Carbon isotope discrimination in the C4 shrub Atriplex confertifolia along a salinity gradient

Journal ArticleCarbon isotope discrimination (∆) was measured for leaves of Atriplex confertifolia along a salinity gradi~ ent in northern Utah. Over this gradient, the variation of ∆ values was high for a C4 species, and the ∆ values were positively correlated with salinity in both years of the study. Of the possible explanations for this pattern, the ∆ results are consistent with the notion that salinity induces an increase in the bundle sheath leakiness of these C4 plants

Population- and family-level variation of brittlebush (Encelia farinosa, Asteraceae) pubescence: its relation to drought and implications for selection in variable environments

Journal ArticleBecause leaf pubescence of the desert shrub Encelia farinosa increases in response to drought and influences photosynthesis and transpiration, we hypothesized that differences in water availability across the range of this species may result in genetic differentiation for pubescence and associated productivity traits. We examined maternal family variation of pubescence-moderated light absorption (absorptance) in three populations of E. farinosa

A Bayesian model for predicting local El Niño events using tree ring widths and cellulose δ18O

This is the peer reviewed version of the following article: Nippert, Jesse B.; Hooten, Mevin B.; Sandquist, Darren R.; Ward, Joy K. (2010). "A Bayesian model for predicting local El Niño events using tree ring widths and cellulose δ18O." Journal of Geophysical Research: Biogeosciences, 115(G1):G01011. http://www.dx.doi.org/10.1029/2009JG001101., which has been published in final form at http://doi.org/10.1029/2009JG001101. This article may be used for non-commercial purposes in accordance with Wiley Terms and Conditions for Self-Archiving.The oxygen stable isotopic composition (δ18O) of cellulose recorded in annual tree rings reflects the climate and precipitation history experienced during tree growth and development. Here, we show proxy evidence of El Niño events over the past 30 years using juniper tree rings from southern California, United States. The relationship between tree ring δ18O in α cellulose and annual ring width was negative during most years, reflecting amount-driven fractionation during precipitation. During El Niño years, the relationship between δ18O and ring width was positive with the largest ring widths correlated to the heaviest δ18O. Warmer sea surface temperatures during vapor formation and the strengthening of vapor transport from the eastern Pacific Ocean inland is the most likely mechanism driving heavier δ18O in precipitation during El Niño years. Based on this varying relationship between tree ring width and climate-dependent δ18O values, we created a model to estimate the probability that a given annual tree ring was formed during an El Niño or non–El Niño year. The methods used in this analysis differ from standard dendrochronological technique because we explicitly account for the varying relationship between climate and tree ring characteristic during an El Niño or non–El Niño year. Moreover, our approach accommodates uncertainty in model parameters and predictions better than traditional classification methods. The application of this model to prehistory tree samples or samples of unknown age may allow for El Niño detection and subsequent determination of changes in El Niño frequency

COMPENSATORY FOLIAGE GROWTH RESPONSE TO PARTIAL DEFOLIATION IN THE DESERT PERENNIAL SHRUB ENCELIA FARINOSA (ASTERACEAE)

Volume: 51Start Page: 301End Page: 30

POTENTIAL ADAPTABILITY AND CONSTRAINTS OF RESPONSE TO CHANGING CLIMATES FOR ENCELIA FARINOSA VAR. PHENICODONTA FROM SOUTHERN BAJA CALIFORNIA, MEXICO

Volume: 43Start Page: 465End Page: 47

CARBON ISOTOPE DISCRIMINATION IN THE C₄ SHRUB ATRIPLEX CONFERTIFOLIA ALONG A SALINITY GRADIENT

Volume: 55Start Page: 135End Page: 14

Native Species Regeneration Following Ungulate Exclusion and Nonnative Grass Removal in a Remnant Hawaiian Dry Forest.

v. ill. 23 cm.QuarterlyHawaiian lowland dry forests have been reduced by >90% since first human contact. Restoration has focused on protection from fire and ungulates, and removal of invasive grasses as ways to stimulate native forest regeneration. Despite these efforts, natural regeneration of native plants has been infrequent. To assess effects of previous restoration treatments on natural regeneration, we monitored seed rain and dynamics of seedlings and juveniles for a period of 3 yr (2004–2007) within three restoration units within a remnant forest on the island of Hawai‘i. All units had been protected from fire for many decades but differed in time since ungulate exclusion and grass removal. The units were as follows: (1) long-term restoration (fenced 1956, grass removal initiated 1995), (2) shortterm restoration (fenced and grass removal initiated 1997), and (3) unmanaged (fenced 1997, no grass removal). Overall juvenile plant abundance was highest in the short-term unit, but native abundance was highest in the long-term unit. Native woody seedlings established in all units, but recruitment into larger size classes was restricted to units with grass removal, primarily the long-term unit. Native seed rain explained much of the variation in native seedling abundance between units with grass removal. Nonnative grass seed rain was extensive but was reduced by an order of magnitude with grass removal. This study suggests that natural regeneration may enhance restoration actions in sites with native canopy, but this is likely only when restoration activities have been maintained for several years to coincide with favorable rainfall conditions that are highly unpredictable over time

