Peter Hartsough

A 14.6 kyr record of nitrogen flux from desert soil profiles as inferred from vadose zone pore waters

A 14.6 kyr chronology of infiltration developed from deep vadose (unsaturated) zone cores in southern Nevada is presented to assess the chronology of nitrogen loss from the active rooting zone. Soil water chemistry in the deep vadose zone is first used to develop a chronology of paleohydrology and subsequently nitrogen fluxes. While elevated nitrogen concentrations (as NO3) are commonly found in desert vadose zones, NO3/Cl ratios indicate that nitrate behaved conservatively beneath the active rooting zone. Mean nitrogen fluxes from the active root zone range from 103 to 108 mg/m²/yr and appear relatively constant over time, in spite of dramatic climate and vegetation changes. The long-term loss of nitrogen from the active rooting zone implies that nitrogen cycling processes have been relatively constant since the Pleistocene/Holocene transition, that the biological community may not be primarily limited by the availability of nitrogen, and that the loss of nitrogen into the vadose zone should be considered in desert ecosystem productivity studies.

Daily Weather and Tree Growth at the Tropical Treeline of North America

Abstract We present here the 2001–2004 results of observational field studies aimed at quantifying tropical timberline climate and radial increment of Pinus hartwegii Lindl. trees on Nevado de Colima, in the middle of the North American Monsoon region. An automated weather station was installed at 3760 m a.s.l., 19°34.778′N latitude, 103°37.180′W longitude, within a forest where multi-century tree-ring records had been previously developed. At the same time, automated electronic sensors for recording tree growth at 30-min intervals were set up at two sites within a 1-km radius from the weather station. Meteorological observations recorded every 30 min were summarized on a daily basis. Time-series patterns are reported for atmospheric pressure, precipitation, incoming solar radiation, air and soil temperature, relative humidity, soil moisture, and wind speed and direction. Of particular interest is the sudden decrease in air temperature after the onset of the monsoon season, which determines very high relative humidity over the summer and results in air temperature having a spring maximum. Despite sub-freezing air temperatures in most months, soil temperatures never drop below 0°C. Dendrometer data show that the timberline growing season begins in March–April as temperature increases, then radial growth continues throughout the cool-wet summer monsoon, and ends in October–November. As an unexpected result, it was also possible to measure the progressive decline of Pinus hartwegii stem increment in response to an outbreak of roundheaded pine beetle (Dendroctonus adjunctus Blandford), which ultimately killed most trees at one of our two experimental sites.

Using Automated Point Dendrometers to Analyze Tropical Treeline Stem Growth at Nevado de Colima, Mexico

The relationship between wood growth and environmental variability at the tropical treeline of North America was investigated using automated, solar-powered sensors (a meteorological station and two dendrometer clusters) installed on Nevado de Colima, Mexico (19° 35′ N, 103° 37′ W, 3,760 m a.s.l.). Pure stands of Pinus hartwegii Lindl. (Mexican mountain pine) were targeted because of their suitability for tree-ring analysis in low-latitude, high-elevation, North American Monsoon environments. Stem size and hydroclimatic variables recorded at half-hour intervals were summarized on a daily timescale. Power outages, insect outbreaks, and sensor failures limited the analysis to non-consecutive months during 2001–2003 at one dendrometer site, and during 2002–2005 at the other. Combined data from the two sites showed that maximum radial growth rates occur in late spring (May), as soil temperature increases, and incoming short-wave radiation reaches its highest values. Early season (April–May) radial increment correlated directly with temperature, especially of the soil, and with solar radiation. Stem expansion at the start of the summer monsoon (June–July) was mostly influenced by moisture, and revealed a drought signal, while late season relationships were more varied.

Reconstructing relative humidity from plant δ18O and δD as deuterium deviations from the global meteoric water line

Cellulose delta18O and deltaD can provide insights on climates and hydrological cycling in the distant past and how these factors differ spatially. However, most studies of plant cellulose have used only one isotope, most commonly delta18O, resulting in difficulties partitioning variation in delta18O of precipitation vs. evaporative conditions that affect leaf water isotopic enrichment. Moreover, observations of pronounced diurnal differences from conventional steady-state model predictions of leaf water isotopic fractionation have cast some doubt on single isotope modeling approaches for separating precipitation and evaporation drivers of cellulose delta18O or deltaD. We explore a dual isotope approach akin to the concept of deuterium-excess (d), to establish deuterium deviations from the global meteoric water line in leaf water (deltad(l)) as driven by relative humidity (RH). To demonstrate this concept, we survey studies of leaf water delta18O and deltaD in hardwood vs. conifer trees. We then apply the concept to cellulose delta18O and deltaD using a mechanistic model of cellulose delta18O and deltaD to reconstruct deuterium deviations from the global meteoric water line (deltad(c)) in Quercus macrocarpa, Q. robur, and Pseudotsuga menziesii. For each species, deltad(c) showed strong correlations with RH across sites. deltad(c) agreed well with steady-state predictions for Q. macrocarpa, while for Q. robur, the relationship with RH was steeper than expected. The slope of deltad(c) vs. RH of P. menziesii was also close to steady-state predictions, but deltad(c) were more enriched than predicted. This is in agreement with our leaf water survey showing conifer deltad(l) was more enriched than predicted. Our data reveal that applications of this method should be appropriate for reconstructing RH from cellulose delta18O and deltaD after accounting for differences between hardwoods and conifers. Hence, deltad(c) should be useful for understanding variability in RH associated with past climatic cycles, across regional climates, or across complex terrain where climate modeling is challenging. Furthermore, deltad(c) and inferred RH values should help in constraining variation in source water delta18O.

