CO2. Ion-exchange resins placed in the soil profile for approximately 1 year revealed that potential N availability at the water table was almost 3 × greater than found elsewhere in the profile, and we found direct evidence using a 15N tracer study that plants took up N from the water table. Increased microbial respiration and shorter mean residence times of inorganic N at shallower depths suggests that enhanced SOM decomposition may promote a sustained supply of inorganic N at elevated CO2. Deep soil N availability at the water table is considerable, and provides a readily available source of N for plant uptake. Increased plant growth at elevated CO2 in this ecosystem may be sustained through greater inorganic N supply from shallow soils and N uptake from deep soil.
Article
To determine the long-term impact of elevated CO2 on primary production of native tallgrass prairie, we compared the responses of tallgrass prairie at ambient and twice-ambient atmospheric CO2 levels over an 8-year period. Plots in open-top chambers (4.5 m diameter) were exposed continuously (24 h) to ambient and elevated CO2 from early April to late October each year. Unchambered plots were monitored also. Above-ground peak biomass was determined by clipping each year in early August, and root growth was estimated by harvesting roots from root ingrowth bags. Plant community composition was censused each year in early June. In the last 2 years of the study, subplots were clipped on 1 June or 1 July, and regrowth was harvested on 1 October. Volumetric soil water content of the 0–100 cm soil layer was determined using neutron scattering, and was generally higher in elevated CO2 plots than ambient. Peak above-ground biomass was greater on elevated CO2 plots than ambient CO2 plots with or without chambers during years with significant plant water stress. Above-ground regrowth biomass was greater under elevated CO2 than under ambient CO2 in a year with late-season water stress, but did not differ in a wetter year. Root ingrowth biomass was also greater in elevated CO2 plots than ambient CO2 plots when water stress occurred during the growing season. The basal cover and relative amount of warm-season perennial grasses (C4) in the stand changed little during the 8-year period, but basal cover and relative amount of cool-season perennial grasses (C3) in the stand declined in the elevated CO2 plots and in ambient CO2 plots with chambers. Forbs (C3) and members of the Cyperaceae (C3) increased in basal cover and relative amount in the stand at elevated compared to ambient CO2. Greater biomass production under elevated CO2 in C4-dominated grasslands may lead to a greater carbon sequestration by those ecosystems and reduce peak atmospheric CO2 concentrations in the future.
Article
Elevated atmospheric carbon dioxide (CO2) often stimulates the growth of fine roots, yet there are few reports of responses of intact root systems to long-term CO2 exposure. We investigated the effects of elevated CO2 on fine root growth using open top chambers in a scrub oak ecosystem at Kennedy Space Center, Florida for more than 7 years. CO2 enrichment began immediately after a controlled burn, which simulated the natural disturbance that occurs in this system every 10–15 years. We hypothesized that (1) root abundance would increase in both treatments as the system recovered from fire; (2) elevated CO2 would stimulate root growth; and (3) elevated CO2 would alter root distribution. Minirhizotron tubes were used to measure fine root length density (mm cm−2) every three months. During the first 2 years after fire recovery, fine root abundance increased in all treatments and elevated CO2 significantly enhanced root abundance, causing a maximum stimulation of 181% after 20 months. The CO2 stimulation was initially more pronounced in the top 10 cm and 38–49 cm below the soil surface. However, these responses completely disappeared during the third year of experimental treatment: elevated CO2 had no effect on root abundance or on the depth distribution of fine roots during years 3–7. The results suggest that, within a few years following fire, fine roots in this scrub oak ecosystem reach closure, defined here as a dynamic equilibrium between production and mortality. These results further suggest that elevated CO2 hastens root closure but does not affect maximum root abundance. Limitation of fine root growth by belowground resources – particularly nutrients in this nutrient-poor soil – may explain the transient response to elevated CO2.
