The University of Arizona Campus Repository: Recent submissions
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Utility of Herbaceous Annual Rings as Markers of Plant Response to Disturbance: A Case Study Using Roots of a Common Milkweed Species of the US Tallgrass PrairieHerb-chronology, the study of annual growth rings in the root to shoot transitional zone of perennial forbs, involves efforts mostly devoted to finding correlations between growth increment and annual climate. The potential of using growth rings as markers of plant growth response to more ecological phenomena such as periodic disturbance still remains mostly untapped. By monitoring the 2016 growth increment of 64 individuals of a common milkweed species (Asclepias viridis) from the US tallgrass prairie system we investigate plant response to factorial treatments of early season shoot removal and neighbor thinning. These treatments simulate bouts of herbivory, grazing, and dormant-season fire, each of which should have differential effects on plant growth. Neighbor thinning had the strongest effects of the study, moderately increasing ring widths. Conversely, ring widths were moderately decreased by shoot removal. Individual age had negative effects on ring width. These results are the first evidence of herbaceous annual ring sensitivity to sudden amelioration of resources as well as a significant loss of aboveground biomass. Herb-chronology could be useful in future studies monitoring the effects of disturbance on plant growth, increasing our understanding of these phenomena and their overall effect on grassland composition.
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Sapwood Rings Estimation for Pinus Sylvestris L. in Lithuania and LatviaPinus sylvestris L. is the predominant tree species used for wooden constructions in the Baltic area. Accordingly, the timber of Pinus is the most important object for investigation and dating carried out by dendrochronologists of the Baltic countries. However, the dating of historical Pinus is often challenging when the outer sapwood rings are missing in the wood samples. In Pinus, sapwood rings increase in number as the tree ages, and therefore calculating the approximate number of missing outer rings from a set range, a technique used for oak, is not possible. In Norway, a simple method for estimating the number of sapwood rings has been developed for some native species of conifers. The aim of this small-scale study was to assess the validity and the practical suitability of the Norwegian method for estimating the number of missing sapwood rings of mostly historical Pinus wood samples obtained in the southeastern part of the Baltic region. Our findings indicate that this method is not acceptable for estimating the number of missing sapwood rings for individual trees, but suggest that it may be applicable when dating tree-ring chronologies for a minimum of 20 trees, containing individuals up to 200 years old.
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No Significant Increase in Tree Mortality Following Coring in a Temperate Hardwood ForestThe collection of tree-ring data from living trees is widespread and highly valuable in ecological and dendro-climatological research, yet there is concern that coring injures trees, potentially contributing to mortality. Unlike resinous conifers that can quickly compartmentalize wounds, less decay-resistant angiosperms may face more pronounced risk of injury from coring. To test if coring increases mortality rates in temperate hardwood trees, we leverage a unique dataset tracking the mortality of cored and uncored hardwood trees representing 19 species from 10 genera in a 26-ha plot in Virginia, USA. We compare mortality rates between 935 cored trees and 8605 uncored trees for seven years following coring. Annual mortality rates did not differ between cored trees (1.71% yr(-1); 95% CI 1.40 to 2.20) and uncored trees (1.85% yr(-1); 95% CI 1.70 to 2.28) across the full dataset, nor were there differences by genus or size class. These results indicate that the risk of hardwood mortality caused by increment coring is probably lower than previously assumed. However, these results cannot rule out the possibility that coring elevates hardwood mortality in other climates or when trees facemultiple additional stressors that were not influential over the course of our study.
