Fine root biomass, production and its proportion of NPP in a fertile middle-aged Norway spruce forest: Comparison of soil core and ingrowth core methods

Fine root bio- and necromass, net primary production (NPP) of fine roots and its proportion of the NPP of trees, as well as turnover rate were investigated in a fertile middle-aged Norway spruce (Picea abies (L.) Karst) stand by sequential core and ingrowth core methods. The stand's site type is Oxalis, the site quality class is Ia and the soil type is Umbric Luvisol (FAO classification). Twenty soil cores (volumetric samples, core diameter 38 mm) were taken monthly during the period June-1996 to November 1996 and in June-1997. Ingrowth cores were collected, 15 at a time, during the growing seasons from 1997 to 1999, once after first year and three times in the second and third years. Spruce roots from samples collected by both methods were separated into living and dead roots (two diameter classes: <1 and 1 mm ≤ d <2 mm). The fine root NPP was calculated according to the decision matrix, and root turnover rate was calculated as annual root production divided by mean fine root biomass. The mean biomass of fine (<2 mm) and finest (<1 mm) roots in ingrowth cores collected in the third year after installation was two times smaller than that in soil cores. The mean fine root biomass was 1420 ± 170 kg ha −1 in soil cores and 700 ± 105 kg ha−1 in the third year ingrowth cores. The finest roots formed ca. 2/3 of fine root biomass. The fine root NPP estimated by the sequential core method was 2510 and 965 kg ha−1 year−1 by the ingrowth core method (third year after installation). The fine root turnover rate was 1.8 year−1 for sequential cores and 1.4 year−1 for third-year ingrowth cores. The inverse of the root turnover rate is, in turn, a measure of average root longevity; it was smaller for the finest roots in both cases. In the investigated spruce stand the annual NPP of trees at the age of 40 years is estimated as 21.4 t ha−1 year−1, the share of the belowground part forming 31%. Fine roots accounted for 13% of the NPP, which is a relatively small value compared to the results revealed in most studies.