Date of Award

Spring 1-1-2013

Document Type

Thesis

Degree Name

Master of Arts (MA)

Department

Geography

First Advisor

Thomas T. Veblen

Second Advisor

Holly R. Barnard

Third Advisor

Duncan Christie

Abstract

Treeline is considered to be an ecosystem highly sensitive to climate variability and therefore an excellent environment to test the possible effects of global warming on forest ecosystems. Treeline control theory postulates that low temperatures determine the altitudinal position of treeline worldwide. Based on this theory climate warming should enhance tree radial growth near altitudinal treeline. The present study of the influence of climate variability on tree radial growth of Nothofagus pumilio forest near altitudinal treeline on Choshuenco volcano (40° S) has as objectives: a) To examine the variation in tree radial growth in relation to interannual climate variability and determine growth relationships to monthly and seasonal temperature and precipitation variables; and b) to examine possible relationships of variability of tree radial growth to variability in El Niño Southern Oscillation (ENSO) and the Southern Annular Mode (SAM) at interannual and decadal time scales. Tree cores were collected from old forest located at the upper forest limit along 24 belt transects (20 m x 5 m) in the transition from tall subalpine N. pumilio forest to alpine shrubland. Standard dendrochronological procedures were used to develop chronologies of tree-ring width variations (Cook et al 1990). Nearby climate stations to the sampling site were used to determine relationships between climate variables and radial growth. The analyses include: 1) correlation of radial growth to monthly climate variables using R program package bootRes; 2) correlation of radial growth with large-scale climate drivers using multiple indices of ENSO and SAM; and 3) determination of teleconnections of local climate variability to variability in climate drivers. A chronology of 99 tree-ring series from 74 trees (mean intercorrelation 0.48) was developed from samples in the old forest. Tree growth is positively correlated with early spring precipitation (September) and negatively correlated with late spring (November and December) precipitation. Temperature and tree growth are positively correlated during early summer of the current growing season (December). Monthly indices indicating the warm phase of ENSO (El Niño conditions) of the previous growing season are positively correlated with tree-ring growth. Tree growth tends to be positively correlated with indices of SAM during spring (October through December) of the current growing season. Correlations of regional temperature with ENSO indices indicate warmer summers in association with El Niño-like conditions. Negative correlations between SAM indices and regional precipitation were found for spring of the current growing season (October through December) and again during late summer (February- March) and early fall (April-May) of the current growing season. The findings of the current study imply that year-to-year variability is likely to continue to be strongly associated with ENSO variability, but that multi-decadal scale trends in tree growth are more likely to be predicted by the positive trend in SAM. Overall, the current study shows that at sites near treeline even in a west coast maritime climate known for its high annual precipitation, that global warming cannot be assumed to enhance tree growth.

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