How to Extract Climate Variability from Tree-Rings
Summary
Tree-ring climate reconstructions can be helpful to better understand past climate variability beyond instrumental records. This protocol shows how to reconstruct past climate using tree rings and meteorological instrumental records.
Abstract
Tree rings have been used to reconstruct climatological variables in many locations around the world. Moreover, tree-rings can provide valuable insights into climatic variability of the last few centuries and, in some areas, several millennia. Despite the important development, that dendrochronology has had in recent decades to study the dendroclimatic potential of a large number of species present in different ecosystems, much remains to be done and explored. In addition to this, in the last few years more people (students, teachers and researchers) around the world are interested in implementing this science to extend the timeline of climate information backwards and understand how climate has changed on scales of decades, centuries or millennia. Therefore, the objective of this work is to describe the general aspects and basic steps needed to conduct a tree-ring climate reconstruction, from site selection and field sampling to laboratory methods and data analysis. In this method’s video and manuscript, the general basis in tree-ring climatic reconstructions is explained so newcomers and students can use it as an available guide into this field of research.
Introduction
Tree rings are fundamental to our understanding of how trees respond to their environment. In addition, because climate affects tree growth, trees serve as environmental gauges recording the temporal variations during their lifespan. Thus, tree rings have been valuable to reconstruct past climates far beyond any instrumental climate record.
Growth processes in roots, stems, branches, leaves, and reproductive strategies in trees are regulated by environmental factors such as water, light, temperature, and soil nutrients1. For example, stems grow radially and the vascular cambium controls radial growth2. The vascular cambium is a meristematic tissue that will actively produce new functional cells such as xylem and bark located at the outer boundary of the stem. Additionally, the vascular cambium is primarily active during seasonal cycles. However, this growth activity can be interrupted during dormancy periods and during particular seasons of the year. This dormancy period usually happens when environmental variables are not optimal (e.g., shorter diurnal cycles, extended drought periods, cold winters, or floods). Furthermore, the growth and dormancy cycles translate in changes in the cambium activity resulting in anatomically distinct concentric boundaries in the stem called tree rings3.
Trees generally produce one tree ring every year since climatic seasonality occurs annually. Thus, tree rings are the visual manifestation of the ecophysiological response of the vascular cambium to the intra-annual climatic conditions during tree growth3. The early cluster of xylem cells formed on a tree ring during the wet season will be characterized by larger cells called earlywood4. In contrast, during the dry season and in response to water scarcity, vascular cambium produces smaller xylem cells (tracheids or vessels) with thicker cell walls called latewood. This variation in anatomical structures is more noticeable in conifers, where the earlywood shows a lighter color than latewood, showing a darker color5. The space between the beginning of the earlywood and the end of the latewood is defined as one tree ring (Figure 8F).
Trees growing on locations with a well-defined rainy and dry season could expect years with a higher or lower amount of precipitation. This variability will lead trees to produce wider rings during wet years and narrower rings during dry years. These temporal patterns of wide and narrow rings can be seen as a barcode. This tree-ring width temporal variation is the basis for applying the process of cross-dating, one of the most critical principles in tree-ring research6. The process of cross-dating is satisfactory when the patterns of wide and narrow rings in all samples are successfully synchronized in time to assign the corresponding year of formation.
In many regions of the world where seasonal climate occurs, the most dominant signal recorded in tree rings is likely related to climate variability7. However, tree rings also contain additional information related to age (young trees grow faster than older ones), competition for resources with surrounding trees, and internal and external disturbances (e.g., mortality events, pest outbreaks, or fire)8. Thus, before attempting to reconstruct past climates using tree ring widths, non-climatic signals need to be removed through several statistical procedures explained in this manuscript.
The main goal of this protocol is to show how to develop a climatic reconstruction based on tree-ring data to understand past climatic variability. Thus, this manuscript will showcase the essential field and laboratory methods such as sampling, sample preparation, cross-dating, and measuring tree-ring widths required to develop a climatic reconstruction. In addition, this protocol will also explain the fundamental statistical analyses used to extract the common variability from tree-ring widths and construct a tree-ring chronology that will be correlated with climatic data. Finally, using a simple linear regression model the protocol will show how to reconstruct past climate using the tree-ring chronology as the predictor variable and the climate data as the predictand.
