Applications of Dendrochronology in Agroforestry
and Climate Change Research
Aster Gebrekirstos
World Agroforestry Centre, Nairobi, Kenya ([email protected])
What is dendrochronology and what can it tell us?
Our laboratory primarily focuses on the following
aspects:
Dendrochronology or examining tree rings as a method for scientific dating provides
accurate dating because in principle each ring represents a year in a trees life.
1) Assessment of climate growth relationships
in the tropics are growing under a seasonal climate and hence experiencing cambial
on tree growth, agroforestry systems and forest
from indistinct to distinct from annual to bi annual.
2) Using tree ring data for proxy reconstruction of
Seasonality in climate is the main driver for growth-ring formation and many tree species
and evaluation of the effect of a changing climate
dormancy. Depending on climate seasonality and species, formation of rings can range
development
past climate conditions beyond the period where
instrumental records are available, proxy evidence of past variation in the El Nino/
Southern Oscillation and other large-scale atmospheric circulations
3) The analyses of radial growth increment and stable isotopes at the inter-annual and
intra-annual scale together with tree physiological parameters to better understand the
ecological relationships between tree growth, climate and site conditions
4) Determining cambial dynamics, carbon allocation patterns and changes in wood
Tree rings are history books and they can tell us many things about past climates,
anatomy, and hydraulic conductivity at different spatial and temporal resolutions
rise and fall of civilization, among many others.
climate change
tree growth and vegetation dynamics, history of people and their landscapes, and the
5) Changes in river flow, source of water and hydrological fluctuations in relaton to
6) The study of past and present changes in wild fire by dating the fire scars left in
Knowledge of the range of natural climate variability and tolerance range of tree
tree rings to determine the frequency of fire occurrences
species to climatic stress is scarce especially in the tropics and subtropics.
From daily to seasonal dynamics: does
the species form annual rings ? When and
how big does it grow and how sensitive is
it to climate variability?
sample preparation
• stem disks from dead trees or
increment cores can be used
• Careful sample preparation
(polishing the surface with sanding
paper of different grit size is
important)
• for δ13C and δ18O series
measurements were done using
mass spectrometry.
Carbon turn over
holes indicate sample preparations for δ13C and δ18O
series measurements using mass spectrometry.
how was past climate and its impacts? Were there
extreme events and how frequent ?
Ring width ( mm) 800 scle Tug 700 scle nub 600 500 Sahel drought ?
400 300 200 100 0 1926 1931 1936 1941 1946 1951 1956 1961 1966 1971 1976 1981 1986 1991 1996 2001 2006 year Ring width cross correlation of S. birrea between Tugure and Nubere showed a similar pattern
and positive correlation (r=0.54)
Reconstruction of precipitation and
source of water?
Location specific climate
reconstructions?
δ13C and δ18O of S. birrea along a climate gradient follow a similar pattern, showing that external factors affect isotope fractionations in a similar way. However, δ13C
from the Sahel region is highly enriched compared to the transition Sudan Sahel zone. In contrast its δ18O differences are not quite so obvious. The annual δ13C and
δ18O patterns of S. birrea correlate negatively with precipitation, PDSI (Palmer Drought Sensitivity Index) (of August up to r=-0.65), and show positive correlations with
temperature (r=0.72). High temperature (evaporative demand) in drought years would cause stomatal closure and preferential loss of the light oxygen isotope resulting
in high concentration of δ13C and δ18O, respectively. S birrea is sensitive to climate variability. The strong correlation of δ18O with rainfall also indicates that S. birrea is
using soil water (precipitation). (Gebrekirstos et al. 2011).
Acknowledgements
We are grateful to CCAFS for funding the lab set up, all our project collaborators and Prof. Dr.
Achim Braeuning
Intra-annual stable carbon isotope (δ13C) study related to a labeling experiment to illustrate differences in temporal pattern of recent carbon allocation to wood structure
of two functional types of trees. A proportion of labeled δ13C is stored in reserves of wood parenchyma and/or needles for up to three years in the late successional
conifer species Podocarpus falcatus. In contrast, the pioneer broad-leaf tree (Croton macrostachyus) shows a high turnover and a carry-over effect of δ13C that is only
detectable in the subsequent year. This shows the consequences of forest disturbance in carbon turn over (Krepkowski et al. 2013 New Phytologist).
Hydrological fluctuations and
river flow
Conclusions
The results highlight the potential of tree ring width and stable isotopes for reconstructing
past climates and extreme events, determining water use efficiency and water sources and
evaluating resilience of species to climate variability, as well as reconstructing hydrological
fluctuations and river flow.