Absence of net long-term successional facilitation by alder in a boreal Alaska floodplain.

Autor: Stuart Chapin F 3rd; Institute of Arctic Biology, University of Alaska Fairbanks, Fairbanks, Alaska, 99775, USA., Conway AJ; Department of Biology, University of Saskatchewan, 112 Science Place, Saskatoon, Saskatchewan, S7N 5E2, Canada., Johnstone JF; Department of Biology, University of Saskatchewan, 112 Science Place, Saskatoon, Saskatchewan, S7N 5E2, Canada., Hollingsworth TN; USDA Forest Service, Pacific Northwest Research Station, Fairbanks, Alaska, 99775, USA., Hollingsworth J; Institute of Arctic Biology, University of Alaska Fairbanks, Fairbanks, Alaska, 99775, USA.
Jazyk: angličtina
Zdroj: Ecology [Ecology] 2016 Nov; Vol. 97 (11), pp. 2986-2997. Date of Electronic Publication: 2016 Oct 04.
DOI: 10.1002/ecy.1529
Abstrakt: Long-term experiments provide a way to test presumed causes of successional or environmentally driven vegetation changes. Early-successional nitrogen (N)-fixing plants are widely thought to facilitate productivity and vegetation development on N-poor sites, thus accounting for observed vegetation patterns later in succession. We tested this facilitative impact on vegetation development in a 23-yr field experiment on an Interior Alaska (USA) floodplain. On three replicate early-successional silt bars, we planted late-successional white spruce (Picea glauca) seedlings in the presence and absence of planted seedlings of an early-successional N-fixing shrub, thinleaf alder (Alnus incana). Alder initially facilitated survivorship and growth of white spruce. Within six years, however, after canopy closure, alder negatively affected spruce survivorship and growth. Our three replicate sites followed different successional trajectories. One site was eliminated by erosion and supported no vegetation development during our study. The other two sites, which differed in site moisture, diverged in vegetation composition. Structural equation modeling (SEM) suggested that, in the drier of these two sites, alder inhibited spruce growth directly (presumably by competition) and indirectly through effects mediated by competition with other woody species. However, at the wetter site, alder had both positive and negative effects on spruce growth, with negative effects predominating. Snowshoe hares (Lepus americanus) in alder thickets further reduced height growth of spruce in the wetter site. We conclude that net effects of alder on white spruce, the late-successional dominant, were primarily inhibitory and indirect, with the mechanisms depending on initial site moisture. Our results highlight the importance of long-term research showing that small differences among initial replicate sites can cause divergence in successional trajectories, consistent with individualistic distributions of species and communities along environmental gradients. This divergence was detectable only decades later.
(© 2016 by the Ecological Society of America.)
Databáze: MEDLINE
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