Protocol
Representative Results
Discussion
Proxy records are natural systems that depend on the weather, which were present in the past and still exist, such as lake and marine sediments, pollen, coral reefs, ice cores, packrat middens, and tree rings, so information can be derived from them24. However, from most climate-sensitive proxies, tree rings represent the proxy with the highest precision and interannual resolution, allowing the dating of climatic and ecological events to the exact year of occurrence, spanning for centuries, and so…
Disclosures
The authors have nothing to disclose.
Acknowledgements
The research project was carried out thanks to the financing through the projects CONAFOR-2014, C01-234547 and UNAM-PAPIIT IA201621.
Materials
| ARSTAN Software | https://www.ldeo.columbia.edu/tree-ring-laboratory/resources/software | ||
| Belt Sander | Dewalt Dwp352vs-b3 3×21 PuLG | For sanding samples | |
| Chain Saw Chaps | Forestry Suppliers | PGI 5-Ply Para-Aramid | https://www.forestry-suppliers.com/Search.php?stext=Chain%20Saw%20Chaps |
| Chainsaw | Stihl or Husqvarna for example | MS 660 | Essential equipment for taking cross sections samples (Example: 18-24 inch bar) |
| Clinometer | Forestry Suppliers | Suunto PM5/360PC with Percent and Degree Scales | https://www.forestry-suppliers.com/Search.php?stext=Clinometer |
| COFECHA Software | https://www.ldeo.columbia.edu/tree-ring-laboratory/resources/software | ||
| Compass | Forestry Suppliers | Suunto MC2 Navigator Mirror Sighting | https://www.forestry-suppliers.com/Search.php?stext=compass |
| Dendroecological fieldwork programs | Programs where dating skills can be acquired or honed | http://dendrolab.indstate.edu/NADEF.htm | |
| Diameter tape | Forestry Suppliers | Model 283D/10M Fabric or Steel. | https://www.forestry-suppliers.com/Search.php?stext=Diameter%20tape |
| Digital camera | CANON | EOS 90D DSLR | To take pictures of the site and the samples collected (https://www.canon.com.mx/productos/fotografia/camaras-eos-reflex) |
| Digital camera for microscope | OLYMPUS | DP27 | https://www.olympus-ims.com/es/microscope/dp27/ |
| Electrical tape or Plastic wrap to protect samples | uline.com | https://www.uline.com/Product/Detail/S-6140/Mini-Stretch-Wrap-Rolls/ | |
| Field format | There is no any specific characteristic | To collect information from each of the samples | |
| Field notebook | To take notes on study site information | ||
| Gloves | For field protection | ||
| Haglöf Increment Borer Bit Starter | Forestry Suppliers | https://www.forestry-suppliers.com/Search.php?stext=Increment%20borer | |
| Hearing protection | Forestry Suppliers | There is no any specific characteristic | https://www.forestry-suppliers.com/Search.php?stext=Hearing%20protection |
| Helmet | Forestry Suppliers | There is no any specific characteristic | https://www.forestry-suppliers.com/Search.php?stext=Wildland%20Fire%20Helmet |
| Increment borer | Forestry Suppliers | Haglof | https://www.forestry-suppliers.com/Search.php?stext=Increment%20borer |
| Large backpacks | There is no any specific characteristic | Strong backpack for transporting cross-sections in the field | |
| Safety Glasses | Forestry Suppliers | There is no any specific characteristic | https://www.forestry-suppliers.com/Search.php?stext=Safety%20Glasses |
| Sandpaper | From 40 to 1200 grit | ||
| Software Measure J2X | Version 4.2 | http://www.voortech.dreamhosters.com/projectj2x/tringSubscribeV2.html | |
| STATISTICA | Kernel Release 5.5 program (Stat Soft Inc. 2000) | Statistical analysis program | |
| Stereomicroscope | OLYMPUS | SZX10 | https://www.olympus-ims.com/en/microscope/szx10/ |
| Topographic map, land cover map | Obtained from a public institution or generated in a first phase of research | ||
| Tube for drawings | There is no any specific characteristic | Strong tube for transporting samples in the field | |
| Velmex equipment | Velmex, Inc. | 0.001 mm precision | www.velmex.com |
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