Glacial Tree Physiology: Using Stable Isotopes to Reconstruct Plant Responses to Environmental Change Since the Last Glacial Period
My dissertation research focused on how changes in atmospheric CO2 concentrations over the last 50,000 years have affected tree physiology. To address this, I studied Juniperus (juniper, left) wood from the Rancho La Brea tar pits in Los Angeles, California and Agathis australis (kauri, right) wood from peat bogs in North Island, New Zealand. Juniper remains were provided by the Page Museum of Natural History curator Dr. John Harris and range in age from 15-50,000 years old. Kauri remains were provided by Ancientwood, Ltd. president Robert Tiesberg and range in age from 45-55,000 years old (or perhaps older, as these dates push the limits of 14C dating).
I compared these specimens to modern juniper and kauri in each area. Comparing the physiological functioning of glacial and modern trees allows us to assess plant responses to long-term climatic change, incorporating both natural and anthropogenic climatic change.
In both cases, tree trunks are very well preserved, and individual growth rings were clearly visible (as even these low-resolution images show). Consequently, we could not only look at differences between glacial and modern trees, but also assess similarities or differences between trends throughout an individual tree’s lifetime in both glacial and modern periods. In both the tar pits and peat bogs, glacial trees were preserved in their original organic state with no mineralization, meaning these remains are some of the oldest non-fossil (ie. not mineralized) remains on Earth.
Atmospheric CO2 levels have changed quite drastically over the past million years (see figure below). Historically, CO2 concentrations oscillated between ~170-270 parts per million (ppm) corresponding to glacial (low CO2) and interglacial (higher CO2) periods. The most recent minimum occurred during the Last Glacial Maximum (LGM), approximately 20,000 years ago, when atmospheric CO2 was ~180 ppm. Following the LGM, CO2 levels rose to about 270 ppm just prior to the Industrial Revolution. In the last 250 years, CO2 levels have risen rapidly, to a modern level of just under 400 ppm. The modern level is more than double the levels seen during the LGM, and atmospheric CO2 concentrations this high have not occurred for potentially as long as 14 million years.
Atmospheric CO2 is the substrate for photosynthesis, the process through which plants turn energy from the sun into chemical energy for growth and other functions. Consequently, such drastic changes in atmospheric CO2 over such short evolutionary time scales have the potential for serious impacts on plant physiology.
There have been many studies looking at the effects of low atmospheric CO2 characteristic of the last glacial period on plant physiology, such as those illustrated below (click on figures for larger image). In many of these studies, modern plants are grown in experimental growth chambers, where environmental conditions (such as light, temperature, humidity, water and nutrient availability, and CO2 concentrations) can be experimentally manipulated. These studies report serious and drastic impacts of low CO2 on plant function, growth, reproduction, and survival (for a review of these effects, see Gerhart & Ward, 2010).
Since these studies focus primarily on modern plants grown under glacial conditions, and often only for a single generation (not allowing time for genetic adaptation), their findings may represent a more short-term acclimation response. This begs the question “How representative are these findings of actual glacial plant function 20-50,000 years ago?” It is for this reason I focused my dissertation research on glacial plant remains, in order to analyze the physiological function of plants that actually lived during the last glacial period, and had ~14 million years to adapt to low CO2 conditions.
When dealing with living plants, a researcher can measure many parameters relating to plant physiology: absolute growth (including growth of specific organs, like leaves or roots), growth rates, gas exchange (CO2 uptake and water loss), etc, etc, etc. When dealing with dead remains of trees, our options are more limited: we have only partial trunks (no leaves, no roots, and often no branches) and since they are no longer living, we cannot measure any sort of rate (growth rate, or gas exchange rate). We can, however, use stable isotopes analysis of each individual growth ring to back-calculate physiological function during the time that ring was laid down.
For my dissertation research, I focused on carbon isotopes – Carbon-12, Carbon-13, and Carbon-14 (see figure below). All carbon molecules contain 6 electrons and 6 protons. Carbon-12 also has 6 neutrons in the nucleus with the protons (6 neutrons + 6 protons = 12). Carbon-13 has an extra neutron in the nucleus; this means Carbon-13 binds with other atoms and molecules the same as Carbon-12, but is simply a bit larger and heavier. Carbon-14 has two extra neutrons in the nucleus and is radioactive, meaning it breaks down into a different molecule over time. This behavior is why Carbon-14 is used to date old material – and was used to date the glacial trees used in this research!
Biological processes tend to fractionate against the heavier atom, meaning these processes ‘prefer’ the lighter atom. The amount of Carbon-13 is already quite low – 99% of carbon is Carbon-12, ~1% is Carbon-13 (only trace amounts of Carbon-14 exist), meaning fractionation against Carbon-13 reduces the number of these atoms in plant tissue even further below their already low natural abundance.
Using mass spectrometry, we can measure the amount of Carbon-12 and Carbon-13 atoms in plant tissue to determine their relative abundances, and using the ratio of the two (Carbon-12/Carbon-13), we can calculate plant physiological function even 50,000 years after the tree has died. The parameter which is most useful is the ratio of intercellular to atmospheric CO2 concentration (called ci /ca, see diagram below). Changes in ci /ca operate similarly to a supply-and-demand function, where stomatal conductance (how open or closed the pores on the leaf are at a given moment) represents the supply function and photosynthetic capacity (how quickly CO2 in the leaf is taken up for photosynthesis) represents the demand function. The relative levels of these two processes determine how high or low the ci /ca ratio is at a given point in time. Additionally, any time the stomata are open to allow CO2 into the leaf, water is also evaporating out of the leaf. Consequently, ci /ca ratios of interannual rings give us a broad, integrated view of plant function throughout the growing season, including both carbon and water relations.
My research showed that both juniper and kauri exhibited constant ci /ca throughout the last 50,000 years, despite significant changes in CO2 and other environmental parameters across this time. Constant ci /ca resulted in drastically lower intercellular CO2 concentrations (ci) in glacial trees of both species – so low, in fact, as to be almost entirely outside the range of modern ci levels. Lower ci would predict lower levels of growth in ice age trees compared to modern trees, since ci determines the amount of CO2 available to the plant for photosynthesis. Interestingly, neither species shows significant reductions in growth in glacial trees compared to modern.