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Identifying Old Trees to Inform Ecological Restoration in Montane Forests of the Central Rocky Mountains, USAOld trees (defined here as >= 150 years old) can be rare in many forests because of past timber harvest, uncharacteristically severe wildfires, and-increasingly-climate change. Old trees provide unique structural, ecological, scientific, and aesthetic values missing in forests containing only younger trees. Here we compile crossdated ages from over 10,000 living and dead trees sampled in montane forests of the central Rocky Mountains in Colorado and southern Wyoming, USA, to examine changes in age structure of the oldest trees since Euro-American settlement and to provide guidelines to aid in identification of old trees for retention during ecological restoration treatments. Eroded stumps (containing only heartwood) were found in over 93% of 179 randomly sampled plots. Number of stumps found in each plot was proportional to reconstructed historical (1860 C. E.) stand basal area. The regional median date of maximum plot tree recruitment was over 150 years older when including stumps versus only living trees, suggesting that if all those harvested trees had survived to the present, the ages of oldest trees would be substantially greater than it is today. However, the regional median age of oldest trees in 1860 before harvesting was not different from the median age of oldest living trees in the current forest (246 vs. 248 years), which alternatively suggests that the regional population of oldest trees has recovered to near historical levels in the time since early Euro-American harvests. Each living tree at the time of sampling was assigned to one of three potential age classes based on a subjective assessment of tree morphology: old (likely >= 150 years old), young (likely <150 years old), or transitional (containing a mixture of young and old tree characteristics). Trees assigned to the old and young morphology categories were classified correctly 88% to 96% of the time depending on species as confirmed by their crossdated ages. Regression tree analysis revealed that tree diameter at breast height was not as reliable a predictor of tree age as were morphological characteristics. A measure of site productivity was a significant variable to use to separate transitional morphology trees into old and young age classes, but classification accuracy was not high because of large variability in ages of these trees. Our results suggest that residual live old trees in the current forest, although perhaps not rare compared to historical age distributions, should be retained during restoration treatments, and that using simple morphological and environmental criteria to identify old trees is more reliable than tree size alone.
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Nitrogen Can Limit Overstory Tree Growth Following Extreme Stand Density Increase in a Ponderosa Pine ForestExtreme stand density increases have occurred in ponderosa pine forests throughout the western U.S. since the early 20th Century, with adverse implications for growth, physiological functioning, and mortality risk. Identifying primary stressors on large, old overstory trees in dense forests can informmanagement decisions to promote resilience and survival. We tested the impact of stand density increase on overstory tree-ring growth, and the relative influence of water and nitrogen, in an old-growth ponderosa pine forest in northern New Mexico subject to variable density increase. We measured annual tree-ring growth and carbon discrimination in trees before stand density increased, in a climatically-similar period post-density increase, and in recent transition to drought. We expected density-driven water stress to drive reduced tree-ring growth in overstory trees in dense stands. We found reduced growth and higher mortality in dense stands, but nitrogen rather than water constrained growth, as determined by carbon isotope discrimination in tree rings, leaf nitrogen concentration, and soil nitrogen supply. In dense stands, less available nitrogen limited photosynthetic rate, leading to reduced assimilation of intracellular C-13 and higher discrimination with low tree-ring growth and a reduced relationship with climate. This unexpected result illustrates that a variety of limiting factors can influence forest dynamics, as density-driven nitrogen limitation interacts with water stress to influence tree growth and physiological functioning.
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Dating the Methuselah Walk Bristlecone Pine Floating ChronologiesTwo floating, ring-width chronologies predate the long bristlecone pine (Pinus longaeva) absolutely-dated, ring-width chronology from the Methuselah Walk (MWK) site in the White Mountains of California. The two non-overlapping floating chronologies were derived from samples that crossdate internally but are temporally unconnected to each other and to the nearly 9000-year, ring-width sequence that is crossdated to the calendar year. We used radiocarbon wiggle-matching and crossdating to place the two floating sequences more accurately in time and to better understand the temporal relationships between the three time series. The trees from the oldest floating sequence were alive near the beginning of the Pleistocene/Holocene boundary and they do not overlap with the other chronologies because of a gap of two-to-three centuries between the two floating series. However, the trees from the younger floating sequence likely do overlap with the long, calendar-dated MWK chronology. We find a possible 57-year overlap that connects these two. If confirmed with additional work, the resulting tree-ring dated annual record from this single location will span 10,359 years, a unique accomplishment in dendrochronology.
