Biotic interactions are an unexpected yet critical control on the complexity of an abiotically driven polar ecosystem.

Abiotic and biotic factors control ecosystem biodiversity, but their relative contributions remain unclear. The ultraoligotrophic ecosystem of the Antarctic Dry Valleys, a simple yet highly heterogeneous ecosystem, is a natural laboratory well-suited for resolving the abiotic and biotic controls of community structure. We undertook a multidisciplinary investigation to capture ecologically relevant biotic and abiotic attributes of more than 500 sites in the Dry Valleys, encompassing observed landscape heterogeneities across more than 200 km2. Using richness of autotrophic and heterotrophic taxa as a proxy for functional complexity, we linked measured variables in a parsimonious yet comprehensive structural equation model that explained significant variations in biological complexity and identified landscape-scale and fine-scale abiotic factors as the primary drivers of diversity. However, the inclusion of linkages among functional groups was essential for constructing the best-fitting model. Our findings support the notion that biotic interactions make crucial contributions even in an extremely simple ecosystem.

Trends in entropy production during ecosystem development in the Amazon Basin.

Understanding successional trends in energy and matter exchange across the ecosystem-atmosphere boundary layer is an essential focus in ecological research; however, a general theory describing the observed pattern remains elusive. This paper examines whether the principle of maximum entropy production could provide the solution. A general framework is developed for calculating entropy production using data from terrestrial eddy covariance and micrometeorological studies. We apply this framework to data from eight tropical forest and pasture flux sites in the Amazon Basin and show that forest sites had consistently higher entropy production rates than pasture sites (0.461 versus 0.422 W m(-2) K(-1), respectively). It is suggested that during development, changes in canopy structure minimize surface albedo, and development of deeper root systems optimizes access to soil water and thus potential transpiration, resulting in lower surface temperatures and increased entropy production. We discuss our results in the context of a theoretical model of entropy production versus ecosystem developmental stage. We conclude that, although further work is required, entropy production could potentially provide a much-needed theoretical basis for understanding the effects of deforestation and land-use change on the land-surface energy balance.

Plasticity in mesophyll volume fraction modulates light-acclimation in needle photosynthesis in two pines.

Acclimation potential of needle photosynthetic capacity varies greatly among pine species, but the underlying chemical, anatomical and morphological controls are not entirely understood. We investigated the light-dependent variation in needle characteristics in individuals of Pinus patula Schlect. & Cham., which has 19-31-cm long pendulous needles, and individuals of P. radiata D. Don., which has shorter (8-17-cm-long) stiffer needles. Needle nitrogen and carbon contents, mesophyll and structural tissue volume fractions, needle dry mass per unit total area (M(A)) and its components, volume to total area ratio (V/A(T)) and needle density (D = M(A)/(V/A(T))), and maximum carboxylase activity of Rubisco (V(cmax)) and capacity of photosynthetic electron transport (J(max)) were investigated in relation to seasonal mean integrated irradiance (Q(int)). Increases in Q(int) from canopy bottom to top resulted in proportional increases in both needle thickness and width such that needle total to projected surface area ratio, characterizing the efficiency of light interception, was independent of Q(int). Increased light availability also led to larger M(A) and nitrogen content per unit area (N(A)). Light-dependent modifications in M(A) resulted from increases in both V/A(T) and D, whereas N(A) changed because of increases in both M(A) and mass-based nitrogen content (N(M)) (N(A) = N(M)M(A)). Overall, the volume fraction of mesophyll cells increased with increasing irradiance and V/A(T) as the fraction of hypodermis and epidermis decreased with increasing needle thickness. Increases in M(A) and N(A) resulted in enhanced J(max) and V(cmax) per unit area in both species, but mass-based photosynthetic capacity increased only in P. patula. In addition, J(max) and V(cmax) showed greater plasticity in response to light in P. patula. Species differences in mesophyll volume fraction explained most of the variation in mass-based needle photosynthetic capacity between species, demonstrating that differences in plastic adjustments in mass-based photosynthetic activities among these representative individuals were mainly associated with contrasting investments in mesophyll cells. Greater area-based photosynthetic plasticity in P. patula relative to P. radiata was associated with larger increases in M(A) and mesophyll volume fraction with increasing irradiance. These data collectively demonstrate that light-dependent increases in mass-based nitrogen contents and photosynthetic activities were associated with an increased mesophyll volume fraction in needles at higher irradiances. They also emphasize the importance of light-dependent anatomical modifications in determining needle photosynthetic capacity.

