A cross-biome comparison of daily light use efficiency for gross primary production

Vegetation light use efficiency is a key physiological parameter at the canopy scale, and at the daily time step is a component of remote sensing algorithms for scaling gross primary production (GPP) and net primary production (NPP) over regional to global domains. For the purposes of calibrating and validating the light use efficiency ( ε _g) algorithms, the components of ε _g– absorbed photosynthetically active radiation (APAR) and ecosystem GPP – must be measured in a variety of environments. Micrometeorological and mass flux measurements at eddy covariance flux towers can be used to estimate APAR and GPP, and the emerging network of flux tower sites offers the opportunity to investigate spatial and temporal patterns in ε _g at the daily time step. In this study, we examined the relationship of daily GPP to APAR, and relationships of ε _g to climatic variables, at four micrometeorological flux tower sites – an agricultural field, a tallgrass prairie, a deciduous forest, and a boreal forest. The relationship of GPP to APAR was close to linear at the tallgrass prairie site but more nearly hyperbolic at the other sites. The sites differed in the mean and range of daily ε _g, with higher values associated with the agricultural field than the boreal forest. ε_g decreased with increasing APAR at all sites, a function of mid‐day saturation of GPP and higher ε _g under overcast conditions. ε _g was generally not well correlated with vapor pressure deficit or maximum daily temperature. At the agricultural site, a ε _g decline towards the end of the growing season was associated with a decrease in foliar nitrogen concentration. At the tallgrass prairie site, a decline in ε _g in August was associated with soil drought. These results support inclusion of parameters for cloudiness and the phenological status of the vegetation, as well as use of biome‐specific parameterization, in operational ε _g algorithms.     

Regeneration of Hydra from Reaggregated Cells

Separated cells of hydra reaggregate and develop into normal animals. The regeneration of serially grafted aggregates derived from different parts of hydra tissue demonstrates that the polarity of morphogenesis in hydra is the result of the cellular composition of the tissue, not cellular orientation.     

Age determination of Acacia tortilis (Forsk.) Hayne from northern Kenya

Age determination on the basis of ring count holds great potential for the study of dry woodland ecosystem dynamics. Discs from stems of Acacia tortilis from Marsabit District, northern Kenya, were collected for dendrochronological analysis. Problems in the identification of growth rings are summarized. Regression analysis is used to develop age–ring count and ring count–size relationships. The time seedlings spend in a browse‐suppressed state is also estimated. It is concluded that there is insufficient evidence for the development of statistically valid age–size relationships. Problems that current techniques have yet to overcome are identified. Future studies are recommended using cambial injury techniques, particular emphasis being placed on the potential effects of browsing on cambial activity.     

A Fatigue-Based Model of Disuse Osteoporosis

A theoretical model has been developed for predicting the extent of bone adaptation as a function of exercise level. The model works by estimating the amount of fatigue damage which will occur for a given cyclic strain. This allowed us to predict that there would be a difference in the strengths of bones taken from sedentary individuals compared to those from more active individuals. The predicted difference, expressed as the ratio of the fatigue strengths of the two bones, was 1.53.

Experimental tests were carried out using bones from chickens, comparing those kept in battery cages (who are unable to engage in any strenuous activity) with those kept in normal free-range conditions. A finite element model was used to ensure that the in vitro fatigue tests were conducted at strain levels within the physiological range. Significant differences in both static and fatigue strength were found for the radius and tibia bones. These differences could be accurately predicted by the theoretical model, after a slight difference in weight between the two groups was allowed for. Additional evidence for these predictions can also be found in the literature. This work supports the concept that bone adaptation is driven by the need to optimise fatigue damage accumulation.     

Geographic differentiation and speciation in Cerion–a preliminary discussion of patterns and processes

Cerion's exuberant morphological diversity (600 described 'species') combined with the extreme rarity of reproductive isolation among morphotypes (only one unambiguous case of sympatry in the Bahamas) has long made this genus an object of fascination for evolutionists and of frustration for taxonomists. We have pursued an integrated approach of genetic and morphometric study based on field investigations of ecology and biogeography in the Bahamas. Cerion's morphotypes are not distributed haphazardly, but show definite patterns of correlation with habitat and geography. Although all morphotypes interbreed, hybrid zones tend to be narrow and characterized by highly local genetic anomalies–unique alleles present in neither parental population. Different patterns of covariance in ontogeny, and habitat preferences, also indicate mat the morphotypes are distinctive, non-amalgamating entities (despite little difference in the frequencies of structural genes among them) that may be called species once a definition based on strict reproductive isolation is abandoned. Variation in structural genes, anatomy and morphology is non-concordant, but orderly for each criterion. Similar morphologies are often polyphyletic and evolved repeatedly as one possible ontogenetic route within a developmental program common to all Cerion. Although we cannot always distinguish among competing causes for observed patterns, we can establish genetic, morphological, anatomical, and biogeographic criteria for decisions when adequate evidence is available. The species of Cerion will be reduced by more than an order of magnitude from a list currently described.