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Soil Science Society of America Journal 63:1424-1429 (1999)
© 1999 Soil Science Society of America

DIVISION S-7-FOREST & RANGE SOILS

Revegetation and Nitrate Leaching from Lake States Northern Hardwood Forests Following Harvest

Thomas M. Isemana, Donald R. Zaka, William E. Holmesa and Amy G. Merrilla

a School of Natural Resources & Environment, Univ. of Michigan, Ann Arbor, MI, 48109-1115 USA

drzak{at}umich.edu


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 Methods
 Results and discussion
 REFERENCES
 
The sugar maple (Acer saccharum Marshall)–red oak (Quercus rubra L.) and sugar maple–basswood (Tilia americana L.) ecosystems are Lake States forests that differ in net nitrification (5 and 15 g N m-2 yr-1, respectively), but experience equivalent rates of NO-3 leaching following clear-cut harvest ({approx}5 g N m-2 yr-1). Our objectives were to determine whether high rates of N leaching are sustained following harvest and whether ecosystem-specific patterns of biomass accumulation influence NO-3 loss. We studied two stands in each ecosystem and established four research plots in each stand; two plots were clear-cut in 1991 and two were controls. In 1996, we measured soil solution NO-3 concentration (1-m depth) and calculated areal losses by a water balance method. We used allometric equations to estimate woody biomass in clearcut plots; herbaceous biomass was clipped. In the sugar maple–red oak ecosystem, NO-3 leaching from 5-yr-old clear-cut plots (0.56 g N m-2 yr-1) was significantly greater than leaching from control plots (0.05 g N m-2 yr-1). In contrast, NO-3 leaching did not differ between control (0.41 g N m-2 yr-1) and 5-yr-old clear-cut (0.02 g N m-2 yr-1) in the sugar maple–basswood ecosystem; however, loss from these clear-cut plots was significantly lower than that from clear-cut sugar maple–red oak plots. Five years after harvest, 7.1 Mg ha-1 of aboveground biomass accumulated in clear-cut sugar maple–basswood plots, almost twice that of clear-cut sugar maple–red oak plots (3.9 Mg ha-1). Five years after harvest, the highest rates of NO-3 loss occurred in the sugar maple–red oak ecosystem, in which aboveground biomass accumulation was least.


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
Methods
 Results and discussion
 REFERENCES
 
CLEAR-CUT HARVEST can dramatically alter nutrient dynamics in forest ecosystems, particularly in forests where rates of N cycling are rapid. Because of the wide geographic extent of northern hardwood forests in eastern North America and their rapid rates of N cycling, the effects of clear-cut harvest on nutrient cycling and loss from these ecosystems have been intensively studied, especially the leaching of NO-3 following overstory harvest (Bormann and Likens, 1979; Vitousek and Melillo, 1979; Hornbeck and Kropelin, 1982). Immediately following clear-cut harvest, direct insolation causes an increase in soil temperature, while a decline in evapotranspiration results in an increase in soil water status. These conditions promote net N mineralization and nitrification (Matson and Vitousek, 1981), while plant N uptake is simultaneously reduced by overstory removal (Vitousek et al., 1979). In the absence of plant uptake, NO-3 accumulating in soil solution can be lost from forest ecosystems through leaching below the rooting zone, a process that is exacerbated by augmented flux of water resulting from reduced evapotranspiration (Vitousek, 1977; Likens et al., 1978).

