Understanding erratic germination in perennial grasses using physiological models and field experimentation.

Abstract

Year-to-year variability in field seedling emergence in arid rangelands poses questions about the viability of perennial grasses threatened by overgrazing. There are practical difficulties in estimating long-term seedling emergence for perennial species with erratic behavior (high temporal variability). We present a novel approach combining seed germination studies, modeling, and field experiments to estimate long-term germination probability using Poa ligularis as a perennial grass model. Additional objectives were to evaluate: (i) germination requirements of different P. ligularis populations, (ii) field emergence behavior at two extreme locations within the distributional range in the southern region, and (iii) the influence of microsite or local soil on field emergence. Germination parameters were obtained from 8 populations collected across a 12° latitude transect. Germination parameters were combined with a soil water model to estimate long-term germination (used as a proxy of field emergence) in the southernmost location. Germination requirements differed across populations and correlated with environmental conditions of collection sites, demonstrating ecological significance. Field experiments showed very low emergence. Simulations for the southern location indicated erratic emergence. Fifty percent of the years had nil simulated field emergence and the magnitude of the events, when occurred, was small. The magnitude of these events depended on the population, demonstrating an additional genetic constraint. Erratic field emergence arises from the mismatch between environmental conditions and germination requirements. Knowing favorable climatic conditions triggering seed germination (and thus emergence) events are key for opportunistic management of species with this erratic behavior.