Tree seedling establishment in response to climate warming and increased nitrogen deposition

Background/Question/Methods

In some northern temperate regions, areas previously used for agricultural production have been abandoned, leading to increased old field habitat. While in the absence of disturbance these old fields will develop into secondary forest, climate change and increased atmospheric nitrogen deposition over the next century may alter the resulting plant community composition. Specifically, increased temperature and nitrogen deposition may alter the ability of woody species to germinate and compete with grasses and forbs. This study investigated the interactive effects of warming and nitrogen addition on the germination, survival and growth of four early successional tree species in an old field habitat.  We transplanted newly germinated seedlings (in spring) and mesh bags filled with seeds (in fall) into plots of a field experiment designed to simulate predicted levels of warming and nitrogen deposition expected by the year 2050. We monitored survival, health, and growth of the seedlings as well as productivity of the surrounding grasses over one growing season. Based on the assumption that the treatments would enhance the growth of competing grasses, we predicted seedling survival would decrease with warming and nitrogen addition and surviving seedlings in the treatment plots would have decreased growth, leaf area and biomass.

Results/Conclusions

Overall seedling survival was low, likely as a result of drought conditions over the course of the summer.  As we predicted, Malus coronaria (crab apple) seedlings had higher survival, leaf number, leaf area, and total biomass in control plots than in treatment plots.  However, the other three species had higher survival and growth in warmed or nitrogen addition plots than in control plots. Both the nitrogen addition and warming treatments increased grass biomass and percent cover, despite decreased soil moisture in the warmed plots. Our results suggest that increases in temperature and nitrogen deposition may exert strong effects on seedling survival and productivity in temperate old fields, particularly for early successional tree species that are shade intolerant or drought sensitive. However, for the seedlings of shade tolerant species, increased aboveground grass biomass may provide a beneficial microclimate during stressful hot and dry periods.