Guanghui Lin

Assessing the Response of Terrestrial Ecosystems to Potential Changes in Precipitation

Abstract Changes in Earths surface temperatures caused by anthropogenic emissions of greenhouse gases are expected to affect global and regional precipitation regimes. Interactions between changing precipitation regimes and other aspects of global change are likely to affect natural and managed terrestrial ecosystems as well as human society. Although much recent research has focused on assessing the responses of terrestrial ecosystems to rising carbon dioxide or temperature, relatively little research has focused on understanding how ecosystems respond to changes in precipitation regimes. Here we review predicted changes in global and regional precipitation regimes, outline the consequences of precipitation change for natural ecosystems and human activities, and discuss approaches to improving understanding of ecosystem responses to changing precipitation. Further, we introduce the Precipitation and Ecosystem Change Research Network (PrecipNet), a new interdisciplinary research network assembled to encourage and foster communication and collaboration across research groups with common interests in the impacts of global change on precipitation regimes, ecosystem structure and function, and the human enterprise.

A mechanistic model for interpretation of hydrogen and oxygen isotope ratios in tree-ring cellulose

A mechanistic model is presented to quantify both the physical and biochemical fractionation events associated with hydrogen and oxygen isotope ratios in tree-ring cellulose. The model predicts the isotope ratios of tree-rings, incorporating both humidity and source water environmental information. Components of the model include (1) hydrogen and oxygen isotope effects associated with leaf water enrichment; (2) incorporation of leaf water isotope ratio values into photosynthetic carbohydrates along with the biochemical fractionation associated with autotrophic synthesis; (3) transport of exported carbohydrates (such as sucrose) from leaves to developing xylem in shoots and stems where cellulose is formed; (4) a partial exchange of oxygen and hydrogen isotopes in carbohydrates with xylem sap water during conversion into cellulose; and (5) a biochemical fractionation associated with cellulose synthesis. A modified version of the Craig-Gordon model for evaporative enrichment adequately described leaf water dD and d 18 O values. The leaf water model was robust over a wide range of leaf waters for both controlled experiments and field studies, far exceeding the range of values to be expected under natural conditions. The isotopic composition of cellulose was modeled using heterotrophic and autotrophic fractionation factors from the literature as well as the experimentally derived proportions of H and O that undergo exchange with xylem water during cellulose synthesis in xylem cells of tree-rings. The fraction of H and O from carbohydrates that exchange with xylem sap water was estimated to be 0.36 and 0.42, respectively. The proportions were based on controlled, long-term greenhouse experiments and field studies where the variations in the dD and d 18 O of tree-ring cellulose were measured under different source water isotopic compositions. The model prediction that tree-ring cellulose contains information on environmental water source and atmospheric vapor pressure deficit (related to relative humidity) was tested under both field and greenhouse conditions. This model was compared to existing models to explain cellulose isotope ratios under a wide range of source water and humidity conditions. Predictions from our model were consistent with observations, whereas other models showed large discrepancies as soon as the isotope ratios of source water and atmospheric water deviated from each other. Our model resolves the apparently conflicting and disparate interpretations of several previous cellulose stable isotope ratio studies. Copyright

Partitioning overstory and understory evapotranspiration in a semiarid savanna woodland from the isotopic composition of water vapor

The relative contributions of overstory and understory plant transpiration and soil evaporation to total evapotranspiration (ET) in a semiarid savanna woodland were determined from stable isotope measurements of atmospheric water vapor. The savanna overstory was dominated by the deeply rooted, woody legume Prosopis velutina (“mesquite”), and the understory was dominated by a perennial C4 grass, Sporobolus wrightii. “Keeling plots” (turbulent mixing relationships) were generated from isotope ratios (D and 18 O) of atmospheric water vapor collected within the tree (3–14 m) and understory (0.1–1 m) canopies during peak (July) and post-monsoon (September) periods of 2001. The unique regression intercepts from upper and lower profiles were used to partition the ET flux from the understory layer separately from that of the whole ecosystem. Although ET partitioning was problematic during the first sampling period in July, our results in September provided support to the validity of this method for measuring and understanding the dynamic behavior of water balance components in this semiarid savanna woodland. During the post-monsoon period (22nd September), transpiration accounted for 85% of ecosystem ET. Transpiration by the grass layer accounted for 50% of the understory ET over the same period. The total ecosystem ET estimated by eddy covariance (EC) on 22nd September was 3.5 mm per day. Based on partitioning by the isotope method, 2.5 mm per day (70%) was from tree transpiration and 0.5 mm per day (15%) was from transpiration by the grass layer. Independent estimates of overstory and understory ET partitioning from distributed understory EC measurements were remarkably consistent with our isotope approach.

