Christopher J. Still

Widespread amphibian extinctions from epidemic disease driven by global warming

As the Earth warms, many species are likely to disappear, often because of changing disease dynamics. Here we show that a recent mass extinction associated with pathogen outbreaks is tied to global warming. Seventeen years ago, in the mountains of Costa Rica, the Monteverde harlequin frog (Atelopus sp.) vanished along with the golden toad (Bufo periglenes). An estimated 67% of the 110 or so species of Atelopus, which are endemic to the American tropics, have met the same fate, and a pathogenic chytrid fungus (Batrachochytrium dendrobatidis) is implicated. Analysing the timing of losses in relation to changes in sea surface and air temperatures, we conclude with ‘very high confidence’ (> 99%, following the Intergovernmental Panel on Climate Change, IPCC) that large-scale warming is a key factor in the disappearances. We propose that temperatures at many highland localities are shifting towards the growth optimum of Batrachochytrium, thus encouraging outbreaks. With climate change promoting infectious disease and eroding biodiversity, the urgency of reducing greenhouse-gas concentrations is now undeniable.

The Origins of C4 Grasslands: Integrating Evolutionary and Ecosystem Science

Grassland Emergence The evolution of the C4 photosynthetic pathway from the ancestral C3 pathway in grasses led to the establishment of grasslands in warm climates during the Late Miocene (8 to 3 million years ago). This was a major event in plant evolutionary history, and their high rates of foliage production sustained high levels of herbivore consumption. The past decade has seen significant advances in understanding C4 grassland ecosystem ecology, and now a wealth of data on the geological history of these ecosystems has accumulated and the phylogeny of grasses is much better known. Edwards et al. (p. 587) review this multidisciplinary research area and attempt to synthesize emerging knowledge about the evolution of grass species within the context of plant and ecosystem ecology. The evolution of grasses using C4 photosynthesis and their sudden rise to ecological dominance 3 to 8 million years ago is among the most dramatic examples of biome assembly in the geological record. A growing body of work suggests that the patterns and drivers of C4 grassland expansion were considerably more complex than originally assumed. Previous research has benefited substantially from dialog between geologists and ecologists, but current research must now integrate fully with phylogenetics. A synthesis of grass evolutionary biology with grassland ecosystem science will further our knowledge of the evolution of traits that promote dominance in grassland systems and will provide a new context in which to evaluate the relative importance of C4 photosynthesis in transforming ecosystems across large regions of Earth.

Simulating the effects of climate change on tropical montane cloud forests

Tropical montane cloud forests are unique among terrestrial ecosystems in that they are strongly linked to regular cycles of cloud formation. We have explored changes in atmospheric parameters from global climate model simulations of the Last Glacial Maximum and for doubled atmospheric carbon dioxide concentration (2 × CO2) conditions which are associated with the height of this cloud formation, and hence the occurrence of intact cloud forests. These parameters include vertical profiles of absolute and relative humidity surfaces, as well as the warmth index, an empirical proxy of forest type. For the glacial simulations, the warmth index and absolute humidity suggest a downslope shift of cloud forests that agrees with the available palaeodata. For the 2× CO2 scenario, the relative humidity surface is shifted upwards by hundreds of metres during the winter dry season when these forests typically rely most on the moisture from cloud contact. At the same time, an increase in the warmth index implies increased evapo-transpiration. This combination of reduced cloud contact and increased evapo-transpiration could have serious conservation implications, given that these ecosystems typically harbour a high proportion of endemic species and are often situated on mountain tops or ridge lines.

Forest responses to increasing aridity and warmth in the southwestern United States

In recent decades, intense droughts, insect outbreaks, and wildfires have led to decreasing tree growth and increasing mortality in many temperate forests. We compared annual tree-ring width data from 1,097 populations in the coterminous United States to climate data and evaluated site-specific tree responses to climate variations throughout the 20th century. For each population, we developed a climate-driven growth equation by using climate records to predict annual ring widths. Forests within the southwestern United States appear particularly sensitive to drought and warmth. We input 21st century climate projections to the equations to predict growth responses. Our results suggest that if temperature and aridity rise as they are projected to, southwestern trees will experience substantially reduced growth during this century. As tree growth declines, mortality rates may increase at many sites. Increases in wildfires and bark-beetle outbreaks in the most recent decade are likely related to extreme drought and high temperatures during this period. Using satellite imagery and aerial survey data, we conservatively calculate that ≈2.7% of southwestern forest and woodland area experienced substantial mortality due to wildfires from 1984 to 2006, and ≈7.6% experienced mortality associated with bark beetles from 1997 to 2008. We estimate that up to ≈18% of southwestern forest area (excluding woodlands) experienced mortality due to bark beetles or wildfire during this period. Expected climatic changes will alter future forest productivity, disturbance regimes, and species ranges throughout the Southwest. Emerging knowledge of these impending transitions informs efforts to adaptively manage southwestern forests.

