## 1 Introduction
The timing of precipitation was found to be as important as changes in the total amount of precipitation in controlling vegetation productivity in water-limited ecosystems [23]. A delayed onset of rain is thought to exert a detrimental effect on seasonal GPP by delaying plant growth and preventing full vegetation development [19]. On the other hand, an early onset may enhance initial productivity and water reserve depletion, thus inhibiting later photosynthesis and also reducing overall production [12]. Furthermore, the magnitude and type of effect resulting from the altered timing and amount of a rainfall event can be quite specific for different plant species, functional types and communities [24]. In addition, it is unclear whether a delayed onset of vegetation development has the same effect on crops and natural vegetation seasonal productivity. For example, nearly all crops are sensitive to temperature and some to photoperiod as well. Seeding or planting exactly after the first abundant rains to catch maximum water is a typical risk minimization farming strategy in the south of Sahel and in other semi-arid areas of the world [25]. Because plants experience different temperature and photoperiod conditions as a consequence of variable rain onsets and sowing dates, crop performances are expected to be sensitive to anomalies in the start of the season. For example, when photosensitive crop varieties are planted too late (for instance, because of replanting after a first crop failure due to the late onset of the rainy season), the \"flowering signal\" is received by a plant when it is still very young. As a consequence, it will start flowering when the biomass is not fully developed, with detrimental effects on final grain yield. Sowing delays can therefore translate into yield reduction [26]. However, farmers may rely on multiple cultivars with different lengths of growing cycle to cope with rainfall variability [27], partially compensating for the possible negative effects of late planting [28, 29]. More arid areas in the north of the Sahel are less suitable for crops and are typically exploited for grazing. Such areas enjoy only a one to two month-long growing season, because of the short duration of the West African monsoon. Here, the herbaceous vegetation is also sensitive to the photoperiod, and a delayed rainfall onset season may have an effect on seasonal productivity as well. Present climate change has already altered both the amount and the seasonal distribution of rainfall worldwide and especially in the dry tropics [30], with consequences on vegetation phenology (e.g., [19, 31, 32]). Gaining insights into the possible effect of these changes on crop and pasture production in semi-arid regions is important to develop appropriate adaptation strategies, particularly in such semi-arid areas, where the impact on terrestrial ecosystems is expected to be more dramatic [33, 34]. Several studies analyzed the changes or trends in phenological variables as possible indicators of climate change (e.g., [35, 36, 37]). Others have addressed the long-term Sahel desertification/re-greening trends (see [PERSON] _et al._[38] for a comprehensive introduction and a recent contribution to the debate) and their possible causes (e.g., [39, 40, 41, 42]). So far, only [PERSON] and [PERSON] [43] have addressed the relationship between remote sensing-derived phenology and biomass production. They qualitatively analyzed the start of the season anomaly for a single year in Burkina Faso and found it to be related to partial crop failure. However, large-scale and multi-annual investigations on the relation between different key phenological variables and vegetation biomass production are missing. A better understanding of the role of the timing of the start of season on vegetation development is also important to evaluate, and possibly improve, the fixed phenology settings often used in crop development simulation models and in many RS-based empirical approaches.