The impact of land uses on the recruitment dynamics of tree species

Background/Question/Methods

Patterns of human land use vary as distance from an urban center increases. These changes in land use alter nutrient loads, invasive species pressure and have been associated with altered patterns in plant communities. Additionally, habitat fragmentation increases in proximity to urban centers ushering in further environmental changes (increasing edge effects, dispersal distance between patches, isolation, etc.) that also affect plant communities. The combined impacts of habitat fragmentation and human land use patterns surrounding remnant forest patches may further alter plant communities. Few studies however, have empirically tested the impacts of these combined effects on plant species. Here, I study the impacts of the surrounding landscape on the growth and survival of eight native and two invasive tree species in remnant forest patches. To do so, I planted seedlings of these species at 4 forests along a 40 km urban-rural gradient in southeast Michigan. Seedlings were planted at three different forest habitats, forest edge, middle distance to the edge, and forest interior. Over the course of the summer I measured seedling growth and mortality, in addition to environmental characteristics of the sites, i.e., soil moisture and light availability. To analyze the data, I constructed hierarchical models using a Bayesian framework that reflected the spatial scale of my data. 

Results/Conclusions

My results shows differential growth and survival along this land use gradient for each of the studied species. In general, the invasive species had greater survival closer to urban areas even after edge effects were taken into account, while several large seeded native species had lower survival rates closer to urban centers. Later successional and more shade tolerant species had higher survival in more rural forests. Other species had higher growth and survival at specific landscape-habitat combinations. For example, Acer saccharum tended to have higher growth and survival in the more shaded interior plots than edge plots across the land use gradient. On the other hand, Prunus serotina had higher survival in the edge plots, but only at the two more rural sites. The invasive species Elaeagnus umbellata and Celastrus orbiculatus had high survival and growth across the land use gradient. This could explain why invasive species have higher abundances closer to urban areas. These results show that human land uses impact the recruitment dynamics of tree species along urbanization gradients and suggest that patterns of human land use could shape species composition in remnant forest patches.