Revegetation following clear-cut harvest in northern hardwood forests functions as a sink for N, diminishing N loss from ecosystems following disturbance (Marks and Bormann, 1972; Vitousek and Reiners, 1975). Early successional species, such as red raspberry (Rubus ideaus L.) and pin cherry (Prunus pensylvanica L.), take advantage of reduced competition for light and nutrients to densely colonize clear-cut areas (Marks, 1974; Whitney, 1982; Mou et al., 1993). Loss of N is reduced or minimized by rapid accumulation of biomass, which utilizes available soil N (Vitousek, 1977). In addition, the production of NO-3 often declines as enhanced rates of decomposition reduce labile soil organic matter, thus slowing net N mineralization and nitrification (Likens et al., 1978; Fisk and Fahey, 1990). Bormann and Likens (1979) observed a significant reduction in NO-3 leaching within 2 yr following harvest in northern hardwood forests of New Hampshire. Subsequent studies in the Northeast confirmed that elevated N loss from northern hardwood forests immediately following harvest declines to preharvest levels within 2 to 5 yr (Hornbeck and Kropelin, 1982; Mann et al., 1988).

This study was a continuation of our investigation into the effects of clear-cut harvest on N dynamics in two floristically and functionally distinct northern hardwood ecosystems located in northern Lower Michigan. The sugar maple–red oak/Maianthemum G.H. Weber ex Wiggers and the sugar maple–basswood/Osmorhiza Raf. ecosystems differ in rates of net nitrification, presenting the possibility that they also could differ in rates of NO-3 leaching following harvest (Zak et al., 1986; Holmes and Zak, 1994). In December 1991, plots within each ecosystem were clear-cut harvested and N leaching was measured in the growing season immediately following harvest (1992). Despite initial differences in net nitrification, both the sugar maple–red oak/Maianthemum and sugar maple–basswood/Osmorhiza ecosystems had equally high rates of N leaching the first year following harvest ({approx}5 g m-2 yr-1; Holmes and Zak, 1999). Our objectives were (i) to determine the extent to which high rates of N leaching are sustained following clear-cut harvest in these Lake States northern hardwood forests and (ii) to assess whether ecosystem-specific differences in aboveground biomass accumulation lead to differences in rates of N leaching following clear-cut.


   Methods
TOP
 ABSTRACT
 INTRODUCTION
 Methods
Results and discussion
 REFERENCES
 
Study Area
Our study was conducted in two northern hardwood ecosystems which are widely distributed throughout northern Lower Michigan: the sugar maple–basswood/Osmorhiza ecosystem and the sugar maple–red oak/Maianthemum ecosystem (hereafter referred to as "sugar maple–basswood" and "sugar maple–red oak"). These ecosystems were classified by a multifactor site classification method, using relationships among landform, soil, and vegetation to delineate functionally distinct ecological units (Barnes et al., 1982). The sugar maple–basswood and sugar maple–red oak ecosystems are primarily distinguished from one another by differences in overstory composition, edaphic properties, and patterns of N cycling (Table 1) . Although annual rates of net N mineralization are equivalent between the two ecosystems, annual rates of net nitrification are significantly greater in the sugar maple–basswood ecosystem (14.8 g N m-2 yr-1) than in the sugar maple–red oak ecosystem (5.4 g N m-2 yr-1; Holmes and Zak, 1994).


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  		Table 1 Selected overstory, soil, and N-cycling characteristics of two northern hardwood forests.{dagger}

 
As part of previous work, two stands representative of each ecosystem were randomly located within the Manistee National Forest in northwestern Lower Michigan (Zak et al., 1986). The landscape is characteristic of the Interlobate moraine. Hilly topography and well-drained, sandy soils predominate in this region. All stands are subject to similar climatic conditions throughout the year: mean annual temperature is 6.7°C, growing seasons last {approx}115 d, and annual precipitation averages 77 cm (Albert et al., 1986). Because of its proximity to Lake Michigan and relatively high elevation (300–450 m), this region accumulates substantial snowpack in winter.

