Solomon Z. Dobrowski

Changes in Climatic Water Balance Drive Downhill Shifts in Plant Species' Optimum Elevations

Many mountain plant species in California shifted downhill, tracking regional changes in water balance. Uphill shifts of species’ distributions in response to historical warming are well documented, which leads to widespread expectations of continued uphill shifts under future warming. Conversely, downhill shifts are often considered anomalous and unrelated to climate change. By comparing the altitudinal distributions of 64 plant species between the 1930s and the present day within California, we show that climate changes have resulted in a significant downward shift in species’ optimum elevations. This downhill shift is counter to what would be expected given 20th-century warming but is readily explained by species’ niche tracking of regional changes in climatic water balance rather than temperature. Similar downhill shifts can be expected to occur where future climate change scenarios project increases in water availability that outpace evaporative demand.

Relationships among net primary productivity, nutrients and climate in tropical rain forest: a pan-tropical analysis

Tropical rain forests play a dominant role in global biosphere-atmosphere CO(2) exchange. Although climate and nutrient availability regulate net primary production (NPP) and decomposition in all terrestrial ecosystems, the nature and extent of such controls in tropical forests remain poorly resolved. We conducted a meta-analysis of carbon-nutrient-climate relationships in 113 sites across the tropical forest biome. Our analyses showed that mean annual temperature was the strongest predictor of aboveground NPP (ANPP) across all tropical forests, but this relationship was driven by distinct temperature differences between upland and lowland forests. Within lowland forests (< 1000 m), a regression tree analysis revealed that foliar and soil-based measurements of phosphorus (P) were the only variables that explained a significant proportion of the variation in ANPP, although the relationships were weak. However, foliar P, foliar nitrogen (N), litter decomposition rate (k), soil N and soil respiration were all directly related with total surface (0-10 cm) soil P concentrations. Our analysis provides some evidence that P availability regulates NPP and other ecosystem processes in lowland tropical forests, but more importantly, underscores the need for a series of large-scale nutrient manipulations - especially in lowland forests - to elucidate the most important nutrient interactions and controls.

Steady-state chlorophyll a fluorescence detection from canopy derivative reflectance and double-peak red-edge effects

Abstract A series of experiments carried out in a controlled environment facility to induce steady-state chlorophyll a fluorescence variation demonstrate that natural fluorescence emission is observable on the derivative reflectance spectra as a double-peak feature in the 690–710 nm spectral region. This work describes that the unexplained double-peak feature previously seen on canopy derivative reflectance is due entirely to chlorophyll fluorescence (CF) effects, demonstrating the importance of derivative methods for fluorescence detection in vegetation. Measurements were made in a controlled environmental chamber where temperature and humidity were varied through the time course of the experiments in both short- and long-term trials using Acer negundo ssp. californium canopies. Continuous canopy reflectance measurements were made with a spectrometer on healthy and stressed vegetation, along with leaf-level steady-state fluorescence measurements with the PAM-2000 Fluorometer during both temperature–stress induction and recovery stages. In 9-h trials, temperatures were ramped from 10 to 35 °C and relative humidity adjusted from 92% to 42% during stress induction, returning gradually to initial conditions during the recovery stage. Canopy reflectance difference calculations and derivative analysis of reflectance spectra demonstrate that a double-peak feature created between 688, 697 and 710 nm on the derivative reflectance is a function of natural steady-state fluorescence emission, which gradually diminished with induction of maximum stress. Derivative reflectance indices based on this double-peak feature are demonstrated to track natural steady-state fluorescence emission as quantified by two indices, the double-peak index (DPi) and the area of the double peak (Adp). Results obtained employing these double-peak indices from canopy derivative reflectance suggest a potential for natural steady-state fluorescence detection with hyperspectral data. Short- and long-term stress effects on the observed double-peak derivative indices due to pigment degradation and canopy structure changes were studied, showing that both indices are capable of tracking steady-state fluorescence changes from canopy remote sensing reflectance.

