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0 Department of Forest Resources, University of Minnesota, 115 Green Hall, 1530 N. Cleveland Ave., St. Paul, Minnesota 55108 USA,2,,3
Received for publication August 11, 1999. Accepted for publication February 29, 2000.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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Key Words: Cupressaceae • decayed logs • leaf litter • mineral soil • Minnesota • moisture • safe sites • seedbeds
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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) is a useful framework for examining the regeneration potential of many plant species. Fowler (1988)
explored the notion that several variables interactively constitute the safe site for any given species, concluding that safe sites occur along a continuum of safety. The circumstances that enable individual species to persist in an ecosystem are often most critical during the early stages of germination and seedling establishment (Grubb, 1977
). These early stages may be especially important for the conservation of certain shade-tolerant conifers with a recent history of poor regeneration. Late-successional conifers in general require specialized safe sites for germination and early establishment. Thuja occidentalis is an example of a species that has declined throughout much of its range, a trend attributed by some authors to a land use history that has resulted in inadequate availability of safe sites (e.g., Waller and Alverson, 1997
; Simard, Bergeron, and Sirois, 1998
).
Many authors have demonstrated the importance of decaying wood as a seedbed for conifers, including T. occidentalis, in both lowland and upland forest communities (McCullough, 1948
; Nelson, 1951
; Curtis, 1946
; Holcombe, 1976
; Christy and Mack, 1984
; Scott and Murphy, 1987
; Anderson and Winterton, 1996
; Cornett, Reich, and Puettmann, 1997
). Several characteristics of decayed wood may contribute to safe sites for the germination and establishment of many late-successional species. Among these characteristics are higher water retention by decaying logs than other substrates (e.g., Place, 1950
; Harvey, Larsen, and Jurgensen, 1979
); less buildup of leaf litter (Christy and Mack, 1984
; but see Minore, 1972
); presence of nitrogen-fixing microorganisms (Sharp and Millbank, 1973
); earlier warming to temperatures sufficiently high for germination (Nelson, 1951
); and a greater number of mycorrhizal associations (Maser and Trappe, 1984
). Although many studies have speculated about the relationships between these characteristics and early germination and establishment of tree species, few have explored these links experimentally.
Although important for T. occidentalis regeneration, decaying logs are fairly rare in forests (Scott, 1984
; Cornett, Reich, and Puettmann, 1997
; Cornett, 2000
). Leaf litter is generally the most abundant seedbed available, but is not considered favorable for the germination and establishment of T. occidentalis (Nelson, 1951
; Caulkins, 1967
). On disturbed, post-harvest sites, silvicultural treatments for T. occidentalis regeneration often include mechanical scarification or prescribed burns to create mineral soil seedbeds (Johnston, 1975, 1977
; Verme and Johnston, 1986
). The success of these silvicultural practices presumably results from better access to soil moisture (Scott, 1984
; Davis, Puettmann, and Perala, 1998
).
In addition to gross differences among litter, log, and mineral soil seedbeds, species origin of each of these seedbed types may also contribute to their relative safety. Canopy tree species directly influence the character of available seedbeds through deposition of leaf litter and logs, the decomposition of which shapes soil characteristics such as organic matter content and chemistry (e.g., Beatty, 1984
; Cornett, Puettmann, and Reich, 1998
; Evans, Miller, and Friedland, 1998
). Decay rates also differ among leaf litter and logs of different species, which may partially determine seedbed safety (Sydes and Grime, 1981
; Hale and Pastor, 1999
). Many of these interspecific differences may also interact with moisture availability.
A constant moisture supply is considered critical for the early development and survival of T. occidentalis (Curtis, 1946
; Caulkins, 1967
). Moisture availability results from an interaction between precipitation and the differential ability of seedbeds to retain moisture. Although decaying wood provides a good seedbed for T. occidentalis germination, some authors have questioned whether logs continue to function as safe sites during periods of prolonged drought (Caulkins, 1967
; Scott, 1984
). The degree to which decayed wood functions as a safe site for T. occidentalis may vary substantially with amount of precipitation. For example, in T. occidentalis forests on the Lake Superior Highlands in northeastern Minnesota, USA, rainfall during the growing season has ranged from 20 cm/yr to 43 cm/yr between 1989 and 1997 (Wolf Ridge Environmental Learning Center, Finland, Minnesota, USA). The present study addresses this variation in "safety," which has not been adequately described previously.
