Changes in soil properties and site productivity caused by red alder

~vi tedpaper

CHANGES IN SOIL PROPERTIES AND SITE PRODUCTIVITY

CAUSED BY RED ALDER

Dale W. Cole I, Jana Compton I, Helga Van Miegroet 2, and Peter Homann 3

iUniversity of Washington, AR-10 College of Forest Resources

Seattle, WA 98195 USA

2Oak Ridge National Laboratories Mail Stop 6038-ORNL

P.O. Box 2008 Oak Ridge, TN 37831-6038 USA

3University of Minnesota Soil Science Department

439 Borlaug Hall St. Paul, MN 55108 USA

A~strac~. Red aider (Alnus rubra Borg.) is well recognized as an effective host plant for the

symbiotic fixation of N. While this fixation process leads to the rapid accumulation of N within the

ecosystem, it also enhances nutrient accumulation in biomass and soil organic matter and increases

nitrification and cation leaching. We hypothesized that changes in soil properties resulting from

these processes would decrease site productivity fox second rotation red alder. Adjacent stands of 55

yr old aider and Douglas fir (Pseudotsusa menziesii [Mirb.] France) were studied at the Thompson

Research Center on the Cedar River Watershed in western Washington, USA. The presence of red alder

caused the following soil changes: decreased soil solution pH, increased CEC, increased exchangeable

acidity accompanied by a decreased soil pH and base saturation. This decreased soil and soil solution

pH resulted in increased AI concentration in the soil solution and on exchange sites as well as

decreased P availability. To determine the effect of these changes on the productivity of the 2nd

rotation alder forest, a species conversion experiment was initiated 5 yr ago. Results from this

conversion study clearly indicated that the first rotation red alder forest has caused a relative

decrease in the productivity of the second rotation red alder plantation. Compared to the growth of

red aider on the former Douglas fir site, the second rotation red alder on the former red alder site

exhibited 33% Less height growth and 75% less aboveground biomass accumulation after 5 yr. Future

research will focus on identifying those factors causing this lower productivity including P

availability, soil acidity and A1 toxicity, cation availability, and competition with other vegetation.

i. Introduction

Symbiotic N2-fixing species play an important role in both natural as well as managed forest and agroforest ecosystems (Gordon and Wheeler, 1983). Because they fix atmospheric N2, these species have a competitive advantage on sites with low levels of available N. They are found extensively on sites that have been disturbed, where the surface soil has been lost, and on soils that have been recently deposited or exposed (e.g. alluvial flats, glacial deposits). The use of symbiotic N2-fixing species to biologically enhance the N status of a site is an attractive approach to soil improvement in forests and agroforest ecosystems, while minimizing the need for chemical fertilizers. Nitrogen is added on a continuous basis directly to the host plant, meeting its N needs

Water, Air, and Soil Pollution 54:231-246, 1990/91. © 1990/91 K l ~ e r Academic Publishers. Printed in the Netherlands.

232 D.W. COLE ET AL.

efficiently and effectively. The amount of carbohydrate required for this fixation process is probably less than 20% of the total photosynthate procured by the host plant (Dixon and Wheeler, 1983; Paul and Kucey, 1981). Several studies have shown increased productivity and biological activity of entire ecosystems containing N-fixers (Binkley and Husted, 1983; Mikola et al., 1983; Miller and Murray, 1978). Associated with this increased productivity is a greater accumulation of soil organic matter, which in turn increases cation exchange capacity and moisture holding capacity (Binkley et al., 1984; Bormann and DeBell, 1981; Franklin et al., 1968; Tarrant and Miller, 1963).

Although symbiotic N 2 fixation can provide the benefits described above, one must ask if there are any long- or short-term detrimental consequences caused by such N fixation. Specifically: (i) Is there a cost of N fixation, other than that associated with the

expenditure of energy by the host to support the symbiont, i.e. in terms of deterioration of drainage water quality or decreased availability of nutrients other than N?

