Comparing effects of invasive Acer platanoides (Norway maple) and native Acer rubrum (Red maple) freshly-collected leaves, leaf litter, and soil extract on the primary growth of native Betula lenta (Black birch): A greenhouse study

Background/Question/Methods

Acer platanoides is an aggressive invader in North American forests, responsible for loss of native understory biodiversity, but allelopathic effects on native tree growth have been poorly characterized. In this study, we examined the effects of invasive A. platanoides and its native congener, A. rubrum, on the primary growth of native Betula lenta. Additions to nutrient-poor potting soil of freshly-collected leaves, ground-collected leaf litter and extracts derived from understory soil from stands of A. platanoides were compared to additions of each from A. rubrum in three separate greenhouse experiments. Moreover, each experiment included treatments with and without additions of activated charcoal in potting soil, as well as a sterilization treatment, where freshly-collected leaves, ground-collected leaf litter, soil extracts and potting soil were sterilized or unmanipulated prior to their use (N = 15 pots per experimental treatment with five B. lenta seeds planted per pot). An additional set of 15 pots with five seeds each were used as a control where no additions of leaves, litter, or soil extracts occurred.

Results/Conclusions

Mean B. lenta shoot length was greater with additions of A. platanoides (NM) leaves (FL) and litter (LT) compared to additions from A. rubrum (RM) and controls. Moreover, mean shoot length was greater with additions of FL and LT from A. rubrum compared to controls [FL: mean (± 1 standard deviation) = 6.94 ± 4.15, 6.05 ± 2.92, and 5.11 ± 2.64 for NM, RM, and controls respectively; FL F_1,176 = 4.331, P = 0.039; LT: mean (± 1 SD) = 8.30 ± 4.22, 6.08 ± 2.11, and 5.11 ± 2.64 for NM, RM, and controls respectively; F_1,170 = 20.25, P < 0.001]. No differences were detected in mean shoot length across any soil extract treatment or in mean root length in any of the addition experiments. Seed germination was similar within all experiments, but was increased with addition of soil extracts.

Leaf and litter additions from both A. platanoides and A. rubrum had positive effects on B. lenta, and, because the potting soil used was nutrient poor, enhanced shoot lengths may be due to nutrients contributed by the leaves and litter. Our results contrast with allelopathy as an explanation for reductions in biodiversity of North American forests through A. platanoides invasion.