James R. Gosz

The controls of potential nitrogen mineralization, nitrate production, and nitrate mobilization in a wide range of forest ecosystems were investigated through a combination of field and laboratory experiments. Trenched plot experiments were performed in 17 forests, and laboratory incubation studies of potential ammonium and nitrate production were made on soils from 14 of these sites. The site with the greatest potential for nitrate production in the laboratory was a New Hampshire northern hardwoods forest. Several other sites, including New Hampshire balsam fir, Indiana maple—beech, New Mexico aspen, and Oregon western hemlock forests, also had high potential nitrate production. All of these sites also had rapid nitrate movement to below the rooting zone following trenching in the field. Of nine processes which could be important in preventing or delaying solution losses of nitrate from disturbed forests, two appeared most important among the forests we examined. Low net nitrogen mineralization (caused by either nitrogen immobilization or low gross nitrogen mineralization) and lags in nitrification (probably caused by either low initial populations of nitrifying bacteria or the allelochemic inhibition of nitrification) were identified as important in several sites and in different regions. A direct relationship between the amount of nitrogen in annual litterfall and the proportion of forest floor nitrogen mineralized in laboratory incubations was observed, suggesting that refractory organic nitrogen compounds are produced in nitrogen—poor sites. An inverse relationship was found between the amount of nitrogen in litterfall in these and other sites and the carbon:nitrogen ratio of that litterfall, suggesting that the immobilization capacity of litter is increased in nitrogen—poor sites. The presence and length of lags in nitrification were inversely correlated with the mean concentration of mineral nitrogen in mineral soil. These patterns suggest that nitrogen retention within disturbed forest ecosystems can be caused by low nitrogen availability prior to disturbance.

Rates of weight loss and nutrient release (N, P, S, K, Mn, Ca, Zn, Fe, Mn, Cu, Na) were measured in decomposing leaf and branch tissue form yellow birch, sugar maple, and beech, and in branch tissue from red spruce and balsam fir. Neither leaf nor branch decomposition differed significantly over an elevational range of 220 m. Decomposition rates for leaves varied with yellow birch > sugar maple > beech. The decomposition rate for hardwood branches was greater than that for conifer branches, but differences between hardwoods were not significant. Maximum decomposition rates occurred during the summer for both branch and leaf tissue. The rate of nutrient release from decomposing branch and leaf litter appears to be correlated with nutrient concentration in current litter fall, precipitation, and leaf wash. The concentration and absolute weight of N. S. and P in the leaf litter of all species increased with time. The amount of the increase as well as the initiation of nutrient release was influenced by C: element ratios in the leaf tissue. These studies also indicate that P levels can influence the mineralization or immobilization of other important nutrients. Carbon—to—element ratios in decomposing litter varied between species and elevation at different times of the year, but element: P ratios were much more uniform. In branch tissue the physical loss of N— and P—rich bark and buds offset any increase in concentration that would have occurred through decomposition. Potassium and magnesium were rapidly released from the litter by leaching. Similar minimum concentrations in leaf tissue indicate that critical C: element ratios also exist for these elements. Calcium release was similar to dry weight loss, indicating that it is a structural component primarily released by decomposition. Maximum nutrient release from current litter occurred in the autumn and summer. It was not correlated with the nutrient output from the ecosystem which occurred primarily during the spring. The net output of Ca, Mg, and K from the watershed was very small compared to quantities released from current litter. Factors which contribute to the complex nature of decomposition are: seasonal heterotroph activity, heterotroph nutrient demand, environmental conditions regulations heterotroph activity, species tissue palatability, species composition of litter, tissue composition of litter, nutrient content of litter, nutrient mobility, and nutrient input (i.e., leafwash, litter fall).

We develop a conceptual foundation for the investigation of ecosystem patterns and processes that explicitly considers the spatial patchiness of ecological landscapes. Our emphasis is on the factors determining the location of boundaries between patch types in a landscape mosaic, how boundaries affect ecological processes and the movement of materials over an area, and how imbalances in these transfers in space can affect boundary characteristics and landscape configuration. We suggest that, other things being equal, the edaphic patterns of a landscape will determine the spatial patterning of the biota in the system, primarily through their effects on vegetation. Boundaries in a landscape may be located as direct consequences of the edaphic pattern, or may reflect perturbations of this underlying pattern due to physical disturbances or the actions of various transfer agents or vectors. Abiotic vectors such as wind or surface water flow respond differently to boundary features than do biotic vectors such as beetles or rodents. Both abiotic and biotic vectors may create disturbances through their actions, which in turn may alter patch boundary locations. Vectors also contribute directly to movements of materials, energy, or organisms over the landscape, both within and between patches, and thereby may determine the spatial patterns of the spread of perturbations through the system. If there are net imbalances in fluxes or the spread of disturbances across boundaries in relation to within-patch processes, a directional bias to system fluxes will be established. Over time, this changes the characteristics and locations of patch boundaries. Boundaries thus have dynamics that are superimposed on the edaphic foundation. Consideration of these dynamics offers the potential to gain insight into the spatial configuration of ecosystem functioning and emphasizes the value of a reductionist approach to ecosystem studies.

A systematic examination of nitrogen cycling in disturbed forest ecosystems demonstrates that eight processes, operating at three stages in the nitrogen cycle, could delay or prevent solution losses of nitrate from disturbed forests. An experimental and comparative study of nitrate losses from trenched plots in 19 forest sites throughout the United States suggests that four of these processes (nitrogen uptake by regrowing vegetation, nitrogen immobilization, lags in nitrification, and a lack of water for nitrate transport) are the most important in practice. The net effect of all of these processes except uptake by regrowing vegetation is insufficient to prevent or delay losses from relatively fertile sites, and hence such sites have the potential for very high nitrate losses following disturbance.

Term Ecological Research (LTER) program has now grown to 24 projects involving more than 1100 scientists. This article describes why the program exists and what it does. But the success of a scientific program cannot be measured by the number of sites or scientists involved in a project. Instead, the question must be, What have the LTER program and its longterm data sets contributed to the intellectual progress of ecological and environmental sciences? This special section takes direct aim at this question in six articles focusing on accomplishments of the LTER program in discrete areas of ecological research.The goal of each article is to highlight LTER contributions, not to provide an extensive review of an ecological topic. Readers should keep in mind that LTER projects make many other contributions than those described here.