Anthony Fowler

Multi-centennial tree-ring record of ENSO-related activity in New Zealand

It is unclear how global warming will affect the El Nino/Southern Oscillation (ENSO), in part because the instrumental record is too short to understand how ENSO has changed in the past. Now a 700-year-long tree-ring record indicates that ENSO-related climate variability may increase in New Zealand with continued warming.

Extension of the New Zealand kauri (Agathis australis) chronology to 1724 BC

Long tree-ring chronologies have been constructed in the Northern Hemisphere for dendroclimatology and palaeoenvironmental studies, radiocarbon calibration and archaeological dating. Numerous tree-ring chronologies have also been built in the Southern Hemisphere, primarily for dendroclimatology, but multimillennial chronologies are rare. Development of long chronologies from the Southern Hemisphere is therefore important to provide a long-term perspective on environmental change at local, regional and global scales. This paper describes the extension of the New Zealand Agathis australis (kauri) chronology from AD 911 to 1724 BC. Subfossil (swamp) kauri was collected from 17 swamp sites in the upper North Island. Kauri timbers were also obtained from an early twentieth century house on the University of Auckland campus. Twelve site chronologies and 11 independent tree-sequences were constructed and crossmatched to produce a 3631-yr record, which was calendar dated to 1724 BC-AD 1907 against the modern kauri master chronology. A new long chronology, AGAUc04a, was built by combining the modern kauri data with house timbers and subfossil kauri. This new chronology spans 1724 BC-AD 1998. It is of similar length to chronologies from Tasmania and South America and is the longest tree-ring chronology yet built in New Zealand. The greatest significance of the long kauri chronology lies in its potential as a high-quality palaeoclimate proxy, especially with regard to investigation of the El Ninio-Southern Oscillation phenomenon. The chronology also has application to investigation of extreme environmental events, dendroecology, archaeology and radiocarbon calibration.

Dendroclimatic interpretation of tree-rings in Agathis australis (kauri): 2. Evidence of a significant relationship with ENSO

Part 1 of this investigation (Buckley et al 2000) found consistent significant correlations between tree growth and climate for nine New Zealand kauri tree‐ring chronology sites The nature of these correlations suggests that Agathis australis (kauri) may carry a useful signal of the El Nino‐Southern Oscillation (ENSO) phenomenon We have explored the potential of kauri tor ENSO reconstruction through statistical analysis of the relationship between the Southern Oscillation Index (SOI) and the tree ring indices previously derived Results showed a consistent SOI‐kaun growth relationship for eight of the nine sites A significant negative correlation was found between kauri growth and concurrent seasonal mean SOI, and a positive correlation with the seasonal mean SOI recorded over the preceding two years The former is consistent with a hypothesised ENSO‐kaun growth model, but the two‐year lag suggests an additional relationship, perhaps associated with kauri phenology Decadal‐scale variability was evident in the strength of the SOI‐kaun growth relationships, particularly in autumn (March‐May) and winter (June‐August) Comparison of SOI and extreme kauri growth years indicated general consistency in the growth response to ENSO, but also identified some anomalies, suggesting that kauri ring‐width is an imperfect ENSO proxy However, combined with the spatial scale at which ENSO operates, and the known variability of links with regional climates, we conclude that kauri growth‐rings could provide a useful ENSO proxy, particularly within the context of multi‐proxy spatially distributed studies

Assessment of the validity of using mean potential evaporation in computations of the long-term soil water balance

Multi-decadal modelling of the soil water balance is often limited by the availability of data needed to calculate potential evaporation (PE). A solution sometimes adopted is to use available precipitation data but substitute a climatological representation of PE, the rationale being that PE is an inherently conservative variable. Convincing support for such an approach can be found in the literature but there are grounds for suspecting significant bias under extreme conditions. The validity of using climatological PE in long-term water balance studies is investigated by comparing soil water deficit time series simulated using a daily water balance model. This involves comparing a baseline run of the model using actual PE (based on observations) with alternative simulations using various climatology-based estimates of PE. The modelling experiments are over 13 years for one site (Auckland, New Zealand) with emphasis on comparing model performance during wet and dry years. Results show that substituting climatological PE estimates into the daily water balance produces a soil water regime very similar to that derived using actual PE values, including over relatively extreme periods. Best results were obtained where a PE reduction was applied to account for PE suppression on raindays, but the improvement in performance was minor compared to cruder methods, including PE estimates based on monthly data.

Tree-Ring Studies on Agathis australis (Kauri): A Synthesis of Development Work on Late Holocene Chronologies

The potential of kauri (Agathis australis) for paleoclimate research is well established. Multiple tree-ring chronologies have been derived from living and sub-fossil material and growth-climate relationships have been identified. Work has progressed to the stage where raw ring-width data and chronologies covering the last half of the second millennium can confidently be placed in the public domain, to facilitate multi-proxy paleoclimate studies. This paper outlines progress in deriving kauri tree-ring chronologies, summarises data availability and quality, and explores the scope for developing composite chronologies. Statistical quality control of the available data was undertaken, following application of an “optimum” standardisation technique. Variations in sample depth with time and between sites result in a complex evolving pattern of chronology quality across sites. Analysis of inter-site statistical relationships identified a pervasive regional-scale signal in kauri with some minor secondary patterns. In light of the strong common signal, a kauri master chronology was built by pooling tree-ring series. Analysis of the quality of this chronology indicates that high-quality master chronologies can be derived for A.D. 1597–1996 from as few as 25 trees from seven sites.

