Abstract. The self-thinning rule predicts that for a crowded even-aged plant population a log-log plot of average plant mass vs. plant density will reveal a straight "self-thinning" line of slope -%. The rule is supported by examples from many individual populations, and by the existence of an interspecific relationship that yields a line of slope -% in a loglog plot displaying average mass and density data from many populations of different species. I examined and reanalyzed the evidence to evaluate the strength of support for this widely accepted rule.Some problems in fitting thinning lines and testing agreement with the rule have no truly satisfactory solution, but three improvements on commonly used methods were made: the analysis related stand biomass density to plant density because the alternative of relating average plant mass to plant density is statistically invalid; principal components analysis was used rather than regression, because regression relies on unrealistic assumptions about errors in the data; and statistical tests of hypotheses were used to interpret the results.The results of this reanalysis were that 19 of 63 individual-population data sets previously cited in support of the thinning rule actually showed no significant correlation between stand biomass density and plant density, and 20 gave thinning slopes significantly different ( P < .05) from the thinning rule prediction. Four other analyses provided additional evidence against a single quantitative thinning rule for all plants: slopes of the thinning lines were more variable than currently accepted, differed significantly among plant groups, were significantly correlated with shade tolerance in forest trees, and differed among stands of the same species. The same results held for the intercepts of self-thinning lines.Despite the failure of the thinning rule for individual populations, the combined data for all populations are still consistent with an interspecific relationship of slope -3/,; therefore, the existence of the interspecific relationship does not necessarily support the withinpopulation thinning rule. The within-population and interspecific relationships are apparently different phenomena that may arise from different constraints, so the two relationships should be tested and explained separately. power law.The self-thinning rule (also called the -% power rule or Yoda's law) describes a relationship between size and density in even-aged plant populations that are crowded but actively growing. If competition from other populations and crowding-independent stresses (drought, fire, disease, etc.) are absent, mortality or "thinning" is caused by competition within the population, hence the term "self-thinning." Yoda et al. Two lines of evidence support this relationship as an ecological "rule" or "law" governing even-aged plant populations. First, the slopes of many reported thinning lines are near y = -%, including data from monospecific populations ranging in size from small herbs to large trees , White and Harper 1970, Wh...

Watershed land cover is widely used as a predictor of stream‐ecosystem condition. However, numerous spatial factors can confound the interpretation of correlative analyses between land cover and stream indicators, particularly at broad spatial scales. We used a stream‐monitoring data set collected from the Coastal Plain of Maryland, USA to address analytical challenges presented by (1) collinearity of land‐cover class percentages, (2) spatial autocorrelation of land cover and stream data, (3) intercorrelations among and spatial autocorrelation within abiotic intermediaries that link land cover to stream biota, and (4) spatial arrangement of land cover within watersheds. We focused on two commonly measured stream indicators, nitrate‐nitrogen (NO3–N) and macroinvertebrate assemblages, to evaluate how different spatial considerations may influence results. Partial correlation analysis of land‐cover percentages revealed that simple correlations described relationships that could not be separated from the effects of other land‐cover classes or relationships that changed substantially when the influences of other land‐cover classes were taken into account. Partial Mantel tests showed that all land‐cover percentages were spatially autocorrelated, and this spatial phenomenon accounted for much of the variation in macroinvertebrate assemblages that could naively be attributed to certain classes (e.g., percentage cropland). We extended our use of partial Mantel tests into a path‐analytical framework and identified several independent pathways between percentage developed land and in‐stream measurements after factoring out spatial autocorrelation and other confounding variables; however, under these conditions, percentage cropland was only linked to nitrate‐N. Further analyses revealed that spatial arrangement of land cover, as measured by areal buffers and distance weighting, influenced the amount of developed land, resulting in a threshold change in macroinvertebrate‐assemblage composition. Moreover, distance‐weighted percentage cropland improved predictions of stream nitrate‐N concentrations in small watersheds, but not in medium or large ones. Collectively, this series of analyses clarified the magnitude and critical scales of effects of different land‐cover classes on Coastal Plain stream ecosystems and may serve as an analytical framework for other studies. Our results suggest that greater emphasis should be placed on these important spatial considerations; otherwise, we risk obscuring the relationships between watershed land cover and the condition of stream ecosystems.

