Root dynamics and global change: seeking an ecosystem perspective

Changes in the production and turnover of roots in forests and grasslands in response to rising atmospheric CO₂ concentrations, elevated temperatures, altered precipitation, or nitrogen deposition could be a key link between plant responses and longer-term changes in soil organic matter and ecosystem carbon balance. Here we summarize the experimental observations, ideas, and new hypotheses developed in this area in the rest of this volume. Three central questions are posed. Do elevated atmospheric CO₂, nitrogen deposition, and climatic change alter the dynamics of root production and mortality? What are the consequences of root responses to plant physiological processes? What are the implications of root dynamics to soil microbial communities and the fate of carbon in soil? Ecosystem-level observations of root production and mortality in response to global change parameters are just starting to emerge. The challenge to root biologists is to overcome the profound methodological and analytical problems and assemble a more comprehensive data set with sufficient ancillary data that differences between ecosystems can be explained. The assemblage of information reported herein on global patterns of root turnover, basic root biology that controls responses to environmental variables, and new observations of root and associated microbial responses to atmospheric and climatic change helps to sharpen our questions and stimulate new research approaches. New hypotheses have been developed to explain why responses of root turnover might differ in contrasting systems, how carbon allocation to roots is controlled, and how species differences in root chemistry might explain the ultimate fate of carbon in soil. These hypotheses and the enthusiasm for pursuing them are based on the firm belief that a deeper understanding of root dynamics is critical to describing the integrated response of ecosystems to global change.     

Projected changes in mineral soil carbon of European croplands and grasslands, 1990–2080

We present the most comprehensive pan‐European assessment of future changes in cropland and grassland soil organic carbon (SOC) stocks to date, using a dedicated process‐based SOC model and state‐of‐the‐art databases of soil, climate change, land‐use change and technology change. Soil carbon change was calculated using the Rothamsted carbon model on a European 10 × 10′ grid using climate data from four global climate models implementing four Intergovernmental Panel on Climate Change (IPCC) emissions scenarios (SRES). Changes in net primary production (NPP) were calculated by the Lund–Potsdam–Jena model. Land‐use change scenarios, interpreted from the narratives of the IPCC SRES story lines, were used to project changes in cropland and grassland areas. Projections for 1990–2080 are presented for mineral soil only. Climate effects (soil temperature and moisture) will tend to speed decomposition and cause soil carbon stocks to decrease, whereas increases in carbon input because of increasing NPP will slow the loss. Technological improvement may further increase carbon inputs to the soil. Changes in cropland and grassland areas will further affect the total soil carbon stock of European croplands and grasslands. While climate change will be a key driver of change in soil carbon over the 21st Century, changes in technology and land‐use change are estimated to have very significant effects. When incorporating all factors, cropland and grassland soils show a small increase in soil carbon on a per area basis under future climate (1–7 t C ha^?1 for cropland and 3–6 t C ha^?1 for grassland), but when the greatly decreasing area of cropland and grassland are accounted for, total European cropland stocks decline in all scenarios, and grassland stocks decline in all but one scenario. Different trends are seen in different regions. For Europe (the EU25 plus Norway and Switzerland), the cropland SOC stock decreases from 11 Pg in 1990 by 4–6 Pg (39–54%) by 2080, and the grassland SOC stock increases from 6 Pg in 1990 to 1.5 Pg (25%) under the B1 scenario, but decreases to 1–3 Pg (20–44%) under the other scenarios. Uncertainty associated with the land‐use and technology scenarios remains unquantified, but worst‐case quantified uncertainties are 22.5% for croplands and 16% for grasslands, equivalent to potential errors of 2.5 and 1 Pg SOC, respectively. This is equivalent to 42–63% of the predicted SOC stock change for croplands and 33–100% of the predicted SOC stock change for grasslands. Implications for accounting for SOC changes under the Kyoto Protocol are discussed.     

