The overall theme in this Ph.D. thesis is natural colonization as afforestation practice with focus on drivers of temporal changes in woody species richness, colonization rate and structural variation in the initial three decades of forest succession and how simple management actions can facilitate the colonization.
Forests play a significant role in mitigating the negative effects of climate change and the loss of biodiversity. The ability of forests to sequester and store carbon has led to ambitious goals of increasing the forest cover at national as well as international level. The new forests must however, meet various other societal and economic needs and not least help in mitigating the loss of biodiversity. Natural colonization of previously cultivated fields is a relevant and
inexpensive alternative to traditional afforestation, but has so far not been actively prioritized in afforestation in temperate climates on a larger scale.
Afforestation by natural colonization is unpredictable and influenced by a multitude of different factors and stochastic incidents. Compared to traditional afforestation practice the colonization of trees and bushes is slow and in some cases, the colonization can be arrested for a long time in the initial phase. Unpredictability and a slow colonization are some of the reasons why the natural colonization has not been given higher priority in afforestation, where focus has been on meeting socio-economic values. However, the integration of natural colonization in afforestations can be a mean of increasing biodiversity.
This Ph.D. study aims to widen the understanding of the woody vegetation dynamics in the young phase of natural colonization. I compare woody species richness and selected funtional traits as proxies for biodiversity in 33 areas laid out for natural colonization in Danish state forests from 1990-2018 with planted forest from the same period. In addition, I investigate the influence of five different factors on colonization rate, species richness and variation in
vegetation structure as a proxy/potential for biodiversity. I examine temporal changes in vegetation dynamics over the three decades and the importance of neighboring existing forest and simultaneous afforestation, as a facilitator of the colonization rate.
I found that the number of species of woody plants in the 35.8 ha of natural colonization was high, with 60 different tree and shrub species and that the majority of the most common native tree and bush species in Danish forests and nature areas were represented. The number of species was slightly higher in the planted forests due to a higher share of non-native coniferous species, whereas the proportion of native, insect-pollinated and bird dispersed species was higher in the natural colonization and the proportion of vegetation cover of insect-pollinated woody species was more than three times higher. There was a difference in species richness related to soil type, with significantly lower species richness in natural colonization on sandy soil and higher on loamy soil. The same pattern was not observed in the planted forest. These results suggest that natural colonization benefits biodiversity, especially on nutrient-rich soil.
I compared the influence of five different drivers on the natural colonization and found that woody vegetation cover, number of species and variation in the vegetation structure increased significantly with the age of colonization. Neighboring existing forest significantly increased the colonization rate and there were significantly higher species richness in sloping areas relative to flat. Soil type played a role in terms of both colonization rate and species richness.
Natural colonization on sandy soils was significantly slower than on loamy soil and had lower species richness than found on loamy and clay soils.
Natural colonization was dominated by wind dispersed species in the first three decades, but the proportion of bird dispersed vegetation cover was greater in the latter part of the period, which supports other research, that the colonizing vegetation facilitates recruitment of bird dispersed species. Furthermore, I found that forests planted at the same time as areas were laid out for natural colonization increased colonization rate in the edge zone of colonization areas.
The results of the analyzes can form the basis for planning afforestation with natural colonization as an integral part of the forest by utilizing the knowledge that not only existing forest but also simultaneous afforestation, topography and soil type act as facilitators of colonization rate, species richness and structural variation and can thereby promote biodiversity in future forests.