Abstract. During a 1-year period we measured discharges of water, suspended solids, and nutrients from 27 watersheds having differing proportions of cropland in the Piedmont and Coastal Plain provinces of the Chesapeake Bay drainage. Annual flow-weighted mean concentrations of nitrate and organic N and C in stream water correlated with the relative proportions of base flow and storm flow. As the proportion of base flow increased, the concentration of nitrate increased and the concentrations of organic N and C decreased. This suggests that discharge of nitrate is promoted by groundwater flow but discharges of organic N and C are promoted by surface runoff. Concentrations of N species also increased as the proportion of cropland increased. We developed a statistical model that predicts concentrations of N species from the proportions of cropland and base flow. P concentrations did not correlate with cropland or base flow but correlated with the concentration of suspended solids, which differed among watersheds. IntroductionOver the past few decades, rivefine discharges of plant nutrients have increased in response to enormous increases in anthropogenic inputs.

We measured annual discharges of water, sediments, and nutrients from 17 Chesapeake Bay watersheds with differing proportions of agricultural lands on the inner, central, and outer Coastal Plain. In all regions of the Coastal Plain, the flow-weighted mean concentrations of N species in watershed discharge increased as the proportion of cropland in the watershed increased. In contrast, the concentrations of P species did not correlate with any land use. Instead, P concentrations correlated with the concentration of suspended particles, which differed greatly among watersheds in different regions of the Coastal Plain. Consequently, the ratio of NIP in discharges differed widely among watersheds, potentially affecting N or P limitation of phytoplankton growth in the receiving waters. Concentrations of dissolved silicate, organic C, pH, and alkalinity in discharges did not differ greatly among watersheds or correlate with land use. Nitrogen discharge correlated with net anthropogenic inputs of N to the watershed, but usually less than one-third of the net anthropogenic inputs were discharged.GRICULTURAL activities can increase fluvial dis-

To investigate the ability of riparian forest to intercept nutrients leaving adjacent cropland, we examined changes in the chemistry of groundwater flowing from a corn (Zen mays L.) field through a riparian forest. This study provided a comparison to previous studies of a different forest. We sampled groundwater from a transect of wells, and used a Br-tracer to confirm that groundwater moved laterally along the transect through the forest. As groundwater flowed through the forest, NO; concentrations decreased from about 8 mgl L at the edge of the corn field to <0.4 mg/L halfway through the forest. Dissolved organic N and NH: increased by less than 0.1 mg/ L, and dissolved organic C did not change with distance. Sulfate remained constant with distance until midway through the forest, where it began to increase. Chloride concentration rose until midway through the forest, then fell. Values of pH increased from under 5 at the edge of the corn field to over 7 at the stream bank, perhaps as a result of the NO; consumption. Most of the change in NO; occurred abruptly at the edge of a floodplain within the forest. There the water table was closest to the surface and soil Eh below the water table was less than -90 mV. Such strongly reducing conditions may have promoted denitrification in the floodplain. In contrast, soil Eh on the adjacent hill slope was above 500 mV, too high to support denitrification. There were only slight seasonal changes in groundwater chemistry. We also studied the net annual accretion of sediment in the riparian forest by measuring changes in the elevation of the soil surface. There was little or no accretion in the forest, but along a path of overland storm flow there was net erosion. Thus, nutrient retention by this forest, in contrast with the forest we previously studied, was entirely a below ground process. Functional differences within sections of this forest and among different riparian forests suggest a need for research on the factors that control nutrient retention. SEVERAL STUDIES have shown that riparian forests can take up N, P, and sediment from discharges leaving adjacent croplands (e.g., Lowrance et al., 1984a,b;Peterjohn and Correll, 1984, 1986;Jacobs and Gilliam, 1985;Pinay and Decamps, 1988;Correll and Weller, 1989;Haycock and Pinay, 1993). This finding has important implications for land use management. Removal of NO; reduces the hazards of high NO3 concentrations in drinking water. Nitrate removal also raises the pH of the groundwater, thus reducing acidification of downstream aquatic ecosystems. Removal of N and P also reduces eutrophication of aquatic ecosystems. Removal of sediment from surface runoff reduces turbidity and sediment deposition downstream.Strong effects of riparian forests on agricultural runoff have consistently been reported for coastal plain forests with shallow underlying aquicludes. The aquicludes force cropland groundwater flowing toward the stream to move laterally through the near-surface layers of the riparian forest. Such forests can remov...