Stand age and fine root biomass, distribution and morphology in a Norway spruce chronosequence in southeast Norway
Isabella Børja (1, 2), Heleen A. De Wit (3), Arne Steffenrem (1) and Hooshang Majdi (4)
1. Norwegian Forest and Landscape Institute, P.O. Box 115, NO-1431 Ås, Norway / 2. Corresponding author () / 3. Norwegian Institute for Water Research (NIVA), Gaustadalléen 21, 0349 Oslo, Norway / 4. Department of Ecology and Environmental Research, Swedish University of Agricultural Sciences, SE-750 07 Uppsala, Sweden / Received April 8, 2007; accepted October 11, 2007; published online March 3, 2008
Summary
We assessed the influence of stand age on fine root biomass and morphology of trees and understory vegetation in 10-, 30-,
60- and 120-year-old Norway spruce stands growing in sandy soil in southeast Norway. Fine root (< 1, 1–2 and 2–5 mm in diameter)
biomass of trees and understory vegetation (< 2 mm in diameter) was sampled by soil coring to a depth of 60 cm. Fine root
morphological characteristics, such as specific root length (SRL), root length density (RLD), root surface area (RSA), root
tip number and branching frequency (per unit root length or mass), were determined based on digitized root data. Fine root
biomass and morphological characteristics related to biomass (RLD and RSA) followed the same tendency with chronosequence
and were significantly higher in the 30-year-old stand and lower in the 10-year-old stand than in the other stands. Among
stands, mean fine root (< 2 mm) biomass ranged from 49 to 398 g m–2, SLR from 13.4 to 19.8 m g–1, RLD from 980 to 11,650 m m–3 and RSA from 2.4 to 35.4 m2 m–3. Most fine root biomass of trees was concentrated in the upper 20 cm of the mineral soil and in the humus layer (0–5 cm)
in all stands. Understory fine roots accounted for 67 and 25% of total fine root biomass in the 10- and 120-year-old stands,
respectively. Stand age had no affect on root tip number or branching frequency, but both parameters changed with soil depth,
with increasing number of root tips and decreasing branching frequency with increasing soil depth for root fractions < 2 mm
in diameter. Specific (mass based) root tip number and branching density were highest for the finest roots (< 1 mm) in the
humus layer. Season (spring or fall) had no effect on tree fine root biomass, but there was a small and significant increase
in understory fine root biomass in fall relative to spring. All morphological characteristics showed strong seasonal variation,
especially the finest root fraction, with consistently and significantly higher values in spring than in fall. We conclude
that fine root biomass, especially in the finest fraction (< 1 mm in diameter), is strongly dependent on stand age. Among
stands, carbon concentration in fine root biomass was highest in the 30-year-old stand, and appeared to be associated with
the high tree and canopy density during the early stage of stand development. Values of RLD and RSA, morphological features
indicative of stand nutrient-uptake efficiency, were higher in the 30-year-old stand than in the other stands.
