Does nitrogen fertilization increase stand resistance to storm damage?

Background/Question/Methods

Storms are the most significant natural disturbance in the forests of eastern North America. Storm damage can increase forest light levels by more than 45%, and cause lasting changes in tree recruitment, diversity, and species composition. We hypothesized that nitrogen fertilization might increase resistance to storm damage. To test this hypothesis, damage from Superstorm Sandy (2013) was compared between a fertilized (WS 3) and unfertilized watershed (WS 7) at the Fernow Experimental Forest, near Parsons, WV. Both watersheds have a stand age of ca. 43 years, but WS 3 has received 35 kg N/ha/yr as ammonium sulfate ((NH₄)₂SO₄) since 1989. Measurements of stand damage in the watersheds were also compared to measurements from a previous winter windstorm (2009) in a nearby long-term soil productivity experiment (LTSP) – where there are both fertilized (same rate as WS 3) and unfertilized plots in a forest re-growing since 1996.  Damage was estimated from 18 plots in each watershed, and in each plot in LTSP. Stand destruction was assessed in the watersheds using a classification system with seven categories of damage and in LTSP with a similar system. Comparisons between treatments and tree families were performed using a one-way analysis of variance.

Results/Conclusions

Heavy snowfall by Superstorm Sandy damaged 42% of the total basal area (BA) in both watersheds – 38% of canopy BA and 63.2% of sub-canopy BA. The majority of stand damage was classified as minor crown damage (class 2; 30.4% of BA), followed by bent trees (class 3; 6.2% of BA). The mean BA damaged was lower in the fertilized watershed (p = 0.0090). Specifically, 35.9% of BA was damaged in WS 3 vs. 48.6% in WS 7. The overall differences in stand damage were driven by the differential damage to both the canopy BA (WS 3 – 30%; WS 7 – 44.1%; p = 0.0249) and among the major tree families (p = 0.0450). Differential damage among tree families indicates that watershed differences in overall storm damage may have resulted from differences in species composition. However, damage to trees in the Rosacaea and Sapindaceae families was lower in WS 3, indicating the likelihood of a direct watershed effect. The watershed results contrast with findings from the 2009 windstorm that damaged LTSP plots, where fertilized treatments were damaged more than unfertilized. This suggests the effect of fertilizer may depend on the nature of the storm event (snow loading vs. strong winds).