dc.description.abstract
This research presents a multi-scalar work on the effects of human disturbance
on high Andean forests in Colombia. It focuses on the analysis of vegetation
composition and structure and on the dynamics of forest cover change and connectivity
through time in selected localities around Bogotá. Additionally, a distribution model
exercise is carried out to further the understanding of the environmental constraints
driving the present distribution of a distinctive Andean treeline species and to examine
its response to climate change in the future.
High Andean forests are a unique and highly biodiverse ecosystem, located at
high elevations in one of the world’s biodiversity hotspots, the Tropical Andes.
Unfortunately, these forests are extremely fragmented and particularly vulnerable to
climate change. Despite the fact that they provide essential ecosystem services to
neighboring high Andean cities, such as the capital of Colombia, Bogotá, overall, high
Andean forests have been poorly studied. Their fragility and their great value in terms
of environmental services, together with their outstanding diversity, make them
ecosystems with high priority for conservation. Nevertheless, especially around
Bogotá, the vegetation studies carried out often have been rather circumscribed, rarely
incorporating more complex parameters such as landscape fragmentation and
anthropic disturbance metrics nor assessing functional and phylogenetic diversity
attributes and ecosystem services. Also, there is a lack of studies that consider multiple
vegetation strata at once and that address long-term changes in the extent and
structure of high Andean forests, pairing plot-based work with forest cover change
analysis. Lastly, few efforts have been made so far to model the response to climate
change of high Andean forests and their species. Investigating these aspects could
provide meaningful insights into species’ responses to anthropic influences and
community assembly mechanisms, broadening our understanding of high Andean
forests and strengthening strategies to conserve them.
The aim of the present study is to explore the relationship between human
disturbance and the condition of high Andean forest in the surroundings of Bogotá, the
growing capital city of Colombia. Therefore, three parallel approaches were developed
and implemented. On one hand, permanent plots establishment and plot-based
fieldwork was carried out in several localities in the surroundings of Bogotá. Here, tree
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layer and understory vegetation were assessed and analyzed in terms of their
taxonomic, functional and phylogenetic diversity, but also as to their structure and
above-ground biomass production. A broad array of environmental factors and
anthropic disturbance-related indicators was compiled and subsequently used to
characterize the tree layer and understory plant communities using a cluster analysis
in combination with ordination techniques (Non-metric Multidimensional Scaling and
partial Redundancy Analysis). Finally, environmental and disturbance predictors were
related to tree and understory diversity metrics and above-ground biomass by means
of Generalized Linear Models (GLMs). As a result, a noticeable effect of anthropic
related predictors on both tree and understory layer community composition and also
a direct negative effect on tree layer diversity and above-ground biomass production
have been documented.
On the other hand, a thorough analysis of aerial pictures from the studied
localities of the first study has been conducted, covering the period between the 1940s
to early 2000s, in order to deepen the understanding of the history of forest
fragmentation in the area. The images were classified into three land cover classes
(Forest, Grasslands and Bare ground), utilizing texture analysis and object-based
image segmentation. Then, the extent of forested areas was assessed through time.
Additionally, forest connectivity metrics were calculated by performing a Morphological
Spatial Pattern Analysis (MSPA) and correlated with the forest cover dynamics over
the same period. For the period 1940-2000, in all except one of the studied localities a
general restoration of forest cover and connectivity was determined. The highlighted
forest cover and connectivity trends were related to the individual history of each
locality in terms of population displacement and urban expansion.
Lastly, to further expand the geographical and temporal coverage of this study,
species distribution models were performed for Weinmannia fagaroides
(Cunoniaceae), a tree species that was found to be distinctive of the higher elevation
forest fragments that were studied in this project. This species is found throughout the
Central and Northern Andes, and is a typical component of high elevation treelines,
ecotonal zones that are believed to be highly vulnerable to climate change impacts.
The main aim was to understand which are the focal drivers of the current distribution
of W. fagaroides, and to hypothesize its response to climate change in the future. At
the same time, a comparison of the efficacy of environmental predictors pertaining to
three different climate datasets (Chelsa 1.2, Worldclim 1.4 and Worldclim 2) and of
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three different modelling algorithms (GLMs, Random Forests - RF, MaxEnt) was
carried out for predicting the current distribution of W. fagaroides. The current species
distribution was found to be driven mostly by one temperature, one precipitation and
one soil-related variables. In regards to the performance of the three different climate
datasets no significant difference was found, although Chelsa 1.2 retrieved slightly
higher model evaluation scores. As to the algorithms, RF and MaxEnt retrieved the
highest model evaluation scores. Moreover, the modeled distribution of W. fagaroides
was predicted to decrease by 38% and to shift towards higher elevations in the period
2060-2080 for a high CO2 emission scenario. Considering that at present only around
22% of the modeled species distribution is situated in protected areas, an adaptation
of currently implemented conservation efforts is strongly recommended to protect this
and other fragile high Andean treeline species.
Overall, this study contributes to the characterization of forest fragments in the
vicinity of Bogotá and is an important addition for a better understanding of the effects
of anthropic disturbances on high Andean forests and their species. Also, several
methodological approaches are provided that may serve as a basis to better
understand the situation of other species and ecosystems in similar regions.