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Climate-Growth Responses from Pinus Ponderosa Trees Using Multiple Measures of Annual Radial GrowthWhen using old-growth trees from semiarid, open-canopy environments, basal area increment (BAI), an absolute measure of radial growth, is sometimes used instead of the more commonly used 'conservative techniques' (negative exponential or linear regression with a negative slope; NegX) because narrow rings have been shown to potentially bias results. In this study we explore the relationship between radial growth of ponderosa pine from four study sites in Montana and climate (temperature, precipitation, drought severity) using unstandardized raw ring width and BAI values, and standardized values generated via Friedman Super Smoother and NegX. All sites are minimally disturbed, and our selection criteria are limited to older (interior dates pre-A.D. 1850 at breast height) trees growing in open-canopy environments free of visible disturbance such as lightning strikes. We found the strongest relationships (r > 0.60) for radial growth with July and prior-year October Palmer Drought Severity Index values. Our results show that radial growth-climate responses generally fall within a narrow range regardless of the representation of annual growth (e. g. for July temperature r-values are largely -0.3 to 0.4) and that site conditions determine which radial-growth values (i. e. unstandardized or standardized) optimize climate-growth responses.
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Two Reconstructions of August-July Precipitation for Central Northern Arizona from Tree RingsThis study reports two multi-century regional reconstructions of annual precipitation based on Pinus ponderosa and P. edulis from four sites in central northern Arizona. It compares standard regional and time-nested methods to generate reconstructions from 1581-2016 C.E. and 1529-2016 C.E., respectively. The strongest climate relationship is a positive correlation between total ring width and 12-month total precipitation ending in July of the growth year. The chronologies account for 50% of the variance of observed annual precipitation in the regional model and 59%, 60%, and 47% and 35% in the nested models. The two reconstructions are highly correlated (Pearson's correlation r > 0.97, p < 0.001) demonstrating that the reconstructions are highly similar over the period common to both reconstructions, with the nested-model's advantage of extending the range of the reconstruction. The precipitation reconstructions are significantly correlated (r = 0.66, p < 0.001) with the North American Drought Atlas (NADA).
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Tree-Ring Expeditions (Trex): Online Labs that Guide Undergrads to Think Like ScientistsHere we describe five publicly available online labs, geared to undergraduate students, which focus on foundational tree-ring research. Students are introduced to basic dendrochronological concepts and practices (Lab 1) while learning about research that has implications for human well-being. Students learn about the way scientists use tree-ring records to reconstruct drought in the Hudson Valley in New York (Lab 2), how tree-ring science began through its utility in putting exact calendar dates on ancestral pueblos (Lab 3), how tree-ring records can be used to put drought into a long-term context, reconstruct streamflow, and better manage water resources (I ab 4), and how tree rings have been used to reconstruct temperatures in the northern latitudes (Lab 5). These labs have the dual aim of guiding students to use many of the same tools as tree-ring scientists, while also giving them a sense of the nature-of-science and how scientists work. Throughout the labs, students are guided to explore virtual field sites, navigate public databanks, observe and measure tree-ring samples, and describe trends and extremes in paleoclimate records. Labs are designed for a 2 to 3-hour lab class and have been classroom-tested and assessed by faculty teams and students.
