Changing woody plant density across agricultural landscapes of the Sudano-Sahelian region is a debated issue. This paper reports the results of an investigation on the contribution of field hedges to overall woody plant density. Hedges separating individual cropped parcels were studied within village agropastoral territories in the Dantiandou district in western Niger. In 1992, field hedges were mapped over the study area using aerial photography and in 2016, using high resolution Google Earth imagery. In 1992, field hedge length was equal to 1006 km within 435 km2 of croplands, equivalent to 2.3 km km−2. In 2016, 17.5% of these hedges had disappeared, but 1591 km of new hedges were observed resulting in an increased density of hedges to 5.6 km km−2. In 24 years, hedges had increased at a mean annual rate of 3.7% likely associated with the splitting of crop fields by inheritance. The composition and productivity of hedges were also monitored in 1996, 2010 and 2015. All trees (maximum height ≥ 4 m), shrubs (<4 m) and tussock perennial grasses were recorded within twelve field hedge samples of 200 m each. Field measurements were used to estimate basal area, crown area, foliage mass and wood mass of each woody plant within the sampled hedges. No significant trends were found between 1996 and 2015 in woody plant density, basal area, crown cover, wood and foliage masses across all monitored hedges. However, overall means hide a slight decrease in tree contributions, while shrub contributions first increased and then decreased. They also mask contradictory trends among sites most likely related to different rates of shrub coppicing and tree cutting. The woody species composition of the hedges is poor with an increase of Combretum glutinosum to the detriment of Guiera senegalensis over the study period. Multiplying the hedge sample statistics by the changing lengths of field hedges in the study area provides estimates of the contribution of the hedge woody plants to the woody plant population at a landscape scale. Between 1992 and 2016, field hedges contributed to increases of woody plant density by 3.9%, basal areas by 5.4%, crown cover by 2.7%, leading to 6.1% annual increase in foliage mass and 8.8% increase in wood mass.
Tag: hedgerow
Light interception and evaporation in hedgerow agroforestry systems
Quantifying water lost through evaporation and transpiration in a cropping system is an important tool in adapting a system for semi-arid conditions. During two cropping seasons in eastern Kenya, light interception and soil water content were measured in several different cropping systems: monocultures of cowpea (Vigna unguiculata (L.) Walp.), maize (Zea mays cv. Katumani), Senna spectabilis cv. Embu managed as a hedge, and hedge intercrops of cowpea and maize. These systems differed with respect to plant population density, maximum light interception (44–75%) and canopy height (0.5–2.0 m). Parameters in the EPIC model for leaf area development were derived from periodically-measured light interception in the different systems. Daily light interception predicted from the leaf area was used to partition potential evapotranspiration into potential evaporation and potential transpiration. A simple water balance model was used to predict actual transpiration and actual soil evaporation. Predicted values of water loss during the two seasons correlated closely with measured values (r2 = 0.85 and 0.91; slope = 1.00 and 1.01). In both seasons, the model predicted that soil evaporation comprised approximately half (42–58%) of the estimated evapotranspiration. This study suggests that evapotranspiration can be predicted for a variety of cropping systems when light interception measurements are used in conjunction with a simple model of plant water uptake. It also demonstrates the difficulty of maximizing plant water use in agroforestry systems in semi-arid environments when the canopies of both annual crops and hedges develop simultaneously.