Seasonal and episodic moisture controls on plant and microbial contributions to soil respiration

Moisture inputs drive soil respiration (SR) dynamics in semi-arid and arid ecosystems. However, determining the contributions of root and microbial respiration to SR, and their separate temporal responses to periodic drought and water pulses, remains poorly understood. This study was conducted in a pine forest ecosystem with a Mediterranean-type climate that receives seasonally varying precipitation inputs from both rainfall (in the winter) and fog-drip (primarily in the summer). We used automated SR measurements, radiocarbon SR source partitioning, and a water addition experiment to understand how SR, and its separate root and microbial sources, respond to seasonal and episodic changes in moisture. Seasonal changes in SR were driven by surface soil water content and large changes in root respiration contributions. Superimposed on these seasonal patterns were episodic pulses of precipitation that determined the short-term SR patterns. Warm season precipitation pulses derived from fog-drip, and rainfall following extended dry periods, stimulated the largest SR responses. Microbial respiration dominated these SR responses, increasing within hours, whereas root respiration responded more slowly over days. We conclude that root and microbial respiration sources respond differently in timing and magnitude to both seasonal and episodic moisture inputs. These findings have important implications for the mechanistic representation of SR in models and the response of dry ecosystems to changes in precipitation patterns.

The contribution of C3 and C4 plants to the carbon cycle of a tallgrass prairie: an isotopic approach.

The photosynthetic pathway composition (C3:C4 mixture) of an ecosystem is an important controller of carbon exchanges and surface energy flux partitioning, and therefore represents a fundamental ecophysiological distinction. To assess photosynthetic mixtures at a tallgrass prairie pasture in Oklahoma, we collected nighttime above-canopy air samples along concentration and isotopic gradients throughout the 1999 and 2000 growing seasons. We analyzed these samples for their CO2 concentration and carbon isotopic composition and calculated C3:C4 proportions with a two-source mixing model. In 1999, the C4 percentage increased from 38% in spring (late April) to 86% in early fall (mid-September). The C4 percentages inferred from ecosystem respiration measurements in 2000 indicate a smaller shift, from 67% in spring (early May) to 77% in mid-summer (late July). We also sampled daytime CO2 concentration and carbon isotope gradients above the canopy to determine ecosystem discrimination against 13CO2 during net uptake. These discrimination values were always lower than corresponding nighttime ecosystem respiration isotopic signatures would suggest. After accounting for the isotopic disequilibria between respiration and photosynthesis resulting from seasonal variations in the C3:C4 mixture, we estimated canopy photosynthetic discrimination. The C4 percentage calculated from this approach agrees with the percentage determined from nighttime respiration for sampling periods in both growing seasons. Isotopic imbalances between photosynthesis and respiration are likely to be common in mixed C3:C4 ecosystems and must be considered when using daytime isotopic measurements to constrain ecosystem physiology. Given the global extent of such ecosystems, isotopic imbalances likely contribute to global variations in the carbon isotopic composition of atmospheric CO2.

Influence of clouds and diffuse radiation on ecosystem‐atmosphere CO2 and CO18O exchanges

The influence of clouds and diffuse radiation on ecosystem-atmosphere CO 2 and CO 18 O exchanges Still, C.J., 3 Riley, W.J., 3 Biraud, S.C., 4 Noone, D.C., 4 Buenning, N.H., 5 Randerson J.T., 3 Torn, M.S., 6 Welker, J., 7 White, J.W.C., 7 Vachon, R., 8 Farquhar, G.D., and 9 Berry, J.A. 1. Geography Department, University of California, Santa Barbara, CA USA 2. Institute for Computational Earth System Science, University of California, Santa Barbara, CA USA 3. Earth Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA USA 4. Department of Atmospheric and Oceanic Sciences, and Cooperative Institute for Research in Environmental Science, University of Colorado, Boulder, CO USA 5. Earth System Science Department, University of California, Irvine, CA USA 6. Environment and Natural Resources Institute, University of Alaska, Anchorage, AK USA 7. INSTAAR, and Cooperative Institute for Research in Environmental Science, University of Colorado, Boulder, CO USA 8. Research School of Biological Sciences, Australian National University, Canberra, ACT Australia 9. Department of Global Ecology, Carnegie Institution of Washington, Stanford, CA USA Index terms: 0426, 0321, 0414, 0454, 0428 Keywords: clouds, oxygen isotope discrimination, diffuse radiation, photosynthesis, isofluxes

