Altitudinal Treelines in Northern Patagonia

The Dynamics of Altitudinal Treelines in Northern Patagonia:
Spatio-Temporal Influences of Climate

L.D. Daniels
PhD Dissertation, University of Colorado at Boulder

ABSTRACT

This research examined altitudinal treelines dominated by Nothofagus pumilio in northern Patagonia near 40°S latitude in Chile and Argentina. It examined the classical paradigm that low temperature is the dominant climatic factor limiting altitudinal treeline against the alternative that moisture availability is dominant.

The three principal research objectives were: (1) to quantify regional climate in northern Patagonia and relationships between El Nino-Southern Oscillation (ENSO) and climate, (2) to compare biophysical attributes of treeline under the influence of disturbance and contrasting regional and local climates, and (3) to examine the influence of inter-annual to decadal climate variation on tree seedling demography and radial growth rates of krummholz trees of Nothofagus pumilio growing near treeline.

Treelines were sampled at three spatial scales corresponding to regional, local and micro-climates. Three study areas were positioned across the Andes: one in Chile (Antillanca) and two in Argentina (Cerro Bayo and Challhuaco). Within study areas, treelines on slopes with contrasting aspects were compared. At the finest scale, the treeline ecotone was sampled from the erect forest below timberline and through the krummholz forest to treeline.

This research revealed significant spatial and temporal variation in Nothofagus pumilio treelines related to disturbance, differences in regional, local, and micro-climates, as well as climate variability over time. Both physical and biological attributes of treeline differentiated non-disturbed treelines from those affected by rapid mass movements and seasonal soil creep. Variation of treeline elevation, krummholz growth rates and seedling densities at the regional and local scales, implied that climate factors influence treeline differently west and east of the Andes. Higher temperature had a positive influence on these treeline attributes in the wet climate of Antillanca, Chile but its influence was negative in the dry climate on the Argentine side of the Andes.

Analysis of instrumental climate records identified significant spatial and temporal differences in regional climates west and east of the Andes. The relationship between ENSO and climate parameters in northern Patagonia has been unstable over time. In 1976-77, climate on the Argentine side of the Andes shifted from cool, wet to warm, dry conditions. This facilitated comparisons of the relationships of radial growth and seedling establishment of Nothofagus pumilio during two climatically distinct periods: 1957-76 versus 1977-96.

Four aspects of the relationships between climate and Nothofagus pumilio radial growth and seedling establishment demonstrated the complexity of treeline dynamics in northern Patagonia. (1) The relationships of radial growth to temperature variation were non-linear. (2) Moisture availability was the dominant climatic factor influencing seedling establishment, although temperature-precipitation interactions resulted in variability among study areas. (3) Radial growth of krummholz and seedling demography responded differently to climate variation. (4) The relationships of krummholz radial growth and seedling demography with climate and ENSO differed among study areas and were unstable over the past 40 years.

The spatial and temporal instabilities in vegetation-climate relationships documented in this study exemplify the need to exercise caution when conducting historical reconstructions of climate-vegetation relationships that may be unstable through time. Retrospective research methods inherently assume temporal stability of climate-vegetation relationships. However, this research illustrated instabilities both in seedling establishment and tree radial growth of Nothofagus pumilio over time and through space. Understanding these instabilities contributes to knowledge of the responses of forests in northern Patagonia to past, present, and future climate variation. When relationships are unstable, short-term intensive analyses may provide a more reliable understanding of treeline dynamics in relation to climate change than alternative research approaches that assume stability.