J. Adam Langley

MYCORRHIZAL CONTROLS ON BELOWGROUND LITTER QUALITY

Plant productivity and ecosystem productivity are strongly influenced by nutrient availability, which is largely determined by the decomposition rate of plant litter. Belowground litter inputs (dead roots, mycorrhizae, and exudates) are larger than above- ground litterfall in many systems. Chemical quality and diameter primarily control decom- position for coarse roots, but these patterns do not hold for finer classes of roots, which are frequently colonized by mycorrhizae. Though mycorrhizal status is known to drastically alter root chemistry, morphology, life span, and exudation, it has never been explicitly considered as a factor affecting root decomposition. We hypothesize that mycorrhizal status substantially influences fine root decomposition rates. Both ectomycorrhizal (EM) and arbuscular mycorrhizal (AM) fungi can change root properties but do so in different ways. Dominant tree species of most cold and temperate forests rely heavily on EM associations. EM fungi form massive structures that envelop fine roots. Roots infected by ectomycorrhizae have higher nitrogen concentrations than nonmycorrhizal roots, which would be expected to increase decomposition rates, but much of this nitrogen is bound in recalcitrant forms, such as chitin, so the net effect on decom- position is difficult to predict. AM fungi lack elaborate, macroscopic structures and may not alter root chemistry as profoundly. In addition to mycorrhizal roots, external fungal hyphae can contribute significantly to ecosystem carbon budgets and thereby influence rates of soil carbon turnover. Hyphae have commonly been considered a rapidly decomposing carbon pool, though this has never been demonstrated experimentally. If hyphae are produced at the expense of rapidly decomposing root exudates, then the net effect of hyphal litter production might be to reduce soil microbial activity and overall carbon cycling rates. Based on known differences in morphology and chemistry, EM hyphae may be more recalcitrant than AM hyphae. In summary, we submit that mycorrhizal status could substantially influence fine root decomposition and soil carbon processing rates, potentially explaining some of the variation observed within and among individual plant species and ecosystems.

Elevated CO2 stimulates marsh elevation gain, counterbalancing sea-level rise

Tidal wetlands experiencing increased rates of sea-level rise (SLR) must increase rates of soil elevation gain to avoid permanent conversion to open water. The maximal rate of SLR that these ecosystems can tolerate depends partly on mineral sediment deposition, but the accumulation of organic matter is equally important for many wetlands. Plant productivity drives organic matter dynamics and is sensitive to global change factors, such as rising atmospheric CO2 concentration. It remains unknown how global change will influence organic mechanisms that determine future tidal wetland viability. Here, we present experimental evidence that plant response to elevated atmospheric [CO2] stimulates biogenic mechanisms of elevation gain in a brackish marsh. Elevated CO2 (ambient + 340 ppm) accelerated soil elevation gain by 3.9 mm yr−1 in this 2-year field study, an effect mediated by stimulation of below-ground plant productivity. Further, a companion greenhouse experiment revealed that the CO2 effect was enhanced under salinity and flooding conditions likely to accompany future SLR. Our results indicate that by stimulating biogenic contributions to marsh elevation, increases in the greenhouse gas, CO2, may paradoxically aid some coastal wetlands in counterbalancing rising seas.

Partitioning direct and indirect effects reveals the response of water-limited ecosystems to elevated CO2

Significance Elevated levels of atmospheric carbon dioxide affect plants directly by stimulating photosynthesis and reducing stomatal aperture. These direct effects trigger several more subtle, indirect effects via changes in soil moisture and plant structure. While such effects have been acknowledged, they have never been assessed quantitatively, partly due to the fact they are inseparable in field experiments. Here we show that the indirect effects of elevated CO2 explain, on average, 28% of the total plant productivity response, and are almost equal to the size of direct effects on evapotranspiration. This finding has major implications for our mechanistic understanding of plant response to elevated CO2, forcing us to revisit the interpretation of experimental results as well as simulations of future productivity. Increasing concentrations of atmospheric carbon dioxide are expected to affect carbon assimilation and evapotranspiration (ET), ultimately driving changes in plant growth, hydrology, and the global carbon balance. Direct leaf biochemical effects have been widely investigated, whereas indirect effects, although documented, elude explicit quantification in experiments. Here, we used a mechanistic model to investigate the relative contributions of direct (through carbon assimilation) and indirect (via soil moisture savings due to stomatal closure, and changes in leaf area index) effects of elevated CO2 across a variety of ecosystems. We specifically determined which ecosystems and climatic conditions maximize the indirect effects of elevated CO2. The simulations suggest that the indirect effects of elevated CO2 on net primary productivity are large and variable, ranging from less than 10% to more than 100% of the size of direct effects. For ET, indirect effects were, on average, 65% of the size of direct effects. Indirect effects tended to be considerably larger in water-limited ecosystems. As a consequence, the total CO2 effect had a significant, inverse relationship with the wetness index and was directly related to vapor pressure deficit. These results have major implications for our understanding of the CO2 response of ecosystems and for global projections of CO2 fertilization, because, although direct effects are typically understood and easily reproducible in models, simulations of indirect effects are far more challenging and difficult to constrain. Our findings also provide an explanation for the discrepancies between experiments in the total CO2 effect on net primary productivity.