Carbon Isotopic Fractionation Does Not Occur during Dark Respiration in C3 and C4 Plants.

The magnitude of possible carbon isotopic fractionation during dark respiration was investigated with isolated mesophyll cells from mature leaves of common bean (Phaseolus vulgaris L.), a C3 plant, and corn (Zea mays L.), a C4 plant. Mesophyll protoplasts were extracted from greenhouse-grown leaves and incubated in culture solutions containing different carbohydrate substrates (fructose, glucose, and sucrose) with known [delta]13C values. The CO2 produced by protoplasts after incubation in the dark was collected, purified, and analyzed for its carbon isotope ratio. From observations of the isotope ratios of the substrate and respired CO2, we calculated the carbon isotope discrimination associated with metabolism of each of these substrates. In eight of the 10 treatment combinations, the carbon isotope ratio discrimination was not significantly different from 0. In the remaining two treatment combinations, the carbon isotope ratio discrimination was 1[per mille (thousand) sign]. From these results, we conclude that there is no significant carbon isotopic discrimination during mitochondrial dark respiration when fructase, glucose, or sucrose are used as respiratory substrates.

Hydrogen Isotopic Fractionation by Plant Roots during Water Uptake in Coastal Wetland Plants

Publisher Summary Hydrogen and oxygen isotope analyses of plant stem water are used to quantitatively determine the use of different water sources by various plants in different environments—such as white pines, desert plants, streamside trees, and coastal plants. There are differences in isotopic composition among possible water sources. There is no isotopic fractionation by plant roots during water uptake. The study discussed in this chapter compared δD and δ 18Ο values between stem and source water from several plant species growing naturally in coastal wet-land habitats. Both hydrogen and oxygen isotope ratios of stem water matched those of source water under field and greenhouse conditions when there was no isotopic fractionation during water uptake. On the other hand, differences in isotopic compositions between stem and source water indicate isotopic fractionation during water uptake. The results indicate that there is a significant hydrogen isotopic fractionation during water uptake in coastal wetland plants. Oxygen isotope ratios of stem water from coastal wetland plants, however, match those of source water.

Monosoonal precipitation responses of shrubs in a cold desert community on the Colorado Plateau

South-eastern Utah forms a northern border for the region currently influenced by the Arizona monosoonal system, which feeds moisture and summer precipitation into western North America. One major consequence predicted by global climate change scenarios is an intensification of monosoonal (summer) precipitation in the aridland areas of the western United States. We examined the capacity of dominant perennial shrubs in a Colorado Plateau cold desert ecosystem of southern Utah, United States, to use summer moisture inputs. We simulated increases of 25 and 50 mm summer rain events on Atriplex canescens, Artemisia filifolia, Chrysothamnus nauseosus, Coleogyne ramosissima, and Vanclevea stylosa, in July and September with an isotopically enriched water (enriched in deuterium but not 18O). The uptake of this artificial water source was estimated by analyzing hydrogen and oxygen isotope ratios of stem water. The predawn and midday xylem water potentials and foliar carbon isotope discrimination were measured to estimate changes in water status and water-use efficiency. At. canescens and Ch. nauseosus showed little if any uptake of summer rains in either July or September. The predawn and midday xylem water potentials for control and treatment plants of these two species were not significantly different from each other. For A. filifolia and V. stylosa, up to 50% of xylem water was from the simulated summer rain, but the predawn and midday xylem water potentials were not significantly affected by the additional summer moisture input. In contrast, C. ramosissima showed significant uptake of the simulated summer rain (>50% of xylem water was from the artificial summer rain) and an increase in both predawn and midday water potentials. The percent uptake of simulated summer rain was greater when those rains were applied in September than in July, implying that high soil temperature in midsummer may in some way inhibit water uptake. Foliar carbon isotope discrimination increased significantly in the three shrubs taking up simulated summer rain, but pre-treatment differences in the absolute discrimination values were maintained among species. The ecological implications of our results are discussed in terms of the dynamics of this desert community in response to changes in the frequency and dependability of summer rains that might be associated with a northward shift in the Arizona monsoon boundary.

Plant growth in elevated CO2 alters mitochondrial number and chloroplast fine structure.

With increasing interest in the effects of elevated atmospheric CO2 on plant growth and the global carbon balance, there is a need for greater understanding of how plants respond to variations in atmospheric partial pressure of CO2. Our research shows that elevated CO2 produces significant fine structural changes in major cellular organelles that appear to be an important component of the metabolic responses of plants to this global change. Nine species (representing seven plant families) in several experimental facilities with different CO2-dosing technologies were examined. Growth in elevated CO2 increased numbers of mitochondria per unit cell area by 1.3–2.4 times the number in control plants grown in lower CO2 and produced a statistically significant increase in the amount of chloroplast stroma (nonappressed) thylakoid membranes compared with those in lower CO2 treatments. There was no observable change in size of the mitochondria. However, in contrast to the CO2 effect on mitochondrial number, elevated CO2 promoted a decrease in the rate of mass-based dark respiration. These changes may reflect a major shift in plant metabolism and energy balance that may help to explain enhanced plant productivity in response to elevated atmospheric CO2 concentrations.

