Theme for 2018:
Paleoclimate Insights for Planning Future Natural Resources in California

Historical Perspective on California Dendrohydrology

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Abstract:

Water resources development in California has long drawn on tree-ring studies, beginning in the early 20th century to address concern about the reliability of imported water supply from the Colorado River Basin. When advancements in computers and statistical methods enabled assimilation of large data sets in dendrohydrology, quantitative basin-scale reconstructions for California rivers soon followed. A rich variety of California tree species with great age and moisture sensitivity has enabled accurate reconstructions of full natural flow for the Sacramento River, San Joaquin River, as well as for rivers in the southern Sierra Nevada and along the southern California coast. Streamflow reconstruction studies sponsored by the California Department of Water Resources since the 1980s highlight the value of the long view to water resources planning in California, and contribute to an understanding of modes of climate variability on the larger spatial scale. Historical aspects of California dendrohydrology are discussed in the framework of the water balance.

How Unusual Was the 2012-2014 California Drought?

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From 2012 through 2016, California experienced the most severe drought conditions in more than a century. But how unusual was this event and how did precipitation and temperature anomalies each contribute to the characteristics of this event? Here we use paleoclimate reconstructions of drought and precipitation for Central and Southern California to place the drought in the context of the last millennium of hydroclimate in the region.

Updated tree ring chronologies reveal that precipitation during the drought was indeed anomalously low but not outside the range of natural variability reconstructed for prior centuries. The drought was however exceptionally severe due to the additional influence of record high temperatures — a 'hot drought'. We use our tree-ring reconstructions to examine the frequency and return interval of short-term severe droughts as well as the 'whiplash' transitions between short droughts and exceptionally wet years, as occurred at the end of the 2012 to 2016 event.

Reconstructing Proxies of Sierra Nevada Snowpack

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Tree rings are increasingly being used to reconstruct historic snowpack fluctuations and put recent changes into a long term context. Here I present the Western Cordilleran Snow Atlas, a 4x4 km resolution gridded snowpack product that spans the last 2000 years within California. Methods used to develop the dataset as well as some potential uses will be described.

Snowpack Reconstruction for the American River Watershed

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Abstract:

Snowpack in the Sierra Nevada Mountains accounts for around one third of California's water supply. Melting snow provides water into dry summer months characteristic of the region's Mediterranean climate. As climate changes, understanding patterns of snowpack, snowmelt, and biological response is critical in this region of agricultural, recreational, and ecological value. Tree rings can be used as proxy records to inform scientists and resource managers of past climate variability where instrumental data are unavailable.

Here we investigate relationships between tree rings of high-elevation, snow-adapted conifer trees (Tsuga mertensiana, Abies magnifica, Abies concolor, Calocedrus decurrens, Juniperus occidentalis, and Pinus ponderosa) and regional climate indices with the goal of reconstructing April 1st snow-water equivalent (SWE) in the North Fork American River watershed of the Sierra Nevada Mountains.

Chronologies are significantly positively correlated with April 1 SWE of the year prior to ring formation. Tsuga mertensiana ring growth is correlated negatively with April 1 SWE of the year of ring formation. Additionally, temporal trends in correlation between tree-ring chronologies and climate indices indicate strengthening tree-growth response to climate over time. We developed a skillful, nested reconstruction for April 1 SWE, 1661-2013. Variability of the reconstruction is within the envelope of 20th and 21st century variability; however, the 2015 record low snowpack is unprecedented in the tree-ring record, as in results from previous studies. Future research should focus on integrating modern snow-sensor data into paleoclimate research and determining mechanistic linkages between climate and tree growth response.

Red Cones of the John Muir Trail: An Ideal Field Site for Dendro Reconstruction of Sierra Snowpack

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Abstract:

At about mile 60 of the John Muir Trail, two young (<8500 years old) cinder cones exist amidst the expanse of Sierra Nevada granite. Dubbed Red Cones, they support tree growth and constitute an intriguing opportunity for dendroclimatology in that they conform to site selection theory that underlies tree-ring science generally. Trees occupy different aspects and hillslope positions of both cones, so tree selection theory is also in consideration. Trees growing on the Red Cones might be uniquely sensitive to annual variability in snowpack and therefore contribute to prior dendro reconstructions of that all-important feature of California hydro-climate.

