Josiane Seghieri

Plant phenology in relation to water availability: herbaceous and woody species in the savannas of northern Cameroon

Phenological patterns of herbaceous and woody plants well represented in the Sudano-Sahelian savannas of northern Cameroon were studied in relation to water availability in three contrasted soil types. Mean annual precipitation in this region is 800 mm, entirely restricted to the period May-June to September-October, but redistribution of rainwater by varying soil substrates leads to considerable heterogeneity in available water resources for plants. Results are discussed in terms of probable competition for water resources among co-occurring plants and the adaptations shown by observed patterns to prevailing environmental conditions. Water stress is seen to be a limiting factor to plant growth but does not constitute the sole trigger for the phenolo- gical phases observed, nor is it the principal factor responsible for the preponderance of annual species in the herbaceous stratum.

Differences between banded thickets (tiger bush) at two sites in West Africa

. This paper deals with the influence of edaphic conditions on the spatial structure of banded thickets or tiger bush (brousse tigree). It is based on two sites in West Africa, with similar climatic conditions but located on contrasting substrates. The spatial structure was described with standardized characteristics including thicket spacing, thicket/inter-thicket contrast, upslope/downslope asymmetry and species zonation throughout the vegetation band. Recruitment and senescence features of woody stands were emphasized in order to understand current dynamics. Data were collected on transects oriented perpendicular to the contours and so to the thickets as well. A standardized analytical procedure was applied to data from both sites to ensure consistent and thorough delineation of thickets. n n n nThe overall periodicity of thickets, the woody flora and the dominant species Combretum micranthum were similar at the two sites. However, thicket spacing, thicket/inter-thicket contrast and upslope/downslope floristic asymmetry of the thickets were higher in the less favourable site. Also seedlings were less abundant, with a greater dependence on pre-existing thickets. n n n nNot all banded vegetation systems show sharp contrasts and are strongly asymmetric, since such characteristics are likely to be reinforced by adverse environmental conditions. As a consequence, current dynamics may be more diverse than expected. Quantified inter-site comparisons can greatly help to classify African banded vegetation systems and to discuss potential dynamic outcomes.

Development of an herbaceous cover in a sudano-sahelian savanna in North Cameroon in relation to available soil water

Dynamics of annual communities are studied in Sudano-Sahelian savannas of the north of the Cameroon. The vegetation composition of three types of soil (a vertisol, a degraded vertisol and a ferruginous soil) under two rainfall conditions is compared. The study supports the idea that intense fluctuations in water availability at the beginning of the rainy season determine which species germinate, grow and survive under conditions of water stress. However soil conditions can accentuate or reduce the influence of the rainfall fluctuations while the land use history of each station tends to buffer the variations of the overall pattern determining potential vegetation.

Variations in phenology of a residual invasive shrub species in Sahelian fallow savannas, south-west Niger

The aim of this work was to analyse the phenological variation of a residual invasive semi-evergreen shrub in Sahelian fallows in Niger. This species, Guiera senegalensis (Combretaceae), has benefited from the elimination of other species thanks to its high resistance to the increasing cutting pressure for cultiva- tion. We sought to identify traits of its life cycle that could contribute to its persist- ence and widespread dominance under such a disturbance regime. Leafing, flower- ing and fruiting were monitored on seven fallows differing in topographic location, age, shrub density and degradation state, during two rainy seasons and one dry season. Phenological spectra showed similar timing of the three phenophases of the seven populations. Differences were observed mainly in magnitude of the phases. However, there were both a high intra-fallow variability (64.6% of the whole variability) and a statistically significant inter-fallow variability, despite only 35.4% of the total variability being due to differences between fallows. A statistical classification of the fallows was proposed based on shrub phenological course over the two cycles monitored. This classification corresponds to a gradient of life-cycle achievement primarily linked to the topographic situation of the stations, and then to a combination of other factors. The flexibility of the phenology at the individual, population and metapopulation scales, inherent to its specific semi-evergreenness property, appeared as an essential phenological attribute contributing to success- ful widespread persistence of G. senegalensis in the study area.

