Beyond tree-ring widths: Stable isotopes sharpen the focus on climate responses of temperate forest trees

Tree rings provide an indispensable tool for assessing a tree's response to variability in its environment, oftentimes also providing a means of reconstructing that variability beyond instrumental records. The wood that trees produce is laid down sequentially, creating an archive of temporally ordered material that is rich in physiological and environmental information. This is made all the more useful because trees are globally distributed, can live for thousands of years and in some cases remain intact long after they die

Identifying drivers of leaf water and cellulose stable isotope enrichment in Eucalyptus in northern Australia

Several previous studies have investigated the use of the stable hydrogen and oxygen isotope composi- tions in plant materials as indicators of palaeoclimate. However, accurate interpretation relies on a detailed understanding of both physiological and environmental drivers of the variations in isotopic enrichments that occur in leaf water and associated organic compounds. To pro- gress this aim we measured δ18O and δ2H values in euca- lypt leaf and stem water and δ18O values in leaf cellulose, along with the isotopic compositions of water vapour, across a north-eastern Australian aridity gradient. Here we compare observed leaf water enrichment, along with pre- viously published enrichment data from a similar north Australian transect, to Craig–Gordon-modelled predic- tions of leaf water isotopic enrichment. Our investigation of model parameters shows that observed 18O enrichment across the aridity gradients is dominated by the relation- ship between atmospheric and internal leaf water vapour pressure while 2H enrichment is driven mainly by variation in the water vapour—source water isotopic disequilibrium. During exceptionally dry and hot conditions (RH 37 °C) we observed strong deviations from Craig– Gordon predicted isotope enrichments caused by partial stomatal closure. The atmospheric–leaf vapour pressure relationship is also a strong predictor of the observed leaf cellulose δ18O values across one aridity gradient. Our find- ing supports a wider applicability of leaf cellulose δ18O composition as a climate proxy for atmospheric humidity conditions during the leaf growing season than previously documented

Effects of forest thinning on soil-plant carbon and nitrogen dynamics

Aims: Corymbia spp. (previously included in the genus Eucalyptus) are common species in sub/tropical Australia and produce high quality timber and round logs. Thinning of native forests helps to preserve native tree species and is more sustainable than replacing native forest stands with mono-species plantations to produce timber. This study aimed to explore the effects of native forest thinning on soil-plant carbon (C) and nitrogen (N) dynamics in two experimental sites, Esk (5 years post-thinning) and Herberton (7 years post-thinning), situated in Queensland, Australia. Methods: The two sites had different thinning regimes. The final stocking rates varied between 75 and 200 stems ha−1at Esk and between 250 and 400 stems ha−1at Herberton. The thinned plots were compared to un-thinned plots. Soil samples were collected to measure labile C and N. Leaf samples were collected from C. variegata and C. citriodora in Esk and Herberton respectively. Results: Thinning did not change soil total C, total N, δ15N and inorganic N at either Esk or Herberton. However, at Esk, intensive thinning resulted in decreases in water soluble total N (WSTN). Foliar δ13C did not vary with respect to thinning whereas foliar δ15N values were more enriched in thinned areas than those of un-thinned plots. The stepwise linear regression indicated that both foliar total N and δ15N were explained mainly by soil TN and WSTN. Conclusions: Thinning did not change soil C and N most likely due to the retention of thinned materials on site and their incorporation into soil. Foliar δ13C was not thinning-dependent due to homeostatic maintenance of the ratio of intercellular to ambient CO2concentrations during photosynthesis. In our study, soil N was not a limiting factor for foliar N, however, foliar N was mainly driven by WSTN which may foreshadow a possible N limitation in severely thinned plots in the long term. We conclude that forest thinning does not decrease soil C and N availability in native Corymbia forests for several years post-thinning if the thinned materials are retained on site. © 2016, Springer International Publishing Switzerland

Stable isotopes in leaf water of terrestrial plants

Leaf water contains naturally occurring stable isotopes of oxygen and hydrogen in abundances that vary spatially and temporally. When sufficiently understood, these can be harnessed for a wide range of applications. Here, we review the current state of knowledge of stable isotope enrichment of leaf water, and its relevance for isotopic signals incorporated into plant organic matter and atmospheric gases