Timberline meadows along a 1000-km transect in NW North America: species diversity and community patterns

Authors


corresponding author, wagner@sci.muni.cz

Abstract

Question

The mountains of NW North America are home to natural meadows that cover large areas at the timberline. The vegetation of these meadows has been the topic of few local studies, and little is known of species composition turnover across mountain ranges and along the latitudinal gradient. We sampled and analyzed community composition along a 1000-km gradient to answer the following questions: (1) what are the main community types in timberline meadows; (2) what are the most important abiotic gradients underlying their composition; and (3) how is species diversity related to environmental factors and how does it change along the latitudinal gradient?

Location

SE and central British Columbia (Canada), NW Montana (USA).

Methods

We recorded data on cover and presence/absence of vascular plant, bryophyte and lichen species, as well as structure and environmental properties in 4 m × 4 m plots (= 112). Using the OptimClass approach, we classified plots into distinct community types. We used NMDS and multiple regressions to analyse the relationship between composition, species richness and environmental and geographic gradients.

Results

Despite drawing from a large geographic area, our classification revealed only four meadow communities. Mesic meadows were the most frequent and had a pronounced species turnover along latitude, resulting in two distinct community types. In addition, a three-dimensional ordination showed that vegetation structural properties, macroclimate and geographic predictors were important underlying gradients for species composition. Vascular species richness increased with soil pH, and exhibited a quadratic relationship with litter cover and latitude. Furthermore, in the latter case, it increased towards the southern and northern ends of our gradient.

Conclusions

Detection of only four meadow communities along the 1000-km sampling gradient suggests a relatively recently established flora and a lack of regional differentiation consistent with the low level of endemism in the region. Higher vascular species diversity at the northern and southern edges of our gradient may reflect the geographic proximity to areas unglaciated during the Pleistocene, and suggests that areas farthest from the unglaciated edge may be experiencing colonization debt.

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