Is water availability really the main environmental factor controlling the phenology of woody vegetation in the central Sahel

Rainfall distribution and the soil moisture regime have been recognized to be the key drivers of the phenological rhythms in Sahelian woody plants, although different climate triggers have been assumed to be involved in determining the date of the onset of the phenophase. However, almost no comparisons have been made of the actual relative predictive power of these environmental factors. The aim of our study was to quantify the ability of several factors to predict phenophase occurrence in the dominant woody populations of northern Mali. Canopy leafing, flowering and fruiting were monitored from May 2005 to July 2007. Multiple logistic regressions were used to test the predictive power of cumulative rainfall, soil moisture, air temperature, air humidity and day length, with time lags of up to 2xa0months. Artificial variables derived from time lags observed in phenophases were included as predictors to account for possible auto-correlation and cross-correlation among phenophases. Surprisingly, a decrease in temperature associated with different time lags was most often found to be the strongest predictor of both leafing and reproductive phenophases. In Sahelian shrubs, morphological and physiological adaptations strongly contribute to the relative independence of their activity from water availability, leaf phenology being a way to adjust the plant water balance to current water availability and atmospheric water content. This study provides insight towards the development of a mechanistic understanding of phenological control in the Sahel, which is becoming increasingly important in the context of expected climate changes.

Adaptative above-ground biomass, stand density and leaf water potential to droughts and clearing in Guiera senegalensis, a dominant shrub in Sahelian fallows (Niger).

Guiera senegalensis tolerates repeated cutting and pruning to which it is increasingly subjected at the onset of each cropping period, and one to three times during the annual cropping period (June-September). It responds by profusely branching at the shrub base. The topographic and geomorphic influences, and the effect of clearing on the relationships between shrub density, mean individual above-ground biomass and leaf water status were analysed for seven fallow sites. They were sampled 75 km east of Niamey in Niger. Surprisingly, shrub density and mean individual above-ground biomass were highest in the sites that had been recently in fallow and intensively cultivated prior to crop abandonment. Stand above-ground biomass was also positively related to mean predawn and daily minimum leaf water potentials. Consequently, in the studied G. senegalensis stands, above-ground biomass appeared to be controlled by water availability - rainfall, runoff, infiltration - rather than by cropping intensity. Anisohydric stomatal regulation, resulting in large safety margins from critical transpiration, was inferred from the diurnal amplitude of leaf water potential during the dry season. The plant physiological resistance to water stress combined with its population adjustment in density and growth contributes to the sustainable dominance of G. senegalensis.

Phenology of Woody Species Along the Climatic Gradient in West Tropical Africa

West tropical Africa (WTA) is known to be one of the most exposed regions in the world to climate change. The seasonality of rainfall and of atmospheric conditions, including a dry season lasting for several months in most of the area, strongly influences vegetation activity (Bourliere & Hadley, 1983; Breman & Kessler, 1995). Indeed, this region was already affected by severe and prolonged droughts in the 1970s-1980s. These events represent one of the major climate variations of the 20th century recorded at the global scale (Giorgi, 2002; Neelin et al., 2006; Redelsperger et al., 2006). The average annual rainfall deficit varied by ± 20% in higher rainfall zones and by 50% in lower rainfall zones. However, while these droughts were relatively uniform over most of WTA, the 1990–2007 period was characterised by a more complex pattern including large spatial variability (Lebel & Ali, 2009). Current projections of global change predict higher temperatures and lower rainfall, although opinions on temperatures are contradictory (Mearns et al., 2001; Haarsma et al., 2005; IPCC, 2007; Funk & Brown, 2009). More frequent extreme events are also expected in some parts of Africa (Hely et al., 2006; Frappart et al., 2009; Lebel & Ali, 2009).