(2) If there is a cost, what is its nature, when will it occur, is it reversible, and does it outweigh the immediate benefits to productivity that the system receives from the added N?

We typically associated nutritional stresses with forests growing on inherently low fertility sites, caused by site manipulations that result in nutrient losses (such as burning or intensive biomass harvesting), or triggered by the interaction of atmospheric acid deposition with forest canopies and/or soils. In this article we will discuss a situation in which nutritional imbalances may have been triggered by the vegetation itself. We examine some changes in the soil caused by N fixation and evaluate the potential effect of these changes on the productivity of the second rotation forest.

If N 2 fixation exceeds the capacity of the ecosystem to accumulate N, nitrification ensues (Van Miegroet and Cole, 1984). Established concepts of soil chemistry (e.g. Bohn et al., 1985; Reuss and Johnson, 1986) suggest that introducing mobile nitrate in the soil solution can drastically alter soil and soil solution properties by:

o a decrease in the soil solution pH (caused by H + generation during the nitrification process);

o soil acidification expressed by an increase in exchangeable acidity, a decrease in base saturation, and a decrease in soil pH (caused by displacement of exchangeable cations by H + produced during the nitrification process, and downward cation movement through the soil profile by NO 3- driven leaching);

o an increase in A13+ activity in the soil solution (caused by the increase in ionic strength and the preferential displacement of A1 from the exchange complex, especially in soils with low base saturation); and

o decreased P availability (caused by the decrease in soil pH and subsequent increase in AEC, precipitation of A1 phosphates, and P accumulation in biomass).

If these changes progress to the point of nutrient limitation or imbalance, or if toxic conditions develop, the long-term productivity of the site could be negatively affected. These dramatic changes to the soil under the red alder raise a series of questions:

o Does N accumulation necessarily lead to greater productivity, especially in the case of red alder?

CHANGES IN SOIL PROPERTIES AND PRODUCTIVITY CAUSED BY RED ALDER 233

o Has nitrification and nitrate leaching resulted in changes in soil properties that could be detrimental to future productivity of red

alder? In short, a nutritional stress condition caused by the vegetation

present could evolve. We hypothesized that changes in soil properties resulting from N 2 fixation by red alder decrease site productivity for second rotation red alder.

To address these questions, a study was initiated at the Thompson Research center in which an area within the 55 yr old stand of alder and Douglas fir was harvested and converted to new plantations of alder and

Douglas fir.

2. Materials and Methods

2.1 Site Description

To evaluate the effect of N2-fixing red alder on soil properties, adjacent stands of 55 yr old alder and Douglas fir were studied at the Thompson Research Center, Cedar River Watershed, Washington. This site is located 56 km SE from Seattle at 220 m elevation in the foothills of the Cascade Mountains. The climate is maritime with cool, dry summers and moderate, wet winters. The recorded mean annual temperature for the area is 9.8°C, with average monthly temperatures of 2.8°C and 16.8°C in January and July. Mean annual precipitation averages 130 cm, 75% of which falls as rain between October and March (Cole and Gessel, 1968).

The soil under the red alder and the Douglas fir stands belongs to the Alderwood series (Dystric Entic Durochrept). It has developed from ablation till overlying compacted basal till and has a gravelly, sandy loam texture (Cole and Gessel, 1968), Due to the presence of the compacted basal till, drainage is restricted in the lower parts of the soil profile, and a perched water table over this compacted basal till is common during the winter months.

The Douglas fir stand was planted in 1931 after a series of wildfires following logging of the original old growth forest between 1910 and 1920. The understory vegetation mainly consists of salal (Gauitheria shallon), Oregon grape (Berberis nervosa), and bracken fern (Pteridium aquilinum). There are also several species of mosses, predominantly Hylocomium spp. and Eurynchium oreEanum. The adjacent red alder forest established naturally a few years later where conifer planting ceased in the burnt area. The understory under this forest type is more prominent and consists mainly of a dense growth of sword fern (Polystichum munitum) and bracken fern intermixed with some Oregon grape.

