Amplification of the hydrological cycle as a consequence of global warming is forecast to lead to more extreme intra-annual precipitation regimes characterized by larger rainfall events and longer intervals between events. We present a conceptual framework, based on past investigations and ecological theory, for predicting the consequences of this underappreciated aspect of climate change. We consider a broad range of terrestrial ecosystems that vary in their overall water balance. More extreme rainfall regimes are expected to increase the duration and severity of soil water stress in mesic ecosystems as intervals between rainfall events increase. In contrast, xeric ecosystems may exhibit the opposite response to extreme events. Larger but less frequent rainfall events may result in proportional reductions in evaporative losses in xeric systems, and thus may lead to greater soil water availability. Hydric (wetland) ecosystems are predicted to experience reduced periods of anoxia in response to prolonged intervals between rainfall events. Understanding these contingent effects of ecosystem water balance is necessary for predicting how more extreme precipitation regimes will modify ecosystem processes and alter interactions with related global change drivers.

No abstract

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.forest mortality | climate change | drought | fire | tree rings I ntense droughts in recent decades have been linked with extensive tree mortality in many temperate regions globally (1). Attribution of the causes of such mortality within forest ecosystems is challenging because wildfire, insect pests and diseases, invasive species, and land-use effects (e.g., grazing, fire suppression) all interact with climatic effects, confounding interpretation of mortality drivers. Forest vegetation is particularly sensitive to changes in moisture availability in semiarid landscapes (2). Within the southwestern (SW) United States (US), rapid and extensive changes in forests and woodlands recently have been associated with drought-related tree mortality, including attacks by cambium-feeding beetles and severe wildfires (1,(3)(4)(5). Landuse effects, however, probably also have contributed to changing wildfire regimes, particularly in SW ponderosa pine forests (6). High severity wildfires in SW forests can result in a multidecadal (or longer) failure of some forest stands to regenerate, converting large patches to grassland or shrubland (7). If warming continues and further amplifies severities and extent of climate-related disturbances in the SW US, the ability of forests to recover to previous species composition may be limit...

ABSTRACT

Summary• Atmospheric CO 2 concentration is expected to increase by 50% near the middle of this century. The effects the free air CO 2 enrichment (FACE) is presented here on growth and development of field-grown grain sorghum ( Sorghum bicolor ) at ample (wet) and limiting (dry) levels of irrigation water at Maricopa, AZ, USA.• Daytime CO 2 mole fractions were 561 and 368 µ mol mol -1 for the FACE and control treatments, respectively. Irrigation plus precipitation averaged 1132 mm for the wet plots and 396 mm in the dry plots.• During the growing season, FACE increased biomass accumulation in the dry plots but the effects in the wet plots were inconsistent. At final harvest, FACE increased total yield from 999 to 1151 g m -2 in the dry plots and had no effect in the wet plots.• If atmospheric CO 2 continues to increase, total sorghum yield is likely to be higher in the future in areas where water is limited.