Nick Betson, PhD Student
Project Title: Temporal variation in the stable isotope signals of trees with contrasting growth strategies (2004)
A long term study of the variation in d13C of bulk leaf material was undertaken under 3 light regimes (3 %, 12 % and 100 % sunlight) using 2 different species displaying contrasting shoot phenologies: Betula pendula Roth. an early successional species with a rapid leaf turnover ("successive" phenology) and Fagus sylvatica L. a late successional species with a single flush of long-lived leaves ("flush" phenology).
It was hypothesised that there would be greater seasonal changes in the isotopic composition of the leaves of B. pendula, within a single leaf cohort (due to rapid leaf turnover of that species) and that the different light regimes and shade tolerances of the two species would further extend variation. Physiological and phenological parameters, alongside bulk leaf d13 C, were measured over the first growth season(2002) on 4 leaf cohorts in the B. pendula and the single flush of the F. sylvatica. The 13 C of the initial leaf was consistently more enriched in the heavier isotope than bud material. Further changes were observed both within (up to 2 %) and between (up to 4 %) leaf cohorts in both species as well as substantial shifts between light regime (up to 6%).
During the following growth season, measurements were focused at 2 time points: early season and mid season to attempt to explain the variation within leaf cohort and in the change in isotopic signal between the bud and initial leaf. Alongside the continuation of measurements from the previous season, the δ 13 C of the soluble carbohydrates and starch fractions were measured, as well as the d18 O of leaf organic material to follow the mobile carbon fractions as the organic signal changed and to partition the d13 C signal into stomatal and biochemical influences. Carbohydrate d13C values showed that the within cohort variation could be explained by fractionation processes during phloem unloading and, later in the season, the progressive incorporation of new carbohydrate with a more negative delta. The d18O measurements allowed the partitioning of the δ 13 C signal: a positive correlation between the two isotope species in the 100% light regime of the B. pendula indicated that, in accordance with theory, stomatal conductance (gs) was driving the d13 C. The lack of significant correlations elsewhere indicated that under well-watered, but light-limited conditions, the d13 C was being primarily driven by biochemical constraints.
Alongside this experiment, a short-term study was undertaken to study the response of the same species to drought. Moreover, the temporal changes in the δ 13 C of soil respiration as driven by alterations to gs during drought and subsequent enrichment in the carbon isotope pool, was investigated. Gas exchange parameters decreased and leaf carbohydrate d13 C became more enriched over the course of the experiment as the soil moisture deficit increased as well as a result of the substantial atmospheric vapour pressure deficit (VPD) that occurred during the experiment. The B. pendula was found to display a higher water use efficiency (WUE, the ratio of carbon gained to water lost) but was more sensible to both VPD and drought, in terms of speed and magnitude of response. Instead, the F. sylvatica generally had higher gs values (a lower WUE), but was able to maintain the more negative midday d that resulted. Significant correlations were observed between the d13C of soil respired CO2 and the 6-8 day averages of both gs and VPD, well within the timeframe found in theliterature.
Current research interests:
Canopy-soil cycling of carbon and nitrogen using stable isotopes
Dr Nicholas Betson
Dept. of Medical Biochemistry and Biophysics
S-901 87 UMEÅ