Peatland plant communities under global change: negative feedback loops counteract shifts in species composition

Abstract

Mires (bogs and fens) are nutrient-limited peatland ecosystems, the vegetation of which is especially sensitive to nitrogen deposition and climate change. The role of mires in the global carbon cycle, and the delivery of different ecosystem services can be considerably altered by changes in the vegetation, which has a strong impact on peat-formation and hydrology. Mire ecosystems are commonly open with limited canopy cover but both nitrogen deposition and increased temperatures may increase the woody vegetation component. It has been predicted that such an increase in tree cover and the associated effects on light and water regimes would cause a positive feed-back loop with respect to the ground vegetation. None of these effects, however, have so far been confirmed in large-scale spatiotemporal studies. Here we analyzed data pertaining to mire vegetation from the Swedish National Forest Inventory collected from permanent sample plots over a period of 20 yr along a latitudinal gradient covering 14°. We hypothesized that the changes would be larger in the southern parts as a result of higher nitrogen deposition and warmer climate. Our results showed an increase in woody vegetation with increases in most ericaceous dwarf-shrubs and in the basal area of trees. These changes were, in contrast to our expectations, evenly distributed over most of the latitudinal gradient. While nitrogen deposition is elevated in the south, the increase in temperatures during recent decades has been larger in the north. Hence, we suggest that different processes in the north and south have produced similar vegetation changes along the latitudinal gradient. There was, however, a sharp increase in compositional change at high deposition, indicating a threshold effect in the response. Instead of a positive feed-back loop caused by the tree layer, an increase in canopy cover reduced the changes in composition of the ground vegetation, whereas a decrease in canopy cover lead to larger changes. Increased natural disturbances of the tree layer due to, for example, pathogens or climate is a predicted outcome of climate change. Hence, these results may have important implications for predictions of long-term effects of increased temperature on peatland vegetation.

Half a century of multiple anthropogenic stressors has altered northern forest understory plant communities

“…We analyzed change in mean SLA between the old and modern periods using a GLM with a gamma error distribution and log link. Dwarf shrubs are a relatively homogeneous group and important indicators of environmental changes (Hedwall et al 2017), and cover estimates are available from NFI data. To estimate the change in dwarf shrub cover, we calculated the aggregate cover of all species of Calluna, Empetrum, Erica, and Vaccinium for the two time periods by using vegetation type-specific values computed from the cover estimates of those species in the modern data.…”

A pore-size classification for peat bogs derived from unsaturated hydraulic properties

“…The resulting strong decrease in saturated hydraulic conductivity, K s (L T −1 ) (Boelter, 1969;Ingram, 1978;Quinton et al, 2008), is positively correlated with the state of decomposition as shown by Ingram (1978), Clymo (1970Clymo ( , 1984, Clymo and Hayward (1982), Hayward and Clymo (1982), and Morris et al (2015). Exceptions to this rule have been observed in cases where pipe flow (Holden, 2005), fire disturbances (Sherwood et al, 2013), and rapid climate change resulting in changes in vegetation and subsequent peat deposition history (Rydin and Jeglum;Hedwall et al, 2017) occur. In summary, the afore-mentioned processes constitute the entire continuous pedogenesis shaping the soil profile as an ongoing process (Blume et al, 2016).…”

With Ellenberg indicator values towards the north: Does the indicative power decrease with distance from Central Europe?

“…Ecologist Heinz Ellenberg was one of the first to develop a quantitative indicator system for vascular plants, and in 1974, he published lists of indicator values (Ellenberg, ) for most species occurring in Central Europe. Ellenberg indicator values (EIV) have been used widely by plant ecologists, mainly in Central Europe (Diekmann, ) and also in other regions, such as Britain (Hill, Mountford, Roy, & Bunce, ) and Northern Europe (Hannus & Von Numers, ; Hedwall & Brunet, ; Hedwall, Nordin, Brunet, & Bergh, ; Hedwall et al., ; Robbins & Matthews, ; Strengbom & Nordin, ) as well as, exceptionally, along large geographic gradients (Verheyen et al., ). EIV have been criticized for being subjective estimates based on expert opinions, that is, not being systematically derived from measurements (Økland, ), and for implying circular reasoning when used as variables substituting real measurements in explaining, for example, plant species composition (plant species in that way only explain their own occurrence).…”