We study the statistics of the return intervals between extreme events above a certain threshold in long-term persistent records. We find that the long-term memory leads (i) to a stretched exponential distribution of the return intervals, (ii) to a pronounced clustering of extreme events, and (iii) to an anomalous behavior of the mean residual time to the next event that depends on the history and increases with the elapsed time in a counterintuitive way. We present an analytical scaling approach and demonstrate that all these features can be seen in long climate records. The phenomena should also occur in heartbeat records, Internet traffic, and stock market volatility and have to be taken into account for an efficient risk evaluation.

Long-term memory: a natural mechanism for the clustering of extreme events and anomalous residual times in climate records.

The RESISTOGRAPH device measures the drill resistance of a fine needle as it penetrates wood. The mean levels of the RESISTOGRAPHIC charts closely correlate with the gross density of dry wood (r 2 >0.8). The charts even reveal density variations within and between tree rings and density variations caused by decay. The density parameters which RESISTOGRAPH charts reveal, e. g. the earlywood and latewood areas in coniferous and deciduous wood correspond to those found in X-ray density charts. But the RESISTOGRAPH resolution is smaller by about one order of magnitude. The average lowest possible limit for tree ring identification from RESISTOGRAPH charts is approximately 0.5 mm. This limit depends on anatomic properties of the wood, on the local geometry of the tree-ring contours, and on the tree-ring density differentiation and is therefore hard to estimate exactly.

Resistograph and X-ray density charts of wood : comparative evaluation of drill resistance profiles and X-ray density charts of different wood species

[1] Tree rings have been used to both reconstruct past climate, and to estimate and project carbon uptake of forest ecosystems. Here we show that large groups of trees of the dominant tree species within widely-distributed circumpolar forest sites show opposing growth trends during recent warming. These opposing growth trends are present at a sub-chronology level and, if averaged into chronologies, may have contributed to the widely reported overall decreased temperature sensitivity of high-latitude chronologies over recent decades. Unlike previous studies, we find that temperature sensitivity has actually increased for most individual trees at these sites. This recent, widespread divergence in growth response seems unique over the past three centuries, and may relate to different microsite responses of individual trees to temperature-induced drought stress or other factors. This divergence needs to be taken into account in dendroclimatic reconstructions, estimations of global warming impacts, and carbon uptake projections.

https://www.ldeo.columbia.edu/res/fac/trl/downloads/Publications/2005WILMKGL023331.pdf

Increased temperature sensitivity and divergent growth trends in circumpolar boreal forests

Stable carbon isotopes in tree rings are a promising tool in palaeoclimate research, provided attempts are made to disentangle climatic from local effects (e.g. soil properties, competition, light). The 13C/12C variations in cellulose of tree rings of beech (Fagus sylvatica) were determined at several sites in the Swiss Central Plateau covering the last 50 years. We chose sites which differ in moisture conditions and sampled cores from four to six trees per site. The mean 13C/12C series from the different dry sites (distant by up to 40 km) are closely interrelated suggesting a common external cause. Correlation analysis with climate data proved the total precipitation in the months May, June and July to have the strongest effect on the carbon isotopes (r =  – 0.73). This result is in agreement with the commonly used model which relates the isotope discrimination to the water use efficiency. On the other hand, the isotope series of the wet sites are not as well correlated to the climate. At two of the sites (a dry and a humid) tree ring width suddenly increased. We used this effect as a test-case to study the influence of local growth conditions on the climate-isotope relationship.

/pdf/stable-carbon-isotopes-in-tree-rings-of-beech-climatic-1lu9ax74yw.pdf

Stable carbon isotopes in tree rings of beech : climatic versus site-related influences

Fertilizer effects on terpene production have been noted in numerous reports. In contrast, only a few studies have studied the response of leaf terpene content to naturally different soil fertility levels. Terpene content, as determined by gas chromatography/mass spectrometry/flame ionization detector, and growth of Pinus halepensis, Rosmarinus officinalis, and Cistus albidus were studied on calcareous and siliceous soils under field conditions. The effect of nitrogen (N) and extractable phosphorus (PE) from these soils on terpenes was also investigated since calcareous soils mainly differ from siliceous soils in their higher nutrient loadings. Rich terpene mixtures were detected. Twenty-one terpenes appeared in leaf extracts of R. officinalis and C. albidus and 20 in P. halepensis. Growth of all species was enhanced on calcareous soils, while terpene content showed a species-specific response to soil type. The total monoterpene content of P. halepensis and that of some major compounds (e.g., δ-terpinene) were higher on calcareous than on siliceous soils. A significant and positive relationship was found between concentration of N and PE and leaf terpene content of this species. These findings suggest that P. halepensis may respond to an environment characterized by increasing soil deposition, by allocating carbon resources to the synthesis of terpene defense metabolites without growth reduction. Results obtained for R. officinalis showed high concentrations of numerous major monoterpenes (e.g., myrcene, camphor) in plants growing on calcareous soils, while α-pinene, β-caryophyllene, and the total sesquiterpene content were higher on siliceous soils. Finally, only alloaromadendrene and δ-cadinene of C. albidus showed higher concentrations on siliceous soils. Unlike P. halepensis, soil nutrients were not involved in terpene variation in calcareous and siliceous soils of these two shrub species. Possible ecological explanations on the effect of soil type for these latter two species as well as the ecological explanation of rich terpene mixtures are discussed.

David M. Meko

Papers

From tree rings