Within each stand, four plots (5 by 30 m) were randomly located (Zak and Pregitzer, 1990). Two of the four plots within each stand were randomly selected for clear-cut harvest; the other two plots within each stand were left intact to serve as controls. Clear-cut harvest took place in December 1991, when stems >10 cm diameter at breast height were harvested and removed. Branches and stems <10 cm diameter were distributed throughout the clear-cut area. In order to minimize the influence of adjacent vegetation on the clear-cut plots, a tree-height buffer strip (22.5 m) surrounding each clear-cut plot was also harvested, such that the total harvested area was 50 by 75 m (0.375 ha). A trench (0.25 m wide and 1.3 m deep) was excavated 15 m from the perimeter of each clear-cut plot to prevent N uptake by roots of adjacent overstory trees (Holmes and Zak, 1999). Natural revegetation of clear-cut plots has been allowed to occur.

Nitrogen Leaching
Within each plot, four ceramic cup lysimeters were installed in November 1989 (Merrill, 1991). The lysimeters were located at 10-m intervals along the length of each plot. They were installed to collect soil water at a depth of 1 m beneath the soil surface, a depth which is below the majority of fine roots in many upland forest ecosystems (Gale and Grigal, 1987). A tension of 35 kPa was applied to the lysimeters at each sampling date. Soil water samples were collected monthly during the snow-free months prior to harvest (1990–1991; Merrill, 1991), the year following harvest (1992–1993; Holmes and Zak, 1999), and 5 yr after harvest (1996–1997). Concentration of NO-3–N in soil water was determined using an Alpkem 300 Rapid Flow Analyzer (Astoria-Pacific International, Clackamas, OR).

A water budget method was used to determine the volume of water moving below the rooting zone on a monthly basis (Thornthwaite and Mather, 1957). Mean monthly temperature and latitude were used to calculate monthly potential evapotranspiration. Actual evapotranspiration was estimated on a monthly basis by budgeting the available water (precipitation + soil water storage) vs. potential evapotranspiration. When potential evapotranspiration was satisfied by monthly precipitation and soil water storage, excess water was assumed to move below the rooting zone, transporting NO-3–N from the ecosystems. Areal leaching losses NO-3–N (g N m-2 mo-1) were calculated on a monthly basis as the product of N concentration and volume of water moving below the rooting zone. Climatic data were obtained from the Cadillac weather station, Wexford County, MI, operated by the USDA Forest Service.

Aboveground Biomass
Aboveground biomass was estimated for each clear-cut plot 5 yr following overstory harvest. All aboveground vegetation was clipped by hand from four randomly located sampling frames (0.75 by 0.75 m) within each clear-cut plot. Species-specific biomass equations were used to estimate aboveground biomass of stump-sprouted black cherry (Prunus serotina Ehrh.) that had grown too large to harvest (Smith and Brand, 1983). Due to the highly patchy distribution of vegetation, four additional samples were collected from within each of the clear-cut plots in one sugar maple–red oak stand (total samples per plot = 8). Biomass samples were oven dried at 70°C and weighed in the laboratory. Biomass was expressed in kilograms per hectare and averaged by plot.

Statistical Analyses
Differences in annual rates of NO-3 leaching (g N m-2 yr-1) 5 yr following clear-cut harvest were analyzed using a partially crossed, partially nested analysis of variance. Stands (n = 2 per ecosystem) were nested within ecosystems (n = 2), which were crossed with treatments (n = 2). Ecosystem and treatment were fixed factors and stand was a random factor. Means were compared by Fisher's protected LSD. A repeated measures analysis of variance with a similar design was used to compare temporal changes in annual N leaching rates across three sampling intervals: preharvest (1990–1991), 1 yr following harvest (1992–1993), and 5 yr postharvest (1996–1997). Differences in aboveground biomass in clear-cut plots were compared using a nested analysis of variance (stands nested within ecosystems). Statistical results were accepted as significant at P <= 0.05.