Differences in transpiration between a forest and an agroforestry tree species in the Sudanian belt

Average population growth in the African Sudanian belt is 3xa0% per year. This leads to a significant increase in cultivated areas at the expense of fallows and forests. For centuries, rural populations have been practicing agroforestry dominated by Vitellaria paradoxa parklands. We wanted to know whether agroforestry can improve local rainfall recycling as well as forest. We compared transpiration and its seasonal variations between Vitellaria paradoxa, the dominant species in fallows, and Isoberlinia doka, the dominant species in dry forests in the Sudanian belt. The fallow and dry forest we studied are located in northwestern Benin, where average annual rainfall is 1200xa0mm. Sap flow density (SFD) was measured by transient thermal dissipation, from which tree transpiration was deduced. Transpiration of five trees per species was estimated by taking into account the radial profile of SFD. The effect of the species and of the season on transpiration was tested with a generalized linear mixed model. Over the three-year study period, daily transpiration of the agroforestry trees, V. paradoxa (diameters 8–38xa0cm) ranged between 4.4 and 26.8xa0Lxa0day−1 while that of the forest trees, I. doka, (diameters 20–38xa0cm) ranged from 9.8 to 92.6xa0Lxa0day−1. Daily transpiration of V. paradoxa was significantly lower (15xa0%) in the dry season than in the rainy season, whereas daily transpiration by I. doka was significantly higher (13xa0%) in the dry season than in the rainy season. Our results indicate that the woody cover of agroforestry systems is less efficient in recycling local rainfall than forest cover, not only due to lower tree density but also to species composition.

Is the WBE model appropriate for semi-arid shrubs subjected to clear cutting?

It is crucial to understand the adaptive mechanisms of woody plants facing periodic drought to assess their vulnerability to the increasing climate variability predicted in the Sahel. Guiera senegalensis J.F.Gmel is a semi-evergreen Combretaceae commonly found in Sahelian rangelands, fallows and crop fields because of its value as an agroforestry species. We compared canopy leafing, and allometric measurements of leaf area, stem area and stem length and their relationships with leaf water potential, stomatal conductance (gs) and soil-to-leaf hydraulic conductance (KS-L), in mature and current-year resprouts of G. senegalensis in Sahelian Niger. In mature shrubs, seasonal drought reduced the ratio of leaf area to cross-sectional stem area (ALu2005:u2005AS), mainly due to leaf shedding. The canopy of the current-year resprouts remained permanently leafed as the shrubs produced leaves and stems continuously, and their ALu2005:u2005AS ratio increased throughout the dry season. Their KS-L increased, whereas gs decreased. West, Brown and Enquists (WBE) model can thus describe allometric trends in the seasonal life cycle of undisturbed mature shrubs, but not that of resprouts. Annual clear cutting drives allometric scaling relationships away from theoretical WBE predictions in the current-year resprouts, with scaling exponents 2.5 times greater than those of mature shrubs. High KS-L (twice that of mature shrubs) supports this intensive regeneration process. The adaptive strategy described here is probably common to many woody species that have to cope with both severe seasonal drought and regular disturbance over the long term.

Comparison of the measurements of leaf water potential between a hydraulic press and a pressure chamber in six Sahelian woody species

The aim of this methodological study was to quantify differences between water potential measured with a pressure chamber (PC) and with a hydraulic press (HP) in six north Sahelian dominant species of the woody strata across the range of their local environmental conditions in the Malian Gourma. Mean annual rainfall is 372xa0mm, falling from June to September, followed by 8–10xa0months of dry season. The daily course of Leaf Water Potential (LWP) was monitored in 2–6xa0m tall healthy individuals. Water potential measured with the two instruments were statistically comparable (R²xa0>xa040%) except in A. senegal. However, the HP under-estimated LWP and revealed smaller ranges of water potential than the PC. In the Sahelian shrubs studied here, for the precise measurement of a water potential gradient in the soil–plant-atmosphere continuum and for inter-specific comparisons, the PC is more appropriate than the HP. However, the HP may be useful for intra-species comparison in large sampled fields, since calibrations will be checked across a wider range of dates and a large number of sites.