An inventory of epigeal ants of the western Soutpansberg Mountain Range, South Africa

The distribution, abundance and sensitivity invertebrates to habitat change are largely unknown. Long-term monitoring of ecological gradients with standardised and comparable protocols can form the basis of a better understanding. Altitudinal gradients are particularly relevant within this context. Here we provide a check list and baseline data for ant species collected over a 5-year period across the Soutpansberg Mountain Range, South Africa. Standardised pitfall surveys across 11 sites yielded a total of 133 species in 38 genera and 6 subfamilies. Sample coverage of epigeal ants was 0.98 for the transect as a whole. Of these species, 21% were restricted to the southern slope of the mountain and 14% to the northern slope. Extrapolated richness estimates reached an asymptote for all, except for three sites. These were the only sites impacted by bush encroachment. Observed richness was the highest at a low-altitude mesic site that is exposed to considerable disturbance by megaherbivores and mechanical clearing of woody vegetation. Structural classification of vegetation was predictive of a broad-scale ant assemblage structure. On a smaller scale, however, structure was a function of elevation, space and temperature.Conservation implications: Future monitoring should target indicator taxa associated with bush encroachment, particularly with reference to their impacts on grasslands. Bush encroachment could endanger several ant species associated with mesic grasslands and woodlands on the mountain, as well as ant diversity, as these were the habitats with the highest ant diversity.

Phenology, Growth, and Fecundity as Determinants of Distribution in Closely Related Nonnative Taxa

Invasive species researchers often ask: Why do some species invade certain habitats while others do not? Ecological theories predict that taxonomically related species may invade similar habitats, but some related species exhibit contrasting invasion patterns. Brassica nigra, Brassica tournefortii, and Hirschfeldia incana are dominant, closely related nonnative species that have overlapping, but dissimilar, distributions. Brassica tournefortii is rapidly spreading in warm deserts of the southwestern United States, whereas B. nigra and H. incana are primarily limited to semiarid and mesic regions. We compared traits of B. tournefortii that might confer invasiveness in deserts with those of related species that have not invaded desert ecosystems. Brassica tournefortii, B. nigra and H. incana were compared in controlled experiments conducted outdoors in a mesic site (Riverside, CA) and a desert site (Blue Diamond, NV), and in greenhouses, over 3 yr. Desert and mesic B. tournefortii populations were also compared to determine whether locally adapted ecotypes contribute to desert invasion. Experimental variables included common garden sites and soil water availability. Response variables included emergence, growth, phenology, and reproduction. There was no evidence for B. tournefortii ecotypes, but B. tournefortii had a more rapid phenology than B. nigra or H. incana. Brassica tournefortii was less affected by site and water availability than B. nigra and H. incana, but was smaller and less fecund regardless of experimental conditions. Rapid phenology allows B. tournefortii to reproduce consistently under variable, stressful conditions such as those found in Southwestern deserts. Although more successful in milder, mesic ecosystems, B. nigra and H. incana may be limited by their ability to reproduce under desert conditions. Rapid phenology and drought response partition invasion patterns of nonnative mustards along a gradient of aridity in the southwestern United States, and may serve as a predictive trait for other potential invaders of arid and highly variable ecosystems.

Drought-triggered false ring formation in a Mediterranean shrub

Recently, the Mediterranean region has experienced unprecedented drought. Climate models continue to predict an increase in drought frequency and duration, which increases the importance of quantifying the response of already drought-tolerant Mediterranean plants to increased drought. We examined the wood anatomy and dendroecological features of a Mediterranean shrub, Arbutus unedo L., at a xeric and a mesic site on the Italian island of Elba to identify shrub growth response to drought. Cross-sectional microsections of A. unedo stems were stained, described, and crossdated. Annual ring widths of radial microsections were measured and compared with regional climatic variables. False rings (intra-annual bands of latewood typically formed in response to a specific stressor) were visible in the wood samples when viewing radial microsections under high magnification. False ring formation coincided with below-average rainfall in late summer at the xeric site, and below-average rainfall and high temperatures in spring and summer at the mesic site. If increased drought occurs in the Mediterranean region, it is likely that plants in this region will experience a drought-induced growth response similar to that seen in A. unedo, and slight differences in drought tolerance may become more important as plants compete for moisture under drier conditions.

Altering successional trends in oak forests: 19 year experimental results of low- and moderate-intensity silvicultural treatments

Intensive silvicultural treatments can sometimes prevent the conversion of an oak (Quercus spp.) forest to a forest composed of mesophytic competitors following harvest, but the required labor is a disincentive for many private landowners. In this study, shelterwood removal, commercial clear-cutting, understory control, and oak underplanting were conducted on mesic and dry–mesic sites in southwestern Wisconsin to evaluate the effect of these treatments on forest composition and to identify the least intensive combination needed for successful oak regeneration. Commercial clear-cutting, with or without prior herbicide spray of low vegetation and oak underplanting, resulted in nearly complete dominance by a wide array of non-oak species on both mesic and dry–mesic sites. In contrast, 153–903 ha–1 of the oaks that were underplanted on shelterwood – understory removal plots successfully achieved dominant or codominant status by age 19. Control of tall understory saplings was essential for successful oak regeneration on both sites. On the mesic site, oak underplanting was an additional necessary treatment, whereas natural regeneration was adequate in shelterwood plots on the dry–mesic site. The study suggests that successful oak regeneration can be obtained on productive sites in this region after a single application of a moderately intense silvicultural treatment, although the effort required for understory control may still be an obstacle to widespread application without external incentives.

An allozyme study of sexual and vegetative regeneration in Hieracium pilosella

Allozyme markers were used to fingerprint clones of the grassland plant Hieracium pilosella L. and, by inference, to estimate the relative importance of sexual and vegetative reproduction in a population. Field studies in populations of clonal plant species have often reported negligible or absent recruitment from seed. In contrast, studies of genetic markers have found substantial clonal diversity in populations, suggesting recruitment of new genets into established populations. Our results showed that H. pilosella regenerates from seed both within and between dense clonal patches. Two sites differing in environmental conditions were sampled to investigate how the balance between seed-derived and stolon-derived recruitment changes with biotic and abiotic stress. In a relatively drought-prone site on a south-facing slope, the balance was shifted towards recruitment from seed, compared with a mesic site in which vegetative regeneration was more important.

Résistance comparée de l'épinette blanche et du sapin baumier au renversement

Information on eastern Canadian tree species vulnerability to windthrow is scarce. Some statements on relative species vulnerability have been made but they rely on empirical observations, which are often difficult to generalize. In this context, a study was conducted to compare the overturning resistance of balsam fir (Abies balsamea (L.) Mill.) and white spruce (Picea glauca (Moench) Voss) on a mesic site. To establish which tree characteristics would best explain the critical turning moment, simple linear regressions were calculated using tree dendrometric data. The best regressions were obtained with stem weight. With this variable, resistance to overturning did not differ between the two species. Only regressions involving total height showed a significantly greater resistance for white spruce. This difference can be explained by a difference between the species in height–diameter relationships. For a similar height, spruce has a greater diameter, involving a higher stem weight and thus a greater resistance. Decay did not play a major role in our experiment as trees with external defects were excluded. Our results suggest that to minimize losses from windthrow, silvi cultural treatments on mesic sites should try to increase the proportion of trees of either species with the lowest height/diameter ratio.