Dendroclimatology from Regional to Continental Scales: Understanding Regional Processes to Reconstruct Large-Scale Climatic Variations Across the Western Americas

Common patterns of climatic variability across the Western Americas are modulated by tropical and extra-tropical oscillatory modes operating at different temporal scales. Interannual climatic variations in the tropics and subtropics of the Western Americas are largely regulated by El Nino-Southern Oscillation (ENSO), whereas decadal-scale variations are induced by long-term Pacific modes of climate variability such as the Pacific Decadal Oscillation (PDO). At higher latitudes, climate variations are dominated by oscillations in the Annular Modes (the Arctic and Antarctic Oscillations) which show both interannual and longer-scale temporal oscillations. Here we use a recently-developed network of tree-ring chronologies to document past climatic variations along the length of the Western Cordilleras. The local and regional characterization of the relationships between climate and tree-growth provide the basis to compare climatic variations in temperature- and precipitation-sensitive records in the Western Americas over the past 3–4 centuries. Upper-elevation records from tree-ring sites in the Gulf of Alaska and Patagonia reveal the occurrence of concurrent decade-scale oscillations in temperature during the last 400 years modulated by PDO. The most recent fluctuation from the cold- to the warm-phase of the PDO in the mid 1970s induced marked changes in tree growth in most extratropical temperature-sensitive chronologies in the Western Cordilleras of both Hemispheres. Common patterns of interannual variations in tree-ring chronologies from the relatively-dry subtropics in western North and South America are largely modulated by ENSO. We used an independent reconstruction of Nino-3 sea surface temperature (SST) to document relationships to tree growth in the southwestern US, the Bolivian Altiplano and Central Chile and also to show strong correlations between these regions. These results further document the strong influence of SSTs in the tropical Pacific as a common forcing of precipitation variations in the subtropical Western America during the past 3–4 centuries. Common patterns of interdecadal or longer-scale variability in tree-ring chronologies from the subarctic and subantarctic regions also suggest common forcings for the annular modes of high-latitude climate variability. A clear separation of the relative influence of tropical versus high-latitude modes of variability is currently difficult to establish: discriminating between tropical and extra-tropical influences on tree growth still remains elusive, particularly in subtropical and temperate regions along our transect. We still need independent reconstructions of tropical and polar modes of climate variability to gain insight into past forcing interactions and the combined effect on climates of the Western Americas. Finally, we also include a series of brief examples (as ‘boxes’) illustrating some of the major regional developments in dendrochronology over this global transect in the last 10 years.

Stable Isotope Characterization of the Ecohydrological Cycle at a Tropical Treeline Site

ABSTRACT We investigated the seasonal variation in pools of water available to mature trees growing at high elevation in a tropical environment. The study focused on the dominant tree species (Pinus hartwegii) at about 3800 m a.s.l. on Nevado de Colima, Mexico, where climate is typical of the North American Monsoon System. Stable isotope ratios of hydrogen and oxygen in water extracted from soil, xylem, and leaves were measured through a cycle of two dry and two wet seasons in 2003–2004. Isotopic ratios were also measured in accumulated precipitation, a few single precipitation events, and in spring water over the two-year period. Based on evidence from water, stable isotopes in soil, and xylem samples, trees utilized water from relatively shallow soil depths, which are representative of current conditions, rather than tapping groundwater, which is more representative of long-term trends. While the stable isotope signature in environmental waters showed a slightly different pattern before and during the monsoon, the more pronounced differences in leaf water isotopes between the two seasons, due to drought stress, will lead to a clear seasonal isotopic signal in tree ring cellulose. This study represents a unique snapshot of water cycling in a tropical treeline ecosystem, where our understanding of eco-hydrological pathways is limited. This type of analysis is also useful for proper calibration of stable isotopic signals in tree ring records.