Most of the existing information about safe sites for T. occidentalis seedlings has been generated by field surveys (e.g., Nelson, 1951
; Scott, 1984
; Scott and Murphy, 1987
). However, some experimental work has compared T. occidentalis germination on different seedbeds. In a greenhouse experiment, St. Hilaire and Leopold (1995)
found that T. occidentalis germinates better on seedbeds below shorter than taller mosses from a New York forested minerotrophic peatland. Work from upland T. occidentalis ecosystems is scarce, although a seeding experiment in a cliff escarpment ecosystem determined that T. occidentalis germinates and grows beneath deciduous canopy types provided leaf litter is removed before sowing (Bartlett, Reader, and Larson, 1991
). A better understanding of safe sites for T. occidentalis germination and establishment is needed to understand fully the tremendous decline in its regeneration rates over the last several decades.
The objective of the study was to examine the combined influence of seedbed and moisture retention on the germination, survival, and growth of T. occidentalis seedlings under different levels of moisture availability. Although this experiment was conducted in a greenhouse, a general understanding of safe site complexity was sought within the context of a human-dominated landscape in northeastern Minnesota, USA. Experimental seedbeds were obtained from stands of upland T. occidentalis forest embedded within a matrix of early-successional deciduous forest (Betula papryifera—Populus sp.) to refine the definition of safe sites available beneath each of these canopy types for early T. occidentalis regeneration. We predicted that no differences among germination, survival, or growth responses will arise among different seedbeds as long as water is constantly abundant. When water is limiting, however, we hypothesized that germination, survival, and growth of T. occidentalis seedlings will be higher on seedbeds with higher moisture retention. Finally, we predicted that germination, survival, and growth of T. occidentalis on litter, log, and soil seedbeds may differ on seedbeds of different canopy origin (deciduous or conifer) under different watering regimes.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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The experimental design consisted of six replicates (pots) of seven seedbeds subjected to three water treatments, for a total of 126 pots. In each pot, 50 seeds of T. occidentalis were sown. Seeds and seedbeds were collected in the fall of 1996 within the natural range of T. occidentalis from a Thuja-Betula stand at Tettegouche State Park in northeastern Minnesota.
Seeded pots were arranged in a stratified-random design. The 126 seeded pots were placed on a central greenhouse bench, 18 pots long and seven pots wide. Another block of 36 unseeded pots served as reference pots for water content. Although each row of pots received the same water treatment, the arrangement of seedbeds within a row was random, as were the water treatments assigned to each row. To compensate for the heterogeneity that occurs even within a controlled greenhouse environment, rows of pots were rotated every 7 d.
Water treatments
High, medium, and low treatments consisted of water applied in different frequencies and amounts to simulate natural patterns, which range from 1.29 cm/mo to 26.4 cm/mo in the field during the growing season. A fine-mist nozzle was used to water the pots. The rate of water flow was calibrated with a beaker and timer before each watering session to determine watering rate in millilitres per second, generally 10 mL/s. All pots received 500 mL (0.38 cm) of water on the day of sowing. High water treatment pots received 200 mL (0.15 cm) on a daily basis thereafter. Medium water treatment pots received 500 mL water every 7 d, and were given 125 mL (0.1 cm) of water in the middle of the week. Low water treatment pots received 500 mL of water every 14 d and were given 125 mL of water on the 7th d.
A 2 cm3 core of seedbed material was extracted from each unseeded reference flat once a week prior to watering. Gravimetric water content (e.g., Pratt, 1986
) was determined for each core by weighing immediately after extraction, drying at 80°C for 3 d, and weighing cores again. Reference water retention levels were determined for six of the seven seedbed materials within each water regime, with insufficient soil from beneath the birch canopy to use for reference pots (Table 1).
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Pots (11.5 x 11.5 x 5.5 cm) were filled with a 2-cm layer of 50/50 peat/sand mixture. A 2-cm layer of seedbed material was placed on top of the peat/sand mixture. In the case of leaf litter and soil, the seedbed material was spread over the peat/sand mixture. For decayed logs, small chunks were placed over the peat/sand mixture.
