Biology subjects

Ackerly, D. D.

Publications and source records attributed to Ackerly, D. D..

4 recordsLinked to original sources

bioRxiv2025

Maximum water stress is decoupled from climate, traits and growth in a xeric oak

O_LIWidespread drought-induced forest mortality highlights the ecological consequences of climate change, yet our ability to explain, let alone predict, the spatial patterns of forest mortality remains limited. C_LIO_LIWe conducted a range-wide survey of drought stress, growth, and allocation traits in a widespread oak species (Quercus douglasii, blue oak) to test the predictability of stress across populations and explore how water availability and allocation mediate spatial variation in tree growth. C_LIO_LIAcross 15 sites, we found little relationship between end-of-season water availability (predawn leaf water potentials) or maximum water stress (midday water potentials) and climate or soils. Instead, water availability (within and among sites) was predicted by access to deep water resources inferred from stem water stable isotopes. C_LIO_LIWe also found a remarkable three-way decoupling of water stress, growth, and allocation to leaf tissue, which challenged a data-parameterized mechanistic plant model. C_LIO_LIOur results reveal that deeply rooted trees can be hydrologically decouple from above-ground climate, and that the seasonality of growth, trait development and hydraulic risk are phenologically disconnected. The complicated relationship between carbon gain and hydraulic risk in seasonal environments and limited data on critical zone hydrology are key challenges to predicting drought vulnerability. C_LI Plain Language SummaryBelow-ground structure complicates our understanding of plant water stress. We show that below-ground complexity and highly seasonal environments can both disconnect trees from their above-ground climate and disconnect drought mortality risk from growth and trait development.

ecology↗

bioRxiv2023

Topography influences diurnal and seasonal microclimate fluctuations in hilly terrain environments of coastal California

Understanding the topographic basis for microclimatic variation remains fundamental to predicting the site level effects of warming air temperatures. Quantifying diurnal fluctuation and seasonal extremes in relation to topography offer insight into the potential relationship between site level conditions and changes in regional climate. The present study investigated an annual understory temperature regime for 50 sites distributed across a topographically diverse area (>12 km2) comprised of mixed evergreen-deciduous woodland vegetation typical of California coastal ranges. We investigated the effect of topography and tree cover on site-to-site variation in near-surface temperatures using a combination of multiple linear regression and multivariate techniques. Sites in topographically depressed areas (e.g., valley bottoms) exhibited larger seasonal and diurnal variation. Elevation (at 10 m resolution) was found to be the primary driver of daily and seasonal variations, in addition to local topographic features that measure how depressed a site is compared to the surrounding area, canopy cover and northness. The elevation effect on seasonal mean temperatures was inverted, reflecting large-scale cold-air pooling in the study region, with elevated minimum and mean temperature at higher elevations. Additionally, several of our sites showed considerable buffering (dampened diurnal and seasonal temperature fluctuations) compared to average regional conditions measured by weather station. Results from this study help inform efforts to extrapolate temperature records across large landscapes and have the potential to improve our ecological understanding of fine-scale seasonal climate variation in coastal range environments.

ecology↗

bioRxiv2023

Plasticity drives geographic variation and trait coordination in blue oak drought physiology

Our ability to predict drought stress across the landscape remains limited. This uncertainty stems in part from an incomplete understanding of within-species variation in hydraulic physiology, particularly coordinated variation across multiple traits. This variation reflects genetic differentiation among populations (ecotypic variation) and phenotypic plasticity. We examined among-population differentiation in morphological and hydraulic traits in California blue oak (Quercus douglasii) using a 30 year-old common garden. We compared trait differentiation and trait-trait coordination in the garden to wild phenotypes from the original source populations. We found remarkably limited among-population differentiation in all traits in the common garden but considerable site-to-site variation in the field that could rarely be explained with site climate variables. Trait-trait relationships were also stronger in the field than in the garden, particularly links between leaf morphology, leaf hydraulic efficiency and stem hydraulic efficiency. Only four trait-trait relationship were present in both the wild and garden, but 12 of 45 relationships showed significant wild phenotypic correlations, with strong coordination among leaf and stem hydraulic efficiency apparently mediated by leaf size. Ultimately, we found limited evidence for ecotypic variation but considerable geographic in phenotypic integration in the wild, suggesting considerable acclimation potential in the face of climate change.

ecology↗

bioRxiv2020

Natural selection maintains species despite widespread hybridization in the desert shrub Encelia

Natural selection is an important driver of genetic and phenotypic differentiation between species. A powerful way to test the role of natural selection in the formation and maintenance of species is to study species complexes in which potential gene flow is high but realized gene flow is low. For a recent radiation of New World desert shrubs (Encelia: Asteraceae), we use fine-scale geographic sampling and population genomics to determine patterns of gene flow across two hybrid zones formed between two independent pairs of species with parapatric distributions. After finding evidence for extremely strong selection at both hybrid zones, we use a combination of field experiments, high-resolution imaging, and physiological measurements to determine the ecological basis for selection at one of the hybrid zones. Our results identify multiple ecological mechanisms of selection (drought, salinity, herbivory, and burial) that together are sufficient to maintain species boundaries despite high rates of hybridization. Given that multiple pairs of species hybridize at ecologically divergent parapatric boundaries in the adaptive radiation of Encelia, such mechanisms may maintain species boundaries throughout this group. SIGNIFICANCE STATEMENTIn Baja California, the deserts meet the coastal dunes in a narrow transition visible even from satellite images. We study two species pairs of desert shrubs (Encelia) that occur across this transition. Although these species can interbreed, they remain distinct. Using a combination of genetics, field experiments, 3D-imaging, and physiological measurements, we show that natural selection counteracts the homogenizing effects of gene exchange. The different habitats of these species create multiple mechanisms of selection - drought, salinity, herbivory, and burial, which together maintain these species in their native habitats and their hybrids in intermediate habitats. This study illustrates how environmental factors influence traits and fitness and how this in turn maintain species, highlighting the importance of natural selection in speciation.

evolutionary biology↗