Peters, T. (2009): <b>Struktur und ökologische Merkmale der oberen Waldgrenze in der Andinen Depression</b> Institut für Geographie, FAU Erlangen-Nürnberg, <i>phd thesis</i>
Resource Description
Title:
Struktur und ökologische Merkmale der oberen Waldgrenze in der Andinen Depression
Since 1998, members of the DFG investigation group 402 (Since 2003 DFG investigation<br/>
group 816) devoted themselves to the analysis of a local mountainous rainforest system in<br/>
the Cordillera Real in southern Ecuador. The ?upper treeline? was subject to a closer study<br/>
because of the atypical lowering of the local treeline ecotone at 2600 to 3350 m a.s.l. and<br/>
the missing scientific findings on this topic. Within this thesis, methods of vegetation<br/>
research as well as climatologic and pedologic analysis were used. Vegetation was taken<br/>
up at ten different treeline sites ranging from the northern boundary of the Andes<br/>
Depression near Cuenca in southern Ecuador to Huancabamba in the north of Peru. At<br/>
each investigation area, eight vegetation transects (100 m2 each) were analysed and trees<br/>
with a trunk diameter of >5 cm were recorded to species lists.<br/>
The coverage of climatic parameters was carried out following the North-South-gradient at<br/>
six sites. Three of them were analysed in detail by means of comparative micro-climatic<br/>
measurements between forest and Páramo.<br/>
The pedologic research was carried out at three lower-located sites of the upper treeline<br/>
ecotone in the centre of the investigation area.<br/>
The chapter of results is ordered into three sections, namely, structures of vegetation and<br/>
its characteristics, climate and soil. In the chapter of botanical geography, syntaxonomic<br/>
similarity of each transect and site was calculated by means of statistical classification<br/>
methods and forest species with their particular populations were presented in detail.<br/>
Results show that the upper treeline of the Andes Depression is completely different from<br/>
the outer areas of the Depression in floristic terms. In the centre of the Andean Depression,<br/>
which is rich in endemic species, up to 66 tree species form part of the forest border line<br/>
(concerning an area of only 800 m2!). This number falls drastically at the northern edge of<br/>
the area of examination, where the upper treeline rises to barely more than 4000 m a.s.l.<br/>
Apart from the reduction of species with rising sea level, this fact can be postulated by<br/>
distribution of Polylepis and different treeline structures. Monotypic Polylepis forests<br/>
containing less species only exist at the northern edge of the Andes Depression and at the<br/>
most southern study site in the West of Huancabamba. At all other sites, polytypic forests<br/>
containing many different species are characteristic of the local treeline ecotone.<br/>
The sites? taxonomic differences can be confirmed by ordination methods, which allow a<br/>
clear floristic separation between Polylepis-free areas with populations of lowland taxa and<br/>
Polylepis-forests.<br/>
The different combinations of species are mainly due to variations in altitude, temperature,<br/>
the number of humid months, and latitude. Nevertheless, the existence of an enormous<br/>
range of species itself builds the precondition for a huge potential and a variable mixture of<br/>
available taxa.<br/>
The analysis of climate data shows that soil as well as air temperatures in the centre of the<br/>
study area are clearly above 5.5° C, the postulated threshold value for tree growth.<br/>
Therefore, temperature cannot be considered to cause the lowering of the local treeline<br/>
ecotone of the Andean Depression compared to the Northern and Central Andes. Instead,<br/>
quasi-permanent East winds, high amounts of precipitation throughout the year and maybe<br/>
even extreme incidents of radiation lead to the depression of the local treeline ecotone<br/>
system. While high global radiation leads to radiation stress especially for tree seedlings<br/>
situated outside closed forest stands, strong East winds additionally constrain the<br/>
establishment of forests in the higher-located ridge areas of Páramo formations.<br/>
High precipitation leads to a shortage of nutrients in the more gently inclined ridges where<br/>
the interflow is lower compared to steeper slopes.<br/>
The results of chemical soil analysis show clearly that the three examined sites are<br/>
characterized by low pH-values as well as a lack of nutrients. Furthermore, the upper soil<br/>
layers of the Páramo areas show higher aluminium toxicity due to the type of litter<br/>
decomposed by Páramo plants, a fact hindering the settling of young trees.<br/>
Among the natural factors which prevent the growth of forests in higher areas are high<br/>
rates of radiation, an extreme wind speed, as well as enormous rainfalls. Other sites<br/>
restricted in forest growth are caused by human influence.<br/>
The high diversity of species is remarkable in the upper treeline ecotone in the centre of the<br/>
Andes Depression, a fact that can be ascribed to the lack of nutrients (strongly competitive<br/>
species are locked out, cf. soil nutrient hypothesis) on the one hand. On the other hand,<br/>
high precipitation amounts are also responsible for the lowering of the treeline. The result<br/>
is the combination of a clearly marked forest treeline depression combined with an<br/>
astonishing treeline complexity which is still an almost unknown paradox.