Article
The effect of elevated atmospheric CO2 concentration (Ca) on the aboveground biomass of three oak species, Quercus myrtifolia, Q. geminata, and Q. chapmanii, was estimated nondestructively using allometric relationships between stem diameter and aboveground biomass after four years of experimental treatment in a naturally fire-regenerated scrub-oak ecosystem. After burning a stand of scrub-oak vegetation, re-growing plants were exposed to either current ambient (379 µL L−1 CO2) or elevated (704 µL L−1 CO2) Ca in 16 open-top chambers over a four-year period, and measurements of stem diameter were carried out annually on all oak shoots within each chamber. Elevated Ca significantly increased aboveground biomass, expressed either per unit ground area or per shoot; elevated Ca had no effect on shoot density. The relative effect of elevated Ca on aboveground biomass increased each year of the study from 44% (May 96–Jan 97), to 55% (Jan 97–Jan 98), 66% (Jan 98–Jan 99), and 75% (Jan 99–Jan 00). The effect of elevated Ca was species specific: elevated Ca significantly increased aboveground biomass of the dominant species, Q. myrtifolia, and tended to increase aboveground biomass of Q. chapmanii, but had no effect on aboveground biomass of the subdominant, Q. geminata. These results show that rising atmospheric CO2 has the potential to stimulate aboveground biomass production in ecosystems dominated by woody species, and that species-specific growth responses could, in the long term, alter the composition of the scrub-oak community.
Article
We report the results of a 2-year study of effects of the elevated (current ambient plus 350 μmol CO2 mol−1) atmospheric CO2 concentration (Ca) on net ecosystem CO2 exchange (NEE) of a scrub–oak ecosystem. The measurements were made in open-top chambers (OTCs) modified to function as open gas-exchange systems. The OTCs enclosed samples of the ecosystem (ca. 10 m2 surface area) that had regenerated after a fire, 5 years before, in either current ambient or elevated Ca. Throughout the study, elevated Ca increased maximum NEE (NEEmax) and the apparent quantum yield of the NEE (φNEE) during the photoperiod. The magnitude of the stimulation of NEEmax, expressed per unit ground area, was seasonal, rising from 50% in the winter to 180% in the summer. The key to this stimulation was effects of elevated Ca, and their interaction with the seasonal changes in the environment, on ecosystem leaf area index, photosynthesis and respiration. The separation of these factors was difficult. When expressed per unit leaf area the stimulation of the NEEmax ranged from 7% to 60%, with the increase being dependent on increasing soil water content (Wsoil). At night, the CO2 effluxes from the ecosystem (NEEnight) were on an average 39% higher in elevated Ca. However, the increase varied between 6% and 64%, and had no clear seasonality. The partitioning of NEEnight into its belowground (Rbelow) and aboveground (Rabove) components was carried out in the winter only. A 35% and 27% stimulation of NEEnight in December 1999 and 2000, respectively, was largely due to a 26% and 28% stimulation of Rbelow in the respective periods, because Rbelow constituted ca. 87% of NEEnight. The 37% and 42% stimulation of Rabove in December 1999 and 2000, respectively, was less than the 65% and 80% stimulation of the aboveground biomass by elevated Ca at these times. An increase in the relative amount of the aboveground biomass in woody tissue, combined with a decrease in the specific rate of stem respiration of the dominant species Quercus myrtifolia in elevated Ca, was responsible for this effect. Throughout this study, elevated Ca had a greater effect on carbon uptake than on carbon loss, in terms of both the absolute flux and relative stimulation. Consequently, for this scrub–oak ecosystem carbon sequestration was greater in the elevated Ca during this 2-year study period.
