Développement des unités de croissance de jeunes plants d’essences sahéliennes : Acacia tortilis (Forsk.) Hayne subsp.raddiana (Savi) Brenan, de Balanites aegyptiaca (L.) Del et de Zizyphus mauritiana Lam., (Dakar, Sénégal)

Malgré les usages multiples des plantes ligneuses du Sahel, peu d’études se sont intéressées au suivi de la mise en place des différents segments de tige de jeunes plants de Acacia tortilis (Forsk.) Haynesubsp.raddiana (Savi) Brenan, de Balanites aegyptiaca (L.) Del et de Zizyphus mauritiana Lam. Notre objectif était alors de représenter les différents ordres, leur nombre en fonction de leur période d’apparition et le rapport entre le développement successif des Unités de Croissance (UC) des jeunes plants de A. tortilis, de B. aegyptiaca et de Z. mauritiana a été suivi en milieu semi-contrôlé par une approche visuelle globale à l’échelle du houppier entre octobre 2002 et octobre 2004. Les processus de ramification à l’échelle du houppier révèlent 5 ordres différents d’UC chez A. tortilisà 16 mois, 7 chez B. aegyptiaca à 24 mois et 5 chez Z. mauritiana à 9 mois. Les UC3 et UC4 sont les plus nombreuses chez les jeunes plants de B.aegyptiaca et Z.mauritiana alors que chez ceux de A.tortilis, le nombre des UC2 est nettement supérieur. L’UC1 est la plus longue chez les trois espèces. Le diamètre de l’UC1 à 24 mois est de 79 mm chez B. aegyptiaca, de 39 mm chez Z. mauritiana et de 20 mm chez A. tortilis avec un nombre de noeuds nettement plus important chez A. tortilis. La longueur et le nombre de noeuds feuillés varient d’une espèce à l’autre.Mots-clés : Acaciatortilis, Balanites aegyptiaca, Zizyphusmauritiana, jeune plant, unité de croissance (UC),noeuds

Overyielding of temperate mixed forests occurs in evergreen–deciduous but not in deciduous–deciduous species mixtures over time in the Netherlands

Recent studies show that mixed species forests sometimes have higher stand productivity than monospecific forests, which we refer to as overyielding. Yet, results for temperate forests are ambiguous, possibly because forests differ in local site conditions, thinning history and forest age. In line with the niche complementarity hypothesis, we expect stronger overyielding for forests with species differing in both leaf phenology (evergreen or deciduous) and shade tolerance. We also hypothesize that overyielding will decrease with stand development because of decreasing resource availability. We compared 4 two-species mixtures with their corresponding monospecific stands from long-term field measurements in the Netherlands. The mixtures were Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco)–common beech (Fagus sylvatica L.), Scots pine (Pinus sylvestris L.)–common oak (Quercus robur L.), common oak–common beech, and common oak–silver birch (Betula pendula Roth). Overyielding was observed in 2 of the 4 mixtures: Douglas-fir–common beech mixtures had 35.9% and 36.7% higher volume growth relative to Douglas-fir and common beech monocultures, respectively; Scots pine–common oak mixtures had 20.3% and 31.2% higher volume growth relative to Scots pine and common oak monocultures, respectively, on average over time. Furthermore, overyielding was relatively constant for the two mixtures through stand development. This result was robust after accounting for possible effects of site quality and thinning history, where site quality contributed independently to stand productivity and thinning history had no effect. No significant overyielding effects were observed for the two deciduous mixed stands, i.e. common oak–common beech and common oak–silver birch. Mixing tree species in temperate forests resulted in overyielding for evergreen–deciduous species mixtures, but not for deciduous–deciduous species mixtures. This indicates that leaf phenology contributes to overyielding effects. Overyielding was higher in the Douglas-fir–common beech mixtures than the Scots pine–common oak mixtures, which coincides with a stronger contrast in shade tolerance between Douglas-fir and common beech and thus stronger complementarity. Our results support the complementarity hypothesis and imply that such mechanisms are maintained with stand development. It therefore appears that mixing evergreen with deciduous species with contrasting shade tolerance is a valid management strategy for increasing diversity and productivity of temperate forests in the Netherlands

Variability of key biological parameters of round sardinella Sardinella aurita and the effects of environmental changes

We examined growth rates and reproductive characteristics of Sardinella aurita off Senegal and other coastal areas over a 20 year period (1995-2014) to determine how they relate to variations in environmental characteristics of coastal waters. Based on fish length-frequency data and a coastal upwelling index, we found that S. aurita recruitment tends to occur during the periods of most intensive upwelling (March-April off Senegal). Peak reproduction corresponds to periods of low sea-surface temperature (in February or March). The sex ratio was remarkably consistent during the 30 year study period and so was not affected by environmental changes. We hypothesise that S. aurita takes advantage of the higher zooplankton productivity that occurs in coastal waters when upwelling brings nutrient-rich water to the surface (i.e., it increases its growth rate and accumulates energy reserves for spawning). Growth performance appears to be strongly dependent on environmental conditions. The timing of spawning seems to occur when food (zooplankton) is most available for supplying the energy requirements needed by adults for spawning and early development of larvae. Environmental changes seem to have a significant effect on S. aurita growth and reproduction, which endorses their high phenotypic plasticity