Northern Lower Michigan Research Articles

Effects of Jack Pine Plantation Management on Barrens Flora and Potential Kirtland's Warbler Nest Habitat

AbstractJack pine barrens, once common in northern lower Michigan, mostly have been converted to managed jack pine plantations. Management of the disturbances associated with logging provides the opportunity to maintain the unique plant assemblages of jack pine barrens and nest habitat of the federally endangered Kirtland's warbler. Studies indicate that Carex pensylvanica can develop into dense mats and strongly compete with other barrens species such as Vaccinium angustifolium, which seem to be important species for Kirtland's warbler nest locations. According to forest managers, the most important factors facilitating high cover of V. angustifolium and reducing cover of C. pensylvanica are the amount of shade produced by tree crowns before harvest (pre‐harvest shade), the length of time between harvest and planting (planting delay), and fire. We found that high or low levels of pre‐harvest shade had no effect on cover of either V. angustifolium or C. pensylvanica. Planting delays of at least three years following prescribed burns generally increased cover of V. angustifolium in forest plots, which are important for warbler nesting. Analysis of community composition in openings indicated that burning enhanced the growth of barrens species. We found only weak evidence for a negative correlation between the cover of V. angustifolium and C. pensylvanica on our study sites. The openings created in the jack pine plantation are important refugia for barrens flora that would likely be lost under forests managed strictly for jack pine. Maintenance of jack pine barrens flora and Kirtland's warbler nest habitat is possible within the context of a heavily managed forest plantation system.

Effect of Overstory and Understory Vegetation Treatments on Removal of Planted Northern Red Oak Acorns by Rodents

Abstract We documented patterns in the removal of planted northern red oak (Quercus rubra) acorns by rodents in red pine (Pinus resinosa) stands during a field study of oak regeneration in northern Lower Michigan. Each stand contained four overstory treatments assigned at random: clearcut, 25% cover, 50% cover, and uncut (control). Each level of canopy cover contained four randomly assigned understory treatments: shrub-layer removal, herb-layer removal, litter removal, and a control. Rodents removed acorns from 59% of the seed spots in the uncut control and 12% of the seed spots in the clearcut treatment. This five-fold difference was significant. Acorns were removed from 48% and 40% of the seed spots in the 75% and 50% canopy cover treatments, respectively. Differences in acorn removal between understory treatments were not significant. These results suggest that overstory vegetation treatments can influence the removal of planted acorns by rodents, which has implications for the use of direct seeding for artificially regenerating northern red oak.

The effect of 50 years of landscape change on species richness and community composition

AbstractWe analysed a 50‐year dataset of avian species observations to determine how richness and community composition varied over a period of landscape‐scale environmental change. Our study area, northern lower Michigan, has experienced substantial land‐use and land‐cover change over time. Like much of the northern Midwest, it has shifted from a largely unpopulated, post‐logging shrubland to a moderately populated closed‐canopy forest. Such changes are generally expected to influence overall richness and community composition. We found that regional richness per year remained virtually unchanged over the study period. Year‐to‐year variation in species number was surprisingly low. Richness totals included vastly different species groups as the composition of the regional bird community changed substantially over time. Changes in the types of species present appear to reflect deterministic changes in habitat. The number of grassland and open‐habitat species decreased, for example, while species associated with older forests and urban habitats increased. Our results suggest that habitat changes at the landscape scale do not necessarily lead to changes in the number of species a region can support. Such changes, however, do appear to influence the types of species that will occupy a region, and can lead to substantial changes in community composition.

Paleowind (11,000 BP) directions derived from lake spits in Northern Michigan

We report on the characteristics of several, previously unstudied, late Pleistocene spits in northern lower Michigan. Each spit developed as strong waves eroded a headland or island within the Main and/or later stages of glacial Lake Algonquin. Most have ESE to WNW orientations with prominent recurved hooks at their ends. Some are as long as 20 km. Sedimentologically, the spits contain thick (several meters or more) sequences of sand and well-rounded gravel. We suggest that summertime winds during the spits' formation (11,800 to ≈10,500 BP) were both very strong and dominantly from the E and SE, based on the following data: (1) their ESE (source, or head, of the spit) to WNW orientation, (2) the presence of steep, wave-cut bluffs on the east sides of the islands and headlands, and (3) spit locations (on the northeastern shore of Michigan's lower peninsula, coupled with their absence on the northwestern shore, just 100 km away). Wind direction data provided by these spits agree with paleoclimatic models that suggest that the Laurentide Ice Sheet had developed a strong anticyclonic circulation. The near lack of archeological sites from this time period in the northern lower peninsula, which was surrounded by Lake Algonquin at that time, supports the notion that the climate of northern lower Michigan was inhospitably cold and very windy in the late Pleistocene.

Landscape‐Level Patterns of Microbial Community Composition and Substrate Use in Upland Forest Ecosystems

The composition and diversity of biotic communities are controlled by the availability of growth‐limiting resources. Resource availability for microbial populations in soil is controlled by the amount and types of organic compounds entering soil from plant litter. Because plant communities differ in the amount and type of substrates entering soil, we reasoned that the composition and function of soil microbial communities should differ with the dominant vegetation. We tested this idea by studying two sugar maple (Acer saccharum Marsh.)‐dominated and one oak (Quercus spp.)‐dominated forest ecosystems in northern Lower Michigan that differ in rates of soil N cycling. We used phospholipid fatty acid (PLFA) analysis to gain insight into microbial community composition, and we used a subset of Biolog GN substrates found in root exudate to assess the metabolic capabilities soil microbial communities. Although microbial biomass did not differ among ecosystems, principal components analysis of bacterial, actinomycetal, and fungal PLFAs clearly separated the microbial communities of the three ecosystems. Similarly, principal components analysis separated microbial communities by differences in growth on carbohydrates, organic acids, and amino acids. Discrimination among microbial communities in the three ecosystems by PLFAs and substrate use occurred in spring, summer, and fall, but the individual PLFAs and substrates contributing to discrimination changed during the growing season. Our results indicate that floristically and edaphically distinct forest ecosystems also differ in microbial community composition and substrate use. This pattern was consistent across the growing season and repeatedly occurred across relatively large land areas.

