Characterization of Polylepis tarapacana Life Forms in the Highest-Elevation Altiplano in South America: Influence of the Topography, Climate and Human Uses

Abstract

In the upper vegetation limit of the Andes, trees change to shrub forms or other life forms, such as low scrubs. The diversity of life forms decreases with elevation; tree life forms generally decrease, and communities of shrubs and herbs increase in the Andean highlands. Most of treeline populations in the northwestern Argentina Altiplano are monospecific stands of Polylepis tarapacana, a cold-tolerant evergreen species that is able to withstand harsh climatic conditions under different life forms. There are no studies for P. tarapacana that analyze life forms across environmental and human impact gradients relating them with environmental factors. This study aims to determine the influence of topographic, climatic, geographic and proxies to human uses on the occurrence of life forms in P. tarapacana trees. We worked with 70 plots, and a new proposal of tree life form classification was presented for P. tarapacana (arborescent, dwarf trees, shrubs and brousse tigrée). We describe the forest biometry of each life form and evaluate the frequency of these life forms in relation to the environmental factors and human uses. The results show a consistency in the changes in the different life forms across the studied environmental gradients, where the main changes were related to elevation, slope and temperature.

Keywords: Argentinean highlands; high Andean vegetation; multi-stemmed; scrub; scrublands; stem; tree; tree growth form.

Figure 1

Classification of life forms in P. tarapacana. Ar: Arborescent; Dt: Dwarf tree; Sh: Shrubs; Bt: Brousse tigrée. The red line in the photo indicates the vertical cut that is observed in the graph on the left.

Figure 2

Kruskal–Wallis test for the diameter, height, tree crown and crown spread ratio of P. tarapacana life forms. Different letters indicate significant differences (p < 0.05) by Conover–Iman test.

Figure 3

Topographic, climatic and human use factors classified by the frequency of life forms. Relationship among topographic variables ((A) Elevation and slope, (B) Aspects), climatic ((C) temperature and precipitation) and human use ((D) human footprint and distance to towns). Bars indicate the standard deviation of each axis. Elevation in m a.s.l.; Slope in degree; S|N: North Aspect; W|E: East aspect. The aspects factors were calculated as sine and cosine functions, where sine values range from −1 (west) to 1 (east), while cosine values range from −1 (south) to 1 (north). AMT: Annual mean temperature in °C; AP: Annual precipitation in mm.yr⁻¹; HF: Human footprint; DTT: Distance to towns in km. Ar: Arborescents; Sh: Shrubs; Dt: Dwarf trees; Bt: Brousse tigrée; M: Multiple forms.

Figure 4

Distribution of Polylepis tarapacana forests (green) in the study area (Altiplano, Argentina), showing plots (red circles), towns (black circles) and contour lines, at 4000 m a.s.l. (narrow line), 4500 m a.s.l. (medium line) and 5000 m a.s.l. (tick line). Modified from López et al. [23].

Figure 5

Determination of the height, diameter at the base, crown diameter of the maximum axis and of the axis at 90 degrees for each life forms in P. tarapacana. H: height; DAB: diameter at the base of the tree; Ar: Arborescent; Dt: Dwarf tree; Sh: Shrubs; Bt: Brousse tigrée.