Individual and combined effects of disturbance and N addition on understorey vegetation in a subarctic mountain birch forest

Abstract

Questions: What are the effects of repeated disturbance and N‐fertilization on plant community structure in a mountain birch forest? What is the role of enhanced nutrient availability in recovery of understorey vegetation after repeated disturbance? How are responses of soil micro‐organisms to disturbance and N‐fertilization reflected in nutrient allocation patterns and recovery of understorey vegetation after disturbance? Location: Subarctic mountain birch forest, Finland. Methods: We conducted a fully factorial experiment with annual treatments of disturbance (two levels) and N‐fertilization (four levels) during 1998–2002. We monitored treatment effects on above‐ground plant biomass, plant community structure and plant and soil nutrient concentrations. Results: Both disturbance and N‐fertilization increased the relative biomass of graminoids. The increase of relative biomass of graminoids in the disturbance treatment was over twice that of the highest N‐fertilization level, and N‐fertilization further increased their relative biomass after disturbance. As repeated disturbance broke the dominance of evergreen dwarf shrubs, it resulted in a situation where deciduous species, graminoids and herbs dominated the plant community. Although relative biomass of deciduous dwarf shrubs declined with N‐fertilization, it did not cause a shift in plant community structure, as evergreen dwarf shrubs remained dominant. Both disturbance and N‐fertilization increased the N concentration in vascular plants, whereas microbial biomass N and C were not affected by the treatments. Concentrations of NH4+, dissolved organic N (DON) and dissolved organic C (DOC) increased in the soil after N‐fertilization, whereas concentrations of NH4+ and DON decreased after disturbance. Conclusions: Disturbances caused by e.g. humans or herbivores contribute more to changes in the understorey vegetation structure than increased levels of N in subarctic vegetation. Fertilization accelerated the recovery potential after repeated disturbance in graminoids. Microbial activities did not limit plant growth.

Arctic-alpine vegetation biomass is driven by fine-scale abiotic heterogeneity

“…Our study provides empirical evidence that geomorphological processes affect vegetation structure and biomass, with geomorphologic activity decreasing arctic-alpine plant biomass, probably through mechanical disturbance (Jonasson 1986;le Roux and Luoto 2014). The effect of geomorphology was the strongest for vegetation height, which may be explained by the fact that, generally, taller woody plants are more sensitive to disturbance and soil instability (Jonasson 1986;Haugland and Beatty 2005;Manninen et al 2011). For vegetation height the shared effects of geomorphology and topography are also strong (Fig.…”

“…The effect of geomorphology was the strongest for vegetation height, which may be explained by the fact that, generally, taller woody plants are more sensitive to disturbance and soil instability (Jonasson ; Haugland and Beatty ; Manninen et al . ). For vegetation height the shared effects of geomorphology and topography are also strong (Fig.…”

Outbreaks by canopy-feeding geometrid moth cause state-dependent shifts in understorey plant communities

“…In northern boreal coniferous and mixed forests the dominance of E. nigrum is broken by recurrent fires (Zackrisson et al 1995 , 1996 ), which both remove above-ground plant parts and change soil conditions such as pH, humus layer thickness and nutrient availability. Field experimental studies have confirmed that severe disturbance is needed to break the dominance of E. nigrum , and that the effects of disturbance are enhanced by fertilization (Olofsson et al 2005 ; Manninen et al 2011 ). We have documented here that severe moth outbreaks effectively break the dominance of E. nigrum in a manner qualitatively similar to wild fires in coniferous forest, albeit on a much larger spatial scale.…”

Recent changes in mountain birch forest structure and understory vegetation depend on the seasonal timing of reindeer grazing

“…E. hermaphroditum is unpalatable to most herbivores and relatively sensitive to disturbance (Tybirk et al., 2000). For example, clipping experiments have demonstrated its recovery to be much weaker compared with other species (Manninen et al., 2011), and that, when combined with fertilization, E. hermaphroditum could recover and out‐compete other plant species after a 50% biomass loss, but not after a complete biomass removal (Aerts, 2010). E. hermaphroditum may also decrease in abundance after winter conditions promoting pathogenic outbreaks (Olofsson et al., 2011), due to intense winter disturbance by rodents (Tuomi et al., 2019), and after moth outbreaks (Biuw et al., 2014).…”