Latitudinal gradients in population growth do not reflect demographic responses to climate

Abstract

Spatial gradients in population growth, such as across latitudinal or elevational gradients, are often assumed to primarily be driven by variation in climate, and are frequently used to infer species’ responses to climate change. Here, we use a novel demographic, mixed‐model approach to dissect the contributions of climate variables vs. other latitudinal or local site effects on spatiotemporal variation in population performance in three perennial bunchgrasses. For all three species, we find that performance of local populations decreases with warmer and drier conditions, despite latitudinal trends of decreasing population growth toward the cooler and wetter northern portion of each species’ range. Thus, latitudinal gradients in performance are not predictive of either local or species‐wide responses to climate. This pattern could be common, as many environmental drivers, such as habitat quality or species’ interactions, are likely to vary with latitude or elevation, and thus influence or oppose climate responses.

Climate manipulations differentially affect plant population dynamics within versus beyond northern range limits

“…While Festuca roemeri exhibited leading edge‐trailing edge patterns in this experiment, data from natural populations of A. lemmonii, F. roemeri and D. californica collected between 2015 and 2018 show an opposing pattern of λ decreasing from south to north (Peterson et al, in press). However, that study also showed that local performance of those natural populations decreases with warmer, drier conditions, suggesting that factors other than climate (e.g.…”

The ‘black box’ of plant demography: how do seed type, climate and seed fungal communities affect grass seed germination?

“…All populations, except for D . californica from the Jefferson, Sublimity and Whidbey sites, were part of a demographic study initiated in 2015 to examine fitness across a climatic gradient running from southern Oregon to Northern Washington (Peterson et al ., 2021). For D .…”

“…For D. californica, both cleistogamous and chasmogamous seeds, when present, were collected for each sampled plant. Seeds were collected from 10-25 individuals with multiple stems per site; the plants were selected haphazardly ≥ 1 m apart along the 1-mwide transects used in the demography study (Reed et al, 2020;DeMarche et al, 2021); transect length in the demography study was variable and depended on plant density but contained ≥ 200 plants. The starting and ending latitude and longitude were recorded for each transect (Table S1).…”

“…These populations are on an aridity gradient, with less rainfall to the south and more to the north. All populations, except for D. californica from the Jefferson, Sublimity and Whidbey sites, were part of a demographic study initiated in 2015 to examine fitness across a climatic gradient running from southern Oregon to Northern Washington (DeMarche et al, 2021). For D. californica, both cleistogamous and chasmogamous seeds, when present, were collected for each sampled plant.…”

“…In this study, we investigated seed germination of two important native bunchgrasses (Poaceae), Festuca roemeri (Pavlick) E. B. Alexeev and Danthonia californica Bolander, which dominated presettlement prairies in the Pacific Northwest (PNW) (Noss et al ., 1995; Christy & Alverson, 2011) and are now being widely planted in restorations (Schultz, 2001; Applestein et al ., 2018; Lindh et al ., 2018). To understand the natural rates of seed production and germination, we recently completed a study of the demography of these grasses in remnant natural populations, and found that both measured environmental factors and unmeasured ‘site’ effects, potentially including local fungal communities, influenced population growth rates (Peterson et al ., 2021). Here we begin to open the ‘black box’ of plant demography by examining how different abiotic and biotic drivers affect seed fungal community composition and seed germination of these grasses.…”