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Towards a More Ecological DendroecologyThe use of tree-ring methods to study ecological processes, known as dendroecology, has been booming over the last decade. We believe that the incredible methodological strides in this subdiscipline over the last half century will be further advanced by purposefully integrating with other ecological subdisciplines and broadening the scope of dendroecology both in terms of methods and theory. Simultaneously, these efforts will greatly benefit a broad range of ecological disciplines through the incorporation of one of the greatest strengths of dendrochronology: highly-resolved ecological data that spans from seasons to centuries. Because these data are still alarmingly scarce in ecology but are crucial to understand the ecology of long-living organisms, we believe better integrating dendroecology and mainstream ecology will benefit both disciplines. We discuss five actions that can be readily embraced by the dendrochronological community to further advance the field while also making it more open for non-dendroecologists. These actions include: (i) promoting diverse or multi-discipline scientific collaborations and partnerships, (ii) diversifying dendroecological data sources, (iii) incorporating inference-based and hierarchical models to the dendroecological toolbox, (iv) improving and updating the global tree-ring databases, and (v) increasing the focus on ecological and evolutionary mechanisms in tree-ringdriven papers. We believe these actions will help facilitate a broad discussion on how to better integrate tree-ring-based ecology within mainstream ecology. We believe this has the potential to trigger major advancements in dendroecology, help resolve long-standing ecological questions and, ultimately, bring a new perspective and scale to ecological theory.
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Seasonal Precipitation Signal in Earlywood and Latewood Ring Width Chronologies of Pinus RoxburghiiThe growth response of earlywood and latewood to precipitation in chir pine (Pinus roxburghii) was studied by examining a series of core samples from the Garhwal Himalaya, India. Earlywood and latewood were observed to contribute about equal proportions towards the total ring width. Comparison of tree-ring data with the CRU TS3.22 (land) precipitation dataset indicates that earlywood is positively correlated with spring and early summer precipitation, whereas latewood is negatively correlated with pre-monsoon and early monsoon precipitation. This seasonally-reversed climate signal is confirmed when regional weather station precipitation data were used. A similar seasonally reversed climate response was found in earlywood and latewood of two datasets obtained from core samples from two other sites located in Nepal and Bhutan. Because chir pine is a light-demanding species, light limitation during the monsoon season could be an important factor behind the negative correlation between latewood and precipitation. NOAA NCEP-NCAR low cloud data were used to test this hypothesis, and the preliminary results support the hypothesis; however, further analysis will be needed to fully validate this hypothesis.
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Meteorological Factors Associated with Frost Rings in Rocky Mountain Bristlecone Pine at Mt. Goliath, ColoradoThe meteorological factors involved in the formation of earlywood frost rings in Rocky Mountain bristlecone pine (Pinus aristata) have not been described in detail. This study used 51 tree-ring dated Rocky Mountain bristlecone pine trees growing at ca. 3500 m a.s.l. on Mt. Goliath, Colorado, to develop earlywood and latewood frost ring chronologies dating from 1930 to 2010 for investigation of the regional and large-scale weather anomalies responsible for these unusual growing season freeze events. The high-elevation meteorological station at Niwot Ridge, Colorado, was used to document the daily temperature anomalies most likely associated with these frost-damaged rings. NCEP-NCAR Reanalysis data were used to examine the synoptic meteorological conditions that tend to prevail during these unusual growing season temperature conditions. Earlywood frost rings occur during anomalous late-May and June freeze events in the Colorado Rockies associated with unseasonal mid-latitude circulation, including the penetration of a deep upper-level low pressure system and cold surface air temperatures into the west-central United States. The three latewood frost rings all occurred during September freeze events also associated with unseasonal and highly amplified mid-latitude circulation. The chronology of these early and late growing season freeze events may provide a useful independent check on daily temperature minima estimated with reanalysis techniques, and they can be extended into the pre-instrumental era thanks to the great age of Rocky Mountain bristlecone pine. Frost damage in Mt. Goliath bristlecone pine appears to be most frequent and severe in young trees found in the depressed tree line below a large cirque subject to intense cold air drainage. The development of the most detailed tree-ring records of past freeze events may therefore benefit from site selection in these cold air drainages, along with age-stratified tree sampling to ensure that the young and most frost susceptible age classes are well represented throughout the chronology.
