Dendrochronology in climatology – the state of the art

The use of orthogonalized climatic variables in regression to specify treegrowth /climate relationships, commonly known as response function analysis, involves several a priori decisions and a posteriori interpretations, any of which maybe open to question. Decisions about the number of climatic variables to include, confidence limits, number of eigenvectors to allow as candidate predictors in regression, etc., can affect the response function in unpredictable ways and lead to possible errors in interpretation. To demonstrate the nature of these effects, we compared response functions for particular chronologies with the correlation function, which is simply the series of correlation coefficients between a tree -ring chronology and each of several sequential monthly climatic variables. The results indicate that response functions including high -order eigenvectors should be interpreted cautiously, and we recommend using the correlation function as an interpretive guide. Prior tree -growth variables in regression can mask climatic effects, and the correlation function can also be useful in detecting this masking. Statistical significance is more often attained in response functions than in correlation functions, possibly due to differences in the statistical testing procedures, to the statistical efficiency of eigenvectors in spending degrees of freedom, or to the filtering effects on the climatic data that result from eliminating high -order eigenvectors (noise) from the response function. These filtering effects plus the orthogonalization make response function analysis an efficient method for specifying tree -growth /climate relationships. The examples and guidelines presented here should enhance the usefulness of the method. Die Benutzung von orthogonalisierten Klimavariablen zur regressionsanalytischen Bestimmung von KlimaWachstumsBeziehungen -Bekannt als Responsefunktionsanalyseerfordert mehrere aprioriEntscheidungen und aposterioriInterpretationen, die fragwurdig sein konnen. Entscheidungen uber die Anzahl der einzubeziehenden Klimavariablen, Vertrauensgrenzen, die Anzahl der Eigenvektoren als Prediktoren in der Regression, usw. konnen die Responsefunktion in unvorhersehbarer Weise beeinflussen und zu Fehlinterpretationen fuhren. Um derartige Wirkungen aufzuzeigen, wurden die Responsefunktionen fur bestimmte Chronologien mit den entsprechenden Korrelationsfunktionen verglichen. Die Korrelationsfunktion besteht aus den Korrelationskoeffizienten zwischen einer Jahrringchronologie und den einzelnen, aufeinanderfolgenden Klimavariablen. Die Ergebnisse zeigen, das Responsefunktionen aus Eigenvektoren hoher Ordnung vorsichtig interpretiert werden sollten. In solchen Fallen wird die Korrelationsfunktion als Interpretierungschilfe empfohlen. In der Regressionsberechnung konnen Vorjahreszuwachse Klimaeinflusse uberdecken. Die Korrelationsfunktion kann derartige Uberdeckungen erkennen helfen. Bei Responsefunktionen wird haufiger eine statistische Signifikanz erreicht als bei Korrelationsfunktionen, vermutlich aufgrund unterschieldlicher Testverfahren, wegen der positiven Eigenschaft der Eigenvektoren beim Verbrauch von Freiheitsgraden und wegen der Filterwirkung auf die Klimadaten, die dadurch hervorgerufen wird, das Eigenvektoren hoher Ordnung (Gerausche) eliminiert werden. Diese Filterwirkungen zusammen mit der Orthogonalisierung machen die Responsefunktionsberechnung zu einer wirksamen Methode zur Bestimmung von Klima -Wachstums -Beziehungen. Die vorgestellten Beispiele und Empfehlungen sollten die Brauchbarkeit dieses Verfahrens hervorheben. L'utilisation de variables climatiques orthogonalisees dans une regression destinee a definir les relations cernes -climat, communement connues comme analyse de la

Response Functions Revisited

To understand how tree growth has responded to recent climate warming, an understanding of the tree-climate-site complex is necessary. To achieve this, radial growth variability among 204 trees established before 1850 was studied in relation to both climatic and site factors. Seventeen forest stands were sampled in the Spanish Central Pyrenees. Three species were studied: Pinus uncinata, Abies alba, and Pinus sylvestris. For each tree, a ring-width residual chronology was built. All trees cross-dated well, indicating a common influence of the regional climate. For the 1952-1993 period, the radial growth of all species, especially P. uncinata, was positively correlated with warm Novembers during the year before ring formation and warm Mays of the year the annual ring formed. Differences in species-stand elevation modulated the growth-climate associations. Radial growth in P. uncinata at high elevation sites was reduced when May temperatures were colder and May precipitation more abundant. In the 20th century, two contrasting periods in radial growth were observed: one (1900-1949) with low frequency of narrow and wide rings, low mean annual sensitivity, and low common growth variation; and another (1950- 1994) with the reverse characteristics. The increased variability in radial growth since the 1950s was observed for all species and sites, which suggests a climatic cause. The low shared variance among tree chronologies during the first half of the 20th century may result from a ''relaxation'' of the elevation gradient, allowing local site conditions to dominate macroclimatic influence. These temporal trends may be related to the recently reported increase of climatic variability and warmer conditions. This study emphasizes the need to carefully assess the relationships between radial growth and site conditions along ecological gradients to improve dendroclimatic reconstructions.

