Canopy cover effects on nitrogen and phosphorus status in understory vegetation in oak and pine stands

The rate of change of leaf mass, N and P levels in understory vegetation at various levels of canopy cover were measured for 2 years following canopy cover manipulations in northern red oak (Quercus rubra L.) and red pine (Pinus resinosa Ait.) stands in northern Lower Michigan, USA. Canopy cover treatments consisted of clearcut, 25% (50% during first sampling year), 75%, and uncut. Leaf mass, and N and P contents were significantly higher in the clearcut treatment than in other canopy cover treatments, except for the 25% treatment in red pine stands. Leaf N concentrations in understory vegetation were also significantly higher in the clearcut (1991, 20.8mgg^–1; 1992, 22.4mgg^–1) than in the uncut treatment (1991, 16.5mgg^–1; 1992, 16.9mgg^–1). Canopy type (northern red oak and red pine) had little influence on understory nutrient status and leaf mass. In addition, fronds of bracken ferns in all canopy cover treatments in both northern red oak and pine stands were a major sink of nutrients in the understory. The results of this study showed that partial canopy removal generally had only a minor impact on understory leaf production and nutrient status compared with clearcuts during the 2-year period following canopy removal.     

Canopy cover effects on cellulose decomposition in oak and pine stands

Cellulose mass loss was measured for four levels of canopy cover,i.e., clearcut, 25%, 75%, and uncut, in northern red oak (Quercus rubra) and red pine (Pinus resinosa) stands in northern Lower Michigan, USA. Cellulose mass loss was more rapid in the clearcut and 25% canopy cover treatments than in the 75% canopy cover and uncut treatments. After 4 month incubation of cellulose filter papers, mass loss rates averaged 75.2% in the clearcut, 56.3% in the 25% canopy cover, 46.9% in the 75% canopy cover, and 45.7% in the uncut stands. For the clearcut and the 25% canopy cover treatments, cellulose mass loss in the mineral soil layer was significantly higher than in the forest floor after 2 and 4 months of incubation, while cellulose mass loss of the uncut treatment was significantly lower in the soil layer than in the forest floor after 4 months of incubation. Cellulose mass loss was not significantly different between the oak and the pine stands (p > 0.05), but cellulose mass loss rates in other canopy cover treatments except for the clearcut were generally higher in red oak stands than in red pine stands. These results suggest that canopy manipulation increases cellulose decomposition and may stimulate nutrient cycling process in canopy removal stands. This study was supported in part by USDA Forest Service and Michigan Technological University.     

Decomposition of needle/leaf litter from Scots pine, black cherry, common oak and European beech at a conurbation forest site

Litter decomposition was studied for 2 years in a mixed forest serving as a water protection area (Rhine-Neckar conurbation, SW Germany). Two experiments differing in initial dry weight equivalent in litterbags were set up: one to compare decomposition of European beech leaves (Fagus sylvatica) with common oak leaves (Quercus robur), and the other comparing decomposition of Scots pine needles (Pinus sylvestris) with black cherry leaves (Prunus serotina Ehrh.), respectively. Mass losses were greater for oak litter than for beech (75.0 versus 34.6%), and for cherry litter than for pine (94.6 versus 68.3%). In both experiments, a strong initial loss of soluble compounds occurred. The changes in litter N and P concentrations and the decrease in C-to-N ratio coincided with changes in residual mass. However, neither tannin and phenolic concentrations nor NMR could explain the pronounced variation in mass loss after 2 years. Differences in litter palatability and toughness, nutrient contents and other organic compounds may be responsible for the considerable differences in residual mass between litter types. The fast decay of black cherry leaves appears to play a major role in the present humus dynamics at the studied site. Since black cherry has a high N demand, which is mainly met by root uptake from the forest floor, this species is crucial for internal N cycling at this conurbation forest site. These effects together may significantly contribute to prevent nitrate leaching from the forest ecosystem which is subject to a continuous N deposition on an elevated level.