   Results and discussion
TOP
 ABSTRACT
 INTRODUCTION
 Methods
 Results and discussion
REFERENCES
 
Nitrogen Leaching
Temporal patterns of NO-3 leaching in both ecosystems were similar during the 3 yr. The majority of NO-3 export occurred during fall and early spring (Fig. 1 and 2) , periods when water inputs exceeded soil water storage capacity and evapotranspiration. Leaching in April occurred as snowmelt released water before overstory trees had leafed out. During the growing season, leaching losses were minimal due to increased evapotranspiration, but NO-3 accumulated in soil solution (Fig. 1a and 2a). Later in the season as temperatures cooled and leaf fall occurred, evapotranspiration was reduced and leaching losses increased in October and November (Fig. 1b and 2b). This seasonal pattern of NO-3 leaching is consistent with observations in other northern hardwood forests (Bormann and Likens, 1979; Vitousek et al., 1982).



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  		Fig. 1 (A) Nitrate concentration in soil water and (B) areal NO-3 leaching loss in intact and clear-cut plots within the sugar maple–red oak ecosystem in northern Lower Michigan. Values are ecosystem means in intact and clear-cut plots, the bar is one standard deviation, and n = 2 stands per ecosystem

 


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  		Fig. 2 (A) Nitrate concentration in soil water and (B) areal NO-3 leaching loss in intact and clear-cut plots within the sugar maple–basswood ecosystem in northern Lower Michigan. Values are ecosystem means in intact and clear-cut plots, the bar is one standard deviation, and n = 2 stands per ecosystem

 
Prior to the initiation of our harvest treatment, rates of NO-3 leaching did not differ among plots (Fig. 3) . One year following harvest, rates of NO-3 leaching dramatically increased in clear-cut plots of both ecosystems (Fig. 3; Holmes and Zak, 1999). However, five years following overstory harvest, rates of NO-3 leaching were similar to those prior to harvest (Fig. 3). Rates of NO-3 leaching after harvest and the return to preharvest rates of NO-3 leaching in these Lake States northern hardwood forests are similar to responses of harvested northern hardwood forests of the northeastern USA. Bormann and Likens (1979) reported a decline in N loss 2 to 3 yr after harvest, following very high rates of N loss in the first year after harvest. Mann et al. (1988) also observed significantly elevated NO-3 leaching immediately following harvest in hardwood ecosystems, which declined to control levels within 4 yr.



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  		Fig. 3 Annual NO-3 leaching prior to clear-cut harvest and 1 and 5 yr following clear-cut harvest in two Lake States northern hardwood forests. Values are ecosystem means in intact and clear-cut plots, the bar is one standard deviation, and n = 2 stands per ecosystem

 
In the forests we studied, rates of NO-3 leaching from intact and clear-cut plots 5 yr after harvest differed substantially. In the sugar maple–red oak ecosystem, NO-3 leaching from clear-cut plots (0.56 g N m-2 yr-1) was significantly greater than losses from intact plots (0.05 g N m-2 yr-1; Fig. 3). In the sugar maple–basswood ecosystem, NO-3 leaching did not differ between intact (0.41 g N m-2 yr-1) and clear-cut plots (0.02 g N m-2 yr-1; Fig. 3). Nitrate export from clear-cut plots of the sugar maple–red oak ecosystem was significantly greater than from clear-cut plots of the sugar maple–basswood ecosystem (Fig. 3). These ecosystem-specific patterns of NO-3 leaching following harvesting are probably due to differences in biomass accumulation between ecosystems.

Aboveground Biomass and Nitrate Leaching
Five years postharvest, clear-cut plots of the two ecosystems differed significantly in aboveground biomass accumulation. Aboveground biomass was almost two times greater within clear-cut plots of the sugar maple–basswood ecosystem (7.1 ± 2.72 Mg ha-1) compared with clear-cut plots within the sugar maple–red oak ecosystem (3.9 ± 3.09 Mg ha-1). In the sugar maple–basswood ecosystem, clear-cut plots were evenly colonized by a dense stand of red raspberry ({approx}100% of aboveground biomass), whereas a sparse cover of red raspberry (55% of aboveground biomass) and a few black cherry stump sprouts (6–10 m tall) characterized clear-cut plots of the sugar maple–red oak ecosystem (W.E. Holmes, 1994 and 1995, personal observation). Thus, establishment of red raspberry is primarily responsible for the differences in aboveground biomass accumulation between clear-cut plots of the two ecosystems.

