Vincent P. Gutschick

Root system adjustments: regulation of plant nutrient uptake and growth responses to elevated CO2

Nutrients such as nitrogen (N) and phosphorus (P) often limit plant growth rate and production in natural and agricultural ecosystems. Limited availability of these nutrients is also a major factor influencing long-term plant and ecosystem responses to rising atmospheric CO2 levels, i.e., the commonly observed short-term increase in plant biomass may not be sustained over the long-term. Therefore, it is critical to obtain a mechanistic understanding of whether elevated CO2 can elicit compensatory adjustments such that acquisition capacity for minerals increases in concert with carbon (C) uptake. Compensatory adjustments such as increases in (a) root mycorrhizal infection, (b) root-to-shoot ratio and changes in root morphology and architecture, (c) root nutrient absorption capacity, and (d) nutrient-use efficiency can enable plants to meet an increased nutrient demand under high CO2. Here we examine the literature to assess the extent to which these mechanisms have been shown to respond to high CO2. The literature survey reveals no consistent pattern either in direction or magnitude of responses of these mechanisms to high CO2. This apparent lack of a pattern may represent variations in experimental protocol and/or interspecific differences. We found that in addressing nutrient uptake responses to high CO2 most investigators have examined these mechanisms in isolation. Because such mechanisms can potentially counterbalance one another, a more reliable prediction of elevated CO2 responses requires experimental designs that integrate all mechanisms simultaneously. Finally, we present a functional balance (FB) model as an example of how root system adjustments and nitrogen-use efficiency can be integrated to assess growth responses to high CO2. The FB model suggests that the mechanisms of increased N uptake highlighted here have different weights in determining overall plant responses to high CO2. For example, while changes in root-to-shoot biomass allocation, r, have a small effect on growth, adjustments in uptake rate per unit root mass,

Biological Extreme Events: A Research Framework

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Functional Biology and Plant Strategies

, and photosynthetic N use efficiency, p*, have a significantly greater leverage on growth responses to elevated CO2 except when relative growth rate (RGR) reaches its developmental limit, maximum RGR (RGRmax).

Coordinated responses of plant hydraulic architecture with the reduction of stomatal conductance under elevated CO2 concentration

New versions of evapotranspiration (ET) algorithms based on the Surface Energy Balance Algorithm for Land (SEBAL) are being published, each containing slightly different equations to calculate the energy balance. It is difficult to determine what impact changing one or more of the equations or coefficients in the series of equations of SEBAL has on the final calculation of ET. The objective of this article is to conduct a sensitivity analysis of ET estimates in SEBAL to identify the most sensitive variables and equations. A remote sensing ET model based on SEBAL was programmed and validated against eddy-covariance data. A sensitivity analysis was conducted for three contrasting land surface conditions: full, half, and sparse canopy cover in pecan orchards. Results were most sensitive to the selection (according to temperature) of the dry (~zero ET) reference pixel and to c (the estimated ratio of soil heat flux to net solar radiation). At all the three degrees of canopy cover, estimated ET changed by 40% to 270% (1 to 2 mm d-1) when either variable changed from its baseline value by ±50% of the permissible range. Estimated ET was also sensitive to the selection of the wet (full ET) reference pixel and to dT (aerodynamic difference of air and land temperatures). Changes in ET estimates were 47% to 72% (1.3 to 3.7 mm d-1) at both the full and half canopy areas under changes from baseline values equal to 50% of the permissible range for either variable. In addition, ET was sensitive to the roughness length in areas of half canopy cover (ET changed by 61% [1.5 mm d-1]) and to the value of the normalized difference vegetation index (NDVI) in areas of sparse canopy cover (ET changed by 118% [0.35 mm d-1]). Future research on ET algorithm improvement should focus on the above variables and relative equations. The selection of the wet- and dry-spots should be automated to avoid subjective errors from manual selection.

Closeout technical report for DOE award number DE-FG02-97ER62332 [Nitrogen budget under elevated CO{sub 2} levels: regulation by absorption and assimilation]

Efforts designed to understand and predict adaptation responses of organisms and populations to global climate change must make a clear distinction between responses to changes in average conditions (e.g., doubling of atmospheric carbon dioxide concentration accompanied by an average increase of 1°–3°C in global air temperature by the end of this century) and responses resulting from increased incidence of extreme events [Loehle and LeBlanc, 1996; Easterling et al., 2000; Garrett et al., 2006]. Such distinction is critical because, unlike changes in average conditions, extremes (e.g., megadroughts, fire, flooding, hurricanes, heat waves, and pest outbreaks) are typically short in duration but challenge organisms and populations considerably further beyond their ability to acclimate than those expected from average trends in climate changes.

