Print PageGap formation due to large tree mortality and concomitant changes in light and soil resources are traditionally regarded as dominant processes governing dynamics of northern hardwood forests. However, crown defoliation by insects may be an equally, or more, important influence of understory resource levels and thus forest dynamics. Forest tent caterpillar (FTC), a native forest insect with approximately decadal outbreak periodicity, defoliates a range of host tree species early in the growing season. Defoliated canopy trees and seedlings typically produce new leaves within a few weeks after defoliation, in mid growing season. FTC outbreaks increase understory irradiance, but also could increase soil water availability due to decreased transpirational demand of canopy trees. Similarly, soil nutrient availability could increase due to lower foliar demand, decreased uptake, and short-circuiting of litter decomposition by frass inputs. In turn, changes in resources should influence seedling growth. We compared resource availability and seedling height growth of eight tree species during the FTC outbreak in 2008-2009 with earlier non-outbreak years across a soil fertility gradient in northern lower Michigan.
Results/Conclusions
Resource availability increased tremendously during the FTC outbreak. Canopy openness increased from 2% during non-outbreak years to up to 40% during the outbreak. Soil ammonium, nitrate, and potassium were higher during the outbreak than in non-outbreak years at most of the defoliated sites. For soil phosphorus, magnesium, and calcium, no difference was found. Soil water content increased slightly at the defoliated sites. The seedling growth response to defoliation varied with the level of canopy and seedling defoliation of the sites, and with soil resource availability. In lightly defoliated sites, for most species seedling growth did not increase. In moderately and heavily defoliated sites, where many seedlings also were defoliated, most species increased growth. For example, Fraxinus americana seedlings, which were not defoliated, showed a strong growth increase. In contrast, Quercus rubra seedlings were defoliated, and did not respond. Acer saccharum seedlings, which also were defoliated, increased growth only at sites with higher soil fertility. Thus, the FTC outbreak strongly influenced resource availability and seedling growth. Although canopy openness returns to non-outbreak levels, our results to date support that FTC outbreaks could promote the persistence of relatively shade intolerant species even in the absence of canopy gaps.