New germinants were counted for each pot twice a week over 67 d. Each germinant was marked with a colored toothpick. Successful germination was defined as the point at which radical elongation surpassed the length of the seedcoat. At the end of the study, seedling mortality was recorded for each flat. To test the germinative energy of the T. occidentalis seeds, 50 seeds were sown in each of five petri plates kept continuously moist. Each day, new germinants were recorded and removed.
At the end of the study period, all seedlings in each flat were counted and washed. For each pot, one seedling of average size was selected at random. Total root length was measured using AgVision®, a monochrome system for root and leaf analysis (Decagon Devices, Inc., Pullman, Washington, USA).
To determine whether seedling size, in addition to water and substrate treatments, influenced root:shoot ratios, a small, medium, and large seedling were selected from each pot. Each of these seedlings was divided into root and shoot, dried at 80°C for 4 d, and weighed. The remaining seedlings from each pot were divided into roots and shoots. The separate portions were combined, and dried and weighed similarly. From this group of seedlings, mean biomass/seedling, mean root mass/seedling, mean shoot mass/seedling, and root mass/shoot mass were calculated within each pot. When sufficient numbers of seedlings for the above measurements were unavailable, (i.e., within the low water treatment), the biomass of the randomly selected seedling of average size was also determined.
Data analyses
Data presented are based on effective, rather than absolute, seedling emergence. Percentage effective emergence (%EE) was determined by dividing the number of emerged seedlings by the mean number of viable seeds, calculated from the five petri plates, and converting to percentage.
Logistic regression models were used to examine the importance of seedbed, water treatment, and their interaction for %EE and survival. Logistic regressions were followed by likelihood ratio tests, calculated as twice the difference of the negative log likelihoods between a saturated model and a model without a particular effect or interaction (JMP 3.0.2; SAS Institute Inc., Cary, North Carolina, USA). Differences among seedbed and water treatments were further explored with nonparametric Wilcoxon tests. Pairs of seedbeds within each water treatment were compared, as well as pairs of water treatments with seedbeds pooled.
Seedling growth data were analyzed in different ways. For each of the five growth variables, an initial ANOVA was conducted examining the importance of seedbed, water treatment, and their interaction as predictors. Response variables were normalized with square root or natural log transformations where appropriate. Means separation tests for important predictors of growth response were conducted with a Tukey-Kramer test where the assumptions of normality and homoscedasticity were met, or by a nonparametric Wilcoxon test if this was not the case. To determine whether differences in allocation resulted from experimental treatments or differences in seedling size, an analysis of covariance was conducted for each seedbed, with water treatment, ln(seedling biomass), and their interactions as predictors of ln(root mass).
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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Survival
Seedbed and the interaction of seedbed and water treatment were important predictors of survival, but water treatment alone had little effect (Table 2). Under the high water treatment, 95% of an average of 16.5 initial germinants per flat survived until the end of the experiment. No differences in survival occurred among seedbed types within the high water treatment (Table 4, Fig. 2A). For seedlings receiving the medium water treatment, survival was slightly lower than for those receiving the high water treatment. Ninety percent survived of an average of 13 initial germinants per flat (Fig. 2B). Differences in percent survival among the seven seedbed types within the medium water treatment were modest in magnitude (Fig. 2B). Cedar litter and the control had survival rates of 100%, with rates on birch log and birch soil seedbeds low by comparison (Table 4).
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Survival was similar on seedbeds of the same type but of different canopy origin under the high and medium water treatments. Under the low water treatment, survival rates for seedlings growing on birch soil (0%) were lower than on cedar soil (100%), but for a sample size of only two seedlings each (Table 4, Fig. 2).
Growth
Among most of the seedbeds within the high water treatment, seedling growth responses did not differ (Table 5). However, seedlings growing on birch litter had shorter roots than those growing on birch logs (Table 5). The trend of larger roots on logs than on litter seedbeds was observed even after adjusting for seedling size (P = 0.03, Wilcoxon) (Table 6). Additionally, root : shoot ratios were highest for seedlings on birch logs relative to seedlings rooted in soil and litter seedbeds regardless of canopy origin (Table 5).