Soil properties were determined for a 1 ha area in each forest type prior to harvesting and stand conversion described below. Each reported value is the mean of 16 samples for a given soil depth. Bray P was determined by extraction with NH4F/HCI (Bray2 solution), and is an estimate of available P. Soil solutions were collected in a nearby unharvested control plot with 4 replicate ceramic tension lysimeters at -i0 kPa placed at 3 soil depths: forest floor, A and B horizons (0, i0 and 40 cm, respectively) (Van Miegroet and Cole, 1985); solutions from October 1987 through July 1988 were analyzed for NO 3- and total AI.


2.2 Conversion study experimental design

To test the influence of N2-fixing species and accelerated nitrification on soil and soil solution properties including acidification and NO 3- leaching, and on the productivity of subsequent tree plantations, a forest conversion experiment was initiated at the Thompson Research Center in September 1984. All overstory vegetation was removed from a 2 ha area in the red alder and Douglas fir forest using a cable system so as to prevent forest floor and soil disturbances associated with conventional logging operations. All debris remaining after the logging operation (slash) was removed. Half of each harvested plot was then replanted with Douglas fir and half with red alder seedlings, yielding the following four 0.5 ha forest conversion plots, the layout of which is illustrated in Figure i:

Original Vegetation Conversion plot

Red alder Red alder Red alder Douglas fir Douglas fir Red alder Douglas fir Douglas fir

Douglas-fir Forest ii . i,ioiei iiiisi, i i

i: ALDER TO FIR !:i:i:i:i [

B O U N D A R Y ~ FOf~STTYPES

Figure i. Douglas fir and red alder conversion plots at the Thompson Research Center, Cedar River Watershed, Washington, USA. Each of the four individual plots is 0.5 ha in size (50 m x i00 m).


Soil solutions were collected continuously since the initial installation of these plots with 4 replicate ceramic tension lysimeters (-I0 kPa tension) at 3 depths (0, i0 and 40 cm) in each plot. Subsamples are removed monthly and solutions are analyzed for pH, alkalinity, and major anions and cations. Eight 15 m x 15 m subplots were delineated in each conversion plot, and the height of all seedlings within these subplots is measured annually. The seedlings were planted in winter 1985; alder seedlings were obtained from a nearby area where they were growing in a coarse soil. Tree diameter (at 1.3 m) was measured in 1988. Eight seedlings from each alder plot were destructively sampled in 1988 in order to develop a height/diameter-biomass regression. This article will address primarily those aspects regarding the alder conversion plots.

3. Results

3.1 Divergence in soil properties under these two forest types

The 55 yr of N 2 fixation has led to a significant accumulation of N within the alder ecosystem as compared to the adjacent Douglas fir stand. In the soil profile this differential accumulation of N amounts to over 3000 kg ha -I. The distribution of this accumulated N within the soil profile is illustrated in Figure 2, showing greatest N-enrichment in the upper part of the soil profile. Soil C parallels this N accumulation (Figure 3).

This accumulation of N within the soil under alder has stimulated soil N mineralization and nitrification (Van Miegroet et al., 1989) resulting in far higher NO 3- concentration in the soil solution than those found under Douglas fir as evident in Table I.

Table I

Average soil solution NO 3- concentrations (#mol L -I) and standard deviations about the mean (in parentheses) collected beneath the forest floor (FF), A and B horizons from the 55 yr old red alder and Douglas fir forest sites at the Thompson Research Center.

