Impacts of the seasonal distribution of rainfall on vegetation productivity across the Sahel

Abstract

Abstract. Climate change in drylands has caused alterations in the seasonal distribution of rainfall including increased heavy-rainfall events, longer dry spells, and a shifted timing of the wet season. Yet the aboveground net primary productivity (ANPP) in drylands is usually explained by annual-rainfall sums, disregarding the influence of the seasonal distribution of rainfall. This study tested the importance of rainfall metrics in the wet season (onset and cessation of the wet season, number of rainy days, rainfall intensity, number of consecutive dry days, and heavy-rainfall events) for growing season ANPP. We focused on the Sahel and northern Sudanian region (100-800 mm yr −1 ) and applied daily satellite-based rainfall estimates (CHIRPS v2.0) and growing-season-integrated normalized difference vegetation index (NDVI; MODIS) as a proxy for ANPP over the study period: 2001-2015. Growing season ANPP in the arid zone (100-300 mm yr −1 ) was found to be rather insensitive to variations in the seasonal-rainfall metrics, whereas vegetation in the semi-arid zone (300-700 mm yr −1 ) was significantly impacted by most metrics, especially by the number of rainy days and timing (onset and cessation) of the wet season. We analysed critical breakpoints for all metrics to test if vegetation response to changes in a given rainfall metric surpasses a threshold beyond which vegetation functioning is significantly altered. It was shown that growing season ANPP was particularly negatively impacted after > 14 consecutive dry days and that a rainfall intensity of ∼ 13 mm day −1 was detected for optimum growing season ANPP. We conclude that the number of rainy days and the timing of the wet season are seasonal-rainfall metrics that are decisive for favourable vegetation growth in the semiarid Sahel and need to be considered when modelling primary productivity from rainfall in the drylands of the Sahel and elsewhere.

Ecosystem structural changes controlled by altered rainfall climatology in tropical savannas

“…Here, we found a strong relationship between increases in woody cover and increases in the frequency of heavy rainfall events, which has also been demonstrated at experimental sites 2,19 . Since herbaceous vegetation is known to be vulnerable to an uneven seasonal distribution of rainfall 34 , altered rainfall climatology is likely to affect the coexistence of woody and herbaceous vegetation, e.g., favouring the expansion of the ranges of woody plants 2,13 . This effect would imply that large-scale woody encroachment reported in savanna ecosystems 2 is at least partly driven by changes in the rainfall regime.…”

Impact of temporal precipitation variability on ecosystem productivity

“…The hierarchical structure of tree-based models naturally supports the expression of dependencies among variables and can flexibly model nonlinear relationships and interactions (Breiman, 2001). For example, Zhang et al (2018) analyzed the impacts of rainfall variability on vegetation productivity using random forest in Sahel, Africa, and found that productivity has a nonlinear relationship with variables such as the start and end time of the wet season and consecutive dry days.…”

“…Use spatial gradients to study the long-term impact. Long-term manipulation experiments are time-consuming and costly, so analyzing vegetation productivity along spatial gradients of precipitation variability statistics such as rainfall intensity and rainy season length can be an effective approach for studies at large spatial scales (Fang et al, 2005;Zhang et al, 2018). However, attention should be given to the modifiable areal unit issue (MAUP, Dark & Bram, 2007), which means that the obtained conclusions may be different depending on the shape and scale chosen for analysis.…”

Conceptual Links between Landscape Diversity and Diet Diversity: A Roadmap for Transdisciplinary Research

“…Temporal aspects of food–forest dynamics require forest tracking across different timeframes from the short term to the longer term. Throughout much of Africa, vegetation greenness varies dramatically across the year (Zhang et al 2018 ) affecting forest resources. For example, in Burkina Faso, edible leaves from trees (such as the baobab tree, Adansonia digitata) provide up to 60% of consumed vegetables.…”