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A few differences in growth and allocation were apparent among seedlings growing on seedbeds of the same type but of different canopy origin under the high and medium water treatments. Under the high water treatment, slightly higher root : shoot ratios occurred on cedar than on birch soil (Table 5). Under the medium water treatment, slightly higher root : shoot ratios occurred on birch litter than cedar litter (Table 5). Despite comparable seedling growth on birch and cedar logs, seedlings on birch logs had lighter foliage and a chlorotic appearance. Insufficient sample sizes obscured any differences in growth variables on like seedbeds of different canopy origin within the low water treatment.
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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; Ahlgren and Ahlgren, 1981
; Sydes and Grime, 1981
; Knapp and Smith, 1982
; Bartlett, Reader, and Larson, 1991
; Cornett, Puettmann, and Reich, 1998
; but see Williams et al., 1990
), possibly the result of blocked access to otherwise available soil moisture. Control seedbeds may have produced the highest %EE by providing the easiest access to soil moisture. The pure sand/peat mixture was probably also relatively pathogen-free compared with some of the field-collected seedbeds.
In terms of survival, water treatment was also the most important component of the T. occidentalis safe site. Survival patterns within the high water treatment generally supported the prediction that few differences occur among seedbeds receiving abundant water. Although birch and cedar logs absorbed several times their mass in water, abundant moisture did not affect survival on these seedbeds. Conversely, early survival of T. occidentalis depends on unsaturated soil conditions (Chimner and Hart, 1996
). The similarity of survival rates among seedbeds with different water contents indicated that saturation was not reached under the high water treatment.
Limited water
Conditions of abundant water as described above are unlikely to persist under typical field conditions. Although water treatment was the strongest component of the T. occidentalis safe site, continua of safety for germination, survival, and growth became more prominent within treatments for which water was limited, upholding the second hypothesis. Many of these differences coincided with the capacity of seedbeds to retain water.
Higher %EE on logs than on cedar soil in the low water treatment suggested that greater water retention by these seedbeds conferred an advantage for %EE. Under field conditions, decayed wood maintains consistently higher moisture levels than other seedbeds (Place, 1950
; Harvey, Larsen, and Jurgensen, 1976
). Anderson and Winterton (1996)
suggest that for some conifers, such as Picea engelmannii, better germination on logs than other types of seedbeds is linked to higher moisture retention. Within the low water treatment, although the maximum %EE was achieved on log seedbeds, the low response overall suggested that no seedbed is unequivocally safe for germination under severe drought conditions.
The highest root : shoot ratios overall occurred for the medium water treatment, suggesting that moisture availability as a safe site component produces different effects at different life stages. An increase in root : shoot ratio is both a consequence of and coping strategy for water stress. During times of moderate water stress, shoot growth is reduced while photosynthesis remains relatively stable, resulting in increased allocation of photosynthates to roots (Kozlowski, Kramer, and Pallardy, 1991
).
Similar continua of safety observed for germination and survival on different seedbeds also occurred for early growth. The smaller root length, root mass, and root : shoot ratios for seedlings grown on leaf litter in comparison with log seedbeds under all three water treatments was consistent with Nelson (1951)
and may have additional explanations beyond access to moisture, such as more mycorrhizal associations on logs (Harvey, Larsen, and Jurgensen, 1976, 1979
) or physical barriers to root elongation posed by leaf litter (e.g., Ahlgren and Ahlgren, 1981
; Sydes and Grime, 1981
). Although seedlings on cedar litter had good survival rates over the study period, the less vigorous roots could result in higher rates of seedling mortality in the field.
Comparing like seedbeds of different canopy origin
The differences between %EE on like seedbeds of different canopy origin supported the third hypothesis under conditions of limited water (i.e., birch vs. cedar logs within the low water treatment). The differences observed may have been linked to the higher rate of moisture retention of birch logs compared with cedar logs. It is possible that decomposition of birch logs was more advanced than that of cedar logs in the present study, leading to higher moisture retention. This result contrasted with data from a field experiment in upland Thuja-Betula forests, in which germination rates were higher on average on cedar than on birch logs (Cornett, 2000
). Cornett (2000)
used numerous logs for the field study, while seedbeds in the present study were obtained from only a few birch and conifer logs, not allowing for the diversity of conditions in a natural setting. Other artifacts of the present study may also have resulted from the controlled environment of the greenhouse or timing of seedbed collection.
