The effect of nutrition on the seasonal course of needle respiration in Norway spruce stands

Respiration of 1-year-old needles of 30-year-old Norway spruce trees [Picen abies (L.) Karst.] was studied in a nutrient optimisation experiment in northern Sweden. Respiration rates of detached needles, from ten control (C) and ten irrigated-fertilised (IL) trees, were measured on 16 occasions from June 1992 to June 1993. The aim of the study was to determine the influence of temperature on the seasonal course of needle maintenance respiration, and the effect of nitrogen concentration [N] and carbohydrate content on needle respiration in young Norway spruce trees subjected to long-term fertilisation. The IL treatment significantly affected needle size, in terms of dry mass and length, but not specific needle length (SNL). There was, however, a strong tree-specific effect on SNL (P < 10(-9), R-2 = 0.75). Needle starch content varied markedly with season (0-25% of total dry mass). This, unless accounted for, would cause erroneous estimates of nutrient concentrations, and of rates of needle respiration, within and between treatments. There was considerable seasonal variation in needle respiration, both in terms of maintenance respiration and temperature dependence (Q(10)). Q(10) had its highest value (2.8) during winter and its lowest (2.0) in the middle of summer. In early autumn (August, September), respiration rate and needle [N] were significantly related (C: P = 0.001, IL: P < 0.0005). There was no significant difference in the slope between the two regression lines, but a difference in intercept. At the same needle [N], needles from IL-plots always had a lower respiration rate than needles from control plots. No obvious explanation for the observed difference in intercept was found, but some plausible assumptions are put forward and discussed.