Ecohydrologic Changes Caused by Hydrologic Disconnection of Ephemeral Stream Channels in Mojave National Preserve, California

Emplacement of highways and railroads has altered natural hydrologic systems by influencing surface-water flow paths and biotic communities in Mojave National Preserve. Infiltration experiments were conducted along active and abandoned channels to evaluate changes in hydrology and related effects on plant water availability and use. Simulated rainfall infiltration experiments with vegetation monitoring were conducted along an active channel upslope and a comparable abandoned channel downslope of the transportation corridor. We also conducted 90 single-ring, ponded infiltration experiments in adjacent channels to evaluate field-saturated hydraulic conductivity and particle size distributions. The abandoned channels are still morphologically evident but are disconnected from runoff sources at higher elevations. Infiltration test results show that water infiltrates twice as fast in the active channels. Excavation showed weak soil development with fewer plant roots beneath the abandoned channel. Scanning electron microscopy of surface samples showed the presence of cyanobacteria only in abandoned channels. Plants up to 3 m away from both channels showed physiological responses to channel water applied in a simulated pulse of rain. The response was short lived and less pronounced for plants adjacent to the abandoned channel, whereas those adjacent to the active channel showed responses up to 2 mo after the pulse. These responses may explain observed lower plant densities and fewer deep-rooted species along abandoned channels compared with active channels. We infer that the deeper rooting plants are more abundant where they are able to take advantage of the increased soil-water storage resulting from greater infiltration and flow frequency in active stream channels

Soil microbial responses to nitrogen addition in arid ecosystems

The N cycle of arid ecosystems is influenced by low soil organic matter, high soil pH and extremes in water potential and temperature that lead to open canopies and development of biological soil crusts (biocrusts). We investigated the effects of N amendment on soil microbial dynamics in a Larrea tridentata-Ambrosia dumosa shrubland site in southern Nevada USA. Sites were fertilized with a NO3-NH4 mix at 0, 7, and 15 kg ha-1 yr-1 from March 2012 to March 2013. In March 2013, biocrust (0-0.5 cm) and bulk soils (0-10 cm) were collected beneath Ambrosia canopies and in the interspaces between plants. Biomass responses were assessed as bacterial and fungal SSU rRNA gene copy number and chlorophyll a concentration. Metabolic responses were measured by five ecoenzyme activities (EEA) and rates of N transformation. By most measures, nutrient availability, microbial biomass and process rates were greater in soils beneath the shrub canopy compared to the interspace between plants, and greater in the surface biocrust horizon compared to the deeper 10 cm soil profile. Most measures responded positively to experimental N addition. Effect sizes were generally greater for bulk soil than biocrust. Results were incorporated into a meta-analysis of arid ecosystem responses to N

Precipitation pulses and carbon fluxes in semi-arid ecosystems

In the arid and semiarid regions of North America, discrete precipitation pulses are important triggers for biological activity. The timing and magnitude of these pulses may differentially affect the activity of plants and microbes, combining to influence the C balance of desert ecosystems. Here, we evaluate how a pulse of water influences physiological activity in plants, soils and ecosystems, and how characteristics, such as precipitation pulse size and frequency are important controllers of biological and physical processes in arid land ecosystems. We show that pulse size regulates C balance by determining the temporal duration of activity for different components of the biota. Microbial respiration responds to very small events, but the relationship between pulse size and duration of activity likely saturates at moderate event sizes. Photosynthetic activity of vascular plants generally increases following relatively larger pulses or a series of small pulses. In this case, the duration of physiological activity is an increasing function of pulse size up to events that are infrequent in these hydroclimatological regions. This differential responsiveness of photosynthesis and respiration results in arid ecosystems acting as immediate C sources to the atmosphere following rainfall, with subsequent periods of C accumulation should pulse size be sufficient to initiate vascular plant activity. Using the average pulse size distributions in the North American deserts, a simple modeling exercise shows that net ecosystem exchange of CO2 is sensitive to changes in the event size distribution representative of wet and dry years. An important regulator of the pulse response is initial soil and canopy conditions and the physical structuring of bare soil and beneath canopy patches on the landscape. Initial condition influences responses to pulses of varying magnitude, while bare soil/beneath canopy patches interact to introduce nonlinearity in the relationship between pulse size and soil water response. Building on this conceptual framework and developing a greater understanding of the complexities of these eco-hydrologic systems may enhance our ability to describe the ecology of desert ecosystems and their sensitivity to global change