Although %EE differed between cedar and birch litter for all water treatments, consideration of these differences related to canopy origin was limited to log seedbeds. In a forest setting, birch litter is thought to hinder T. occidentalis germination even more than cedar litter (Ahlgren and Ahlgren, 1981
; Scott, 1984
). In the present study, however, the timing of seedbed collection caused the birch litter to curl and function as less of a barrier than the compacted, partially decomposed leaf litter typically present during spring germination of T. occidentalis.
Differences in growth response on like seedbeds of different canopy origin did not appear to be related to water retention. The higher root : shoot ratios on cedar soil than on birch soil within the high water treatment supported Noble (1972)
, who found that even small differences in soil chemistry can cause clear differences in seedling survival and root elongation for Picea engelmannii seedlings in their first year of growth under the same water treatment.
Chlorosis of T. occidentalis seedlings growing on birch logs indicated that continued survival over a longer period might be higher on cedar logs. Jurgensen et al. (1989)
found that higher rates of bacterial nitrogen fixation occur in deciduous logs hosting white rot than in conifer logs hosting brown rot fungi. Decay induced by white rot fungi happens more rapidly than that of brown rot, possibly resulting in more rapid immobilization of nitrogen in deciduous than conifer logs. Lower nitrogen availability may have contributed to the chlorosis observed in T. occidentalis seedlings. Thus, although birch logs retained moisture better than cedar logs in the present study, cedar logs may be safer for long-term survival on the basis of nutrient availability. This question could be addressed by examining plant nitrogen content over a longer experimental period.
The broad patterns that emerged during the present study were generally linked to the water treatments, with some exceptions. Much of the variation in %EE, survival, and growth within the water treatments also appeared to correspond to differences in gravimetric water content among the seedbeds. Gravimetric water content was used as a comparative index among seedbeds in this study, but water availability to the plant may differ among seedbeds of different texture with similar water content (Rundel and Jarrell, 1991
). Comparing the water content of substrates in the present study with those of other studies suggested that within the high water treatment, water availability was similar between log and soil substrates, but was much lower for litter. Within the high water treatment, the water potential of the two litter types was likely quite negative, given that Dix (1985)
reported a potential of -2.8 MPa for undecomposed leaf litter at a similar water content (Table 1). Dix (1985)
also reported a water potential of -2 MPa for decayed birch logs with a water content of 125 mg H2O/cm3, similar to the value for decayed birch logs in the low water treatment of the current study (132 mg H2O/cm3). Hence, the much higher water contents of decayed birch logs in the medium and high water treatments may represent much less negative water potentials.
Safe sites for T. occidentalis occurred along a continuum of moisture availability and seedbed type. With a few exceptions, the extremely divergent water treatments were the strongest dimension of safe site in this study, but within water treatments seedbed type and canopy origin also contributed to early regeneration success. Among field-collected seedbeds, maximum %EE and survival occurred under higher moisture and nonlitter seedbed conditions. When moisture was moderately limiting, the safest seedbed for T. occidentalis (i.e., logs) had high moisture content relative to other seedbeds. However, under conditions of extreme drought as frequently occur in the field, decayed wood may be no safer than other types of seedbed for T. occidentalis regeneration.
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FOOTNOTES |
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2 Current address: Division of Forestry, Minnesota Department of Natural Resources, 1200 Warner Rd., St. Paul, Minnesota 55106 USA.
3 Author for reprint requests (e-mail: meredith.cornett{at}dnr.state.mn.us
)
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LITERATURE CITED |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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Ahlgren, C. E., and I. F. Ahlgren. 1981 Some effects of different forest litters on seed germination and growth. Canadian Journal of Forest Research 11: 710–714.[CrossRef]
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Beatty, S. W. 1984 Influence of microtopography and canopy species on spatial patterns of forest understory plants. Ecology 65: 1406–1419.[CrossRef][ISI]
Caulkins, H. L. 1967 The ecology and reproduction of northern white-cedar. Master's thesis, University of Michigan, Ann Arbor, Michigan, USA.
