Effects of planting date and species choice on the fate of planted warm-season perennial grass seeds: Implications for revegetation

Abstract

Attempts to establish native grasses in revegetation projects in semidesert grasslands in the Southwestern United States often fail whereas revegetation of non-native lovegrasses (Eragrostis spp.) is frequently more successful. The reasons for differential establishment are unclear. Species may be differentially vulnerable to variable patterns of soil moisture availability during germination and seedling growth. Field experiments described within this dissertation investigated the effects of planting date and species on germination, emergence, mortality, survival, and seedling growth of native and non-native warm-season perennial grasses seeded in southeastern Arizona. Native species [Sideoats grama (Bouteloua curtipendula (Michx.) Torr.), cane beardgrass (Bothriochloa barbinodis (Lag.) Herter), green sprangletop (Leptochloa dubia (H.B.K.) Nees), and Arizona cottontop (Digitaria californica (Benth.) Chase)] germinated rapidly, produced a few, large cohorts of seedlings, and retained limited residual germinability following initial rain events. In contrast, Lehmann lovegrass (Eragrostis lehmanniana Nees) germinated more slowly, produced more, smaller cohorts throughout the growing season, and retained more residual germinable seeds following initial rains. Mortality of all species was highest in the first week following emergence. Early development of adventitious roots and relatively high rates of biomass accumulation exhibited by Lehmann lovegrass are potentially advantageous under variable environmental conditions. Establishment of fast-germinating native species is favored by rainfall patterns that support early seedling growth subsequent to initial rains; the risk of seeding failure for these species increases when lengthy dry periods follow initial rain events. Years in which summer soil moisture conditions are highly variable would tend to favor Lehmann lovegrass establishment. Gradual depletion of the seedbank, early seedling growth characteristics, and rapid production of seed in response to drought increase the probability that at least one cohort will establish or will survive long enough to replenish the seedbank for subsequent years. In southeastern Arizona, the probability of intermittent dry periods decreases as rainstorm frequency increases near the end of July, yet the recommended time to reseed is early summer. Adjusting the planting date to late July or early August may improve the potential for successful revegetation of native species that germinate rapidly and produce few cohorts following initial rains.