Complex biotic interactions drive long-term vegetation dynamics in a subarctic ecosystem

Abstract

Predicting impacts of global warming requires understanding of the extent to which plant biomass and production are controlled by bottom-up and top-down drivers. By annually monitoring community composition in grazed control plots and herbivore-free exclosures at an Arctic location for 15 years, we detected multiple biotic interactions. Regular rodent cycles acted as pulses driving synchronous fluctuations in the biomass of field-layer vegetation; reindeer influenced the biomass of taller shrubs, and the abundance of plant pathogenic fungi increased when densities of their host plants increased in exclosures. Two outbreaks of geometrid moths occurred during the study period, with contrasting effects on the field layer: one in 2004 had marginal effects, while one in 2012 severely reduced biomass in the control plots and eliminated biomass that had accumulated over 15 years in the exclosures. The latter was followed by a dramatic decline of the dominant understory dwarf-shrub Empetrum hermaphroditum , driven by an interaction between moth herbivory on top buds and leaves, and increased disease severity of a pathogenic fungus. We show that the climate has important direct and indirect effects on all these biotic interactions. We conclude that long time series are essential to identify key biotic interactions in ecosystems, since their importance will be influenced by climatic conditions, and that manipulative treatments are needed in order to obtain the mechanistic understanding needed for robust predictions of future ecosystem changes and their feedback effects.

Ecosystem change and stability over multiple decades in the Swedish subarctic: complex processes and multiple drivers

“…Although recovery occurred, tree growth in areas of the greatest damage was limited compared with that outside areas of intense damage (compare tree growth in figure 3 with 2004 insect damage in figure 4). The 2012 event had more severe impacts on the understory than the 2004 defoliation, but its effect on the treeline and forest structure is not known yet [41] whereas the 2004 outbreak had no lasting effect on the sub-alpine birch forest or treeline (Rik Van Bogaert 2012, personal communication). Recent large-scale observations show that O. brumata, associated with a relatively moderate coastal climate, and Argyresthia retinella, currently not important in the area, are spreading northeastwards into traditionally colder regions [156,157].…”

“…The rodent cycles drive corresponding cycles in the biomass of field-layer vegetation. Plant biomass in tundra heath and forest understorey vegetation was between 12 and 24 per cent lower during the year after a vole peak than in the year before, and the combined vole and lemming peaks are visible as a reduced normalized difference vegetation index in satellite images over a 770 km 2 area in the following year ( [13,41]; figure 3). Studies from other regions suggest that rodents should have even stronger effects on the vegetation in high altitude snowbeds [120,165], but no studies have been published in these habitats in Abisko.…”

Neighborhood functions alter unbalanced facilitation on a stress gradient

“…Johnson and Adkisson (1985) revealed the relationship between Fagus grandifolia (Fagaceae) and Cyanocitta cristata (Corvidae) in the upper midwest after wondering how apparently K-selected beech persisted in increasingly fragmented forest remnants. Other plant-animal interactions may also add dimensions to the C-S-R framework (e.g., Chaneton et al, 2010;Grau et al, 2013;Olofsson et al, 2013).…”

“…Spatial feedbacks are at the core of many interactions among individuals, and thus central to self-organized complexity in ecology (cf. Levin, 2005), and are particularly important at boundaries, such as ecotones, where local scale feedbacks can determine the population dynamics and larger scale change (or stability) in biogeography (e.g., Loehle et al 1996;Milne et al, 1996;Li, 2002;D'Odorico et al, 2013;Olofsson et al, 2013).…”

Advancing the long view of ecological change in tundra systems

“…Olofsson et al [42] provide a compelling example in this issue of a shift in long-term vegetation dynamics from ostensible regulation by rather predictable dynamics towards more complex dynamics once a new component of the ecosystem becomes more important. In this case, field-layer vegetation accumulated over 15 years was eliminated by the combination of a major caterpillar outbreak and an increase in disease severity of a pathogenic fungus infecting the dwarf-shrub Empetrum hermaphroditum.…”