Horizon Forest Type Red Alder Douglas fir

FF (0 cm) 625 (420) 0.2 (0.I) A (0 to i0 cm) 674 (244) 0.I (0.0) B (i0 to 40 cm) 515 (32.5) 0.I (0.0)

Nitrification is a strongly acidifying process affecting both soil and solution properties. The H + release associated with this nitrification process is reflected by a significant decline in soil solution pH in the


Soil N (mg/kg)

ne,,,,4,h 0 2 4 6 8

(cm) ' '' '' ' ' '

0-7

7-15 i-

1 5 - 3 0 "

30 -45 -

Figure 2. Comparison of soil N under alder and Douglas fir (adapted from Van Miegroet and Cole, 1988).

Soil C (mg/kg)

Depth o, 40, 80, 120, 160,

(cm) Do~r~s* red aider 0-7

7-15

15-30 -

30-45 -

Figure 3. Comparison of total soil C under the alder and Douglas fir.


upper part of the soil profile. The increase in soil solution NO 3" concentration is accompanied by a concurrent increase in cation leaching (Van Miegroet and Cole, 1984). This process has also decreased the surface soil pH beneath the alder compared to the Douglas fir by over one-half pH unit. This decrease in pH extends to the 45 cm depth in the soil profile, although differences are less pronounced than in the upper soil horizons (Table II).

Table II

Difference in soil acidity between alder and conifer forests expressed by exchangeable base concentration, exchangeable AI, and % base saturation (mean with standard deviation in parentheses, followed by significance of difference between alder and conifer soil at a given depth: *p = 0.01; **p = 0.05). (Adapted from Van Miegroet and Cole, 1988)

Stand pH CEC Exch. Exch. BS Soil depth (cm) (H20) Bases AI %

< ...... mmolc/kg soil ...... >

Douglas fir 0 to 15 5.3 (0.2)** 99** 13 89 (15)** 12.5 (5)**

15 to 30 5.3 (0.2)** 89** ii 81 (16)** 11.5 (6)* 30 to 45 5.3 (0.i) 79 7 72 (15)** 9.0 (5)

Red alder 0 to 15 4.5 (0.3)** 149"* ii 136 (25)** 7.0 (3)**

15 to 30 4.8 (0.2)** 128"* 9 119 (25)** 7.0 (3)** 30 to 45 4.9 (0.2) 118"* II 107 (29)** 8.5 (5)

The surface soil horizons under alder also have lower base saturation than those under Douglas fir. Although there appears to be some displacement of base cations from the upper soil (Table II, Van Miegroet and Cole, 1985), the reduction in soil base saturation is primarily due to an increase in CEC (Table II) associated with greater organic matter accumulation (Figure 3), and increased occupancy of these sites by A1 or exchangeable acidity (Table II). Such changes in the chemistry of the exchange complex have also been observed in other studies (e.g. Franklin et al., 1968; Van Cleve and Viereck, 1972).

The lack of a significant exchangeable base depletion despite elevated cation leaching rates, and the greater role of A1 on the exchange complex under alder, suggest increased weathering, potentially stimulated by H + release during nitrification. A slight increase in amorphous (oxalate- extractable) A1 in the upper horizons of the alder vs the Douglas fir soil (Johnson et al., 1986) suggests the breakdown of A1 minerals, which may further contribute to an increase in CEC (D.W. Johnson, Pers. Comm).

Associated with the acidification of the alder soil are changes in soil and solution properties that can affect tree nutrition. Soil


solution equilibrium models indicate that in soils with low base saturation, an increase in solution NO 3- concentration results in the preferential displacement of A1 over divalent and monovalent base cations from the exchange complex (Reuss and Johnson, 1986). Such A1 mobilization is evident in the alder A horizon (Figure 4). A limited amount of research on the subject indicates that actinorrhizal (McCormick and Steiner, 1978) and leguminous (Thornton et al., 1986) N2-fixing tree species may be fairly sensitive to A1 toxicity at 50 #mol L -I A13+, a value much less than the average A1 concentrations measured in the forest floor and A horizons soil solution in the 55 yr old alder stand (Figure 4). It is important to note, however, that our values are for total dissolved AI, not just for A13+ . 1 1

