Aims The karst vegetation in southwestern China, growing in a harsh environment, is a specific ecosystem that can lead to land degradation and rocky desertification under human disturbances. The root biomass (RB) of the karst vegetation is very difficult to measure due to the karst geomorphology and rigorous habitat and is therefore rarely reported. However, RB is a key factor in understanding ecosystem stability in this region and is an indicator of vegetation restoration in different rocky desertification stages. Our objective was to estimate the RB of karst plant communities in each restoration stage in Maolan National Natural Reserve of Guizhou Province, southwestern China.

Methods We investigated 40 plots. The karst vegetation was divided into five restoration stages according to its community physiognomy, dominant species, habitat types, mean tree diameter at breast height (DBH), mean tree height and tree density, as well as disturbances of rocky desertification: herb community, herb-scrub community, scrub-shrub community, sub-climax community and climax community of evergreen-deciduous broadleaved mixed forests. We divided the karst plant communities into rock-dominated and soil-dominated stands. We measured root biomass in different soil layers and in various root diameter classes and estimated it in 10 representative plots using the Specific Sampling Method of Averaged Standard Tree.

Important findings Forward restoration of karst vegetation significantly increases its RB from 2.63 Mg·hm^-2 in rock-stand herb community to 58.15 Mg·hm^-2 in rock-stand climax evergreen-deciduous broadleaved mixed forest (p < 0.001). In the same restoration stage, RB of rock-stands and RB of soil-stands are not significantly different (p > 0.05). Total RB in rock-stand climax and sub-climax communities are higher than that in soil-stand ones, but it is reversed in the other three restoration stages. In the same restoration stage, coarse RB in the rock-stand community is not significantly different from soil-stand community (p > 0.05). The fine and small RB in the soil-stand community is significantly higher than in the rock-stand community (p < 0.05). The biomass ratio of coarse versus total roots in rock-stand and soil-stand communities increases gradually with forward restoration. The RB of rock-stand communities is mainly distributed on the rock surface or in the rock gaps, and has no obvious vertical pattern of distribution. The distribution of RB in soil-stand communities, however, is concentrated in the top 10 cm of the soil. In different soil layers the ratio of coarse to total RB is ca. 80%; it decreases with increasing soil depth.