Potential and actual yield of african fodder cane (Saccharum spontaneum ssp. aegyptiacum) on areas affected by biophysical constraints

The Joint Research Center (JRC) has set a series of thresholds to define marginal lands in terms of biophysical constraints. We focus on climate limitation given by the ratio between precipitations and potential evapotranspiration (P/PET). Indeed, the Mediterranean climates are characterized by long drought periods during summer, with low rainfall and high evapotranspiration, what limits plant CO2 assimilation and biomass production, particularly of spring-summer crops. The present study ascertained the potential and actual yield of African fodder cane (Saccharum spontaneum ssp. aegypticum), a perennial, herbaceous, rhizomatous perennial grass, native from North Africa and widespread in South Mediterranean regions. Saccharum was grown under different water regimes (I0 - rainfed, I50 – 50% ETm and I100 – 100% ETm restoration) for six successive growing seasons, namely from the 7th to the 12th. Throughout the experimental period, the dryness index greatly changed among the six growing seasons: Three out of the six (2012, 2013 and 2014) were much lower than the threshold of 0.6 set in the JRC report, indicating severe drought seasons, two were quite similar to the threshold value (2015 and 2016), while the 2011, which was the wettest season overall, had a dryness index higher than the threshold. Actual biomass yield was mostly driven by meteorological conditions through the growing seasons. However, even in the driest seasons, Saccharum was able to maintain satisfactory biomass yield and good yield persistence. As compared to the potential yield (I100), the relative yield reduction over the six years was in the range of 31% in the most stress condition (I0), but the energy productivity and the water footprint improved by 62% and 32%, respectively, indicating a higher sustainability of the cropping system when irrigation water was not provided. When the irrigation level was raised to the 50% of the maximum evapotranspiration restoration (ETm), the relative yield, over the six growing seasons, reduced by 16.5%; the energy productivity and the water footprint improved of only 14 and 22%, respectively. This study underlines the importance for strategic selection of crops for a given environmental condition dominated by a specific biophysical constraint and the agronomic practices leading to increase the energy productivity while reducing the pressure on Mediterranean freshwater.