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Spatiotemporal variability in tree growth in the central pyrenees: climatic and site influences

. A study of the forest lines, tree lines and the structures of the sub-alpine forest was performed in Vallone Vallanta and in Aleve forest in the Varaita Valley (Cottian Alps, Piedmont, Italy). Forest- and tree lines were analysed over 1728 ha while forest structures were studied on six 3000-m2 plots located at the tree line (2), at the forest line (2) and inside the sub-alpine forest (2). Dendro-ecological analysis of living plants and stumps showed that Larix decidua was more abundant in the past than today and that Pinus cembra has expanded, both upwards and within sub-alpine forests. Age structure analysis revealed that the current sub-alpine forest stands were established 200–220 yr ago, probably following a clearcut. At the forest lines the tree density decreases, and some trees are more than 500 yr old, whereas at the tree lines most of the trees (almost exclusively Pinus cembra) are younger than 100 yr. Growth dynamics were investigated both by observing Basal Area Increment (BAI) in the old and dominant trees, and by comparing the BAIs of classes of trees with a given cambial age range in different time periods. The results showed that the growth rates of mature Pinus cembra and Larix decidua had increased. These increments are more substantial for Pinus than for Larix. The growth rate of young trees (< 100 yr) of both species has decreased over recent decades. This could be due to competition caused by increased tree densities that have resulted from a decrease in grazing.

Growth trends and dynamics in sub-alpine forest stands in the Varaita Valley (Piedmont, Italy) and their relationships with human activities and global change

Summary We analyze interannual to multi-decadal growth variations of 555 oak trees from Central-West Germany. A network of 13 pedunculate oak (Quercus robur L.) and 33 sessile oak (Quercus petraea (Matt.) Liebl.) site chronologies is compared with gridded temperature, precipitation, cloud-cover, vapor pressure and drought (i.e., Palmer Drought Severity Index, PDSI) fluctuations. A hierarchic cluster analysis identifies three groups for each oak species differentiated by ecologic settings. When high precipitation is primarily a characteristic for one Q. robur and one Q. petraea cluster, the other clusters are more differentiated by prevailing temperature conditions. Correlation analysis with precipitation and vapor pressure reveals statistically significant (P 0.05) correlations for June (r = 0.51) and annual (r = 0.43) means. Growth of both species at dry sites correlates strongly with PDSI (r = 0.39, P 0.05), and weakly with temperature and cloud-cover. In natural stands, Q. robur responds more strongly to water depletion than Q. petraea. Twenty-one-year moving correlations show positive significant growth response to both PDSI and precipitation throughout the 20th century, except for the 1940s – an anomalously warm decade during which all oak sites are characterized by an increased growth and an enhanced association with vapor pressure and temperature. We suggest that the wider oak rings that are exhibited during this period may be indicative of a nonlinear or threshold-induced growth response to drought and vapor pressure, and run counter to the general response of oak to drought and precipitation that normally would result in suppressed growth in a warmer and drier environment. As the wide rings are formed during the severe drought period of the 20th century, a complex model seems to be required to fully explain the widespread oak growth. Our results indicate uncertainty in estimates of future growth trends of Central European oak forests in a warming and drying world.

/pdf/complex-climate-controls-on-20th-century-oak-growth-in-2mws79c1r1.pdf

Complex climate controls on 20th century oak growth in Central-West Germany

INTERPRETATION of proxy records of past environmental conditions derived from dated geological or biological materials is of great importance for the extension of the climatic record1,2. Precisely dated, replicated tree-ring series have been particularly useful as they provide records (usually ring-widths) dated to the individual year, for hundreds or thousands of years. Each such series, or chronology, is derived from a particular known location. Thus a network of such chronologies may be developed for a particular region and used as a proxy record of spatial and temporal climatic variations. This has been achieved in North America by Fritts et al.3,4, using principally chronologies from semi-arid areas or from near altitudinal or polar tree-lines. We report evidence here that tree-ring chronologies from sites in the British Isles will provide suitable proxy records for the reconstruction of historical temporal and spatial variation of climate.

P. Leggett

Papers

Climatic signals in British Isles tree-ring chronologies

Climatic signals in tree-ring chronologies for British oaks (reply)