Adaptive variation, including local adaptation, requires decades to become evident in common gardens

Abstract

Population‐level adaptation to spatial variation in factors such as climate and soils is critical for climate‐vulnerability assessments, restoration seeding, and other ecological applications in species management, and the underlying information is typically based on common‐garden studies that are short duration. Here, we show >20 yr were required for adaptive differences to emerge among 13 populations of a widespread shrub (sagebrush, Artemisia tridentata ssp wyomingensis) collected from around the western United States and planted into common gardens. Additionally, >10 yr were required for greater survival of local populations, that is, local adaptation, to become evident. Variation in survival was best explained by the combination of populations’ home ecoregion combined with grouping of minimum temperature and aridity. Additional reductions in survival were explained by ungrouped (i.e., continuous) measures of garden‐to‐population‐origin separation in geographic distance (5% decrease in survival per 100 km increase in separation; R2 = 0.22) and especially in minimum temperature in younger plants (−4% per + °C difference, R2 = 0.56 vs. 0.29 in the 14th vs. 27th post‐planting years, respectively). Longer‐term common garden studies are needed. While we await them, uncertainty in adaptive variation resulting from short‐term observations could be quantitatively estimated and reported with seed‐transfer guidelines to reduce risks of introducing maladapted provenances in restoration.

Transient population dynamics impede restoration and may promote ecosystem transformation after disturbance

“…S22). Adding to this, restoration seed sources used after disturbance could be maladapted to a given location and reduce demographic performance and population growth (Germino et al 2019). Alternatively, sagebrush, like many dryland species, experiences infrequent recruitment pulses that are in some cases decades apart (Maier et al 2001).…”

Spatial grain of adaptation is much finer than ecoregional‐scale common gardens reveal

“…While adaptive variation in big sagebrush across broad, ecoregional scales is well known (Brabec et al., 2015; Richardson & Chaney, 2018), site‐specific or local adaptation at this scale can require decades to become evident (Germino et al., 2019). Broadscale adaptive variation in survival (Chaney et al., 2017) and flowering phenology (Richardson et al., 2017) were largely explained by climate‐of‐seed origin.…”

Freezing resistance, safety margins, and survival vary among big sagebrush populations across the western United States

“…Additionally, Germino et al. () found that the minimum temperature of populations’ origins similarly explained their differences in survival in both short‐ and long‐term observations. Still, our results may be more relevant to the establishment phase, an important demographic bottleneck for big sagebrush populations (James et al., ).…”

“…Although big sagebrush is a desert species, its highest rates of growth and carbon gain occur during periods of abundant water availability (DePuit and Caldwell, ; Miller and Shultz, ; Ryel et al., ; Germino and Reinhardt, ), which exposes those processes to the freezing nights that co‐occur in spring. A survival cost to freezing resistance related to carbon gain may help explain why Wyoming big sagebrush in two 27‐yr‐old common gardens showed greater mortality in populations that originated in places with lower minimum winter temperatures than the gardens (Germino et al., ).…”

“…Our measurements were taken after only 5 years of growth, and additional mortality of maladapted populations of big sagebrush in our gardens is likely according to the findings and guidelines of Germino et al. (). However, measuring threshold freezing responses in addition to survival allowed us to capture potential influences of climate extremes that might take decades to shape survival patterns.…”