Delayed growth response of Mountain Birch seedlings to a decrease in fertilization and temperature

1. The effects of previous-year environment on current growth response were tested in seedlings of Mountain Birch (Betula pubescens Ehrh. ssp. czerepa novii [Orlova] Hamet-Ahti), a subarctic tree species with indeterminate sho ot growth. 2. Mountain Birch seedlings were pot-grown outdoors in subarctic Sweden for 2 years and showed a clear delay in growth response when fertilization and temperature were reduced after the first year. The seedlings were grown un der four experimental treatments (two temperatures and two nutrient availab ilities) in 1994, and under low-temperature/low-nutrient conditions in 1995 . 3. When nutrient supply and/or temperature was reduced in 1995 compared to 1994, the seedlings maintained the high relative growth rates (RGR) of the previous growing season, although the internal plant N accumulation rate wa s lower than in 1994. This resulted in decreasing plant N concentration (PN C), and a poor relationship between RGR and PNC during 1995. The high RGR i n 1995 was achieved in response to phenotypic adjustments (e.g. number of f oliar buds) to a more favourable environment in the past, and by dilution o f the internal nutrient storage. 4. The effects of delayed responses found in this study indicate problems f or the interpretation of results from growth studies performed under any cl imate with great year-to-year variability, such as the subarctic, because a delay in growth response could distort the relationships between plant gro wth, resource availability and climate. Predictions of current growth are t herefore meaningless if the previous history of the plants is not taken int o account.