Dry mixed conifer forests of southwestern North America are projected to be particularly vulnerable to ongoing persistent warm drought conditions, and related increases in wildfire frequency, size and severity, due in part to consequences of over a century of fire exclusion. Prescribed fire is applied actively in many landscapes to reduce hazardous fuel loads and continuity, restore forest community composition and structure, and increase tree resilience to drought stress. However, fire can also adversely affect tree growth by damaging cambial, root, and canopy tissues, leading to tradeoffs in the use of fire as a tool for forest resilience. Radial growth is an indicator of climatic and ecological stress and can thus provide a relative measure of resilience to stress and disturbances; but, the mechanisms driving tree resilience to prescribed fire and concurrent drought are poorly understood. Thinning effects of prescribed fire may increase tree resilience to drought by increasing water, light and nutrient availability and production of defense mechanisms. However, trends over the last century indicate warming temperatures are increasing tree sensitivity to fire by reducing post-fire growth (lower resilience) and increasing the likelihood of mortality. Trees can be resistant to fire exposure, and where growth changes occur they can be transient or persistent. We studied the interactions between tree- and stand-level fire effects on the growth responses of surviving Abies concolor, Pinus jefferyi, Pinus ponderosa, and Pseudotsuga menziesii over 24 years of variable climatic conditions in ten National Parks across the western and southwest United States. We used linear mixed effects models to identify mechanisms influencing resistance and resilience responses to fire and interannual climate, using climatic water deficit (CWD) as an index of climatic stress. Compared to pre-fire growth, trees exposed to fire increased growth during periods of greater water deficits. Tree growth responses were variable among and within species and size classes, but contingent on time-since-fire and the climate during the recovery period. Negative fire effects on tree resistance were generally transient, while climate and pre-existing stand conditions were persistent controls on tree resilience. These results suggest that antecedent and subsequent climate conditions modulate post-fire forest response. Consideration of climate variation could improve the strategic use of prescribed fire for tree resilience to drought, and a deeper understanding of factors contributing to prefire growth may elucidate the mechanisms driving post-fire growth responses.

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This research examines the influence of western dwarf mistletoe (Arceuthobium campylopodum) infection on the radial growth response of mature ponderosa pines (Pinus ponderosa) and its effects on dendroclimatic reconstructions. I hypothesize that trees with mistletoe have lower annual growth rates than uninfected trees, but exhibit higher mean sensitivities and stronger relationships between growth and climate variation. I tested these hypotheses using correlation and regression analyses to compare 100-year crossdated and standardized tree-ring chronologies from 26 infected and 29 uninfected trees. I compared both chronologies to climate variation as measured by changes in total precipitation, minimum, mean, and maximum temperature, and the Palmer Drought Severity Index (PDSI). Results show that trees infected with dwarf mistletoe have higher radial growth rates, exhibit greater sensitivity, and respond more strongly to climate variation. Both infected and uninfected chronologies are significantly correlated with the respective climate variables, but exhibit different patterns. The strongest correlations are between infected trees and PDSI for all months tested; significant correlations between uninfected trees and PDSI are limited to May through December lagged from the previous year. These results suggest mistletoe-infected trees are more sensitive to climatic factors than uninfected trees and may be useful for dendroclimatic analyses.

Identification of effects that climate teleconnections, such as El Nino-Southern Oscillation (ENSO), Pacific Decadal Oscillation (PDO), and North Atlantic Oscillation (NAO), have on wildfires is difficult because of short and incomplete records in many areas of the world. We developed the first multicentury wildfire chronologies for northeast China from fire-scarred trees. Regional wildfires occurred every 7years from the 1700s to 1947, after which fire suppression policies were implemented. Regional wildfires occurred predominately during drought years and were associated with positive phases of ENSO and PDO and negative NAO. Twentieth century meteorological records show that this contingent combination of +ENSO/+PDO/-NAO is linked to low humidity, low precipitation, and high temperature during or before late spring fire seasons. Climate and wildfires in northeast China may be predictable based on teleconnection phases, although future wildfires may be more severe due to effects of climate change and the legacy of fire suppression.

