CWMs of fine-root diameter were used to determine if shifts in soil hydraulic properties could potentially be explained by roots filling greater fractions of soil pore systems. Species-level root diameter data came from the TRY Plant Trait Database version 5 (38), which contains records from several sources including the Fine-Root Ecology Database (39). TRY was queried for records matching the names of all plant species at Konza Prairie (after taxonomic standardization). The geometric mean was used to aggregate records of fine-root diameter at the species level (n = 1 to 60 per species, 1102 in total); 62 species and 88 ± 4% of cover (mean ± SD across years) were represented in the resulting dataset. Genus-level values were then added for species without fine-root diameter data in TRY and for plant taxa only identifiable to the genus level in the field surveys described below. Genus-level values were likewise computed as geometric means of the relevant diameter data, but the starting point was a previously derived, gap-filled version of the Fine-Root Ecology Database (11). Ninety-three species and 98 ± 2% of cover were accounted for in the final dataset. Next, the relative abundance of each species was computed at the plot scale on an annual basis using data from surveys of plant cover that took place between 1991 and 2015; these served as weights when calculating CWMs. Vegetation was surveyed annually in circular plots (10 m2) that were positioned along the four experimental transects (n = 12 upland and 12 lowland plots, except in 1991–1992, when n = 5 upland and 5 lowland plots). Cover was measured using a modified Daubenmire classification, with the midpoints of each cover class (0 to 1%, 1 to 5%, 5 to 25%, 25 to 50%, 50 to 75%, 75 to 95%, or 95 to 100%) used in calculations of relative abundance. In addition, species cover was measured shortly after soil sampling at the eight locations described above. In this effort, cover was measured to within ~1% of the actual value in four replicate plots. Once the root diameter dataset was joined to each of the cover datasets, species cover values were relativized to the total cover in the relevant plot (table S1). Weighted means of diameters were then computed at the plot level for each year of the study.

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