Drought effects on herbage production of permanent grasslands in northern Germany.

We investigated the influence of sward composition (dry weight by functional group) and nitrogen fertilization on the annual yields of drought-stressed meadows from three sites typical of northern Germany by generating artificial drought with rain-out shelters that held back, on average, 148 of 242 mm rainfall from the beginning of the vegetation period until the end of the last stress treatment. Yields decreased from on average 7510 to 7298 kg ha-1 a-1 when swards were exposed to drought stress, but this effect was enlarged and modified by fertilization, site and sward composition. Nitrogen-fertilization had a positive effect on productivity. The influence of sward composition was site specific and may rather have been a functional-group effect than the influence of diversity. Future productivity losses due to drought stress may be smaller than expected.