Site fertility drives temporal turnover of vegetation at high latitudes

Abstract

Experimental evidence shows that site fertility is a key modulator underlying plant community changes under climate change. Communities on fertile sites, with species having fast dynamics, have been found to react more strongly to climate change than communities on infertile sites with slow dynamics. However, it is still unclear whether this generally applies to high‐latitude plant communities in natural environments at broad spatial scales. We tested a hypothesis that vegetation of fertile sites experiences greater changes over several decades and thus would be more responsive under contemporary climate change compared to infertile sites that are expected to show more resistance. We resurveyed understorey communities (vascular plants, bryophytes, and lichens) of four infertile and four fertile forest sites along a latitudinal bioclimatic gradient. Sites had remained outside direct human disturbance. We analyzed the magnitude of temporal community turnover, changes in the abundances of plant morphological groups and strategy classes, and changes in species diversity. In agreement with our hypothesis, temporal turnover of communities was consistently greater on fertile sites compared to infertile sites. However, our results suggest that the larger turnover of fertile communities is not primarily related to the direct effects of climatic warming. Furthermore, community changes in both fertile and infertile sites showed remarkable variation in terms of shares of plant functional groups and strategy classes and measures of species diversity. This further emphasizes the essential role of baseline environmental conditions and nonclimatic drivers underlying vegetation changes. Our results show that site fertility is a key determinant of the overall rate of high‐latitude vegetation changes but the composition of plant communities in different ecological contexts is variously impacted by nonclimatic drivers over time.

Trait-based responses to land use and canopy dynamics modify long-term diversity changes in forest understories

“…Since the reindeer herding area is generally in higher latitudes than areas outside it, it is possible that the observed effect is due to forests in colder climates having slower growth, smaller species pool, and thus slower community dynamics. Such differences in turnover rates between fertile and less diverse infertile sites have previously been reported for boreal forests (Maliniemi et al, 2019) . A further explanation could be that changes induced by increased reindeer densities are counteracting changes unrelated to them.…”

“…For this reason, the surveys were conducted in relatively undisturbed habitats. These forests harbour higher species richness than the surrounding less fertile and homogenous boreal forests (Maliniemi, Happonen, & Virtanen, 2019), are an important habitat for many threatened species (Kouki et al, 2018), and are thus important to monitor. They occur typically as small patches and cover only a fraction of the forested area in northern Scandinavia reflecting the scattered distribution of calcareous bedrock and soil.…”

“…Several local scale studies have shown reindeer herbivory to cause decreases in biomass and declines in favoured forage species such as herbs, deciduous shrubs, and lichens in the boreal coniferous heath forests and on tundra heaths (Bråthen & Oksanen, 2001;Olofsson et al, 2010;Sundqvist et al, 2019;Väre et al, 1995) . However, there is limited evidence on whether these effects scale to larger geographical extents, and on how reindeer grazing influences vegetation in more productive forests (but see Maliniemi et al, 2019) .…”

Trait‐based responses to land use and canopy dynamics modify long‐term diversity changes in forest understories

“…For this reason, the surveys were conducted in relatively undisturbed habitats. These forests harbour higher species richness than the surrounding less fertile and homogenous boreal forests (Maliniemi et al, 2019), are an important habitat for many threatened species (Kouki et al, 2018), and are thus important to monitor. They occur typically as small patches and cover only a incana and Betula pubescens formed more than 80% of tree cover in our study plots, during both the original survey and the resurvey.…”

Temporal biodiversity change following disturbance varies along an environmental gradient

“…By temporal turnover we mean changes in the identities and/or the abundances of species in a community over time. Observational evidence suggests that temporal turnover is higher in the marine than the terrestrial realm (Blowes et al., 2019), decreases with altitude (Bunn et al., 2010) and increases along productivity (Maliniemi et al., 2019; Virtanen et al., 2010; Zhang et al., 2018) and temperature gradients (Hillebrand et al., 2010). As these environmental gradients influence natural biodiversity change over time, they also have the potential to regulate the magnitude of anthropogenic disturbance‐induced turnover.…”