Multiple Scales of Temporal Variability in Ecosystem Metabolism Rates: Results from 2 Years of Continuous Monitoring in a Forested Headwater Stream

Headwater streams are key sites of nutrient and organic matter processing and retention, but little is known about temporal variability in gross primary production (GPP) and ecosystem respiration (ER) rates as a result of the short duration of most metabolism measurements in lotic ecosystems. We examined temporal variability and controls on ecosystem metabolism by measuring daily rates continuously for 2 years in Walker Branch, a first-order deciduous forest stream. Four important scales of temporal variability in ecosystem metabolism rates were identified: (1) seasonal, (2) day-to-day, (3) episodic (storm-related), and (4) inter-annual. Seasonal patterns were largely controlled by the leaf phenology and productivity of the deciduous riparian forest. Walker Branch was strongly net heterotrophic throughout the year with the exception of the open-canopy spring when GPP and ER rates were co-equal. Day-to-day variability in weather conditions influenced light reaching the streambed, resulting in high day-to-day variability in GPP particularly during spring (daily light levels explained 84% of the variance in daily GPP in April). Episodic storms depressed GPP for several days in spring, but increased GPP in autumn by removing leaves shading the streambed. Storms depressed ER initially, but then stimulated ER to 2–3 times pre-storm levels for several days. Walker Branch was strongly net heterotrophic in both years of the study, with annual GPP being similar (488 and 519 g O₂ m⁻² y⁻¹ or 183 and 195 g C m⁻² y⁻¹) but annual ER being higher in 2004 than 2005 (−1,645 vs. −1,292 g O₂ m⁻² y⁻¹ or −617 and −485 g C m⁻² y⁻¹). Inter-annual variability in ecosystem metabolism (assessed by comparing 2004 and 2005 rates with previous measurements) was the result of the storm frequency and timing and the size of the spring macroalgal bloom. Changes in local climate can have substantial impacts on stream ecosystem metabolism rates and ultimately influence the carbon source and sink properties of these important ecosystems.     

Divergent selection along climatic gradients in a rare central European endemic species,Saxifraga sponhemica

Background and Aims The effects of habitat fragmentation on quantitative genetic variation in plant populations are still poorly known. Saxifraga sponhemica is a rare endemic of Central Europe with a disjunct distribution, and a stable and specialized habitat of treeless screes and cliffs. This study therefore used S. sponhemica as a model species to compare quantitative and molecular variation in order to explore (1) the relative importance of drift and selection in shaping the distribution of quantitative genetic variation along climatic gradients; (2) the relationship between plant fitness, quantitative genetic variation, molecular genetic variation and population size; and (3) the relationship between the differentiation of a trait among populations and its evolvability.Methods Genetic variation within and among 22 populations from the whole distribution area of S. sponhemica was studied using RAPD (random amplified polymorphic DNA) markers, and climatic variables were obtained for each site. Seeds were collected from each population and germinated, and seedlings were transplanted into a common garden for determination of variation in plant traits.Key Results In contrast to previous results from rare plant species, strong evidence was found for divergent selection. Most population trait means of S. sponhemica were significantly related to climate gradients, indicating adaptation. Quantitative genetic differentiation increased with geographical distance, even when neutral molecular divergence was controlled for, and Q_ST exceeded F_ST for some traits. The evolvability of traits was negatively correlated with the degree of differentiation among populations (Q_ST), i.e. traits under strong selection showed little genetic variation within populations. The evolutionary potential of a population was not related to its size, the performance of the population or its neutral genetic diversity. However, performance in the common garden was lower for plants from populations with reduced molecular genetic variation, suggesting inbreeding depression due to genetic erosion.Conclusions The findings suggest that studies of molecular and quantitative genetic variation may provide complementary insights important for the conservation of rare species. The strong differentiation of quantitative traits among populations shows that selection can be an important force for structuring variation in evolutionarily important traits even for rare endemic species restricted to very specific habitats.     

Temperature gradient from the urogenital sinus to the pouch in the pregnant marsupial quoll, Dasyurus hallucatus

1. The mechanisms utilised by the newborn quoll to move from the uterus to the teat within the pouch are unknown. The ability to sense gravity and odour have been suggested and it is possible that temperature may also assist the young in this migration.

