An assessment of seed head and flag leaf contributions to reproductive effort in sagebrush steppe bunchgrasses

Abstract

North American sagebrush-steppe bunchgrasses primarily establish from seed; however, the importance of parental plant carbon from flag leaves or within the seed head itself to reproduction in perennial bunchgrasses is unknown. To address this, we experimentally removed flag leaves and shaded seed heads to assess their importance to reproduction in the exotic bunchgrass crested wheatgrass (Agropyron cristatum), which has high seed head photosynthetic capacity and readily establishes from seed, and the native grass, squirreltail (Elymus elymoides), which has lower seed head photosysnthetic capacity and does not establish as readily from seed. We hypothesized that inflorescence shading would have a greater negative impact on crested wheatgrass reproduction than in squirreltail. In crested wheatgrass, shading did not affect total propagule production but did reduce both total filled seeds and filled seed specific mass (dry mass per unit area). Flag leaf removal stimulated seed filling and increased seed specific mass, especially in unshaded seed heads, suggesting flag leaves are competitive carbon sinks in crested wheatgrass. In contrast, flag leaf removal and shading in squirreltail resulted in similar reductions in total propagules, fewer filled seed produced and lower specific seed mass, indicating similar contributions to reproductive effort by both structures. These results indicate seed head photosynthetic activity is an adaptive reproductive feature in both grasses, but the contrasting effects of flag leaf removal may reflect long-term adaptive responses to grazing pressures that differed in crested wheatgrass's native range compared to those in North American sagebrush steppe.