Enzymic and chemical analysis of particulate organic matter from a boreal river

SUMMARY. 1. Benthic particulate organic matter (POM) was collected from a shallow pool of a fourth order boreal stream and sorted into seven size fractions ranging from 63 to >4000μm. Each size fraction was analysed for fibre, total phosphorus, and total Kjeldahl nitrogen. Microbial activity was measured by oxygen consumption and characterized by assaying for eleven classes of exoenzymes including cellulase, phenol oxidase, peroxidase, phosphatase and sulphatase.
2. Indices of detritus quality such as C/N, C/P, percent lignin, and microbial respiration showed improvement with decreasing particle size. Three covarying exoenzyme groups were identified: a carbohydrase-phosphatase group that included eight enzymes, a phenol oxidase-peroxidase group, and sulphatase. The activity of the carbohydrase-phosphatase group, was significantly correlated with microbial respiration and the carbohydrate content of the POM. Phenol oxidase-peroxidase activity was correlated with lignin content for POM greater than 250 μm, but activity increased markedly in the two smallest size fractions even though the lignin content of the POM continued to decline, Sulphatase activity was inversely related to particle size over the entire range.
3. The changes in microbial activity with particle size were attributed to the increasing surface area to volume ratio of smaller particles and to an ecological succession in the microbial community.     

The influence of elevated temperature,elevated atmospheric CO2 concentration and water stress on net photosynthesis of loblolly pine (Pinus taeda L.) at northern,central and southern sites in its native range

We investigated the effect of elevated [CO₂] (700 μmol mol^?1), elevated temperature (+2 °C above ambient) and decreased soil water availability on net photosynthesis (A_net) and water relations of one‐year old potted loblolly pine (Pinus taeda L.) seedlings grown in treatment chambers with high fertility at three sites along a north‐south transect covering a large portion of the species native range. At each location (Blairsville, Athens and Tifton, GA) we constructed four treatment chambers and randomly assigned each chamber one of four treatments: ambient [CO₂] and ambient temperature, elevated [CO₂] and ambient temperature, ambient [CO₂] and elevated temperature, or elevated [CO₂] and elevated temperature. Within each chamber half of the seedlings were well watered and half received much less water (1/4 that of the well watered). Measurements of net photosynthesis (A_net), stomatal conductance (g_s), leaf water potential and leaf fluorescence were made in June and September, 2008. We observed a significant increase in A_net in response to elevated [CO₂] regardless of site or temperature treatment in June and September. An increase in air temperature of over 2 °C had no significant effect on A_net at any of the sites in June or September despite over a 6 °C difference in mean annual temperature between the sites. Decreased water availability significantly reduced A_net in all treatments at each site in June. The effects of elevated [CO₂] and temperature on g_s followed a similar trend. The temperature, [CO₂] and water treatments did not significantly affect leaf water potential or chlorophyll fluorescence. Our findings suggest that predicted increases in [CO₂] will significantly increase A_net, while predicted increases in air temperature will have little effect on A_net across the native range of loblolly pine. Potential decreases in precipitation will likely cause a significant reduction in A_net, though this may be mitigated by increased [CO₂].     

High rates of net primary production and turnover of floating grasses on the Amazon floodplain: implications for aquatic respiration and regional CO2 flux

We investigated whether rates of net primary production (NPP) and biomass turnover of floating grasses in a central Amazon floodplain lake (Lake Calado) are consistent with published evidence that CO₂ emissions from Amazon rivers and floodplains are largely supplied by carbon from C4 plants. Ground‐based measurements of species composition, plant growth rates, plant densities, and areal biomass were combined with low altitude videography to estimate community NPP and compare expected versus observed biomass at monthly intervals during the aquatic growth phase (January–August). Principal species at the site were Oryza perennis (a C3 grass), Echinochloa polystachya, and Paspalum repens (both C4 grasses). Monthly mean daily NPP of the mixed species community varied from 50 to 96 g dry mass m^?2 day^?1, with a seasonal average (±1SD) of 64±12 g dry mass m^?2 day^?1. Mean daily NPP (±1SE) for P. repens and E. polystachya was 77±3 and 34±2 g dry mass m^?2 day^?1, respectively. Monthly loss rates of combined above‐ and below‐water biomass ranged from 31% to 75%, and averaged 49%. Organic carbon losses from aquatic grasses ranged from 30 to 34 g C m^?2 day^?1 from February to August. A regional extrapolation indicated that respiration of this carbon potentially accounts for about half (46%) of annual CO₂ emissions from surface waters in the central Amazon, or about 44% of gaseous carbon emissions, if methane flux is included.     