Article
This study reports the aboveground biomass response of a fire-regenerated Florida scrub-oak ecosystem exposed to elevated CO2 (1996–2007), from emergence after fire through canopy closure. Eleven years exposure to elevated CO2 caused a 67% increase in aboveground shoot biomass. Growth stimulation was sustained throughout the experiment; although there was significant variability between years. The absolute stimulation of aboveground biomass generally declined over time, reflecting increasing environmental limitations to long-term growth response. Extensive defoliation caused by hurricanes in September 2004 was followed by a strong increase in shoot density in 2005 that may have resulted from reopening the canopy and relocating nitrogen from leaves to the nutrient-poor soil. Biomass response to elevated CO2 was driven primarily by stimulation of growth of the dominant species, Quercus myrtifolia, while Quercus geminata, the other co-dominant oak, displayed no significant CO2 response. Aboveground growth also displayed interannual variation, which was correlated with total annual rainfall. The rainfall × CO2 interaction was partially masked at the community level by species-specific responses: elevated CO2 had an ameliorating effect on Q. myrtifolia growth under water stress. The results of this long-term study not only show that atmospheric CO2 concentration had a consistent stimulating effect on aboveground biomass production, but also showed that available water is the primary driver of interannual variation in shoot growth and that the long-term response to elevated CO2 may have been caused by other factors such as nutrient limitation and disturbance.
Article
Although it is widely accepted that elevated atmospheric carbon dioxide (CO2), nitrogen (N) deposition, and climate change will alter ecosystem productivity and function in the coming decades, the combined effects of these environmental changes may be nonadditive, and their interactions may be altered by disturbances, such as fire. We examined the influence of a summer wildfire on the interactive effects of elevated CO2, N deposition, and increased precipitation in a full-factorial experiment conducted in a California annual grassland. In unburned plots, primary production was suppressed under elevated CO2. Burning alone did not significantly affect production, but it increased total production in combination with nitrate additions and removed the suppressive effect of elevated CO2. Increased production in response to nitrate in burned plots occurred as a result of the enhanced aboveground production of annual grasses and forbs, whereas the removal of the suppressive effect of elevated CO2 occurred as a result of increased aboveground forb production in burned, CO2-treated plots and decreased root production in burned plots under ambient CO2.The tissue nitrogen–phosphorus ratio, which was assessed for annual grass shoots, decreased with burning and increased with nitrate addition. Burning removed surface litter from plots, resulting in an increase in maximum daily soil temperatures and a decrease in soil moisture both early and late in the growing season. Measures of vegetation greenness, based on canopy spectral reflectance, showed that plants in burned plots grew rapidly early in the season but senesced early. Overall, these results indicate that fire can alter the effects of elevated CO2 and N addition on productivity in the short term, possibly by promoting increased phosphorus availability.
Article
Fine root biomass and C content are critical components in ecosystem C models, but they cannot be directly determined by minirhizotron techniques, and indirect methods involve estimating 3-dimensional values (biomass/ soil volume) from 2-dimensional measurements. To estimate biomass from minirhizotron data, a conversion factor for length to biomass must be developed, and assumptions regarding depth of view must be made. In a scrub-oak ecosystem in central Florida, USA, root length density (RLD) was monitored for 10years in a CO2 manipulation experiment using minirhizotron tubes. In the seventh year of the study, soil cores were removed from both ambient and elevated CO2 chambers. Roots from those cores were used to determine specific root length values (m/g) that were applied to the long-term RLD data for an estimation of root biomass over 10years of CO2 manipulation. Root length and biomass estimated from minirhizotron data were comparable to determinations from soil cores, suggesting that the minirhizotron biomass model is valid. Biomass estimates from minirhizotrons indicate the <0.25mm diameter roots accounted for nearly 95% of the total root length in 2002. The long-term trends for this smallest size class (<0.25mm diameter) mirrored the RLD trends closely, particularly in relation to suspected root closure in this system. Elevated CO2 did not significantly affect specific root length as determined by the soil cores. A significant treatment effect indicated smallest diameter fine roots (<0.25mm) were greater under elevated CO2 during the early years of the study and the largest (2–10mm) had greater biomass under elevated CO2 during the later years of the study. Overall, this method permits long-term analysis of the effects of elevated CO2 on fine root biomass accumulation and provides essential information for carbon models.
