Successional processes and global change: tree seedling establishment in response to warming and N addition in a temperate old field

Abstract

Climate change and increased atmospheric nitrogen (N) deposition can alter successional processes, either by directly altering the establishment and growth of later successional species or by altering their interactions with earlier successional species. We examined the effects of these global change factors on post-germination tree establishment in a grass-dominated old field in London, Ontario, Canada, by transplanting tree seedlings into the plots of a warming and N addition field experiment. Transplantation was duplicated over two growing seasons. We also administered warming and N addition treatments to tree seedlings in the absence of grasses in a greenhouse in order to examine the direct seedling responses. In the field, seedling survival was generally low over the first growing season, which was very dry. The only significant treatment effects were those of warming decreasing the survival and biomass of Malus coronaria, and N addition increasing the survival of Elaeagnus umbellata. Grass biomass increased in response to N addition, and seedling condition and survival were positively correlated with increased grass biomass for most species. Seedling survival was much higher in the second growing season, although warming decreased the survival of Rhamnus cathartica and Rhus typhina. N addition increased the biomass and leaf area of M. coronaria, but decreased these responses for R. typhina, and warming increased the leaf area of Fraxinus americana. Grass biomass again increased in response to N addition, but unlike the previous year, seedling condition and survival were generally negatively correlated with increased grass biomass. In the greenhouse (grasses absent), the seedlings generally increased in biomass in response to N addition and were unresponsive to warming. Our results indicate that both warming and N addition can affect the processes determining post-germination tree seedling establishment, and that depending on the environmental conditions, and in particular water availability, the responses of grasses appear to either facilitate or suppress seedling condition and survival.