Seasonal dynamics of the nutritive value of temperate forage trees differ among species

There is growing interest in using temperate forage trees to alleviate the effects of summer drought and heatwaves on herbaceous forage. However, forage trees remain understudied in temperate climates. We studied the seasonal dynamics of the nutritive value of 16 tree species commonly found in Western Europe. We collected 285 samples of tree leaves from 2015-2017 at 14 sites across France in June, August, and October (i.e. from spring to autumn). We measured seven nutritive characteristics: in vitro dry matter digestibility (IVDMD) and the contents of crude protein (CP), dry matter (DM), neutral detergent fibre (NDF), acid detergent fibre (ADF), acid detergent lignin (ADL), and ash. We used linear mixed models to analyse their seasonal dynamics and then clustered the species based on CP and IVDMD. Results showed that CP content and IVDMD generally decreased from spring to autumn, while DM and ash contents increased. Corylus avellana , Morus alba , and Robinia pseudoacacia had the highest CP content, and M. alba had the highest IVDMD. We observed a trade-off between CP and IVDMD among clusters, and the nutritive value of the species decreased less than that of herbaceous forage. The order of clusters based on their nutritive value remained consistent across seasons. This study is one of the first to conduct a multi-site and multi-year assessment of the nutritive value of temperate forage trees. Its results provide new opportunities for farmers to compensate for the lack of herbaceous forage in summer, even though yield and palatability aspects remain to be studied.