Chimner, R. A., and J. B. Hart. 1996 Hydrology and microtopography effects on northern white-cedar regeneration in Michigan's upper peninsula. Canadian Journal of Forest Research 26: 389–393.[CrossRef]
Christy, E. J., and R. N. Mack. 1984 Variation in demography of juvenile Tsuga heterophylla across the substratum mosaic. Journal of Ecology 72: 75–91.[CrossRef][ISI]
Cornett, M. W. 2000 Ecological restoration of upland northern white cedar forests on the Lake Superior Highlands. Ph.D. dissertation, University of Minnesota, Saint Paul, Minnesota, USA.
———, K. J. Puettman, and P. B. Reich. 1998 Canopy type, leaf litter, predation, and competition influence conifer seedling emergence and early survival in two conifer-deciduous forests. Canadian Journal of Forest Research 28: 196–205.[CrossRef]
———, P. B. Reich, and K. J. Puettmann. 1997 Canopy feedbacks and microtopography regulate conifer seedling distribution in two Minnesota conifer deciduous forests. Ecoscience 4: 353–364.[ISI]
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Davis, A., K. Puettmann, and D. Perala. 1998 Site preparation treatments and browse protection affect establishment and growth of northern white-cedar. USDA Forest Service Research Paper NC-330, North Central Forest Experiment Station, St. Paul, Minnesota, USA.
Dix, N. J. 1985 Changes in relationship between water content and water potential after decay and its significance for fungal successions. Transactions of the British Mycological Society 85: 649–653.[ISI]
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Hale, C. M., and J. Pastor. 1998 Nitrogen content, decay rates, and decompositional dynamics of hollow versus solid hardwood logs in hardwood forests of Minnesota, U.S.A. Canadian Journal of Forest Research 28: 1276–1285.[CrossRef]
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———, ———, and ———. 1979 Comparative distribution of ectomycorrhizae in soils of three western Montana forest habitat types. Forest Science 25: 350–358.[ISI]
Holcombe, J. W. 1976 The bryophyte flora of Thuja seedbed logs in a northern white-cedar swamp. Michigan Botanist 15: 173–181.
Johnston, W. F. 1975 Reproducing lowland conifer forests. Journal of Forestry 73: 17–20.[ISI]
———. 1977 Manager's handbook for northern white cedar in the North Central States. USDA Forest Service General Technical Report Number NC-35, North Central Forest Experiment Station, St. Paul, Minnesota, USA.
Jurgensen, M. F., M. J. Larsen, M. Wolosiewicz, and A. E. Harvey. 1989 A comparison of dinitrogen fixation rates in wood litter decayed by white-rot and brown-rot fungi. Plant and Soil 115: 117–122.[CrossRef][ISI]
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Kozlowski, T. T., P. J. Kramer, and S. G. Pallardy. 1991 The physiological ecology of woody plants. Academic Press, San Diego, California, USA.
Maser, C., and J. M. Trappe. 1984 The seen and unseen world of the fallen tree. USDA Forest Service General Technical Report Number PNW-164, Portland, Oregon, USA.
McCullough, H. A. 1948 Plant succession on fallen logs in a virgin spruce-fir forest. Ecology 29: 508–513.[CrossRef][ISI]
Minore, D. 1972 Germination and early growth of coastal tree species on organic seedbeds. USDA Forest Service Research Paper Number PNW-135, Portland, Oregon, USA.
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Noble, D. L. 1972 Effects of soil type and watering on germination, survival, and growth of Engelmann spruce: greenhouse study. USDA Forest Service Research Note RM-216, Rocky Mountain Forest and Range Experiment Station, Fort Collins, Colorado, USA.
Place, I. C. M. 1950 Comparative moisture regimes of humus and rotten wood. Department of Resources and Development, Forestry Branch, Silviculture Leaflet Number 37, Canada.
———. 1955 The influence of seed-bed conditions on the regeneration of spruce and balsam fir. Forestry Branch Bulletin 117, Department of Northern Affairs and National Resources Forestry Branch, Forest Research Division, Ottawa, Canada.
Pratt, C. R., Jr. 1986 Environmental factors affecting seed germination of gray birch (Betula populifolia) collected from abandoned anthracite coal mine spoils in northeast Pennsylvania. Applied Biology 108: 649–658.
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———, and P.G. Murphy. 1987 Regeneration patterns of northern white cedar, an old-growth forest dominant. American Midland Naturalist 117: 10–16.[CrossRef][ISI]
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}2 (in parentheses) for emergence and mortality of T. occidentalis