Article
Abstract The effects of elevated CO2 on plant growth and insect herbivory have been frequently investigated over the past 20 years. Most studies have shown an increase in plant growth, a decrease in plant nitrogen concentration, an increase in plant secondary metabolites and a decrease in herbivory. However, such studies have generally overlooked the fact that increases in plant production could cause increases of herbivores per unit area of habitat. Our study investigated leaf production, herbivory levels and herbivore abundance per unit area of leaf litter in a scrub-oak system at Kennedy Space Center, Florida, under conditions of ambient and elevated CO2, over an 11-year period, from 1996 to 2007. In every year, herbivory, that is leafminer and leaftier abundance per 200 leaves, was lower under elevated CO2 than ambient CO2 for each of three species of oaks, Quercus myrtifolia, Quercus chapmanii and Quercus geminata. However, leaf litter production per 0.1143 m2 was greater under elevated CO2 than ambient CO2 for Q. myrtifolia and Q. chapmanii, and this difference increased over the 11 years of the study. Leaf production of Q. geminata under elevated CO2 did not increase. Leafminer densities per 0.1143 m2 of litterfall for Q. myrtifolia and Q. chapmanii were initially lower under elevated CO2. However, shortly after canopy closure in 2001, leafminer densities per 0.1143 m2 of litter fall became higher under elevated CO2 and remained higher for the remainder of the experiment. Leaftier densities per 0.1143 m2 were also higher under elevated CO2 for Q. myrtifolia and Q. chapmanii over the last 6 years of the experiment. There were no differences in leafminer or leaftier densities per 0.1143 m2 of litter for Q. geminata. These results show three phenomena. First, they show that elevated CO2 decreases herbivory on all oak species in the Florida scrub-oak system. Second, despite lower numbers of herbivores per 200 leaves in elevated CO2, increased leaf production resulted in higher herbivore densities per unit area of leaf litter for two oak species. Third, they corroborate other studies which suggest that the effects of elevated CO2 on herbivores are species specific, meaning they depend on the particular plant species involved. Two oak species showed increases in leaf production and herbivore densities per 0.1143 m2 in elevated CO2 over time while another oak species did not. Our results point to a future world of elevated CO2 where, despite lower plant herbivory, some insect herbivores may become more common.
Article
Portions of a regenerating scrub oak ecosystem were enclosed in open-top chambers and exposed to elevated CO2. The distinct 13C signal of the supplemental CO2 was used to trace the rate of C integration into various ecosystem components. Oak foliage, stems, roots and ectomycorrhizae were sampled over 3 years and were analyzed for 13C composition. The aboveground tissue 13C equilibrated to the novel 13C signal in the first season, while the belowground components displayed extremely slow integration of the new C. Roots taken from ingrowth cores showed that 33% of the C in newly formed roots originated from a source other than recent photosynthesis inside the chamber. In this highly fire-prone system, the oaks re-establish primarily by resprouting from large rhizomes. Remobilization from belowground C stores may support fine roots and mycorrhizae for several years into stand re-establishment and, therefore, may explain why belowground tissues contain less of the new photosynthate than expected. Though it has been shown that long-term cycles of C storage are theoretically advantageous for plants in systems with frequent and severe disturbances, such patterns have not been previously examined in wild systems.
Article
Hurricane disturbances have profound impacts on ecosystem structure and function, yet their effects on ecosystem CO2 exchange have not been reported. In September 2004, our research site on a fire-regenerated scrub-oak ecosystem in central Florida was struck by Hurricane Frances with sustained winds of 113kmh� 1 and wind gusts as high as 152kmh� 1. We quantified the hurricane damage on this ecosystem resulting from defoliation: we measured net ecosystem CO2 exchange, the damage and recovery of leaf area, and determined whether growth in elevated carbon dioxide concentration in the atmosphere (Ca) altered this disturbance. The hurricane decreased leaf area index (LAI) by 21%, which was equal to 60% of seasonal variation in canopy growth during the previous 3 years, but stem damage was negligible. The reduction in LAI led to a 22% decline in gross primary production (GPP) and a 25% decline in ecosystem respiration (Re). The compensatory declines in GPP and Re resulted in no significant change in net ecosystem production (NEP). Refoliation began within a month after the hurricane, although this period was out of phase with the regular foliation period, and recovered 20% of the defoliation loss within 2.5 months. Full recovery of LAI, ecosystem CO2 assimilation, and ecosystem respiration did not occur until the next growing season. Plants exposed to elevated Ca did not sustain greater damage, nor did they recover faster than plants grown under ambient Ca. Thus, our results indicate that hurricanes capable of causing significant defoliation with negligible damage to stems have negligible effects on NEP under current or future CO2-enriched environment.
