Regeneration strategies of tree species in the laurel forest of Tenerife (The Canary Islands)

The laurel-forest of the Canary Islands is a montane cloud-forest. In order to gain some knowledge on the processes that maintain tree species diversity, we conducted an analysis of three different laurel-forest plots of the Anaga massif (Tenerife), varying in canopy composition but growing under similar environmental conditions. For each plot we recorded basal area of the canopy trees (h<1.30 m), the density of suckers and seedlings (h>1.30 m), as well as seed-bank composition. The plots have similar regeneration composition, which appears to be independent of differences in canopy composition. Laurus azorica is the most common seedling species, whereas Prunus lusitanica is the most abundant species among suckers and basal shoots. Neither Erica arborea nor Myrica faya, the two main canopy trees in one of the plots, were found in any of the stands as seedlings or suckers, despite their existence as viable seeds in the seed-bank. The regeneration composition and the canopy composition in one of the plots is remarkable different, revealing differents dynamics processes in the three plots. The results suggest the existence of three well-defined ecological groups: pioneer (regeneration primarily by seedlings), non-pioneer (regeneration by seedlings and suckers) and remnant species (regeneration primarily by suckers).These three groups and the effect of small scale disturbances (natural and human-induced), could help to understand the maintenance of tree species richness.     

Altitudinal variation in fertility and vegetative growth in the invasive plant Rubus alceifolius Poiret (Rosaceae), on Réunion island.

Rubus alceifolius Poiret (Rosaceae) was introduced to the island of Réunion in the southeastern Indian Ocean about 1850 and is now highly invasive. This bramble, native from southeastern Asia and Malaysia, has invaded a wide variety of habitats (lowland rainforest, mountain and submountain rainforest, Acacia heterophylla rainforest) from sea level to 1700 m. It is suspected to be monoclonal so, its remarkable success may be due in part to great phenotypic plasticity. On Réunion, bud, flower, fruit and seed production, the duration of the flowering period and the importance of the seed bank were found to be negatively correlated with elevation (50-1500 m a.s.l.). At a lowland site, fruit production in mature stands averaged between 30 and 80 fruits/m² during 1999 and 2000. No fruit set occurred above 1100 m. This fruit production pattern was similar over two years. Although the number of leaves per unit area is similar along the whole gradient studied, decrease of fruit set in upland areas might be compensated for by an increase in vegetative growth. Temperature variation is very sharp along the elevation gradient and may control the fruit and the seed production. Fruit production allows establishment of new populations all around the island via bird dissemination. Once established, R. alceifolius maintains dense patches that can grow vegetatively. Our results may be relevant for eradication programs that should take into account variation in reproductive strategy in lowland vs. highland habitats.     

Zooplankton succession on seasonal floodplains: surfing on a wave of food

Floodplains represent ecotones with frequent high productivity mediated by regular shifts between aquatic and terrestrial ecosystems. We examined interaction along this intercept on a seasonal floodplain of the Okavango Delta, Botswana. We focused on the zooplankton communities in order to gain knowledge on the energetic and stoichiometric interplay of the two systems involved. A conspicuous horizontal migration and pulsed biomass increase was found for the dominant zooplankton species (Moina micrura, Daphnia laevis, Mesocyclops leuckarti), culminating in truly extreme numbers before a final collapse. There was a distinct succession in the peak abundance of these species, apparently subsidized by hatchlings from the seed bank of resting eggs as the flood proceeded over the savannah. The high productivity of the system seems to be driven by a strong coupling of the terrestrial and aquatic phase of the floodplains via a mobilization of terrestrially derived nutrients, through nutrients from grazing ungulates during dry periods. Carbon of terrestrial origin, however, appeared to be of minor importance for the planktonic part of the food web. Handling editor: S. Dodson     

Methods of estimating seed banks with reference to long-term seed burial

We compared two standard seed-bank estimation techniques using buried seed populations that had been covered to depths of >1 m by volcanic deposits for 20 years. Some seeds were germinated in a greenhouse (germination method [GM]), and other seeds were extracted by flotation using 50% K₂CO₃ solution (floatation method [FM]). In total, FM could detect more species and seeds in the soils than GM. However, many species that were extracted by FM did not germinate by GM and smaller seeds were extracted to a lesser extent by FM. FM and GM have distinct advantages and disadvantages. We concluded that the application of a single method should be avoided in estimating seed banks, in particular for long-lived seed banks, because the seeds cannot be readily germinated and are structurally weak.     

Seasonal variation in the seed bank of a Patagonian grassland in relation to grazing and topography

Abstract. The germinable seed bank of Festuca spp., Carex patagonica, and other annuals and perennials in a semiarid Patagonian grassland was analyzed every three months. The effects of grazing, topography and microsites with respect to established grass tussocks on the germinable-seed bank were also analyzed. The total germinable-seed bank was larger in summer after seed rain. At this time of the year most of the seeds were from annuals and perennial dicots. Seeds of the dominant Festuca spp. were the main components of the graminoid seed bank, which was homogeneously distributed in patches of bare soil. In spring, i.e. some months after the seed rain, the germinable-seed bank of most of the species was greatly reduced, while the seed bank of Carex patagonica did not change significantly. At this time of the year, the graminoid seed bank was heterogeneously distributed in space, with the seeds accumulating in wind-protected microsites. In the grazing treatments, the seed bank of the perennial grasses was reduced proportionally to the decrease of the plant cover. On the other hand, the germinable-seed bank of C. patagonica increased with the grazing treatments, in relation to the increase of vegetation cover. Topographical position had two types of effect on the seed bank of perennial grasses. One was the increase of the germinable-seed bank, after the seed rain, on the relatively warmer northeast facing slopes, which was related to a higher seed rain in these places. The other was an increased seed loss on slopes. The different seed-bank strategies observed in this grassland are compared and discussed in relation to strategies described for other grasslands.