Within-stand and seasonal variations of specific leaf area in a clonal Eucalyptus plantation in the Republic of Congo

Specific leaf area (SLA; m²_leaf kg^?1_leaf) is a key ecophysiological parameter influencing leaf physiology, photosynthesis, and whole plant carbon gain. Both individual tree-based models and other forest process-based models are generally highly sensitive to this parameter, but information on its temporal or within-stand variability is still scarce. In a 2–4-year-old Eucalyptus plantation in Congo, prone to seasonal drought, the within-stand and seasonal variability in SLA were investigated by means of destructive sampling carried out at 2-month intervals, over a 2-year period. Within-crown vertical gradients of SLA were small. Highly significant relationships were found between tree-average SLA (SLA_t) and tree size (tree height, H_t, or diameter at breast height, DBH): SLA_t ranged from about 9 m² kg^?1 for dominant trees to about 14–15 m² kg^?1 for the smallest trees. The decrease in SLA_t with increasing tree size was accurately predicted from DBH using power functions. Stand-average SLA varied by about 20% during the year, with lowest values at the end of the 5-month dry season, and highest values about 2–3 months after the onset of the wet season. Variability in leaf water status according to tree size and season is discussed as a possible determinant of both the within-stand and seasonal variations in SLA.     

Paired comparison of water, energy and carbon exchanges over two young maritime pine stands (Pinus pinaster Ait.): effects of thinning and weeding in the early stage of tree growth

The effects of management practices on energy, water and carbon exchanges were investigated in a young pine plantation in south-west France. In 2009-10, carbon dioxide (CO(2)), H(2)O and heat fluxes were monitored using the eddy covariance and sap flow techniques in a control plot (C) with a developed gorse layer, and an adjacent plot that was mechanically weeded and thinned (W). Despite large differences in the total leaf area index and canopy structure, the annual net radiation absorbed was only 4% lower in plot W. We showed that higher albedo in this plot was offset by lower emitted long-wave radiation. Annual evapotranspiration (ET) from plot W was 15% lower, due to lower rainfall interception and transpiration by the tree canopy, partly counterbalanced by the larger evaporation from both soil and regrowing weedy vegetation. The drainage belowground from plot W was larger by 113 mm annually. The seasonal variability of ET was driven by the dynamics of the soil and weed layers, which was more severely affected by drought in plot C. Conversely, the temporal changes in pine transpiration and stem diameter growth were synchronous between sites despite higher soil water content in the weeded plot. At the annual scale, both plots were carbon sinks, but thinning and weeding reduced the carbon uptake by 73%: annual carbon uptake was 243 and 65 g C m(-2) on plots C and W, respectively. Summer drought dramatically impacted the net ecosystem exchange: plot C became a carbon source as the gross primary production (GPP) severely decreased. However, plot W remained a carbon sink during drought, as a result of decreases in both GPP and ecosystem respiration (R(E)). In winter, both plots were carbon sources, plots C and W emitting 67.5 and 32.4 g C m(-2), respectively. Overall, this study highlighted the significant contribution of the gorse layer to mass and energy exchange in young pine plantations.     

Impact of agroforestry practices on the flowering phenology of <Emphasis Type="Italic">Vitellaria paradoxa</Emphasis> in parklands in southern Mali

The impact of human practices on the phenology of flowering was assessed for an important agroforestry tree species of the Sudano-Sahelian zone: Vitellaria paradoxa. Flowering was monitored at two sites in southern Mali over 2 years. At each site, the impact of agricultural practices on flowering phenology was assessed by comparing field, fallow and forest. The site effect and agricultural practices were significant for all the monitored variables. The proportion of individuals that flowered was 89% at Koumantou and 40% at MPeresso. About 98, 95 and 75% of individuals at Koumantou and 88, 12 and 20% at MPeresso flowered in the field, fallow and forest, respectively. The mean length of flowering ranged from 69 to 81 days at Koumantou and from 45 to 108 days at MPeresso. The mean number of days for the active phase ranged from 36 to 49 days at Koumantou and from 27 to 64 days at MPeresso. Koumantou’s favourable climatic conditions resulted in better flowering ability and a higher probability of abundant flowering. Field appeared to provide better conditions than fallow and forest regarding flowering ability and probability of abundant flowering. However, tree diameter did not affect flowering phenology. Agricultural practices appear to have a noticeable impact on the phenology of flowering of V. paradoxa. Trees flowered abundantly in the parkland and therefore increased gene flow via pollen and/or seeds and the dynamics of genetic diversity.     

BANAD: A farm model for ex ante assessment of agro-ecological innovations and its application to banana farms in Guadeloupe

The ex ante assessment of innovative agro-ecological innovations is a key step in the development of more sustainable crop management systems. To this end, models are useful tools because they make it possible to rapidly assess numerous innovations in different contexts. Whereas many farm optimisation models focusing on the farmer’s strategic decision to adopt new crop management systems have been published, little attention has been given to the ex ante modelling of the dynamic operational impacts of innovation adoption at the farm level. BANAD, a mechanistic model for such applications, is proposed. It allows the ex ante assessment of innovative management systems including new agro-ecological techniques, while taking into account different farming contexts and policy and market conditions. It includes three components: (i) a crop management system model, (ii) a crop model (SIMBA) and (iii) a farming system model. Our results applied to the ex ante assessment of six innovative banana management systems for three contrasted farm types in Guadeloupe showed that the impacts of agro-ecological innovations, which include rotations, improved fallow, intercropping, pest-resistant cultivar, and an integrated organic system, can vary considerably according to (i) the farm type in which the innovation is integrated, (ii) the nature of the agro-ecological innovations, and (iii) the criteria considered and the temporal horizon of the assessment. Innovative intercropping systems that were effective at the field level in terms of the yield improvement and decreased pesticide use could be problematic at the farm level because they increased the workload and decreased income. The adoption of rotations or improved fallow seemed to be relevant for smallholders but could induce a critical period of 1.5-2.5 years during which income decreased drastically. Under certain conditions of markets and subsidies, very environmentally friendly innovations that are less productive can however be economically effective.     

Fluctuations of landings and environmental conditions in the north-western Mediterranean Sea

Most of the monthly catches and CPUE of 13 studied commercial species in the coastal waters of the north-western Mediterranean were significantly positively correlated with run-off of local rivers (Rhône and Muga) and the wind mixing index during the spawning season, with time lags of less than a year (transfer function analyses). Rhône and Ebre interannual fluctuations in run-off were synchronous and were related to the North Atlantic Oscillation (NAO), river run-offs being negatively correlated to high NAO episodes. The fluctuations of river discharges and the wind mixing index were cyclic but not related. The results showed that enhanced hydroclimatic conditions in the NW Mediterranean were favourable for the productivity of the fish and invertebrate stocks, and suggest the presence of linkage between recruitment of Mediterranean species and local (river discharges, wind conditions) and global (NAO) environmental conditions.