Date of Award

Summer 8-1-2025

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Earth & Environmental Systems

First Advisor

James H. Speer

Second Advisor

Alex Badillo

Third Advisor

Greg Bierly

Abstract

Climate change has altered tree-growth patterns and resulted in higher sea-surface temperatures across the planet. The Himalayan biome is one of the most vulnerable areas in the world to climate change because it has caused changes in forests’ growth and glaciers’ retreat. Dendrochronology has been used to understand the impact of climate change on tree growth and for climate reconstruction. In this dissertation, I discuss the growth of trees in the highlands of the eastern, central, and western Himalayas for the last 500 years and relate these patterns to the present global warming scenario and broad-scale climate modes. I reconstructed surface temperatures for the Indian Ocean Basin to better understand the links to the present global warming scenario. Further, I apply the blue intensity technique to the tree-ring chronology of Pinus wallichiana from northern Pakistan. I have investigated the impact of El Niño-Southern Oscillation, Indian Ocean Dipole, global average annual temperature, and northern hemispheric average yearly temperature on the radial growth of trees from the Himalayas. For this, I downloaded tree-ring chronologies collected in Nepal (n=57), Pakistan (n=43), and Bhutan (n=21) from the International Tree-Ring Databank. I also used six chronologies developed by myself with collaborators from Pakistan (n=5) and Nepal (n=1). I created three individual composite chronologies for each country (Pakistan, Nepal, and Bhutan). A chronology from the western Himalayas (Pakistan) showed more variation than the chronologies from the eastern (Bhutan) and the central Himalayas (Nepal). All three chronologies have shown increased annual growth in recent decades. These three chronologies from the eastern, central, and western Himalayas showed a strong positive correlation with the northern hemispheric temperature anomaly and the global average temperature anomaly. A significant positive relationship is observed between El Niño and the composite chronology from the western and central Himalayas for the winter and spring months. The Indian Ocean dipole mode index showed a positive relationship between all of the composite tree-ring chronologies, but a statistically significant relationship is observed with the composite chronologies from the eastern and central Himalayas. Individual chronology from the Himalayas has shown a site-specific relationship to the mode of climate variabilities. The influence of the Atlantic Multidecadal Oscillation is better observed on the individual chronologies for the Himalayas, and the Southern Annular Mode has the least influence on the site-specific tree-ring chronology for the Himalayas. The surface temperature of the oceans significantly impacts global climate patterns. Present global warming has substantially increased sea-surface temperatures. The surface temperature of the Indian Ocean has a considerable influence on global climate, especially in the bordering nations. There is a need for sub-decadal to annual pre-instrumental sea-surface temperature records. I have reconstructed the Indian Ocean Basin sea-surface temperatures anomaly for the last 548 years from 1468 to 2015 CE using tree-ring chronologies from the Himalayas. The data consists of 957 series of 10 different species. The reconstruction depicts a decreasing sea-surface temperature trend from 1468 to 1920 CE and an increasing trend after 1920 CE. Sea-surface temperature has unprecedentedly increased after 1970 CE. The temperature of the Indian Ocean is positively correlated to the temperature of the tropical Atlantic Ocean. A wavelet analysis indicated that the impact of multidecade oscillation has weakened in the Indian Ocean in the present century. Most of the dendrochronology studies in the Himalayas are based on the measurement of ring width, which is a conventional method. Blue intensity-related studies are limited in the region. In this dissertation, I explored blue-intensity research using tree-ring samples of Pinus wallichiana from northern Pakistan. The blue intensity research is a less tedious, less expensive alternative proxy to maximum latewood density and provides a strong climate signal. I developed a latewood blue intensity chronology from 1520 to 2015 CE and showed that the minimum temperature from July to October primarily controls the growth of latewood cells in Pinus wallichiana in the study area. Latewood blue intensity is also strongly associated with sea-surface temperatures of the tropical Atlantic and northern Indian Ocean.

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