Compensatory growth responses to clipping defoliation in Leymus chinensis (Poaceae) under nutrient addition and water deficiency conditions

Compensatory growth responses of Leymus chinensis, a dominant species in Inner Mongolia steppe, to clipping defoliation were evaluated in a pot-cultivated experiment under different nutrient (N and P) and water availability conditions. Leymus chinensis exhibited over-compensatory growth at the light and moderate clipping intensities (20% and 40% aerial mass removed) with a greater accumulated aboveground biomass, higher relative growth rate (RGR), more rhizomatic tillers and a stimulation of compensatory photosynthesis to the remnant leaves as compared with those of the unclipped plants. Intense clipping (80% aerial mass removed), which removed most of the aboveground tissues, greatly reduced the growth of aboveground biomass in comparison with that of the unclipped plants. Nitrogen addition only slightly improved the biomass production and RGR in light and moderately clipped plants, and it did not allow plants in the intense clipping condition to over-compensate. Phosphorus addition had no obvious influences on the growth and physiological responses to clipping defoliation. These results indicated that nutrient addition could not compensate for the negative effects of severe clipping on the defoliated grass. On the other hand, there were no distinct positive responses under water deficiency condition for L. chinensis at all clipping intensities with a significant reduction of aboveground and belowground biomass, lower RGR, fewer rhizomatic tillers, and a lower net photosynthetic rate than other wet treatments. Additionally, the chlorophyll contents of remnant leaves gradually increased with the increase of clipping intensities in each treatment. In conclusion, although L. chinensis could compensate for tissues removal by some morphological and physiological responses, intense clipping and drought can result in a significant decrease of biomass and growth rate, even under enriched nutrition conditions.

DIFFERENCES IN MORPHOLOGY, CARBON ISOTOPE RATIOS, AND PHOTOSYNTHESIS BETWEEN SCRUB AND FRINGE MANGROVES IN FLORIDA, USA

Abstract All three mangrove species in Florida (USA), Rhizophora mangle L., Laguncularia racemosa Gaert. and Avicennia germinans (L.) L., are found frequently in scrub mangrove forests, in which individuals rarely exceed 1.5 m in height. In the present study, the differences in morphological characteristics, leaf carbon isotope ratios and photosynthetic gas exchange between individuals in scrub and fringe mangrove forests in south Florida were investigated quantitatively. Plants in the scrub forests had much lower canopy height, more main stems per tree and smaller leaves, relative to those in the fringe forests. There was a significant correlation between tree height and leaf δ 13 C value, with higher δ 13 C values (1–4% more positive) for plants in the scrub mangrove forests. Correspondingly, scrub mangroves showed significantly lower intercellular carbon dioxide (CO 2 ) concentration and higher intrinsic water use efficiency over long-term carbon assimilation, relative to fringe mangroves. Photosynthetic gas exchange measurements on R. mangle individuals showed a 15.5% lower CO 2 assimilation rate, 6.1% lower intercellular CO 2 concentration and 11.6% higher intrinsic water use efficiency in scrub mangroves, consistent with those estimated from leaf carbon isotope ratios. A higher slope for the linear correlation between CO 2 assimilation rate and stomatal conductance was observed for the individuals in the scrub mangrove forest, which is in agreement with other measurements indicating higher water use efficiency in scrub mangroves. Possible environmental factors responsible for these morphological and physiological differences between scrub and fringe mangroves are discussed.

Tracing Changes in Ecosystem Function under Elevated Carbon Dioxide Conditions

Abstract Responses of ecosystems to elevated levels of atmospheric carbon dioxide (CO2) remain a critical uncertainty in global change research. Two key unknown factors are the fate of carbon newly incorporated by photosynthesis into various pools within the ecosystem and the extent to which elevated CO2 is transferred to and sequestered in pools with long turnover times. The CO2 used for enrichment in many experiments incorporates a dual isotopic tracer, in the sense that ratios of both the stable carbon-13 (13C) and the radioactive carbon-14 (14C) isotopes with respect to carbon-12 are different from the corresponding ratios in atmospheric CO2. Here we review techniques for using 13C and 14C abundances to follow the fate of newly fixed carbon and to further our understanding of the turnover times of ecosystem carbon pools. We also discuss the application of nitrogen, oxygen, and hydrogen isotope analyses for tracing changes in the linkages between carbon, nitrogen, and water cycles under conditions of elevated CO2.