Lower pH and higher Al~els can also reduce anion availability through chemical rea~ns by enhancing adsorption and precipitation. (Bohn et al., 1985; Sposito, 1989). The lower Bray P observed in the alder soil (Figure 5) may be the result of these processes. Biological processes including incorporation into both vegetation and forest floor and soil organic matter might also contribute to lowering the available P. It has been suggested that N2-fixers have higher P requirements than plants taking up solid mineral N especially in acid soils (Dixon and Wheeler, 1983). These higher P demands coupled with reduced P availability may cause P limitations to second-rotation alder growth. A combination of liming and P fertilization have been shown to increase production of both grey (Huss-Dannell, 1986) and black alder (Seiler and McCormick, 1982). Although we did not analyze for Mo, this is another element which could effect alder growth because, like P, it is present in the soil in anionic form, becomes less available as pH decreases, and because it plays a critical role in the nutrition of N2-fixing species (Dixon and Wheeler, 1983).

3.2 Results and discussion of conversion experiment

The conversion experiment has clearly demonstrated that nitrate-driven leaching is drastically reduced when the source of N 2 fixation is removed (Figures 6 and 7). For example, within 1 yr after removal of alder, the soil solution NO 3- levels decreased to approximately 1% of pre-harvesting levels, independent of the tree species planted. This decrease in NO 3" mediated leaching has persisted for more than 3 yr, and by the fourth year, NO 3 - levels on converted alder plots resembled those under established Douglas fir stands. Soil solution - solid phase equilibrium reactions respond rapidly to this reduction in anion solution flux following alder removal by a concomitant reduction in cation leaching, including A1 (Van Miegroet et al., 1990).

It is not known whether cessation of nitrification and NO 3- leaching in previous alder sites will ultimately lead to the recovery of other affected soil properties under alder. Soil properties generally respond more slowly to change in solution acid-base chemistry (Reuss and Johnson, 1986). If cation input (through weathering or mineralization) is maintained at preharvest levels or at a rate that exceeds current cation export rates via leaching, then a reversal of NO 3- induced soil acidification is possible. Such increase recovery of the cation exchange complex in terms of pH and percent base saturation has been observed in Norway within 5 to 6 years after soil acidification with H2SO 4 irrigation was stopped (Abrahamsen et al., 1987). At the present, soil solution pH

Total AI in solution (~mol/L)

0 40 80 120 I , I , I

Horizon Douglas-~ red alder

FF-


A

Figure 4. Comparison of total solution AI concentration under mature red alder and Douglas fir.

Depth (cm)

Bray P (mg/kg) 0 1 oo 200 300

, I , I , I

red alder Douglas-fir

0-7

7-15

15-30"

3 0 - 4 5 "

Figure 5. Comparison of Bray P under 55 yr old red alder and Douglas fir.


A ,,-I m

o E

I.,U I ' - ,,¢ n- I-- Z

10

4"

2 ' OJ

85 86 87 88

FIR TO FIR

c.O

85 86 87 88

FIR TO ALDER

Figure 6. Reduction of soil solution NO 3- concentrations from 1985 to 1988 following the harvesting of Douglas fir and subsequent establishment of alder and Douglas fir conversion plots (expanded from Van Miegroet et al., 1990).

600 -

5 0 0 -,I

O E 400' :=L

v

300 - 1.1.,I I-- ,,¢

200

Z 100

Lt3

03

CO CO

8 5 8 6 8 7 8 8 8 5 8 6 8 7 8 8

ALDER TO FIR ALDER TO ALDER

Figure 7. Soil solution NO 3- concentrations from 1985 to 1988 following the harvesting of Douglas fir and subsequent establishment of alder and Douglas fir conversion plots.