Sex pheromone and visual trap interactions in mate location strategies and aggregation by host-alternating aphids in the field

Abstract. Field observations were made on the responses of males and gynoparae of three host-alternating aphid species, the blackberry-cereal aphid, Sitobion fragariae (Walker), the bird cherry-oat aphid, Rhopalosiphum padi (L.) and the damson-hop aphid, Phorodon humuli (Schrank) to species-specific sex pheromones released from transparent and coloured water traps.Pheromone traps caught significantly more males than did control traps without pheromone, whereas transparent, light green, yellow and orange traps caught most insects.Measurements of the distance over which sex pheromones function indicated that male P.humuli detect the pheromone 2–6 m from the source and can fly upwind to a source in wind speeds of 0.7 m s^-1.In all three species significantly more gynoparae were caught in pheromone traps than in control traps, suggesting that pheromone released by adult sexual females may assist late-flying gynoparae to locate a suitable host plant on which to deposit their progeny.The response is relatively stronger for males than gynoparae, but the pheromones appear to act as both sex and aggregation pheromones.     

Taxonomic and regional patterns in benthic macroinvertebrate elemental composition in streams

1. Ecological stoichiometry has been used to better understand dynamics in consumer growth and the role of consumer‐recycled nutrients because it focuses on more than one element. Most research has focused on pelagic rather than benthic consumers. Variation in elemental composition among benthic consumer taxa would suggest that taxa differ in their susceptibility to nutrient limitation or in their role in recycling nutrients. 2. We collected benthic macroinvertebrates from streams in two regions (Indiana–Michigan and Wisconsin, U.S.A.) to examine taxonomic and regional variation in benthic macroinvertebrate body carbon (C), nitrogen (N), and phosphorus (P) concentrations and ratios. 3. Elemental composition varied little within taxa common to both regions. In contrast, elemental composition differed greatly among taxa and appeared to be related to phylogeny. The elemental composition of macroinvertebrates clustered into three distinct groups: insects, mollusks, and crustaceans. To a lesser extent, insects and mollusks also differed in elemental composition among genera. 4. Functional feeding groups (FFGs) differed in elemental composition, with predators having a higher N content than other groups. Substantial elemental imbalances between C and N were found between most primary consumers and their likely food sources, and the magnitude of the imbalance depended in part on the FFG. 5. Our results support an assumption of most ecological stoichiometry models that, within a species, the elemental composition of aquatic invertebrates is relatively constant. Variation in elemental composition among taxa at various higher taxonomic levels suggests that susceptibility of stream invertebrates to nutrient limitation and their role in nutrient cycling will strongly depend on phylogeny.     

Variation in ultraviolet reflectance by carotenoid-bearing feathers of tanagers (Thraupini: Emberizinae: Passeriformes)

Avian visual sensitivity encompasses both the human visible range (400–700 nm) and also near‐ultraviolet (UV) wavelengths (320–400 nm) invisible to normal humans. I used reflectance spectrophotometry to assess variation in UV reflectance for yellow, orange and red plumage in 67 species of tanager (Passeriformes). Previous chemical studies, and my analysis of reflectance minima, suggest that carotenoids are the dominant pigments in yellow, orange and red tanager plumage. Spectra recorded over the range of wavelengths to which birds are sensitive (320–700 nm) were invariably bimodal, with both a plateau of high reflectance at longer (> 500 nm) wavelengths and a distinct secondary peak at UV (< 400 nm) wavelengths. Within this overall framework, variation in UV reflectance was expressed within well‐defined quantitative limits: (1) peak reflectance was always lower than the corresponding plateau of reflectance at longer visible wavelengths; (2) the intensity of peak reflectance declined steadily below 350 nm; (3) wavelengths of peak reflectance clustered between 350 and 370 nm. Significant correlations were detected between various measures of total reflectance in the UV and visible wavebands, but not between various measures of spectral location of UV and visible reflectance. I propose that the strong absorption band at short visible wavelengths (～ 380–550 nm) responsible for bimodal spectra of carotenoids in vitro is also responsible for bimodal reflectance by carotenoid‐based plumage colours. The construction of the UV and visible reflectance bands from different sides of this same absorbance band provides a mechanism for the observed covariation between UV and visible wavelengths. Lack of an association between the spectral locations of the UV and visible reflectance bands may result from the limited variation in spectral location of the UV band. These patterns suggest that plumage colours are subject to constraints, just as are more traditional morphological characters. © 2005 The Linnean Society of London, Biological Journal of the Linnean Society, 2005, 84 , 243–257.