Soil seed banks in Pinus ponderosa forests in Arizona: Clues to site history and restoration potential

Question: How does the relationship between the viable soil seed bank species composition and the above-ground vegetation in northern Arizona Pinus ponderosa forests differ under varying historical land use disturbances (low, intermediate, high)? Is above-ground vegetation correlated to the viable soil seed bank immediately following soil disturbance from restoration thinning treatments? Location: Northern Arizona, USA. Methods: Soil seed bank samples were taken along replicated transects and collected with a 5-cm diameter bulk density hammer. Samples included a 5 -cm diameter O-horizon sample (at varying depths) plus the underlying mineral soil to a depth of 5 cm. The seedling emergent method was used to quantify seed bank species composition and density. The herbaceous and shrub plant community was quantified along the same transects using the point intercept method. Results: Early-successional or ruderal species were common in the soil seed bank at all three disturbance sites. Non-native species, notably Verbascum thapsus, were more numerous (up to 940 seeds/m²) under high disturbance with overgrazing and logging, and less common or absent under low disturbance. Most viable seeds were found in the O-horizon and the upper 5 cm of mineral soil; there was little correlation between species in the soil seed bank and the above-ground vegetation. Conclusions: We recommend that restoration plans be geared toward minimizing activities, such as severe soil disturbance, that may promote the spread of non-native invasive species, and that manual seeding be explored as an option to restore plant species diversity and abundance.