Background/Question/Methods

Defoliation outbreaks of forest canopies are biological disturbances that preferentially affect growth and mortality rates of host species, producing short and long-term changes in forest productivity, competition and composition. Tree and stand level response to defoliation varies from rapid recovery to abrupt mortality to gradual decline. Simple, quantitative relationships between accumulated defoliation stress, growth suppression and mortality are needed to accurately model outbreak effects on aboveground carbon. I compiled research papers that report defoliation of temperate forests as a percentage of canopy foliage over one to several outbreak years, as well as changes in percent radial growth and mortality. Data were extracted from tables, graphs and text, converted to the above variables and plotted in a consistent manner. Relationship forms and parameters were quantitatively compared among tree species and defoliator systems.

Results/Conclusions

Relative growth decreased in either a linear or curvilinear fashion as defoliation stress accumulated across species. Growth typically decreased by 5-20% in response to 100% defoliation for ring-porous Quercus species, whereas growth of diffuse-porous hardwoods and coniferous species was suppressed by 50-100%. Thus diffuse-porous and coniferous species growth declined more rapidly than ring-porous species. Xylem growth that precedes budburst may constrain ring-porous growth suppression in response to defoliators whose damage must follow foliar development. Mortality increased exponentially with accumulated defoliation stress, with Pinus mortality increasing more rapidly than Quercus mortality. The various growth responses reported here are consistent with explanations stemming from differences in wood growth phenology, anatomy, and non-structural carbohydrate storage. These relationships will be used to parameterize a stochastic forest disturbance and succession model, LANDIS-II, to evaluate long-term forest carbon storage in mixed deciduous landscapes disturbed by periodic outbreaks of multiple defoliators.