Dendroclimatic interpretation of tree‐rings in Agathis australis (kauri). 1. Climate correlation functions and master chronology.

In this paper we analyse nine existing Agathis australis (kauri) chronologies for their response to climate, and compare our results with those of previous studies We update the southernmost chronology, from Katikati, which now extends to the 1997 growth ring (1997–98 growing season in Southern Hemisphere) We also employ recent standardisation procedures that have been demonstrated to eliminate the chance of biasing the chronology indices Climate correlation functions are generated for all nine kauri chronologies, by correlating chronology indices with meteorological datasets In an earlier study only a 12 month response window was analysed, combined with lagging the growth year in order to account for prior‐season growth response Our expanded dendroclimatic response window covers the 21 months from May of the year of growth (t), back to the previous September (year t ‐ 1) There are consistent significant correlations with climate for all nine kauri sites, most pronounced in the form of a positive response in season t to precipitation in the previous season (t ‐ 1), and an inverse response to temperature in the year of growth The most robust climate signal comes from the Katikati chronology, which has been updated by 16 years to the 1997 growth ring The additional years allow for more degrees of freedom and a better estimate of the climate correlation functions Correlation and Principal Component Analyses validate the combining of eight of the nine chronologies into one regional time series The results presented in this paper are encouraging for future dendroclimatic research with Agathis australis, towards the goal of long‐term reconstruction of climate

Seasonal Growth Characteristics of Kauri

Considerable research has occurred in recent years to build Agathis australis (D. Don) Lindley (kauri) tree-ring chronologies for paleoclimate applications and to identify statistical relationships between kauri tree rings and climate. This paper reports on a multi-year study of the seasonal growth of kauri, designed to assist in the interpretation of identified statistical relationships, and to determine if kauris seasonal growth characteristics are dependent on tree size. To achieve this, 43 kauri (0.09–2.00 m diameter) at Huapai Scientific Reserve were fitted with vernier bands to measure circumference change over 3–4 growing seasons. Absolute (mm) and relative (proportion of total ring) monthly growth rates were calculated for each tree and statistics characterizing the timing of growth were calculated (e.g. date corresponding to 50% of growth). Tree size-related differences were assessed by splitting the data into three subsets based on size, then comparing the monthly growth rates and growth timing statistics for the subsets. The growth timing statistics were also correlated with tree diameter. A key finding is the strong dominance of spring growth, with October and November alone accounting for 38–50% of the total ring width. This result is consistent across age cohorts, although the largest trees tended to peak in November, rather than October. This indicates that kauri tree rings are likely to have value in terms of reconstructing spring conditions; consistent with reported statistical relationships between kauri tree rings and the El Niño–Southern Oscillation phenomenon. High inter-tree variance in growth rates characterized the results, but little of this variance was accounted for by tree size. Although relationships between tree size and growth characteristics were generally weak and inconsistent, they are considered sufficient to warrant a precautionary approach in the development of tree-ring chronologies for climate reconstruction purposes.

High-resolution modelling of mid-Holocene New Zealand climate at 6000 yr BP

Palaeoclimate-proxy data provide an invaluable source of evidence for past climatic conditions, which can be compared with data from climate model simulations. This study illustrates how high-resolution regional climate model simulations can be used to estimate the difference in the climate of New Zealand between 6000 years before present (yr BP) and the pre-industrial era (c. ad 1750). Four pairs (pre-industrial and 6000 yr BP) of atmosphere-only global and regional climate model simulations were run with prescribed sea surface temperatures (SST). The SSTs are derived from four different fully coupled ocean–atmosphere general circulation model simulations, resulting in a different lower-boundary forcing in each of the atmosphere-only simulations. We find evidence for generally cooler conditions and for wetter (drier) conditions over eastern (western) New Zealand 6000 yr BP. The work compares well with model and proxy estimates of temperature and precipitation in the New Zealand region between 7000 and 6000 yr BP. The results also highlight the added value of regional model studies in regions with such complex terrain as New Zealand. This study also shows the limitations of applying uniformitarian principles when downscaling global model fields (and ‘up-scaling’ palaeo-proxy data) to infer past climatic conditions.

Comment on Malanson (2017) "Mixed signals in trends of variance in high-elevation tree ring chronologies" published in Journal of Mountain Science

In a recent paper published in Journal of Mountain Science, Malanson (2017) explored variance changes in Rocky Mountains tree-ring chronologies. This commentary points out some methodological issues related to systematic bias in evolving tree-ring chronology variance and suggests that analyzing the slopes of linear regression lines may be suboptimal for identifying temporal changes in variance. The journal editor invited the original article’s authors Dr. Malandson to respond to the comments. Thus Dr. Malandson’s response is attached behind the comments.