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Pinus palustris Miller (longleaf pine), in Virginia, is at the northernmost extent of its range. During presettlement times, this species occurred throughout the Piedmont and Coastal Plain of Virginia in pure and mixed stands, covering 607,000 hectares. This forest type has since been reduced to 81 hectares or 0.01% of its former range. Around 5,000 individual Pinus palustris remain on six sites in the coastal plains. Seacock Swamp and Everwoods are both sites known to have naturally regenerated Pinus palustris native to Virginia occurring in mixed-species stands. At both sites, I measured height and diameter of all Pinus palustris and cored individuals greater than 10 cm in diameter at breast height. A total of 71 trees were cored; the cores were crossdated and measured and crossdating was verified with the COFECHA program. A strong competition signal within the tree ring records at both sites signified the importance of stand dynamics on Pinus palustris in second-growth loblolly pine stands. These results are probably due to the mix of species within these stands and competition from loblolly pine as both the dominant understory and overstory species. Using Black and Abrams (2003) boundary line method, we calculated release and suppression events from the tree-ring record over the last century and found a very dynamic system. During the 1950s and 1960s, Seacock Swamp experienced major and moderate releases (23% moderate release and 18% major release in the 1950s and 33% moderate release and 49% major release in the 1960s) in response to a diameter-limit cut in 1953. Other major and moderate releases varied at both sites and may be attributed to different forest management practices that were in place throughout the last century however locating historical land use records to validate this was not possible at the present. / Master of Science

Dendroecological methods play a critical role in developing our understanding of forest processes by contributing historical evidence of climate variability and the temporal characteristics of disturbance. We seek to contribute to these methods by developing a research protocol for decoupling radial-growth signatures related to climate, fire, and insect outbreaks in central Oregon. Our methods are based on three independent, crossdated tree-ring data sets: 1) a 545-year tree-ring climate reconstruction, 2) a 550-year fire history, and 3) a 250-year pandora moth outbreak history derived from host (Pinus ponderosa) and non-host (Abies grandis-Abies concolor) tree-ring chronologies. Based on these data, we use visual criteria (marker and signature rings), statistical comparisons, and Superposed Epoch Analysis (SEA) to identify the timing of growth anomalies and establish the temporal relationships between drought, climate variation (ENSO and PDO), fire events, and pandora moth (Coloradia pandora) outbreaks. Our results show pandora moth outbreaks generally coincide with periods of below-average moisture, whereas fire in central Oregon often follows a period of wetter than average conditions. Fire events in central Oregon appear to be related to shifts in hemispheric climate variability but the relationship between fire and pandora moth outbreaks remains unclear.

Secondary growth responses of native and nonnative trees exposed to the same climatic conditions can elucidate sensitivities and thus adaptability to a particular region. A long-term mixed-species planting in the pine barrens of northern lower Michigan presented an opportunity to discriminate responses from species commonly planted in this region. Mean ring-width chronologies from living native Pinus resinosa Ait. and P. strobus L. and nonnative P. sylvestris L. and Picea abies (L.) Karst. at this plantation were generated, standardized, and analyzed by correlation analysis against mean monthly climatic variables. The native pine chronologies had the highest mean ring widths and signal-to-noise ratios (SNR), were highly correlated to each other, and exhibited positive responses to years with above-normal April temperatures but no significant relationships to variations in precipitation. The P. sylvestris chronology was highly correlated to the other two pine chronologies and responded similarly to April temperatures but exhibited negative correlations to January and April precipitation and positive correlations to September precipitation. The P. abies chronology had the highest mean sensitivity and was correlated with the P. strobus chronology but only responded positively to precipitation from the previous December. The low SNR (P. sylvestris, P. abies), high mean sensitivity (P. abies), and larger number of significant correlations to variations in monthly climatic variables (P. sylvestris) suggest that these nonnative species are more sensitive to this local climate. These results provide insights to the adaptability, establishment, and geographic distribution of the nonnative Pinaceae.

Coloradia pandora (Blake) is a phytophagous insect that defoliates Pinus ponderosa (Dougl. ex Laws.) in south-central Oregon. Little is known about the extent of damage this insect inflicts upon its host trees during an outbreak. In this paper, we present stem analyses on four dominant Pinus ponderosa trees that enable us to determine the amount of volume lost during each Coloradia pandora outbreak on this site for the past 450 years. We found that on average an outbreak inhibits radial growth so that an individual tree produces 0.057 m³ less wood volume than the potential growth for the duration of an individual outbreak. A total of 0.549 m³ of growth per tree was inhibited by 10 outbreaks during the lifetime of the trees, which, in this stand, equates to 9.912 m³/ha (1,700 board feet/acre) of wood suppressed over the last 450 years throughout the stand. Our results do not support previous findings of a lag in suppression onset between the canopy of the tree versus the base. Crossdating of stem analysis samples is paramount to definitively examine the potential for a lagged response throughout the

No description available.