Recent fieldwork there resulted in a small collection of tree cores to ascertain the dendroclimatic potential of trees of Red Cones. Statistical modeling has yet to be done, but basic chronology stats can already be evaluated to ascertain the utility of this site for adding to dendroclimatology of precipitation of California.

Tree Radial Growth Dependence on the Spatial Variability of Snowpack and Soil Moisture

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Spatial variability of moisture regime in mountain ranges dominated by seasonal snowpack has been often overlooked in dendrohydrology discipline. In this study, we assessed whether tree rings can inform us on the basin scale spatial variability of the snow pack and soil water content. We examined tree-ring radial-growth annual indices (i.e., earlywood and latewood ring width and latewood density) of five conifer species in five sites, within a 60 km radius, at the American River watershed and its vicinity. For each sampling site, a spatially congruent high-resolution land surface model was implemented to simulate 6-hour time series of snow, soil water content, and actual evapotranspiration for 1960- 2016. These simulations were then used to represent key seasonal features that were thought to be important for the growth of the trees. These indices include for example the duration of the dormancy season (winter), the duration of the growth season (spring), the duration of the dry season (summer), and the available seasonal soil moisture at the root zone.

An analysis of these indices with respect to the tree chronologies revealed that although different sites responded differently to these indices, all the sites were relatively insensitive to the winter temperature. Initial results suggest that warming condition and early spring onset as during the recent (2012-2015) drought increase growth in the high elevation that had a short winter with ample moisture while suppressing growth in lower elevation that experiences long dry summers. It is also interesting to note that the growth at the high elevation sites was found to be associated with the available moisture from the previous year, while in lower elevations growth responded to moisture conditions of the current year.

Climate Signal in Giant Sequoia Tree Rings

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A network of 23 well-replicated ring-width chronologies of Sequoiadendron giganteum was investigated for climate signal with precipitation, temperature, snow water equivalent (SWE), California Division-5 Palmer Drought Severity Index (PDSI), and gridded reconstructed June-August PDSI from the North American Drought Atlas (NADA). Pearson correlations were used to gauge the strength of relationship of chronologies with the various types of climate data. Our results show significant positive correlation of tree-growth with precipitation in October-April and negative correlation with precipitation in September preceding the growth year. Tree-ring chronologies are negatively correlated with temperature in April and June of the growth year, and positively correlated with annual SWE. A significant positive correlation was found between the tree-ring chronologies and NADA reconstructed PDSI over a broad part of the West, with highest correlations centered over the sequoia region.

Panel Discussion: California Hydro Climate from Tree Rings

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Using Tree Rings to Inform Water Resource Management

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Water resource planning is often based on records of past precipitation and streamflow. These records, most of which extend 100 years or less, document extreme droughts over the period of record. However, are these droughts representative of the longer-term past and a good indication of what we can expect in the future? Tree-ring records document droughts over past centuries to millennia. Thus, they can be used to address this question, placing recent drought events into a long-term context.

This talk will describe some of the information that tree-ring reconstructions of precipitation and streamflow provide about past droughts, as well as some examples of how this information is being used in water resource management in the western US. Although the climate of the past will not be replicated in the future, natural hydroclimatic variability will continue, underlying warming trends. Consequently, an understanding the range of nature variability that is possible can be useful information for water resource planning.

Assessing Drought Risk in California

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The paleoclimate record provides a tool for qualitatively or quantitatively assessing drought risk based on understanding drought duration and magnitude prior to California's relatively short period of measured record. Reconstructed paleoclimate records allow quantification of key metrics such as the number of droughts of specified duration over a period of many centuries, helping water agencies understand the costs and trade-offs between water supply reliability and shortage risk. Previous state law for urban water management planning required specified water suppliers to plan for three-year droughts; that requirement was extended to a five-year drought duration by 2018 legislation. California's longest droughts in relatively recent times were the six-year 1987-92 event, and the five-year 2012-16 event. Work performed for the Department of Water Resources by the University of Arizona has provided data for northern and southern California showing how these recent events compare to a longer paleorecord.

