The effect of acid rain on Acer rubrum flower and seed production in unglaciated hardwood forests
Anthropogenic acid precipitation has altered soil chemistry in the Ohio River Valley. Notably, acid rain has the ability to acidify soils, decrease soil calcium (Ca) and soil phosphorus (P) availability, while increasing nitrogen (N) availability. At the same time, Ohio’s forest understories are becoming increasingly dominated by Acer rubrum. Could acid rain be contributing to A. rubrum’s increasing dominance? We hypothesized that acid rain would increase A. rubrum flower and seed production because it has low nutritional needs, and thrives on nutrient-poor soils. Forest plots were established in 2009 on unglaciated Southeastern Ohio hardwood forests comprised of 10% A. rubrum, and 60% Quercus spp. These plots received soil amendments to elevate P and Ca levels by adding triple super phosphate and/or lime to plots in a randomized complete block design. The treatments are elevated P, elevated Ca, elevated P + Ca, and ambient conditions. Treatments were found to increase soil Ca or P 3-fold. In the spring of 2012, A. rubrum flowers and seeds were collected in litter traps, and counted or weighed. This number was divided by the basal area of A. rubrum growing in that plot to account for between-plot variation of A. rubrum numbers and sizes.
All treatments were found to have produced from 2 to 3.5 times more flower mass per basal area than the control, although this difference was not found to be statistically significant (P ≅ 0.12). Mean seed production per basal area was from 86% to 175% for the treatments when compared to the control, but were similar (P > 0.5). However, the number of seeds per flower mass of the elevated P, Ca and P + Ca treatments decreased significantly (P < 0.05) by 52%, 81% and 80% , respectively, when compared to the control. Thus, A. rubrum growing in ambient, acidic soils were found to produce more seeds per flower mass than those growing in treatment soil, with the largest treatment effect observed for the elevated Ca soils. These results indicate that perhaps acid rain is increasing A. rubrum’s fecundity, which could be contributing to this species’ observed increase in abundance in Ohio’s forest understories.