The effect of temperature, soil nitrogen and gastropods on Thuja plicata growth and reproduction

Abstract

AbstractHumans are changing the environment. For example, nitrogen deposition, rising temperatures, and non-native species are influencing plant communities. These changes have and will continue to affect trees, thus, understanding the response of species to global change will help conservationists plan for the future. Urban areas already experience higher temperatures, elevated CO~2~, greater nitrogen levels, and more non-native species compared to rural areas. The response of trees to urban parks and park edges can therefore be used as a proxy for the net effects of global change on trees. We investigate the growth and reproduction of Thuja plicata, a native Pacific Northwest conifer, in both urban and rural forest fragments to understand global change impacts. Data collected from sites include hourly temperature, soil NO~3~^-^, seedling counts, and an increment core. After observing extremely low conifer germination rates across sites, we concluded that factors such as herbivory might be important. Gastropods have been shown to impact communities through consumption of plants at the seedling stage, and to limit seedling survival of a European conifer. The effects of soil moisture, soil nitrogen, and gastropod herbivory on Thuja plicata seedlings was analyzed experimentally to determine the relative effect each might be having on the observed germination rate. Our results suggest that urban forest edges resemble future global change conditions of higher temperature and nitrogen. These global change factors appear to be positively influencing tree growth, suggesting that Thuja plicata will benefit from future conditions. However, low seedling recruitment may be a concern, because it appears that urban and rural forests are currently failing to regenerate on their own. Introduced gastropods could play a role, as they have the ability to reduce seedling biomass significantly. The relative impacts of invasive slug herbivory and other global change factors on native conifer recruitment should be prioritized in future study.