Spectral and Structural Measures of Northwest Forest Vegetation at Leaf to Landscape Scales

We report a multiscale study in the Wind River Valley in southwestern Washington, where we quantified leaf to stand scale variation in spectral reflectance for dominant species. Four remotely sensed structural measures, the normalized difference vegetation index (NDVI), cover fractions from spectral mixture analysis (SMA), equivalent water thickness (EWT), and albedo were investigated using Airborne Visible Infrared Imaging Spectrometer (AVIRIS) data. Discrimination of plant species varied with wavelength and scale, with deciduous species showing greater separability than conifers. Contrary to expectations, plant species were most distinct at the branch scale and least distinct at the stand scale. At the stand scale, broadleaf and conifer species were spectrally distinct, as were most conifer age classes. Intermediate separability occurred at the leaf scale. Reflectance decreased from leaf to stand scales except in the broadleaf species, which peaked in near-infrared reflectance at the branch scale. Important biochemical signatures became more pronounced spectrally progressing from leaf to stand scales. Recent regenerated clear-cuts (less than 10 years old) had the highest albedo and nonphotosynthetic vegetation (NPV). After 50 years, the stands showed significant decreases in albedo, NPV, and EWT and increases in shade. Albedo was lowest in old-growth forests. Peak EWT, a proxy measure for leaf area index (LAI), was observed in 11- to 30-year-old stands. When compared to LAI, EWT and NDVI showed exponentially decreasing, but distinctly different, relationships with increasing LAI. This difference is biologically important: at 95% of the maximum predicted NDVI and EWT, LAI was 5.17 and 9.08, respectively. Although these results confirm the stand structural variation expected with forest succession, remote-sensing images also provide a spatial context and establish a basis to evaluate variance within and between age classes. Landscape heterogeneity can thus be characterized over large areas—a critical and important step in scaling fluxes from stand-based towers to larger scales.

Climate refugia: joint inference from fossil records, species distribution models and phylogeography

Climate refugia, locations where taxa survive periods of regionally adverse climate, are thought to be critical for maintaining biodiversity through the glacial-interglacial climate changes of the Quaternary. A critical research need is to better integrate and reconcile the three major lines of evidence used to infer the existence of past refugia - fossil records, species distribution models and phylogeographic surveys - in order to characterize the complex spatiotemporal trajectories of species and populations in and out of refugia. Here we review the complementary strengths, limitations and new advances for these three approaches. We provide case studies to illustrate their combined application, and point the way towards new opportunities for synthesizing these disparate lines of evidence. Case studies with European beech, Qinghai spruce and Douglas-fir illustrate how the combination of these three approaches successfully resolves complex species histories not attainable from any one approach. Promising new statistical techniques can capitalize on the strengths of each method and provide a robust quantitative reconstruction of species history. Studying past refugia can help identify contemporary refugia and clarify their conservation significance, in particular by elucidating the fine-scale processes and the particular geographic locations that buffer species against rapidly changing climate.

The climate velocity of the contiguous United States during the 20th century

Rapid climate change has the potential to affect economic, social, and biological systems. A concern for species conservation is whether or not the rate of on-going climate change will exceed the rate at which species can adapt or move to suitable environments. Here we assess the climate velocity (both climate displacement rate and direction) for minimum temperature, actual evapotranspiration, and climatic water deficit (deficit) over the contiguous US during the 20th century (1916-2005). Vectors for these variables demonstrate a complex mosaic of patterns that vary spatially and temporally and are dependent on the spatial resolution of input climate data. Velocities for variables that characterize the climatic water balance were similar in magnitude to that derived from temperature, but frequently differed in direction resulting in the divergence of climate vectors through time. Our results strain expectations of poleward and upslope migration over the past century due to warming. Instead, they suggest that a more full understanding of changes in multiple climatic factors, in addition to temperature, may help explain unexpected or conflicting observational evidence of climate-driven species range shifts during the 20th century.