Drought reduces water supply reliability, potentially redefining areas that have had adequate water supplies under normal hydrologic conditions as areas of shortage under dry hydrology. The ability of water users to reduce the risk of shortage, or to minimize impacts if a shortage occurs, depends on the value of water to them and their ability to afford a desired level of reliability. Large urban areas typically demand a high level of reliability and have the financial capability to ensure it. It is typically not financially feasible for farming businesses served by agricultural water agencies to make the same level of investment in reliability as is done by urban agencies, and customers of agricultural agencies thus typically must expect a greater risk of shortage. Understanding the big picture of climate risk provides context for making risk management decisions.

Using Paleo-Reconstructions for Vulnerability Assessments and Adaptation Planning for State Water Project (Sacramento & San Joaquin Watersheds)

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Using the hydroclimatic reconstructions from tree rings of the Sacramento and San Joaquin River watersheds (Meko et al., 2014), the California Department of Water Resources developed a process for constructing a paleo-climate analogue timeseries of gridded temperature and precipitation over the Sacramento and San Joaquin River watershed areas. The paleo-climate timeseries reflects the long-term inter-annual variability of the reconstructed streamflow record but adds the spatial (6 km x 6 km resolution) and temporal (daily) detail of observational data. The paleo-climate timeseries was then perturbed to reflect potential climate changes and used to evaluate vulnerabilities to the State Water Project from climate change. The 1,100-year paleo-climate timeseries adds significant additional periods of extreme hydro-climatic stress to what has traditionally been used for water system vulnerability assessment.

On the Use of Lake Sediments for a Deeper Time (Holocene) Understanding of California's Drought, Flood, and Pluvial History

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Global warming is changing Earth's climate. While California will certainly warm (Cayan et al., 2008), it remains uncertain whether precipitation amounts will increase or decrease. Recent studies, however, suggest an increase in the extremes of precipitation (wetness and dryness) is the most likely scenario for CA (Swain et al., 2018). Critical to planning for, and mitigating against, future risks from changes in precipitation requires a geological perspective, longer than our instrumental (<150 years) or tree ring records (<1000-2000 years). Here, we present various lake sediment records from coastal California used to reconstruct the history of droughts, pluvials (periods of above average wetness), and floods back through the entire Holocene (past 11,700 years). These deeper time lake studies will provide a more complete understanding of the amplitude of natural precipitation variability. These studies also allow the identification and examination of past climatic shifts and states, including their climatic forcings. Our results indicate drought, pluvial, and flood conditions of longer duration (and magnitude?) than anything observed in the past 2000 years.

Most recently, our efforts are focused on a deeper time understanding of the California precipitation dipole (Wise, 2010, 2016). The position and strength of this dipole dictates where precipitation occurs in California and how much is received. "Where and how much" are critically important questions for the water distribution network and policies that irrigate California. How the position and strength of the dipole changed during past dry and wet climate states will inform future water management scenarios.

Paleo Applications in Forest Fire Management

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The loss of fire as a keystone process in many plant communities in Sequoia and Kings Canyon National Parks has resulted in changes that have cascaded through the ecosystem after over 100 years of fire exclusion. The parks have a goal of returning fire as a dynamic process to improve ecosystem stability and resilience. While fire's role is being reintroduced, specific restoration objectives have remained elusive because of inadequate baseline information about fire regimes within varied vegetation types that occur across a topographically complex landscape. Objectives can be enhanced by improving our understanding of ecosystem dynamics, such as processes that operated prior to fire exclusion, or species life history traits. Incorporating these factors improves our ability to develop and implement fire management strategies suited to current conditions and helps determine what new objectives may be needed to meet future challenges.

For example, the parks use a GIS based "ecological needs" model based on estimates of fire return interval departures (FRID) derived from tree-ring fire history studies to integrate this information and apply it spatially across the parks. This assists management planning by both highlighting areas where the restoration of fire may be a high or low priority.

Additionally, there is a need to put the large high severity fires that occurred in the Tuolumne and Kings drainages in 2013 and 2015, during extreme drought years, in a historical context. This is being done by reconstructing extent of past fires from a landscape level network of fire history sites. By improving our understanding of historic variation of large fires through time, particularly in relation to the interactions between topography and climate, we will be better able to judge whether future changes in fire or fire effects are within or outside an expected range of variation and potentially whether these are a result of a changing climate or other factors.

Panel Discussion: Paleoclimatology and Natural Resources Planning and Management

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