Polylepis forests are a unique high-elevation forests dominated by trees of the Polylepis (Rosaceae) genus found only in South America. These forests are distinctive not only for growing at some of the highest elevations in the world, but because they are distributed as isolated forest islands inside a landscape otherwise dominated by grasses. This distribution is due in part to human disturbance on the landscape and there is interest in restoring cover of Polylepis. Understanding of regeneration dynamics and seedling ecology of Polylepis can help in the management of forests and the restoration of this forest cover, but there are important gaps in knowledge regarding theses aspects in most species. For the expansion of forest cover into current grassland areas, it is important to understand the ability of seedlings to colonize, survive and grow in these areas and identify important biotic and abiotic barriers to this. In this dissertation, I contribute to our understanding of these ecological aspects for two species of Polylepis (P. sericea and P. weberbaueri) using a series observational studies and manipulative experiments in forests found between 4000–4500 masl in Huascaran National Park (Ancash, Peru).
In Chapter 1, I surveyed patterns of seedling dispersion across forest-grassland boundaries along the elevational gradient and identified significant associations of seedling densities to environmental covariates at local and landscape scales. I found that seedling densities of both species decrease drastically at the edge of the forest canopy, and few seedlings are found in the grassland even within meters of the forest, suggesting significant barriers to seedling establishment in the grassland. Additionally, seedling densities of each species showed patterns with elevation and solar irradiation that mirrored those of adult distributions on the landscape and were different from each other, suggesting that each species has unique environmental tolerances that manifest in the seedling stage and are important to consider in their management and restoration.
In Chapter 2, I used a series of livestock exclusion experiments to examine the influence of domestic livestock on natural regeneration inside and outside the forest and test the hypothesis that livestock are a significant barrier to seedling establishment at forest-grassland edges. After one year of livestock exclusion, I found only partial support for this hypothesis. In P. sericea forest stands, seedling densities increased, and recruitment and seedling growth was higher in fenced grassland areas. However, in P. weberbaueri stands there was little change in juvenile densities and no differences in seedling performance, except that recruitment was lower in fenced forests areas compared to unfenced forest. This showed that the influences of livestock on Polylepis seedling may be different depending on the habitat and species of Polylepis. Additionally, we found that seedling survival was surprisingly high, and similar between forest and grassland habitat.
In Chapter 3, I used sowing and “wildling” transplant experiments within the previous exclosure experiments to tease apart early ontogenetic filters to seedling establishment in the grassland compared to the forest. I found that the seedling emergence stage is a strong bottleneck for seedling establishment in the grassland. Furthermore, I found that P. weberbueri seedling density inside the forest appears to be seed limited, and can be increased by sowing. Transplanted seedlings survival was lower than found previously for natural seedlings, but was also similar between both habitats. However, it appears that livestock depresses P. sericea transplant survival inside the forest. Only P. weberbueri seedlings showed differences in growth between habitats, declining in height in the grassland, but there was only evidence that livestock suppressed growth. I concluded that seedling transplant will be the most efficient way to increase seedlings in the grassland for restoration.
Together, the results of this dissertation suggest that both unassisted and assisted expansion of natural Polylepis forests faces significant barriers, in particular low seedling emergence in grassland is a strong bottleneck to establishment in this habitat. It remains unclear whether livestock are generally a barrier to forest expansion, as their effects appear to be specific to each species of Polylepis. Here, P. sericea appears to be more sensitive in both forest and grassland to livestock activity than P. weberbaueri, but future work should address longer-term effects of livestock exclusion to confirm the population trajectories observed after one year. Finally, wilding transplants of these species can be used in restoration to overcome the barrier emergence poses to Polylepis establishment in grassland, although with further research, seeding schemes may prove a viable strategy
|Advisor:||Young, Truman P., Evans, Richard Y.|
|Commitee:||Laca, Emilio A.|
|School:||University of California, Davis|
|School Location:||United States -- California|
|Source:||DAI-B 79/01(E), Dissertation Abstracts International|
|Subjects:||Ecology, Forestry, Environmental science|
|Keywords:||Ecological restoration, Forest expansion, Forest management, Herbivore effects, Livestock, Polylepis|
Copyright in each Dissertation and Thesis is retained by the author. All Rights Reserved
The supplemental file or files you are about to download were provided to ProQuest by the author as part of a
dissertation or thesis. The supplemental files are provided "AS IS" without warranty. ProQuest is not responsible for the
content, format or impact on the supplemental file(s) on our system. in some cases, the file type may be unknown or
may be a .exe file. We recommend caution as you open such files.
Copyright of the original materials contained in the supplemental file is retained by the author and your access to the
supplemental files is subject to the ProQuest Terms and Conditions of use.
Depending on the size of the file(s) you are downloading, the system may take some time to download them. Please be