Ectomycorrhizal Colonization, Biomass, and Production in a Regenerating Scrub Oak Forest in Response to Elevated CO2

The effects of CO2 elevation on the dynamics of fine root (FR) mass and ectomycorrhizal (EM) mass and colonization were studied in situ in a Florida scrub oak system over four years of postfire regeneration. Soil cores were taken at five dates and sorted to assess the standing crop of ectomycorrhizal and fine roots. We used ingrowth bags to estimate the effects of elevated CO2 on production of EM roots and fine roots. Elevated CO2 tended to increase EM colonization frequency but did not affect EM mass nor FR mass in soil cores (standing mass). However, elevated CO2 strongly increased EM mass and FR mass in ingrowth bags (production), but it did not affect the EM colonization frequency therein. An increase in belowground production with unchanged biomass indicates that elevated CO2 may stimulate root turnover. The CO2-stimulated increase of belowground production was initially larger than that of aboveground production. The oaks may allocate a larger portion of resources to root/mycorrhizal production in this system in elevated rather than ambient CO2.

Plant community feedbacks and long-term ecosystem responses to multi-factored global change

If you add CO2 or nitrogen to a single plant it will likely grow more, but the amount by which each resource stimulates growth differs widely across species. When you add either resource to a whole ecosystem, total plant growth will likely also increase, but there will be winners and losers, causing a change in the relative abundance of plant species, and therefore altering the way the whole ecosystem responds to the added resource, a “community feedback”. These feedbacks are very difficult to predict, especially when multiple resources are added, but a lot of recent experimental evidences suggests that community feedbacks will determine how future ecosystems operate.

Elevated CO2 promotes long-term nitrogen accumulation only in combination with nitrogen addition

Biogeochemical models that incorporate nitrogen (N) limitation indicate that N availability will control the magnitude of ecosystem carbon uptake in response to rising CO2 . Some models, however, suggest that elevated CO2 may promote ecosystem N accumulation, a feedback that in the long term could circumvent N limitation of the CO2 response while mitigating N pollution. We tested this prediction using a nine-year CO2 xN experiment in a tidal marsh. Although the effects of CO2 are similar between uplands and wetlands in many respects, this experiment offers a greater likelihood of detecting CO2 effects on N retention on a decadal timescale because tidal marshes have a relatively open N cycle and can accrue soil organic matter rapidly. To determine how elevated CO2 affects N dynamics, we assessed the three primary fates of N in a tidal marsh: (1) retention in plants and soil, (2) denitrification to the atmosphere, and (3) tidal export. We assessed changes in N pools and tracked the fate of a (15) N tracer added to each plot in 2006 to quantify the fraction of added N retained in vegetation and soil, and to estimate lateral N movement. Elevated CO2 alone did not increase plant N mass, soil N mass, or (15) N label retention. Unexpectedly, CO2 and N interacted such that the combined N+CO2 treatment increased ecosystem N accumulation despite the stimulation in N losses indicated by reduced (15) N label retention. These findings suggest that in N-limited ecosystems, elevated CO2 is unlikely to increase long-term N accumulation and circumvent progressive N limitation without additional N inputs, which may relieve plant-microbe competition and allow for increased plant N uptake.

