Native tree seedlings exhibit greater mortality in urban forests across an anthropogenic land use gradient

Background/Question/Methods

Anthropogenic land use change alters the environment in adjacent undisturbed or less-disturbed areas. These changes alter the environment of remnant vegetation patches. Human development, and in particular urban decelopment, causes changes in local climate, nutrient loads, invasive species, and herbivory pressure and have been associated with altered patterns in plant diversity and abundance. 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 the demographics of 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 in 8 forests along a 40 km urban-rural gradient in southeast Michigan. In each forest, I planted seedlings under closed canopy and in gaps. Additionally, a subset of seedlings were planted within deer exclosures to control for the effect of deer browsing. During 3 growing seasons I measured seedling 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 show differential survival along this land use gradient for each of the studied species. Invasive species exhibited high survival across the gradient. Native seedlings had lower survival rates in the urban forests relative to the rural forests. Large seeded native species (Carya glabra, Quercus rubra, Q. velutina) suffered particularly high mortality in the most urban forests, possibly due to a higher abundance of rodent seed predators at these sites. Interestingly, seedlings planted in deer exclosures exhibited slightly higher mortality than control seedlings. This may be due to increased competition with the herbaceous layer. The higher survival of invasive species relative to natives in the most urban forests could explain why invasive species are so abundant in many urban areas. My 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.