03 April 2023
What tree rings can tell us about climate change
The rings of a tree are like a time capsule recording past environmental conditions and they provide an opportunity for scientists to understand how trees might respond to future climate change.
Niall Farrelly, Hui Xing, Teagasc, and Brian Tobin, UCD, are currently conducting a dendrochronological study (a tree-ring dating technique), as part of the Fit Forests project, to understand how trees respond to climate change.
The team are examining tree rings to determine how tree growth has been effected by past extreme climatic events in Ireland, particularly conditions causing meteorological drought. The project aims to look back at the last 50 years in Ireland, during which there have been some notable drought periods, including a very dry summer in 2018, to investigate to what extent these events have impacted tree ring data.
Dendrochronology is a well-established science pioneered by American astronomer Andrew E. Douglass, who used tree ring dating to examine the impact of historical climatic events. Tree rings are like libraries, which keep a precise record of regional climatic information and environmental cues which can be collated in extensive tree ring chronologies. For long-lived species, such as the bristlecone pines of California, some of which are over 4,000 years old, their life history is preserved by the ring structure which can be precisely dated to reconstruct past climate patterns.
Scientists look for distinctive patterns in the growth of the tree rings to date disruptive climatic events which have affected growth. Events such as volcanic eruptions (such as Krakatau in 1883) affected photosynthesis rates when volcanic ash acted as an atmospheric solar radiation filter.
Disturbances like drought events or forests fires interrupt normal growth patterns causing abnormal features such as narrower rings or fire scar tissue. These anatomical variations in tree rings serve as temporal markers for researchers to date inter-annual, intra-annual and intra-seasonal variations in tree-climate interactions. In addition, such assessments provide an understanding of how trees can cope with changing climates and severe stress.
How trees respond to droughts
The reduction in tree ring growth associated with a drought event is the result of a functional trade-off between carbon gain and water loss during photosynthesis. The process of regulating water loss in response to stressful events takes place in the leaves or needles, where stomata (the microscopic pore-like cell structures) regulate carbon dioxide uptake and water loss during photosynthesis.
As trees cannot absorb carbon dioxide without simultaneously losing water vapour, they regulate water losses by closing their stomata. Indeed, trees often shed whole leaves to minimise water loss during prolonged drought conditions. This reduces photosynthesis rates, causing carbon starvation and a reduction in wood formation.
Annual growth rings are the result of cambial activity forming xylem tissue throughout a growth season. Xylogenesis is the process of xylem cell differentiation in response to various environmental conditions, which is what creates differences in wood quality. The retrospective analysis of tree ring qualities, such as ring width, early or late-wood density and chemical isotopic compositions in wood cellulose, offers an insight into past climatic extremes, such as drought, and their impact on tree growth performance and physiological responses, essentially describing the degree of adaptability.
How we decipher tree ring data
While radial growth is a good indicator of climatic conditions during which trees have grown, it can sometimes be difficult to assign individual rings to specific calendar years. Differences in ring structures can occur due to density fluctuations (causing "false rings") or very subtle transitions from earlywood to latewood in conifers or ring-porous broad-leaved species like ash or oak can cause problems in ring detection.
Dendrochronology research uses software to analyse high-resolution scanned images of tree stem disks or core samples. The age of the outer-most ring (closest to the bark) is dictated by the sampling date, and older internal rings are detected using changes in the composition and pattern of the scanned tree ring image. A library or time series of ring widths is then built, associating each ring with a specific year before cross referencing with detailed meteorological records. Researchers use these time series to examine a tree's immediate physiological response to specific climatic events, and to understand how long the anomalous growth lasts for.
The ability of trees to recover after drought or other disturbance events is particularly important to understand
Of course, competition between trees or management events can also affect the magnitude of radial growth, so careful sampling is required, usually selecting dominant trees whose growth should best reflect climatic conditions rather than localised stand conditions. Cross-dating and other procedures are needed to ensure a consistent growth response across many ring series. Other measures, such as oxygen and carbon isotopic signatures in wood cellulose, can provide more nuanced environmental fingerprinting to disentangle the separate effects of temperature, precipitation or their extremes.
The ability of trees to recover after drought or other disturbance events is particularly important to understand, as radial growth patterns after a drought can be important indicators of resilience. This research shows how trees can respond to adversity and provides us with clues of how management can assist such resilience to best protect our forest and woodland resource.
This study was funded by the Department of Agriculture, Food and the Marine under the FitForests project - grant number 2019R511.
This article was first published by RTE Brainstorm
