Biomass Partitioning in 'Caucasian' and 'WW-Spar' Old World Bluestems

Abstract

Above- and belowground biomass dynamics of 'WW-Spar' (Bothriochloa ischaemum) and 'Caucasian' (B. caucasica) Old World bluestems were monitored under field conditions during spring-summer and summer-fall growing cycles in 1983. The objective was to document biomass partitioning patterns to complement earlier studies of leaf physiology and aboveground growth behavior in these 2 grasses. The standing crop of forage reached 500 and 694 g m-2 in WW-Spar and Caucasian, respectively, by the end of cycle 1 (11 July 1983). However, as in earlier studies, Caucasian was more sensitive to limited soil water so that its forage production and water-use efficiency showed a much greater decline in the second cycle than did these parameters in WW-Spar. At reproductive maturity, both grasses partitioned about 0.4 of total biomass to aboveground compartments (leaves, stems, stem bases, inflorescences) with Caucasian allocating more of its aboveground biomass to leaves and stems and WW-Spar more to stem bases. Excluding stem bases from the aboveground compartment showed that WW-Spar had the higher leaf/stem ratio. Root biomass declined significantly at the start of each growing cycle, but was similar in both grasses (peak root standing crop = 1,900 g m-2 to a depth of 1.2 m). Caucasian tended to partition slightly more of its root biomass to upper soil layers, WW-Spar more to lower layers. Across growing cycles and species, 0.56 of total root biomass was in the 0 to 0.1 m layer, 0.73 between 0 and 0.2 m, and 0.84 between 0 and 0.4 m. Regression analysis indicated that roots should be sampled to 0.4 m to account for 90% of the variation in roots to the 1.2 m depth. Net root production was estimated to be 495 and 753 g m-2 in cycle 1 for WW-Spar and Caucasian, respectively, and 366 and 388 g m-2 in cycle 2. Relative growth rates (RGR) of total plant biomass were similar in both grasses and increased linearly during each growing cycle to values between 0.01 and 0.02 g d-1 g-1. Increases in RGR early and late in a growing cycle were supported by increases in efficiency of the photosynthetic apparatus (unit leaf rate). Mid-cycle increases in RGR were more dependent on expansion in the size of the photosynthetic apparatus (leaf area ratio) as unit leaf rate remained constant during this time. In addition to physiological differences found in earlier studies, this study suggested that variations in drought performance between these 2 grass species may also be related to morphological differences, primarily the tendency of WW-Spar to partition more of its root biomass to lower depths in the soil profile than Caucasian and perhaps the much greater crown or basal area of WW-Spar compared to Caucasian.