Landscape context and long-term tree influences shape the dynamics of forest-meadow ecotones in mountain ecosystems

Abstract

Abstract. Forest-meadow ecotones are prominent and dynamic features of mountain ecosystems.Understanding how vegetation changes are shaped by long-term interactions with trees and are mediated by the physical environment is critical to predicting future trends in biological diversity across these landscapes. We examined 26 yr of vegetation change ) across 20 forest-meadow ecotones spanning a range of landforms/hydrologies and elevations (montane and subalpine) in the Three Sisters Biosphere Reserve, Oregon (USA). We quantified changes in tree structure (cover, density, and basal area) and in the abundance and diversity of ground-layer vegetation based on species' habitat associations and growth forms. To explore the contributions of tree structure, landscape context, and initial vegetation to changes in ecotonal communities, we used a combination of NMDS, PCA, and multiple regression.Despite a long history (50-100 yr) of tree invasion, ecotones were still dominated by meadow species in 1983. Ecotones exhibited significant but varying patterns of change over the study period while adjacent forest and meadow habitats remained stable. Despite a significant increase in summer temperature, we found little evidence of a direct influence of climate on ecotonal changes. Declines in total richness, and in the cover and richness of meadow species, were greater where soil moisture was seasonally limiting (montane mesic slopes and subalpine early snowmelt sites). Forest species showed much greater increases in montane than subalpine ecotones; limited colonization of the latter reflects the depauperate nature of subalpine forest understories in this region. Vegetation changes were related to initial tree structure but not to changes in structure over the study period. Past tree invasions, a legacy of both climate variation and disturbance history, continue to exert strong influences on ecotonal ground-layer communities. However, the consequences for local diversity vary across the landscape. Quantifying the nature of this variation through long-term observations is a critical step toward predicting future changes in the biological diversity of these and other mountain ecosystems.

Climate and landscape drive the pace and pattern of conifer encroachment into subalpine meadows

“…This mirrors findings that climate was the most likely driver of woody plant encroachment into other meadow systems (Woodward et al 1995, Dyer and Moffett 1999, Lepofsky et al 2003, Brandt et al 2013, Durak et al 2015, although Takaoka and Swanson (2008) found fire to be of greater importance while Haugo et al (2011) found little evidence of climate influencing recent meadow encroachment. In combination with seed availability, climate was sufficient to explain most variation in encroachment density among meadows, while climate during early life stages was able to explain nearly all historic variation in recruitment timing.…”

“…In many locations, mountain meadows are being transformed by woody plant encroachment (Woodward et al 1995, Dyer and Moffett 1999, Berlow et al 2002, Lepofsky et al 2003, Takaoka and Swanson 2008, Haugo et al 2011, Brandt et al 2013, Durak et al 2015. Conversion of herbaceous meadows into forest could impact hydrologic services by altering soil properties important to water retention and by altering vegetation water use.…”

Site- and Species-Specific Influences on Sub-Alpine Conifer Growth in Mt. Rainier National Park, USA

“…Climate change is expected to affect alpine and sub-alpine areas [1][2][3][4] and forecasting changes to these areas is critical for natural resource management and conservation. On a warming planet where multiple aspects of climate are changing [5], climate sensitivities that differ among sub-alpine tree species could result in differing growth responses.…”

“…On a warming planet where multiple aspects of climate are changing [5], climate sensitivities that differ among sub-alpine tree species could result in differing growth responses. Since sub-alpine trees set the important forest/meadow ecotone at high elevations [4], the drivers of variability in climate-growth responses of sub-alpine tree species may therefore have particular significance for ecosystem function, resource management and conservation (e.g., the amount of carbon stored in trees, the upslope movement of treeline, conversion of alpine habitat to forests). For example, high mountain meadows are species rich (e.g., [6,7]), and-due to their isolation from each other, and the shape of mountains-are likely to decrease in area if sub-alpine tree growth increases greatly with climate warming.…”

Hydrologic Response to Conifer Removal from an Encroached Mountain Meadow

“…Electrical resistivity tomography is a well-established tool in studies investigating subsurface hydrology (Frohlich, 1994 andRavindran andPrabhu, 2012 (Haugo et al, 2011;Griffiths et al, 2005;Miller and Halpern, 1998). If current climate trends continue, it may become increasingly difficult to restore encroached meadows, as conifer encroachment into meadows may be indicative of a transition to a new stable state (Haugo and Halpern, 2007).…”