A relatively simple procedure is presented for computation of kinetic energy of a rainstorm from information on a recording-raingage chart. An equation is developed de scribing rainfall energy as a function of rainfall intensity. The effects of rainfall energy and its interaction with other variables are evaluated in multiple regression analyses based on data representing four soil types. Application of this information to separate the effects of rainfall from those of physical and management characteristics in plot data is discussed briefly.Soil erosion is a mechanical process that requires energy. Much of this energy is supplied by falling raindrops. The tremendous magnitude of the kinetic energy of rainfall is readily apparent from a few simple calculations. The dead weight of the water falling in 30 min. of a common thunderstorm in the cornbelt states may well exceed 100 tons on each acre. The billions of drops which comprise this 100-ton volume of water strike the soil, if unprotected, at an average velocity of nearly 20 mi/hr. The rainfall energy to be expended during the 30 minutes may well exceed two million foot pounds per acre. If the rain is driven by violent winds, the energy of impact may be even greater.In 1954, the Soil and Water Conservation Re search Division of the Agricultural Research Ser vice initiated a program to summarize all available runoff and soil-loss data on a national basis for further analyses. Studies on the project have pointed out that the accuracy of soil-loss prediction equations on an individual storm basis is consider ably enhanced when a measure of rainfall energy is included as a variable, and that both the ef ficiency and the simplicity of such equations are further improved by terms measuring the inter action effects between variables. In this paper, a relatively simple procedure for computation of the approximate rainfall-energy value of a storm is presented, and the application of this information in more efficient estimates of expected soil loss is discussed.Drop me-Studies of drop-size distribution of natural rainfall have shown a high degree of cor relation between drop size and rainfall intensity. Size distributions of raindrops as a per cent of total volume were published by Laws and Parsons [1943] for intensities ranging from 0.01 to 6.0 inches per Hour. These data are briefly summarized in Figure 1. The mean drop sizes, weighted by per cent of total volume, lie along the solid-line curve. The broken lines in the graph represent the boundaries of the 25-75 percentile and the 5-95 percentile ranges of drop size, respectively.

Forest ecosystems store approximately 45% of the carbon found in terrestrial ecosystems, but they are sensitive to climate-induced dieback. Forest die-off constitutes a large uncertainty in projections of climate impacts on terrestrial ecosystems, climate-ecosystem interactions, and carbon-cycle feedbacks. Current understanding of the physiological mechanisms mediating climate-induced forest mortality limits the ability to model or project these threshold events. We report here a direct and in situ study of the mechanisms underlying recent widespread and climate-induced trembling aspen (Populus tremuloides) forest mortality in western North America. We find substantial evidence of hydraulic failure of roots and branches linked to landscape patterns of canopy and root mortality in this species. On the contrary, we find no evidence that drought stress led to depletion of carbohydrate reserves. Our results illuminate proximate mechanisms underpinning recent aspen forest mortality and provide guidance for understanding and projecting forest die-offs under climate change.carbon starvation | ecosystem shift | biosphere-atmosphere feedbacks | drought impacts | global change ecology

Plant vascular networks are central to botanical form, function, and diversity. Here, we develop a theory for plant network scaling that is based on optimal space filling by the vascular system along with trade-offs between hydraulic safety and efficiency. Including these evolutionary drivers leads to predictions for sap flow, the taper of the radii of xylem conduits from trunk to terminal twig, and how the frequency of xylem conduits varies with conduit radius. To test our predictions, we use comprehensive empirical measurements of maple, oak, and pine trees and complementary literature data that we obtained for a wide range of tree species. This robust intra-and interspecific assessment of our botanical network model indicates that the central tendency of observed scaling properties supports our predictions much better than the West, Brown, and Enquist (WBE) or pipe models. Consequently, our model is a more accurate description of vascular architecture than what is given by existing network models and should be used as a baseline to understand and to predict the scaling of individual plants to whole forests. In addition, our model is flexible enough to allow the quantification of species variation around rules for network design. These results suggest that the evolutionary drivers that we propose have been fundamental in determining how physiological processes scale within and across plant species. U nderstanding the coevolution of plant internal vascular networks and external branching networks is essential to predict botanical form and function (1-4). Seminal studies have attempted to unite these internal and external networks (3,5,6). A decade ago West, Brown, and Enquist (3) proposed a model (WBE) that focuses on the primacy of vascular networks, predicts myriad aspects of plant form and function (3, 7), and has subsequently been tested by the collection of new data for vascular networks (8, 9) and analyses of fluxes through plants (10, 11), forests, and ecosystems (12, 13). Since the publication of the WBE model, several criticisms have been published that question its basic framework, assumptions, and generality (14-16). Indeed, focusing on plant models, several studies have: (i) highlighted how hydraulic safety and efficiency may have shaped the evolution of vascular networks (2, 17), (ii) questioned whether vascular safety and efficiency are adequately described by the WBE model (8,18,19), and (iii) revealed empirical patterns that contradict parts of the WBE model (9,(20)(21)(22)(23). For example, building on earlier work (24, 25), Sperry and colleagues (2) compiled data for the xylem conduits that transport water in plants, and they documented a general principle termed the "packing rule"-the frequency of xylem conduits varies approximately inversely with the square of conduit radius. This packing rule contradicts the WBE model's assumption that conduit frequency remains unchanged as conduit radii taper, decreasing in size from trunk to terminal twig. Safety and efficiency considerations have been pr...

The concept of iso- vs. anisohydry has been used to describe the stringency of stomatal regulation of plant water potential (ψ). However, metrics that accurately and consistently quantify species' operating ranges along a continuum of iso- to anisohydry have been elusive. Additionally, most approaches to quantifying iso/anisohydry require labour-intensive measurements during prolonged drought. We evaluated new and previously developed metrics of stringency of stomatal regulation of ψ during soil drying in eight woody species and determined whether easily-determined leaf pressure-volume traits could serve as proxies for their degree of iso- vs. anisohydry. Two metrics of stringency of stomatal control of ψ, (1) a 'hydroscape' incorporating the landscape of ψ over which stomata control ψ, and (2) the slope of the daily range of ψ as pre-dawn ψ declined, were strongly correlated with each other and with the leaf osmotic potential at full and zero turgor derived from pressure-volume curves.