2. An increasing temperature gradient was observed from the sinus at 28.98 °C increasing to 29.38 °C on the skin between the sinus and the pouch and further increasing to 30.96 °C within the pouch. This temperature gradient was not as apparent during lactation.

3. Hairs may also play an important role in allowing the newborn to leave the gelatinous material emanating from the uterus and travel to the pouch. The hairs form a tunnel between the sinus and the pouch and may assist the young in the moving from uterus to the pouch.     

Effects of body size on the diurnal activity budgets of African browsing ruminants

We compared the diurnal activity budgets of four syntopic species of African browsing ruminant that differ widely in body size. These were concurrently studied through all phases of the seasonal cycle, in the same area, using the same methods. We tested five predictions from the literature on how body size is expected to influence the behaviour of tropical ungulates: the smallest members of the browsing ruminant guild exhibit (1) the lowest allocation of diurnal time to activity; (2) the greatest hour-to-hour variation in activity and resting time; (3) the greatest reduction in activity time during the hottest days; (4) the least change between wet and the dry seasons in the ratio of feeding: ruminating time; and (5) the greatest time budget allocation to vigilance. Prediction 1 was supported in that the smaller species spent less time being active during the day. Prediction 2 was also supported in that the smaller species were more variable in their relative allocations of time to activity and resting through successive hours of the day. Contrary to Prediction 3, however, the greatest reduction in activity with increasing temperature was found for the largest guild member. The smaller species can achieve their daily food intake requirements by feeding at night and in the cool hours of the day, while the larger species have to feed during all hours of the day and are thus more susceptible to thermoregulatory constraints on foraging. Prediction 4 was partially upheld in that the largest species (giraffe) displayed the widest variation in feeding: ruminating time through the seasonal cycle. Prediction 5 was not supported, indicating that multiple factors interact with body size in determining vigilance behaviour.     

An inverted latitudinal gradient of plant diversity in shallow depressions on Ivorian inselbergs

The vegetation of shallow depressions on Ivorian granite inselbergs was studied along a gradient from the savanna zone in the north to the rainforest zone in the south of the country. Short-term inundation and prolonged drought are typical features of this habitat. In total, 64 taxa belonging to 25 families were recorded, with the Poaceae, Cyperaceae and Fabaceae accounting for the greatest proportion of species. Annuals represent the predominant life form and comprise nearly two thirds of all species recorded. DCA ordination of the sample plots illustrates that diversity decreases from north to south, and is accompanied by a gradual transition in the ambient vegetation from savanna to rainforest. This decrease is in marked contrast with diversity of surrounding vegetation types. In the drier northern area, it appears as if the less favourable environmental conditions prevent a state of community equilibrium being attained in the shallow depressions. This enables weak competitors to co-exist along with more vigorous species, which, in the south of the country, form species-poor stands. Furthermore, the fact that inselbergs in the rainforest zone are more isolated enhances the probability of extinction of less competitive associates.     

Antheseabhängige UV-Muster in Blütenständen von Asteraceen

The pseudanthia ofHeliopsis scabra andRudbeckia vulgaris (Asteraceae) were examined during the anthesis for differences in their UV patterns. Distinct changes in the reflectance and absorbance properties could be observed. The results suggest a close correlation between different stages of floral development and pollinator attraction.

Herrn Prof. Dr.Lothar Geitler zum 90. Geburtstag gewidmet.     

Chromosomal evolution withinPiperaceae

Chromosome numbers for 26 different species of the generaPiper, Peperomia andPothomorphe (Piperaceae) are reported. The basic chromosome numbers are 2n = 26, x = 13 (Piper, Pothomorphe) and 2n = 22, x = 11 (Peperomia), polyploid series are characteristic forPiper andPeperomia. Piper has the smallest chromosomes and prochromosomal interphase nuclei,Peperomia the largest ones and mostly reticulate to euchromatic nuclei.Pothomorphe is intermediate in both characters. The karyomorphological differences betweenPothomorphe andPiper underline their generic separation. Interspecific size variation of chromosomes occurs inPiper andPeperomia. Infraspecific polyploidy was observed inPiper betle. C-banding reveals different patterns of heterochromatin (hc) distribution between the genera investigated. The genome evolution is discussed.