Red raspberry is not a component of intact, mature sugar maple–red oak and sugar maple–basswood stands (Host and Pregitzer, 1991, 1992), and it is unclear why red raspberry colonization differed between ecosystems. However, several factors may have affected differences in establishment between clear-cut plots of these ecosystems. We know that red raspberry colonization of disturbed sites: (i) increases with intensity of disturbance (Roberts and Dong, 1993), (ii) increases with greater light availability (Mou et al., 1993; Ricard and Messier, 1996), and (iii) depends on germination of buried seeds (Hughes and Fahey, 1991). The sugar maple–red oak ecosystem has moderately thick Oi and Oe (4–6 cm) horizons overlying the mineral soil, whereas in the sugar maple–basswood ecosystem there is little forest floor development (only a thin, discontinuous Oi horizon) and a thorough mixing of organic matter into mineral soil by fauna. The relatively thick Oi and Oe horizons in sugar maple–red oak ecosystem, although disturbed somewhat by harvesting, may have limited the germination and establishment of red raspberry in clear-cut plots of this ecosystem.

Plant uptake is the primary mechanism of nutrient retention in regenerating forests. Available nutrients are used and retained by accumulating biomass following disturbance (Bormann and Likens, 1979). Red raspberry is an early-successional species, which rapidly colonizes disturbed northern hardwood sites, using available nutrients and growing vigorously under conditions of high soil moisture and light availability soon after harvest (Marks, 1974; Whitney, 1982). This species typically thrives for 2 to 6 yr following harvest, declining as nutrient and light availability diminishes. Tilman (1987) found that Rubus spp. became more abundant in response to increased soil N availability, and Truax et al. (1994) report that red raspberry is particularly adapted to NO-3 uptake. Several studies suggest that red raspberry acts to limit nutrient loss, particularly NO-3, immediately following disturbance, when forests are most susceptible to nutrient loss (Bormann et al., 1974; Hornbeck and Kropelin, 1982; Mou et al., 1993). This pattern is consistent with our observation that clear-cut plots with greater biomass had lower rates of NO-3 leaching. The low rates of NO-3 leaching in clear-cut plots of the sugar maple–basswood ecosystem probably resulted from high demand for NO-3 due to the dense growth of red raspberry. Rates of NO-3 leaching were significantly greater within clear-cut plots of the sugar maple–red oak ecosystem, where red raspberry density and biomass accumulation were lower.

In conclusion, the Lake States northern hardwood forests we studied responded to harvesting in a manner similar to that observed in northern hardwood forests of other regions. Nitrate leaching increased dramatically following clear-cut harvest, but rates in intact and clear-cut plots were similar to preharvest rates after 5 yr. In our study, ecosystem-specific patterns of biomass accumulation appear to control rates of NO-3 leaching. Five years postharvest, clear-cut plots of the sugar maple–red oak ecosystem had greater rates of NO-3 leaching and lower aboveground biomass than clearcut plots of the sugar maple–basswood ecosystem. The density of red raspberry was greater in clear-cut plots of the sugar maple–basswood ecosystem, which may have contributed to the lower rates of NO-3 leaching, since red raspberry has a high demand for NO-3.


   ACKNOWLEDGMENTS
 
This manuscript is based on a portion of the thesis submitted by the senior author in partial fulfillment of the requirements of the Master of Science degree in the School of Natural Resources & Environment, University of Michigan. Our study was supported by funds from the McIntire-Stennis Cooperative Forestry Act (P.L. 87-788). We are grateful for the assistance of Matt Sands and Rose Ingram (USDA Forest Service, Manistee National Forest) and David Rothstein.

Received for publication June 10, 1998.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 Methods
 Results and discussion
 REFERENCES
 




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