Modeling plant ranges over 75 years of climate change in California, USA: temporal transferability and species traits

Species distribution model (SDM) projections under future climate scenarios are increasingly being used to inform resource management and conservation strategies. A critical assumption for projecting climate change responses is that SDMs are transferable through time, an assumption that is largely untested because investigators often lack temporally independent data for assessing transferability. Further, understanding how the ecology of species influences temporal transferability is critical yet almost wholly lacking. This raises two questions. (1) Are SDM projections transferable in time? (2) Does temporal transferability relate to species ecological traits? To address these questions we developed SDMs for 133 vascular plant species using data from the mountain ranges of California (USA) from two time periods: the 1930s and the present day. We forecast historical models over 75 years of measured climate change and assessed their projections against current distributions. Similarly, we hindcast contemporary models and compared their projections to historical data. We quantified transferability and related it to species ecological traits including physiognomy, endemism, dispersal capacity, fire adaptation, and commonness. We found that non-endemic species with greater dispersal capacity, intermediate levels of prevalence, and little fire adaptation had higher transferability than endemic species with limited dispersal capacity that rely on fire for reproduction. We demonstrate that variability in model performance was driven principally by differences among species as compared to model algorithms or time period of model calibration. Further, our results suggest that the traits correlated with prediction accuracy in a single time period may not be related to transferability between time periods. Our findings provide a priori guidance for the suitability of SDM as an approach for forecasting climate change responses for certain taxa.

Spatial variability in wildfire probability across the western United States

Despitegrowingknowledgeoffire-environmentlinkagesinthewesternUSA,obtainingreliableestimatesof relativewildfirelikelihoodremainsaworkinprogress.Thepurposeofthisstudyistouseupdatedfireobservationsduring a 25-year period and a wide array of environmental variables in a statistical framework to produce high-resolution estimatesofwildfireprobability.UsingtheMaxEntmodellingtechnique,point-sourcefireobservationsthatweresampled from area burned during the 1984-2008 time period were related to explanatory variables representing ignitions, flammable vegetation (i.e. fuels), climate and topography. Model results were used to produce spatially explicit predictions of wildfire probability. To assess the effect of humans on the spatial patterns of wildfire likelihood, we built an alternative model that excluded all variables having a strong anthropogenic imprint. Results showed that wildfire probabilityinthewesternUSAisfarfromuniform,withdifferentareasrespondingtodifferentenvironmentaldrivers.The effect of anthropogenic factors on wildfire probability varied by region but, on the whole, humans appear to inhibit fire activity in the western USA. Our results not only provide what appear to be robust predictions of wildfire likelihood, but also enhance understanding of long-term controls on wildfire activity. In addition, our wildfire probability maps provide betterinformationforstrategicplanningofland-managementactivities,especiallywherefireregimeknowledgeissparse. Additional keywords: climate, fuels, ignitions, MaxEnt algorithm, spatial modelling, topography.

Artificial amplification of warming trends across the mountains of the western United States

Observations from the main mountain climate station network in the western United States (U.S.) suggest that higher elevations are warming faster than lower elevations. This has led to the assumption that elevation-dependent warming is prevalent throughout the region with impacts to water resources and ecosystem services. Here we critically evaluate this networks temperature observations and show that extreme warming observed at higher elevations is the result of systematic artifacts and not climatic conditions. With artifacts removed, the networks 1991–2012 minimum temperature trend decreases from +1.16°C decade−1 to +0.106°C decade−1 and is statistically indistinguishable from lower elevation trends. Moreover, longer-term widely used gridded climate products propagate the spurious temperature trend, thereby amplifying 1981–2012 western U.S. elevation-dependent warming by +217 to +562%. In the context of a warming climate, this artificial amplification of mountain climate trends has likely compromised our ability to accurately attribute climate change impacts across the mountainous western U.S.

Managing climate change refugia for climate adaptation

Refugia have long been studied from paleontological and biogeographical perspectives to understand how populations persisted during past periods of unfavorable climate. Recently, researchers have applied the idea to contemporary landscapes to identify climate change refugia, here defined as areas relatively buffered from contemporary climate change over time that enable persistence of valued physical, ecological, and socio-cultural resources. We differentiate historical and contemporary views, and characterize physical and ecological processes that create and maintain climate change refugia. We then delineate how refugia can fit into existing decision support frameworks for climate adaptation and describe seven steps for managing them. Finally, we identify challenges and opportunities for operationalizing the concept of climate change refugia. Managing climate change refugia can be an important option for conservation in the face of ongoing climate change.