Western 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.

The 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.

The 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.

This 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-models’ 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).

Southwestern 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 19^th 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.

Expectations 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.

The 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-ring-driven 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.

Here 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 (Lab 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.