Long term exclusion of small mammal seed predators: Effects on seedling recruitment

Background/Question/Methods

Biotic effects, such as seed predation by small mammals, have been shown to be an important first filter in short-term patterns of seedling recruitment in temperate deciduous forests. However, few, if any, studies have examined if these patterns persist in the long-term. In 1994 we established 36 1x2m hardware cloth small mammal exclosures paired with control plots at Great Mountain Forest in Norfolk, CT, USA. Seed rain was collected at these sites in 1994-1996, and all tree seedlings that established both inside and outside of exclosures were counted in 1995, 1996, 1997, 2005 and 2012.

Results/Conclusions

In 1995, after a large seed crop in 1994, exclosures had significantly more total seedlings than controls (t=2.976, P=0.005). However, in 1996 and 1997, when seed rain was lower, we observed no significant differences between exclosures and controls in total seedlings recruited. After 10 years of protection from seed predators we observed significantly more seedlings inside the exclosures (average = 16) than in control plots (average = 8; t=2.219, P=0.032). Similarly, after 18 years of small mammal exclusion the higher number of seedlings in the exclosures persisted but the difference had declined in magnitude (exclosure mean = 6, control mean = 3; t= -1.983, P=0.052). Species-specific responses to exclosures varied through time; for example, black cherry (Prunus serotina) had significantly more seedlings inside exclosures  in 1996 (average exclosure = 5, average control = 3; t=2.874, P=0.005), 1997 (exclosure: 1.2, control: 0.4; t=2.747, P=0.009) and 2012 (exclosure = 1.8; control = 0.4; t= - 2.839, P= 0.007) whereas red maple had significantly more seedlings in exclosures only in 1995 (average = 32 vs controls = 7; t=2.909, P=0.006). Through time protection from seed predators allows for short-term increases in seedling recruitment, which persisted but appeared to have weakened over 18 years. Whether the large short-term effects of small mammal exclusion result in changes in the composition of mature tree species might depend on the frequency of disturbances such as gap formation that permit long-term seedling survival.