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Identifying and Quantifying Tree-Ring Chronology Variance Artefacts Related to Co-Occurring Changes in Growth RateExpectations that a warming world will be associated with more hydro-climatological extremes has motivated research exploring if an associated signal is evident in paleoclimate archives. Tree-ring chronologies are central to this work because of their high temporal resolution, but they are also potentially compromised by variance artefacts associated with the evolving composition of the chronology and with data processing. Here we present two empirical methods to identify and quantify potential artefacts related specifically to temporally varying growth rate (local level, LL): LL-based partitioning analysis and LL-based chronology stripping. The two methods were developed and tested using a multi-site New Zealand kauri (Agathis australis) living-tree data set. Our results show that the methods are complementary in terms of artefact identification and quantification, and that they can provide useful insight into causal processes when used conjointly. Our results also indicate that data pre-processing to remove LL-related artefacts may be sub-optimal, that there may be an optimal standardization that minimizes bias, and that the evolving variance of kauri master chronologies over the last 500 years is not significantly affected by LL-related artefacts.
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Fire History of an Old-Growth Ponderosa Pine Stand in the Sheep Range, Desert National Wildlife Refuge, Nevada, USASouthwestern ponderosa pine forests have experienced reduced fire frequency since Euro-American settlement generally because of successful fire suppression policies. We report here on the fire history of a ponderosa pine stand located in the Sheep Range, which is part of the Desert National Wildlife Refuge, in the Mojave Desert. A total of 22 dominant, fire-scarred ponderosa pines were sampled by taking 29 partial cross-sections and 18 wood increment cores. Maximum age of ponderosa pines at the study area exceeded 800 years, and sampled trees were often older than 500 years, so that the site tree-ring chronology covered 522 years (1490-2011). Crossdating revealed both extreme sensitivity and highly synchronous patterns, with the expressed population signal (EPS) exceeding 0.9 in 30-year moving windows throughout the length of the chronology. Fire statistics were calculated for the 1565-2011 period, during which at least 10 of the crossdated trees had been scarred and were recording fire. During the recorder period, there were 16 fires that met the two-tree minimum threshold, yielding a mean fire interval (MFI) of 25 years, a median fire interval (MedFI) of 15 years, and a Weibull median probability interval (WMPI) of 18 years; the point mean fire interval (PMFI) was 69 years. The longest fire-free intervals since 1565 occurred in the past two centuries, with 70 years (1862-1931) followed by another 80 years (1933-2012). The stand-wide 1932 fire is the last event recorded by the sampled trees. Overall there was reduced fire frequency from the late 19th Century to present compared to the previous three centuries. Because there is no record of active fire management in the study area, this finding is consistent with similar results obtained in two remote mountains of the Great Basin Desert, and points to a need for greater spatial coverage in fire history information, even for species that have been actively studied in other environments.
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A 307-Year Tree-Ring Spei Reconstruction Indicates Modern Drought in Western Nepal HimalayasWestern Nepal has experienced a severe drought in the past two decades, but observation records across Nepal are too short to place the recent drought in a longer context to understand the full range of natural variability in the climate system. In the present study we have collected tree core samples of Tsuga dumosa from two sites, Chhetti and Ranghadi, in the Api Nampa Conservation Area of the western Nepal Himalayas to understand drought variation for the past three centuries. We have developed a 357-year (AD 1657-2013) tree-ring chronology. The tree growth-climate response analysis revealed a stronger positive correlation with spring (March-May) standardized precipitation evapotranspiration index (SPEI01) (r = 0.523, p < 0.01) than precipitation (r = 0.459, p < 0.01), self-calibrating Palmer drought severity index (scPDSI) (r = 0.250, p < 0.01), or temperature (r = -0.486, p < 0.01). Stronger positive correlation with SPEI01 indicates moisture availability is the limiting factor for the growth of this species on these sites. Based on this growth-climate response we reconstructed spring SPEI from AD 1707 to 2013 for the region. The reconstruction showed several dry and wet episodes indicating no persistent climate trend within the past three centuries. The current drought is one of the four most severe in our 307-year record.