Continental-Scale Distributions of Vegetation Stable Carbon Isotope Ratios

The stable carbon isotope composition (δ13C) of terrestrial vegetation is important for a variety of applications in fields ranging from biogeochemistry to zoology to paleoclimatology. To a large degree, spatial patterns in plant δ13C are imparted by variations in the photosynthetic pathway (C3/C4) composition of vegetation in topical and subtropical regions. Thus, the fractional coverage of each vegetation type must be known in order to predict the spatial distribution of plant δ13C values. Our approach for predicting the C3/C4 composition relies on the strong ecological sorting of C4 plants along temperature gradients, as well as the near-universal restriction of C4 photosynthesis to herbaceous growth forms. We build upon a previous approach to predict C3/C4 vegetation fractions using finer spatial resolution (500 m) MODIS datasets of vegetation growth form (i.e., percent cover of herbaceous, woody, and bare) and crop type coverage, along with precipitation and temperature climatologies. By combining these products, we predict the C3/C4 vegetation fraction at continental-to-global scales. We present a distribution of C3 and C4 vegetation in Africa. The area of land in sub-Saharan Africa covered by C4 vegetation is 6.3 million square kilometre, or approximately 31% of the land surface. The δ13C of vegetation in Africa is estimated from the C3/C4 composition, assuming constant values of −27‰ and −12‰ for C3 and C4 biomass. Strong precipitation gradients in Africa drive the C3/C4 spatial gradients, producing correspondingly strong gradients in vegetation δ13C. These isotopic gradients can be used to infer such information as the migratory connectivity of birds.

A multi‐isotope (δ13C, δ15N, δ2H) feather isoscape to assign Afrotropical migrant birds to origins

A universal challenge in methodology used to study the ecology, conservation and evolutionary biology of migratory species is the quantification of connectivity among breeding, wintering and stopover sites. For the avian Eurasian-Afrotropical migratory system, knowledge of geographical wintering areas used by migrants that breed in Europe remains deficient, despite the advent of satellite transmitters and geolocators. Here we explored the use of theoretical plant δ13C and δ15N landscape distributions coupled with δ2H hydrologic models to construct multi-isotopic avian foodweb clusters for Africa. The cluster analysis identified four distinct regions of Africa based on all three isotopes (13C, 2H, 15N), and five regions based only on 13C and 15N. We applied known isotopic diet-tissue discrimination factors to map equivalent feather isotopic clusters for Africa. The validity of these feather isotopic clusters was tested by examining how well known- and unknown-origin species were placed in regions of Africa using previously published feather isotope data. The success of this multi-isotopic cluster model depended upon the species of interest and additionally on how well potential winter molt origins in Africa were constrained by prior information. Ground-truthing data suggested this approach will be useful for first-order approximation of overwintering regions for Afrotropical migrants and will be improved as our understanding of the nature of isoscapes for Africa is refined.

Evaluating patterns of fog water deposition and isotopic composition on the California Channel Islands

Fog deposition is an important water source for endemic conifer species during the annual summer drought along the California coast ( and in other coastal and montane areas). We present a new design for a passive fog collector that is useful both for characterizing fog regimes ( timing and quantity of deposition) and for collecting fog water for subsequent isotopic analysis. The new collector both mimics vegetation collection efficiency and minimizes isotopic fractionation under a range of fog conditions. Low construction cost and collector durability allow widely distributed installation and greater insight into spatially heterogeneous fog patterns. We installed 21 fog collectors throughout a stand of Bishop pines ( Pinus muricata D. Don) on Santa Cruz Island. In general, there was greater fog deposition with increasing elevation and decreasing frequency farther inland. Within these broad patterns, there was large spatial and temporal variability in fog deposition. Monthly samples of fog and rain waters reveal differences in stable isotope composition ( delta O-18 and delta D) large enough to serve as tracers of different water sources moving through the ecosystem.