Fine root production and turnover in forest ecosystems in relation to stand and environmental characteristics

The production and turnover of fine roots (diameter 2 mm) contributes significantly to carbon cycling in forest ecosystems. We compiled an up-to-date global database covering 186 stands from the literature and estimated fine root production (FRP) and fine root turnover (FRT) for boreal, temperate and tropical forests in order to study the relationships between FRP or FRT and environmental and stand variables. FRP for all plants (trees + understorey) was 311 +/- 259 (n = 39), 428 +/- 375 (n = 71) and 596 +/- 478 g m(-2) a(-1) (n = 32) in the boreal, temperate and tropical forests, respectively, and the corresponding annual FRT rates were 0.77 +/- 0.70, 1.21 +/- 1.04 and 1.44 +/- 0.76, respectively. When the FRP and FRT of trees were estimated separately for boreal and temperate forests the differences between the two biomes were insignificant. The mean FRP of trees for the two biomes combined was 306 240 g m-2 a-1 (n = 86) and the annual FRT was 1.31 +/- 1.43. Fine root biomass (FRB) was the most significant factor explaining the variation in FRP, and more so at the tree level than at the stand level, explaining 53% of the variation in FRP for trees at the tree level. The corresponding proportions at the stand level were 21% for all plants and 12% for trees. Latitude, mean annual temperature and annual precipitation each explained <20% of the variation in FRP or FRT. Fine root production and FRT estimates are highly dependent on the species included in the sampling, the sampling depth and the methods used for estimating FRP or calculating FRT. The results indicate that the variation in FRP on a global scale can be explained to a higher degree if we focus on tree roots separately from the roots of the understorey vegetation and on FRP at the tree level instead of FRP at the stand level or on FRT. (C) 2011 Elsevier B.V. All rights reserved.