has not changed markedly over 5 yr indicating strong buffering by the

soil solid phase. In comparison, the removal of Douglas fir did not result in any major

changes in solution chemistry and N leaching, even on those sites where alder seedlings were planted. There is no increase in NO 3- mediated leaching in the alder plots on the converted Douglas fir site, indicating that N-enrichment through N 2 fixation has not yet reached N saturation

levels. The changes in soil chemical properties caused by 55 yr of alder

occupancy, and the lack of rapid reversal of these changes with the removal of alder has negatively affected the productivity of the site, relative to second-rotation alder growth, as evidenced by the annual height values in Figure 8. Within the first year after planting, height growth of the alder seedlings on the former alder site lagged behind that of the seedlings on the former Douglas fir site. Differences in height growth have become even more pronounced with time and at the end of the fourth growing season (1988) the alder planted on the former alder site were an average of 2 m shorter than those planted on the Douglas fir site, with each year a larger difference becoming evident (Figure 8). This stress condition caused by the original red alder forest is even more evident when considering seedlings biomass on the plots: total biomass of the second rotation alder growing on the former alder site is less than 25% of that growing on the former Douglas fir site (Table III).

Table III

Tree foliage, tree stem and branch, and understory biomass values for the conversion plots 5 yr following harvesSing. Average (and standard deviation) of the eight subplots comprising each conversion plot are presented

Conversion Foliage biomass Stem and branch Understory

Plot (kg ha -1 ) (kg ha -1 ) (kg ha -1 )

Fir to alder 2742 (734) 14202 (3798) 7640 (2400) Alder to alder 503 (125) 3109 (773) 7190 (2420) Fir to fir 277 (35.6) 514 (94.8) 5300 (1540) Alder to fir 317 (19.2) 776 (71) 4480 (976)

When understory vegetation is considered, total aboveground biomass on the former alder site is approximately 50% of that on the former Douglas fir site (Table III). The understory vegetation is more abundant on the former alder conversion plots than in the corresponding former Douglas fir plots, reflecting in part the difference in understory composition and biomass that already existed between the unharvested 55 yr old alder and Douglas fir stands (Van Miegroet et a2., 1990), and possibly a positive response to the improvement in soil N content in the alder stand. The understory vegetation represents a much larger proportion of the total biomass on the alder to alder plot than on the


6 E

Z" 4

t.-

I -

[ ] ALDERTOALDER [ ] FIRTOALDER

1984 1985 1986 1987 1988

Growing season

Figure 8. Initial growth of alder established on sites formerly occupied by Douglas fir and alder. Values represent average value of all trees within a conversion plot; differences between plots were significant since 1985 (p < 0.001).


fir to alder plot, suggesting that competition may also have been a factor contributing to the growth differences between the alder plots, particularly shortly after planting. At this point, however, the direct impact of the understory vegetation on the alder growth should have drastically decreased, as alder trees are now fully occupying the site, gradually shading out the understory vegetation.

Table III indicates that the alder to fir conversion plot supports more biomass than the fir to fir plot. Biomass differences are most pronounced in terms of the understory vegetation, although differences in conifer seedling growth began to emerge in the 1988 growing season. Thus, it would appear that the unfavorable site conditions that red alder has created for itself do not affect the Douglas fir seedlings in the same manner. In fact, the Douglas fir seedlings seem to be benefiting from elevated N availability associated with the former alder stand. The results are not surprising considering that these young glacial soils are typically N-deficient and Douglas fir planted on similar soils generally respond to N fertilization.

Foliar analysis on Douglas fir and understory species present on all conversion plots have also shown significantly higher foliar N levels on the plot previously occupied by alder (Van Miegroet e~ al., 1990). Plant species (such as Douglas fir) that do not have the capability to fix atmospheric N clearly benefit from the elevated N availability associated with former alder sites. That growth differences between conifer seedlings on former alder vs former Douglas fir sites are not more pronounced may be related to the increased competition from the dense understory vegetation. It is expected that in the coming years, as Douglas fir seedlings overtop the understory vegetation, distinct differences in conifer growth between the two sites will become apparent.

