Elevational patterns of Polylepis tree height (Rosaceae) in the high Andes of Peru: role of human impact and climatic conditions

Abstract

We studied tree height in stands of high-Andean Polylepis forests in two cordilleras near Cuzco (Peru) with respect to variations in human impact and climatic conditions, and compared air and soil temperatures between qualitatively defined dry and humid slopes. We studied 46 forest plots of 100 m2 of five Polylepis species at 3560–4680 m. We measured diameter at breast height (dbh) and tree height in the stands (1229 trees in total), as well as air and soil temperatures in a subset of plots. The data was analyzed combining plots of given species from different sites at the same elevation (±100 m). There was no elevational decrease of mean maximum tree height across the entire data set. On humid slopes, tree height decreased continuously with elevation, whereas on dry slopes it peaked at middle elevations. With mean maximum tree heights of 9 m at 4530 m on the humid slopes and of 13 m at 4650 m on the dry slopes, we here document the tallest high-elevation forests found so far worldwide. These highest stands grow under cold mean growing season air temperatures (3.6 and 3.8°C on humid vs. dry slopes) and mean growing season soil temperatures (5.1 vs. 4.6°C). Mean annual air and soil temperature both decreased with elevation. Dry slopes had higher mean and maximum growing season air temperatures than humid slopes. Mean annual soil temperatures did not significantly differ and mean annual air temperatures only slightly differed between slopes. However, maximum air temperatures differed on average by 6.6 K between dry and humid slopes. This suggests that the differences in tree height between the two slopes are most likely due to differences in solar radiation as reflected by maximum air temperatures. Our study furthermore provides evidence that alpine Polylepis treelines grow under lower temperature conditions than global high-elevation treelines on average, suggesting that Polylepis species may have evolved special physiological adaptations to low temperatures.

Elevational Shifts in the Topographic Position of Polylepis Forest Stands in the Andes of Southern Peru

“…The current Polylepis forest distribution in the Andes is highly fragmented, with both natural and anthropogenic causes [1, 2,4,6]. The aim of this study was to find out whether the forest stands of the treeline-forming species Polylepis subsbericans are found at specific topographic positions, and whether their potential associations with topography change with elevation.…”

“…The study area was selected based on the availability of high-resolution and cloud-free remote sensing data and extensive field surveys for ground truthing carried out in the area since 2006 [23]. The climate in the study area is a semi-arid tropical alpine climate, with mean annual growing season air and soil temperatures at 4450 m of 3.9 °C and 4.6 °C, respectively [6], and with low annual but high diurnal variations in temperature [15]. There is a pronounced wet season from October to March.…”

“…The latter determines the likeliness of night-time freezing and exposure to solar radiation. In spite of being located within the tropics, our study area has a colder season coinciding with the southern hemisphere winter, when night temperatures frequently fall below 0 • C in this part of the high Andes [6,15,31]. Polylepis trees can tolerate or avoid freezing for short periods with different physiological adjustments [32,33], but long-term freezing is critical to tree growth.…”

“…A lack of warmth likely also limits Polylepis, although this genus has been found at somewhat lower mean growing-season temperatures than the global average [6]. Also, other abiotic factors such as annual precipitation, soil texture, and hydrological conditions have been shown to play a role in determining Polylepis forest distributions in the Andean landscape [7][8][9].…”

Why tree lines are lower on islands—Climatic and biogeographic effects hold the answer

“…The elevations at which climate parameters such as temperature and water availability limit tree growth (hereafter: tree line) shift in a predictable manner with latitude. The highest tree lines are found at 4,800–4,900 m in tropical and subtropical latitudes in the Andes and Tibet (Hoch & Körner, ; Kessler, Toivonen, Sylvester, Kluge, & Hertel, ), and tree lines decline poleward to sea level at about 75° north (Jobbágy & Jackson, ; Troll, ). Although the underlying physiological causes are still debated (Hertel & Schöling, ; Hertel & Wesche, ; Körner, ; Smith, Germino, Hancock, & Johnson, ), it is well established that at broad spatial scales, the global tree line elevations are closely correlated with climatic variables reflecting the length of the growing season, and temperatures during the growing season (Körner, ; Körner & Paulsen, ; Paulsen & Körner, ).…”