Seed banks of an Arizona Pinus ponderosa landscape: responses to environmental gradients and fire cues

Abstract

We measured soil seed banks in 102 plots within a 110 000 ha Arizona Pinus ponderosa landscape, determined seed-bank responses to fire cues and tree canopy types (open or densely treed patches), compared seed-bank composition among ecosystem types, and assessed the utility of seed banks for ecological restoration. Liquid smoke was associated with increased community-level emergence from seed banks in greenhouse experiments, whereas heating to 100 8C had minimal effect and charred P. ponderosa wood decreased emergence. We detected 103 species in seed-bank samples and 280 species in aboveground vegetation. Erigeron divergens was the commonest seed-bank species; with the exception of Gnaphalium exilifolium, species detected in seed banks also occurred above ground. Although a dry, sandy-textured blackcinder ecosystem exhibited the greatest seed density, seed-bank composition was more ecosystem-specific than was seed density. Native graminoids (e.g., Carex geophila and Muhlenbergia montana) were common in seed banks, whereas perennial forbs were sparse, particularly under dense tree canopies. Our results suggest that (i) smoke may increase emergence from seed banks in these forests, (ii) seed banks can assist establishment of major graminoids but not forbs during ecological restoration, and (iii) seed-bank composition is partly ecosystem-specific across the landscape.Résumé : Nous avons mesuré les banques de graines dans le sol de 102 parcelles établies dans un paysage de 110 000 ha dominé par le Pinus ponderosa en Arizona, déterminé la réponse des banques de graines aux incendies et aux types de canopées (îlots avec couvert dense ou ouvert), comparé la composition des banques de graines entre les types d'écosystèmes et estimé l'utilité des banques de graines pour la restauration écologique. Dans les expériences en serre, la fumée liquide favorisait l'émergence des graines à l'échelle de la communauté, alors qu'une température de 100 8C avait un effet minimal et que le bois carbonisé de P. ponderosa réduisait l'émergence des graines. Nous avons détecté 103 espèces dans les banques de graines et 280 espèces parmi la végétation. Erigeron divergens était l'espèce la plus fréquente dans les banques de graines et, à l'exception de Gnaphalium exilifolium, les espèces détectées dans les banques de graines étaient aussi présentes parmi la végétation. Bien que la plus forte densité de graines ait été associée à un écosystème dont le sol était sec, sablonneux et avait la texture des cendres noires, la composition des banques de graines était plus spécifique à l'écosystème que ne l'était la densité de graines. Les plantes herbacées graminoïdes indigènes (p. ex., Carex geophila et Muhlenbergia montana) étaient communes dans les banques de graines alors que les plantes herbacées non graminoïdes vivaces étaient rares, particulièrement sous des canopées arborées denses. Nous résultats indiquent que (i) la fumée peut augmenter l'émergence à partir des banques de graines dans ces forêts, (ii) les banques de graines peuvent contribuer...

Effects of tree cutting and fire on understory vegetation in mixed conifer forests

“…Studies that relate soil seed bank composition to aboveground vegetation, including before and after disturbance, can aid understanding plant community maintenance and recruitment mechanisms (Archibold, 1989;Stark et al, 2006;Abella et al, 2007). Further work is needed for detailed understanding of the role of seed banks in understory response to treatments (e.g., estimates of what proportion of the seed bank germinates following disturbance, or is lost to disturbance), and several studies have provided a baseline by quantifying soil seed bank composition in untreated mixed conifer forest.…”

“…Some understory species of mixed conifer forests, including Ceanothus integerrimus (deerbrush) that is stimulated by heat (Kauffman and Martin, 1991) and Penstemon spp. stimulated by smoke (Abella et al, 2007), may be favored by fire. Six studies reported groups of native species abundant after prescribed fire or cutting + fire that are apparently fire-dependent, because the species were infrequent or absent in unburned forest, including after tree cutting alone (Lyon, 1966;Huisinga et al, 2005;Stark et al, 2006;Dodson et al, 2007;Knapp et al, 2007;Dodson and Peterson, 2010).…”

Soil seed banks in a mature coniferous forest landscape: dominance of native perennials and low spatial variability

“…For example, using the extraction method in Idaho mixed conifer forests, Kramer and Johnson (1987) extracted 2038 seeds of seven conifer tree species from samples, but only one seed was viable. Viable seeds of Pinus ponderosa were detected in the spring seed-bank sampling of Washington conifer forests by Pratt et al (1984) and the study of northern Arizona P. ponderosa forests by Abella et al (2007), but at low densities averaging , 20 seeds m 22 . Many conifer tree species have short-lived seeds with low dormancy, often germinating in the fall of dispersal or the following spring, thus not forming a persistent seed bank (Chambers et al, 1999).…”

Diverse responses across soil parent materials during ecological restoration

“…As a germination cue associated with restoring natural fire regimes, smoke would be anticipated to increase plant richness and cover (Rokich et al 2002;Flematti et al 2013). Previous research in P. ponderosa forest found that aqueous smoke triggered 21-115% greater emergence from soil seed bank samples in a greenhouse (Abella et al 2007). Grazing also can influence restoration outcomes, and a global synthesis found that grazing effects reversed across soils: grazing increased plant diversity on nutrient-rich soil but decreased diversity on nutrient-poor soil (Proulx & Mazumder 1998).…”