Local landscape position impacts demographic rates in a widespread North American steppe bunchgrass

Abstract

<p>Understanding the environmental drivers of demographic rates and population dynamics over space and time is critical for anticipating how species will respond to climate change. While the influence of temporal environmental variation and large environmental gradients are well recognized, less is known about how local topography and landscape position influence demography over small spatial scales. Here, we investigate how local landscape position (north- vs. south-facing aspects) influence the demographic rates and population growth of a common bunch grass in western North America, bluebunch wheatgrass (Pseudoroegneria spicata), using 6 years of annual censuses measuring growth, survival, and reproductive output. We found notably lower survival on south facing slopes, particularly among smaller individuals. In contrast, south-facing slopes maintained comparatively high reproductive output in most years, measured both as spikes per plant and spikelets per spike. We also found that lower survival among small individuals and greater reliance on reproduction mean south-facing slopes should also be more sensitive to changes in the recruitment rate, and would generally have to maintain higher recruitment for stable population growth. Taken together, our results highlight the important influence that landscape position and local topography can have in driving populations. As conditions warm and dry with climate change (similar to south facing slopes), bluebunch wheatgrass may become more reliant on reproduction to maintain viable populations and more sensitive to variability in recruitment.</p>

Potential demographic collapse in a common perennial bunchgrass on south-facing slopes in a semi-arid grassland via dramatically reduced seedling establishment

“…Many of these declines in perennials are well understood at certain spatial scales, and can often be attributed to disturbances such as increasing fire frequency, over-grazing, herbicide use, and plowing (O'Connor, 1991;Knapp, 1996;Knick & Rotenberry, 1997;DiTomaso, 2000;Keeley et al, 2003;Keeley, 2006;Kulmatiski, 2006;Young & Clements, 2007;Davies et al, 2011;Svejcar et al, 2017). However, perennial success can also vary substantially within a landscape even when disturbance history is similar throughout (Sternberg & Shoshany, 2001;Shriver et al, 2021). Understanding what drives variability in success of perennial rangeland plants in the absence of differences in disturbance is important for understanding how these communities will continue to respond to ongoing environmental change and how management practices might be adjusted.…”

“…One of the most striking drivers of variability in the success of plants within some landscapes is aspect (Armesto & Martinez, 1978;Nobel & Linton, 1997;Sternberg & Shoshany, 2001;Ackerly et al, 2002;Scherrer & Körner, 2010;Shriver et al, 2021). In the Northern Hemisphere, south-facing slopes receive more solar radiation than north-facing slopes, and thus are typically the warmest and driest sites on the landscape (Nobel & Linton, 1997;Bennie et al, 2008).…”

“…In the Northern Hemisphere, south-facing slopes receive more solar radiation than north-facing slopes, and thus are typically the warmest and driest sites on the landscape (Nobel & Linton, 1997;Bennie et al, 2008). In rangelands of Northwestern North America, these south-facing slopes often have lower perennial plant biomass and more invasive annual cover than do north-facing slopes (Harrison, 1999;Gelbard & Harrison, 2003;Shriver et al, 2021). The lower biomass of perennials may result directly from abiotic effects on the demographic rates of perennials (Shriver et al, 2021), but may also be influenced by competition from invasive annuals (Aguirre & Johnson, 1991).…”

“…In rangelands of Northwestern North America, these south-facing slopes often have lower perennial plant biomass and more invasive annual cover than do north-facing slopes (Harrison, 1999;Gelbard & Harrison, 2003;Shriver et al, 2021). The lower biomass of perennials may result directly from abiotic effects on the demographic rates of perennials (Shriver et al, 2021), but may also be influenced by competition from invasive annuals (Aguirre & Johnson, 1991). The demographic impacts of competition from annuals are expected to primarily impact young perennials that are not yet established (Aguirre & Johnson, 1991), but abiotic stresses could impact an array of demographic rates from seedling establishment to growth, reproduction, and survival at any stage (Dalgleish et al, 2011;James et al, 2011;James et al, 2019;Shriver et al, 2021).…”

“…Bluebunch wheatgrass (Pseudoroegneria spicata) is a large perennial bunchgrass with a wide distribution in the semi-arid rangelands of the interior of Western North America, but this species has declined dramatically in concert with changing land use and the spread of invasive annual weeds (Harris, 1967;Pyke, 1990;Rodhouse et al, 2014;Shriver et al, 2021). At our low-elevation study site in the Columbia Basin of Washington State, bluebunch wheatgrass is one of the most abundant large bunchgrasses, and the only large bunchgrass common on both north and south-facing slopes (Shriver et al, 2021). However, bluebunch wheatgrass may be declining on south-facing slopes and is already much less common on south-facing than north-facing slopes.…”