Effects of Late Holocene Forest Disturbance and Vegetation Change on Acidic Mud Pond, Maine, USA

The limnological effects of natural forest disturbance and succession were studied by analyzing the chemistry, charcoal, pollen, chrysophytes, and diatoms in sediments deposited from °3500 to °700 yr BP in Mud Pond, Maine, USA. Mud Pond was chosen for this study because its oligotrophy (mean chlorophyll a = 0.5 mg/L), large ratio of catchment area to lake area (°50:1), and steeply sloping catchment (°12%) are likely to make the lake unusually sensitive to the terrestrial environment. Fine—grained sediments rich in organic matter are interrupted every 100—500 yr (X = 280) by coarser and more minerogenic sediments, reflecting brief pulses of soil erosion. These disturbance horizons contain high concentrations of charcoal, indicating forest fires, low percentages of pollen of late—successional trees such as Tsuga, and high percentage of pollen and spores of early—successional plants such as shrubs and ferns. Within a few decades, pollen percentages of Alnus peak, followed by Betula. Tsuga pollen percentages increase for roughly 200—400 yr after disturbances, and generally stabilize or decrease several decades before the following disturbance horizon. Limnological responses to these terrestrial changes are indicated at disturbance horizons by high percentages of diatoms (e.g., Rhizosolenia sp., Fragilaria spp., and Synedra spp.) and chrysophytes (e.g., Synura spinosa and Mallomonas caudata) of waters of near neutral pH. Diatom—inferred pH typically increases abruptly from °5.0 to °6.1, and chrysophyte—inferred pH from °5.5 to °5.8 at disturbance horizons. These increases may be caused by increased inputs of wood ash and mineral soils rich in base cations after fires or other disturbances. This pulse is followed by a two—step recovery. First, within a few years or a few decades, the inferred pH decreases nearly to the predisturbance condition as PH—circumneutral diatoms are replaced by taxa of waters of pH <5.5 (e.g., Asterionella ralfsii var. americana, and Eunotia spp.) and pH—circumneutral chrysophytes are replaced by taxa of waters of pH <6.0 (largely Mallomonas allorgei and M. duerrschmidtiae). This response likely results from soil stabilization and uptake of base cations by successional vegetation. Second, a slower acidification of °0.1 pH unit per century occurs as early successional shrubs and hardwood forest are replaced by old—growth conifer forest. This process may involve further sequestration of base cations in vegetation, and greater availability of organic acids from the forest floor. Increasing percentages of pollen of Picea, Alnus, and Salix by 1350 yr BP, and decreasing percentages of hemlock and beech around 1350 yr BP imply cooler conditions. Large changes in diatom and chrysophyte assemblages in approximately this period may be related.