Extreme events as shaping physiology, ecology, and evolution of plants: toward a unified definition and evaluation of their consequences

Satellite remote sensing has been used in forest health management as a method for vegetation mapping, fire fuel mapping, fire risk estimation, fire detection, post-fire severity mapping, insect infestation mapping, and relative water stress monitoring. This paper reviews the use of satellite remote sensing in forest health studies, including current research activities; the satellite sensors, methods, and parameters used; and their accuracy. The review concludes that the Moderate Resolution Imaging Spectroradiometer satellite data (MODIS) are more appropriate for most of the remote sensing applications for forest health than other current satellite data when considering temporal and spatial resolutions, cost, and bands. MODIS has a 1-2 day temporal and a 250-1000 m spatial resolution; the data are free and cover more spectral bands than other satellites (up to 36 bands). We recommend that physical and physiological modeling (e.g., evapotranspiration and biomass growth) be developed for remote sensing of forest health. Some additional satellite sensors, such as for high temperature estimates (as high as 1800 K) and sensors of narrow bands, are also needed.

Biotic and abiotic consequences of differences in leaf structure

Functional biology applied to plants explores how they capture and use resources and thus produce usable yield (crops) or ensure the long-term survival of the species and the genes (wild plants). Functional biology examines these results to ask why plants perform as they do, attempting to explain evolved function and to indicate how crops may be improved. The goal of most plant research, either proximately or ultimately, is generally the latter, finding out if and how we may improve crop performance. Functional biology enables us to develop quantitative, testable hypotheses about the optimality of plants’ resource use. Potentially it allows us to determine the upper limits to crop performance. Some limits are known to fairly high specificity, as for leaf photosynthesis (Bassham, 1977; Gutschick, 1986). However, we still have little idea of other limits, such as the theoretical maximum yields for given degrees of salinity in the soil water. Functional biology can also suggest previously unsuspected routes to crop improvement, e.g., breeding for specific leaf mass to optimize canopy photosynthesis (Chap. 3). A particular strength of functional biology is its potential to unify crop improvement efforts of diverse disciplines. Ultimately it promises to determine the single suite of physiological and morphological characteristics of a species that will give the greatest yield or other value indicator for a given environment (Sec. 1.D.i, item 2).

Mechanisms of competition: thermal inhibition of tree seedling growth by grass

Stomatal conductance (gs) generally decreases under elevated CO2 concentration (eCO2) and its sensitivity varies widely among species, yet the underlying mechanisms for these observed patterns are not totally clear. Understanding these underlying mechanisms, however, is critical for addressing problems regarding plant-environment interactions in a changing climate. We examined gs, water transport efficiency of different components along the whole-plant hydraulic system and allometric scaling in seedlings of six tree species grown under ambient and eCO2 treatments (400 and 600 ppm, respectively). Growth under eCO2 caused gs to decrease in all species but to highly variable extents, ranging from 13% (Populus tremuloides Michx.) to 46% (Gymnocladus dioicus (L.)). Accompanying this significant decrease in gs, substantial changes in plant hydraulic architecture occurred, with root hydraulic conductance expressed both on leaf area and root mass bases overall exhibiting significant decreases, while stem and leaf hydraulic efficiency either increased or showed no consistent pattern of change. Moreover, significant changes in allometry in response to eCO2 affected the whole-plant water supply and demand relations. The interspecific variation in gs response among species was not correlated with relative changes in stem and leaf hydraulic conductance but was most strongly correlated with the relative change in the allometric scaling between roots and leaves, and to a lesser extent with the intrinsic root hydraulic conductance of the species. The results underscore that allometric adjustments between root and leaf play a key role in determining the interspecific sensitivity of gs responses to eCO2. Plant hydraulics and their associated allometric scaling are important changes accompanying gs responses to eCO2 and may play important roles in mediating the interspecific variations of leaf gas exchange responses, which suggests that mechanistic investigations regarding plant responses to eCO2 need to integrate characteristics of hydraulics and allometric scaling in the future.