Light-acclimation of cladode photosynthetic potentials in Casuarina glauca: trade-offs between physiological and structural investments.

Many arid and saline habitat species possess sparse canopies with cylindrical foliage that is considered relatively invariable along environmental gradients. However, even in sparse canopies strong gradients of light develop between the canopy top and bottom. We studied structural and photosynthetic acclimation to within-canopy light gradient in Casuarina glauca Sieb. ex Spreng., the photosynthetic organs of which are cylindrical cladodes. Seasonal average integrated quantum flux density (Qint) varied 25-fold between the canopy top and the canopy bottom. Cladode cross-sectional shape was unaffected by irradiance, but cladode dry mass per unit total area (MA) varied 2-fold within the canopy light gradient. This resulted primarily from light-dependent changes in cladode thickness (volume to total area ratio,V / AT) and to a lesser extent from changes in cladode density (D, MA = DV / AT). Nitrogen content, and the volume of mesophyll per unit surface area increased with increasing Qint and V / AT, resulting in positive scaling of foliage photosynthetic potential (capacity of photosynthetic electron transport and maximum Rubisco carboxylase activity per unit area) with light. However, nitrogen content per unit dry mass and the volume fraction of mesophyll decreased with increasing irradiance. This was explained by greater fractional investment in mechanical tissues in cladodes with greater volume to surface area ratio. This trade-off between photosynthetic and support investments reduced the cladode photosynthetic plasticity. Our study demonstrates a significant acclimation potential of species with cylindrical foliage that should be included in larger-scale carbon balance estimations of arid and saline communities.

Is confidence in the monitoring of GE foods justified?

Often the limits of detection of genetically engineered organisms (GEOs, LMOs, GMOs) determine what legislation sets as thresholds of allowable contamination of the human food chain with GEOs. Many countries have legislation that is triggered by certain thresholds of contamination. Importantly, international trade in food and animal feed is becoming increasingly vulnerable to interruptions caused by the ambiguity GEOs can create when shipments are monitored at the border. We examine the tools available for detection. Four key error-generating stages are identified with the aim of prompting a higher uniform standard of routine analysis at export and import points. Contamination of the New Zealand corn crop with GEOs is used as a case study for the application of monitoring tools and vulnerability to errors. These tools fail to meet emerging food safety requirements, but some improvements are in development.

Nocturnal warming increases photosynthesis at elevated CO2 partial pressure in Populus deltoides.

• We measured night-time respiration and daytime photosynthesis of leaves in canopies of 4 m tall cottonwood (Populus deltoides) trees to investigate the link between leaf respiration and photosynthetic capacity. • Trees were grown at three CO2 partial pressures [p(CO2 )a ] (42, 80, 120 Pa) and experimentally exposed to differing nocturnal temperatures (15, 20 or 25°C), but constant daytime temperatures (30-32°C), in a short-term whole-ecosystem environmental manipulation. • Rates of night-time leaf dark respiration (Rd ) increased significantly at all growth CO2 partial pressures when nocturnal temperatures were increased from 15 to 25°C. Predawn leaf nonstructural carbohydrate (soluble sugars and starch) content was significantly lower at the higher night temperatures. Photosynthetic capacity (Amax ) during the day increased significantly between 15 and 25°C at 42 and 80 Pa, but not at 120 Pa. • These findings indicate that the previously determined relationships between elevated night-time temperature, dark respiration and increased photosynthetic capacity may also hold at elevated p(CO2 )a . This response may have a significant influence on plant and ecosystem carbon exchange under global change scenarios.