The differential growth behavior of Douglas fir and red alder seedlings in the two sites suggests that the conifer trees respond first and foremost to an improvement in N status and are relatively less sensitive to the other changes in soil chemistry. On the other hand, red alder, as a N2-fixer , remains generally independent from the soil supply of N, and therefore responds more directly to the proposed negative changes in soil nutrient status, such as suppressed levels of available P (coupled to higher P demands) and potentially toxic levels of AI. Supporting evidence for a decrease in P availability under red alder resulting in growth limitation was further provided in studies by Binkley (1986), in which a positive growth response of alder seedlings to P + S fertilization was observed, and Radwan (1987) where the greatest response was observed with P fertilization alone.

4. Conclusions

It has been demonstrated from this research that although N 2 fixation associated with red alder enhances the N status of a site, changes in soil and solution chemistry caused by the presence of alder may negatively affect the productivity of the second rotation forest. While N 2 fixation by alder results in site improvement for Douglas fir plantations, it is detrimental to the productivity of second rotation red alder. In the first 5 yr after the alder seedlings were planted tree biomass production on the alder to alder conversion plots is less than 25% of that in the fir to alder conversion plots.

244 D.W. COLEETAL.

This research has indicated several changes in soil and solution chemical properties caused by the presence of N 2 fixing red alder that may be responsible for this second rotation alder decline:

o The N additions under alder have stimulated nitrification, a strong acidifying process that contributes to the displacement of nutrient bases from the exchange complex and the mobilization of A1 into solution.

o Soils have developed very differently under red alder as compared to those under Douglas fir: they have a higher organic matter content and CEC, but are more acid in terms of soil pH, exchangeable AI, and percent base saturation. They also have a significantly lower Bray P content.

o The magnitude of nitrate leaching under alder rapidly decreases following harvesting, but it is not yet clear whether and to what extent soil chemical changes caused by N fixation and nitrification can be reversed.

We have not conclusively established the reason for the observed decline in alder productivity. However, it is possible that acidification via nitrification may have resulted in P nutritional stress or A1 toxicity. It may also have decreased the availability of some micronutrients (e.g. Mo) that are essential to the N 2 fixation process. It is also highly likely that the increased competition with the understory on the second rotation stand may reduce productivity and/or aggravate any nutrient deficiency that may be present. The results from this study have led to the following speculations on the ecology, mineral nutrition and management of red alder.

o The negative effect that alder has on the future productivity of alder probably occurred prior to the establishment of the second rotation forest, and was initiated during the 50 yr development of the original stand. This could be partially responsible for the short life span of this species.

o A recovery from the natural stresses caused by alder may be a long- term process depending on whether toxicity or nutritional deficiency is the primary cause for the observed decline in productivity.

o Until the reasons causing this decrease in productivity of second rotation alder is determined and a means for its correction established, a site should not be managed for repeated rotations of this species.

Acknowledgments

The authors wish to thank Robert Gonyea of the University of Washington for his valuable contribution to the field work. We also thank Stan Brozak for the foliage, stem and branch biomass regressions for the fir to fir and alder to fir plots. The research reported in this paper was sponsored by the Electric Power Research Institute as part of the Integrated Forest Study (IFS) on the Effects of Acid Deposition on Forests.


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Van Miegroet, H., Cole, D.W., Binkley, D. and Sollins, P.: 1989, The effect of nitrogen accumulation and nitrification on soil chemical properties in alder forests. Paper presented at the 82nd Annual Meeting of the Air and Waste Management Association, Anaheim, CA, June 25-30, 1989. Preprint 89-134.1.

Van Miegroet, H., Cole, D.W. and Homann, P.S.: 1990, The effect of alder forest cover and alder forest conversion on site fertility and productivity, in S.P. Gessel (ed), Sustained Productivity of Forest Land. Proceedings of the 7th North American Forest Soil Conference, in press.

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Changes in soil properties and site productivity caused by red alder