Allometry data and equations for coastal marsh plants

Coastal marshes are highly valued for ecosystem services such as protecting inland habitats from storms, sequestering carbon, removing nutrients and other pollutants from surface water, and providing habitat for fish, shellfish, and birds. Because plants largely determine the structure and function of coastal marshes, quantifying plant biomass is essential for evaluating these ecosystem services, understanding the biogeochemical processes that regulate ecosystem function, and forecasting tidal wetland responses to accelerated sea level rise. Allometry is a convenient and efficient technique for nondestructive estimation of plant biomass, and it is commonly used in studies of carbon and nitrogen cycles, energy flows, and marsh surface elevation change. We present plant allometry data and models developed for three long-term experiments at the Smithsonian Global Change Research Wetland, a brackish marsh in the Rhode River subestuary of the Chesapeake Bay. The dataset contains 9,771 measurements of stem height, dry mass, and (in 9638 cases) stem width across 11 plant species. The vast majority of observations are for Schoenoplectus americanus (8430) and Phragmites australis (311), with fewer observations for other common species: Amaranthus cannabinus, Atriplex patula, Iva frutescens, Kosteletzkya virginica, Polygonum hydropiper, Solidago sempervirens, Spartina alterniflora, Spartina cynosuroides, and Typha angustifolia. Allometric relationships take the form of linear regressions of biomass (transformed using the Box-Cox procedure) on either stem height and width, or on stem height alone. Allometric relationships for Schoenoplectus americanus were not meaningfully altered by elevated CO2 , N enrichment, the community context, interannual variation in climate, or year, showing that a single equation can be used across a broad range of conditions for this species. Archived files include: (1) raw data used to derive allometric equations for each species, (2) reports and evaluations of the allometric equations we derived from the data, and (3) R code with which our derivations can be replicated. Methodological details of our experiments, data collection efforts, and statistical modeling are described in the metadata. The allometric equations can be used for biomass estimation in empirical and modeling studies of North American coastal wetlands, and the data can be used in ecological studies of terrestrial plant allometry.

Evolutionary history and novel biotic interactions determine plant responses to elevated CO2 and nitrogen fertilization.

A major frontier in global change research is predicting how multiple agents of global change will alter plant productivity, a critical component of the carbon cycle. Recent research has shown that plant responses to climate change are phylogenetically conserved such that species within some lineages are more productive than those within other lineages in changing environments. However, it remains unclear how phylogenetic patterns in plant responses to changing abiotic conditions may be altered by another agent of global change, the introduction of non-native species. Using a system of 28 native Tasmanian Eucalyptus species belonging to two subgenera, Symphyomyrtus and Eucalyptus, we hypothesized that productivity responses to abiotic agents of global change (elevated CO2 and increased soil N) are unique to lineages, but that novel interactions with a non-native species mediate these responses. We tested this hypothesis by examining productivity of 1) native species monocultures and 2) mixtures of native species with an introduced hardwood plantation species, Eucalyptus nitens, to experimentally manipulated soil N and atmospheric CO2. Consistent with past research, we found that N limits productivity overall, especially in elevated CO2 conditions. However, monocultures of species within the Symphyomyrtus subgenus showed the strongest response to N (gained 127% more total biomass) in elevated CO2 conditions, whereas those within the Eucalyptus subgenus did not respond to N. Root:shoot ratio (an indicator of resource use) was on average greater in species pairs containing Symphyomyrtus species, suggesting that functional traits important for resource uptake are phylogenetically conserved and explaining the phylogenetic pattern in plant response to changing environmental conditions. Yet, native species mixtures with E. nitens exhibited responses to CO2 and N that differed from those of monocultures, supporting our hypothesis and highlighting that both plant evolutionary history and introduced species will shape community productivity in a changing world.

Using results from global change experiments to inform land model development and calibration

Formore than two decades, ecologists have studied how ecosystems will respond to environmental changes, such as the ongoing increase in atmospheric carbon dioxide concentration ([CO2]), the accompanying increase in Earth’s surface temperatures, changes in precipitation regimes, and unintended fertilization of the globe with reactive nitrogen (N) compounds, by building experiments that simulate these changes at small scales. While much of this research has been targeted at understanding how the functioning of ecosystemsmay change, and determiningwhich species are likely to be ‘winners’ and ‘losers’ in future conditions, an undercurrent of this research (and also often the stated goal) has been to determine how ecosystem responses themselves may alter the rate of climate change by altering the exchanges of carbon and energy between land and atmosphere. But identifying exactly how small-scale experiments can inform large-scale climate feedbacks is not always simple. Responses measured at the leaf level, for instance, may be quite different from what occurs at the canopy or landscape scale. And landscape-scale processes do not operate in small plots. Integrating results from plot-scale global change manipulations with the Earth system models that now provide state-of-the-art climate projections, and which operate on a scale of c. 1° grid cells, can provide a challenge. To date, few clear examples of such research exist (but see Bonan, 2014). To address these challenges, 48 experimentalists and modelers from around theworld gathered inBeijing for aworkshop on ‘Using results from global change experiments to inform land model development and calibration.’ The workshop, organized by Jeffrey Dukes, Aim ee Classen, and local host and co-sponsor Shiqiang Wan, began with short talks from both the experimental and modeling perspectives, which led into longer, focused small group discussions.

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

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.