Chapter
This chapter discusses the processes that control ecosystem carbon storage and impact of rising atmospheric carbon dioxide. A broad range of ecosystem services is potentially CO2 sensitive. Net primary production (NPP) and carbon storage are but two examples. Other examples that are much less studied include freshwater resources climate, the abundance and distribution of valuable species and genetic resources, and aesthetics or a sense of place. None of these services can be accurately quantified in isolation, and the greatest value of one or more may emerge through their impacts on other services. For example, changes in biome boundaries in response to elevated CO2 could have large impacts on NPP and carbon storage. Subtle changes, like altered flammability or sensitivity to a pest or pathogen, could have major impacts on ecosystem services ranging from carbon storage, to maintenance of soil fertility, to sensitivity to biological invasion. The value of the ecosystem services connected with NPP and carbon storage depends on perspective. Carbon storage or net ecosystem production in response to elevated CO2 is critically dependent on the relative dynamics of production and decomposition. When NPP rises, it tends to get ahead of the equilibrium with carbon losses, resulting in positive NEP.
Article
Elevated atmospheric CO2 tends to stimulate plant productivity, which could either stimulate or suppress the processing of soil carbon, thereby feeding back to atmospheric CO2 concentrations. We employed an acid-hydrolysis-incubation method and a net nitrogen-mineralization assay to assess stability of soil carbon pools and short-term nitrogen dynamics in a Florida scrub-oak ecosystem after six years of exposure to elevated CO2. We found that soil carbon concentration in the slow pool was 27% lower in elevated than ambient CO2 plots at 0–10 cm depth. The difference in carbon mass was equivalent to roughly one-third of the increase in plant biomass that occurred in the same experiment. These results concur with previous reports from this ecosystem that elevated CO2 stimulates microbial degradation of relatively stable soil organic carbon pools. Accordingly, elevated CO2 increased net N mineralization in the 10–30 cm depth, which may increase N availability, thereby allowing for continued stimulation of plant productivity by elevated CO2. Our findings suggest that soil texture and climate may explain the differential response of soil carbon among various long-term, field-based CO2 studies. Increased mineralization of stable soil organic carbon by a CO2-induced priming effect may diminish the terrestrial carbon sink globally.
Article
Elevated carbon dioxide (CO2) caused greater accumulation of carbon (C) and nutrients in both vegetation and O horizons over a 5-yr sampling period in a scrub oak ecosystem in Florida. Elevated CO2 had no effect on any measured soil property except extractable phosphorus (P), which was lower with elevated CO2 after five years. Anion and cation exchange membranes showed lower available nitrogen (N) and zinc (Zn) with elevated CO2. Soils in both elevated and ambient CO2 showed decreases in total C, N, sulfur (S), and cation exchange capacity, and increases in base saturation, exchangeable Ca2+, and Mg2+ over the 5-yr sampling period. We hypothesize that these soil changes were a delayed response to prescribed fire, which was applied to the site just before the initiation of the experiment. In the ambient CO2 treatment, the increases in vegetation and O horizon C, N, and S were offset by the losses of soil total C, N, and S, resulting in no statistically significant net changes in ecosystem C, N, or S over time. In the elevated CO2 treatment the increases in vegetation and O horizon C content outweighed the losses in soil C, resulting in a statistically significant net increase in ecosystem C content. Nitrogen and S contents showed no statistically significant change over time in the elevated CO2 treatment, however. Comparisons of vegetation contents and soil pools of potassium (K), calcium (Ca), and magnesium (Mg) suggest that a substantial proportion of these nutrients were taken up from either groundwater or deep soil horizons. This study demonstrates that changes in ecosystem C sequestration due elevated CO2 or any other factor cannot be accurately assessed in the absence of data on changes in soils.