Mechanisms controlling the impact of multi-year drought on mountain hydrology

Mountain runoff ultimately reflects the difference between precipitation (P) and evapotranspiration (ET), as modulated by biogeophysical mechanisms that intensify or alleviate drought impacts. These modulating mechanisms are seldom measured and not fully understood. The impact of the warm 2012–15 California drought on the heavily instrumented Kings River basin provides an extraordinary opportunity to enumerate four mechanisms that controlled the impact of drought on mountain hydrology. Two mechanisms intensified the impact: (i) evaporative processes have first access to local precipitation, which decreased the fractional allocation of P to runoff in 2012–15 and reduced P-ET by 30% relative to previous years, and (ii) 2012–15 was 1 °C warmer than the previous decade, which increased ET relative to previous years and reduced P-ET by 5%. The other two mechanisms alleviated the impact: (iii) spatial heterogeneity and the continuing supply of runoff from higher elevations increased 2012–15 P-ET by 10% relative to that expected for a homogenous basin, and iv) drought-associated dieback and wildfire thinned the forest and decreased ET, which increased 2016 P-ET by 15%. These mechanisms are all important and may offset each other; analyses that neglect one or more will over or underestimate the impact of drought and warming on mountain runoff.

Diurnal variations of needle water isotopic ratios in two pine species

Diurnal fluctuations of leaf water isotope ratios (δ18O and δD) were measured for Jeffrey (Pinus jeffreyi Balf.) and lodgepole (Pinus contorta Douglas ex Louden) pine. Two trees per species were sampled every few hours on 15–16 October 2005 and 19–20 June 2006. Diurnal gas exchange was measured during the summer sampling. In fall 2005, leaf water δ18O ranged from 0.7 to 9.0‰, and leaf water δD ranged from −70 to −50‰. In summer 2006, leaf water δ18O ranged from 7.7 to 20.7‰, and leaf water δD ranged from −61 to −24‰. Diurnal variation of leaf water isotope values typically reached a maximum in early afternoon, began decreasing around midnight, and reached a minimum in mid-morning. Both periods showed a high degree of enrichment relative to source water, with leaf water–source water enrichments ranging up to 37.8‰ for δ18O, and up to 95‰ for δD. Leaf water enrichment varied by season with summer enrichment being greater than fall enrichment. A steady-state model (i.e., modified Craig–Gordon modeling) for leaf water isotope compositions did not provide a good fit to measured values of leaf water. In summer, a non-steady state model provided a better fit to the measured data than the steady-state model. Our findings demonstrate substantial leaf water enrichment above source water and diurnal variations in the isotopic composition of leaf water, which has application to understanding short-term variability of atmospheric gases (water vapor, CO2, O2), climate studies based on the isotopic composition of tree rings, and ecosystem water fluxes.

Recent warming at the tropical treeline of North America

Tropical treelines are critical zones for observing and understanding regional responses to climatic change(Diaz et al. 2003), especially because the low latitudes play a prominent role in the global climate system(Hoerling and Kumar 2003), andmountain areas regulate downstream availability of water resources (Brad-ley et al. 2004). In North America, tropical treelines are also part of the North American Monsoon System(NAMS); this system’s control over summer precipitation, thunderstorm activity, and lightning patterns in the southwestern US extends to other regions via atmospheric connections (Vera et al. 2006; Dominguez et al.2009). Few weather stations with uninterrupted data series exist above 3000 m in rural areas throughout theentire American Cordillera, from Alaska to Tierra del Fuego (Bradleyet al. 2004), making it difficult to testhypotheses, calibrate models, and detect landscape feedbacks to human activities.

Real-time Alpine Measurement System Using Wireless Sensor Networks

Monitoring the snow pack is crucial for many stakeholders, whether for hydro-power optimization, water management or flood control. Traditional forecasting relies on regression methods, which often results in snow melt runoff predictions of low accuracy in non-average years. Existing ground-based real-time measurement systems do not cover enough physiographic variability and are mostly installed at low elevations. We present the hardware and software design of a state-of-the-art distributed Wireless Sensor Network (WSN)-based autonomous measurement system with real-time remote data transmission that gathers data of snow depth, air temperature, air relative humidity, soil moisture, soil temperature, and solar radiation in physiographically representative locations. Elevation, aspect, slope and vegetation are used to select network locations, and distribute sensors throughout a given network location, since they govern snow pack variability at various scales. Three WSNs were installed in the Sierra Nevada of Northern California throughout the North Fork of the Feather River, upstream of the Oroville dam and multiple powerhouses along the river. The WSNs gathered hydrologic variables and network health statistics throughout the 2017 water year, one of northern Sierra’s wettest years on record. These networks leverage an ultra-low-power wireless technology to interconnect their components and offer recovery features, resilience to data loss due to weather and wildlife disturbances and real-time topological visualizations of the network health. Data show considerable spatial variability of snow depth, even within a 1 km2 network location. Combined with existing systems, these WSNs can better detect precipitation timing and phase in, monitor sub-daily dynamics of infiltration and surface runoff during precipitation or snow melt, and inform hydro power managers about actual ablation and end-of-season date across the landscape.