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 variation of the nutritive value of 16 tree species commonly found in Western Europe. We collected 285 samples of tree leaves between spring and autumn (June, August and October) over three years at 14 sites across France. 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 variation and then clustered the species based on CP and IVDMD. CP content and IVDMD generally decreased from spring to autumn (by 26% and 6 percentage points), while DM and ash contents increased (by 42 and 32%). Corylus avellana, Morus alba, and Robinia pseudoacacia had the greatest CP content (from 138 to 250 g.kg⁻¹), and M. alba had the greatest IVDMD (84.7% on average). We observed a trade-off between CP and IVDMD among clusters. The order of clusters based on their nutritive value remained consistent across seasons. Our findings highlight the importance of carefully planning tree use, as their nutritive value varies substantially among species and across seasons. 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.