Beyond Public Participation: Fairness in Natural Resource Decision Making

Perceptions of justice in decision outcomes and processes increases trust, support for authorities, and satisfaction with unfavorable outcomes (Lind and Tyler 1988). However, very little research has been done to apply justice concepts to natural resource decision making contexts (an exception is Lauber and Knuth 1997; 1998). In the study presented here, two rounds of focus groups with public participants in the Northern Lower Michigan Ecosystem Management Project were conducted. Participants were asked to describe their experiences with and attitudes toward different types of public participation. They were also asked what criteria they use when judging the fairness of a natural resource agency decision. Themes emerging in the answers included the importance of representation, voice, consideration, logic, and desired outcomes. These are very similar to justice principles documented in other fields. This suggests managers would be wise to focus less on public participation techniques and more on achieving fair decision-making processes.

LATE PLEISTOCENE ICE-FLOW DIRECTIONS AND THE AGE OF GLACIAL LANDSCAPES IN NORTHERN LOWER MICHIGAN

Lineation (alignment) of certain landscape features, most of which are glacigenic, and till-fabric data were mapped and used to indicate glacier flow directions in northern lower Michigan. I also reexamine the limit of the Greatlakean (ca. 11,850 14C yr B.P.) ice advance in northern Michigan. My study assumes that reddish-brown (7.5YR and redder) till in the region was deposited by the last ice advance, the Greatlakean, since this color till covers uplands immediately down ice from an established Greatlakean stratigraphic marker bed. Landscape linearity and till-fabric data within the reddish-brown till landscapes of northwestern lower Michigan both show strong northwest-southeast alignment, indicating that Greatlakean ice advanced out of the Lake Michigan basin. Earlier (Port Huron, ca. 13,000 14C yr B.P.) ice deposited brown till of 10YR hue, as evidenced at a borrow pit where reddish-brown Greatlakean till overlies brown drift. Thus, the “7.5YR and redder” vs 10YR hue limit is interpreted as the outer limit of the Greatlakean advance in northern lower Michigan. By mapping this limit only a few tens of km southeast of its only stratigraphically confirmed position at the Cheboygan bryophyte bed, I imply that Greatlakean ice did not advance as far into northeastern lower Michigan as others previously have suggested. Port Huron landforms in brown till show similar alignment and till fabrics to Greatlakean landforms in red till, suggesting that Port Huron ice in northern Michigan (also) advanced primarily out of the Lake Michigan basin. As it advanced from the northwest, flowlines and till fabrics suggest that the Port Huron ice gradually curved, eventually flowing north-to-south in northeastern lower Michigan, nearer the Saginaw Lobe. [Key words: glacial geology, drumlins, landscape fabric, Greatlakean advance, Port Huron advance, till fabric, glacial dynamics.]

Landscape influence on the spatial and temporal distribution of the Kirtland's warbler at the Bald Hill burn, northern Lower Michigan, U.S.A.

The Kirtland's warbler (Dendroica kirtlandii Baird) is an endangered songbird that nests in northern Lower Michigan in ecosystems dominated by young jack pine (Pinus banksiana Lamb.). We used the landscape ecosystem approach to identify two adjacent landform-level ecosystems located at different elevations within the area burned by the 1975 Bald Hill fire in Crawford County, Michigan. Eighty-five percent of the warblers occupied the high-elevation landform from 1982 to 1987 compared with only 25% from 1992 to 1997, indicating a shift in warbler occupancy of the burn from the high- to the low-elevation landform by 1991. Although similar in soil, the landforms have significantly different microclimates as mediated by physiography. Because of a warmer microclimate, jack pines in the high-elevation landform exhibited faster growth and were colonized first by the warbler; the trees in the cooler low-elevation landform grew more slowly and were colonized later. We suggest that the presence of two adjacent landform-level ecosystems within a single burned area prolonged the warbler occupancy of the burn. Thus, a broad-scale ecosystem approach that considers physical site factors as well as vegetative characteristics is useful for understanding the complex interactions that affect spatial and temporal species distributions across the landscape. Such an approach may benefit the management of particular endangered species, especially when the size of management areas is considered to be a crucial element of species preservation.

A 22-Year Study on the Effects of Mammalian Browsing on Forest Succession Following a Clearcut in Northern Lower Michigan

Tree stems were systematically sampled in 1983, 1988 and 1999 in a 1977 clearcut in the Pigeon River State Forest, Michigan. All stems were identified to species and recorded as living or dead and browsed or unbrowsed by mammalian herbivores, based on inspection of twigs and buds. Tree species richness increased in the study area, mostly due to the presence of several wind dispersed species previously not recorded. The abundance of all trees collectively declined over the study period as the clearcut aged and the relative abundance of the most preferred (and most common) browse species in 1983 declined to 0.0 by 1999. There was no evidence that mammalian herbivores were using the clearcut for feeding in 1999, suggesting decline of use as preferred species declined. About half the study area sampled in 1999 was dominated by herbaceous species, whereas the entire area was dominated by young woody growth in earlier years. The results collectively suggest that high degrees of sustained browsing affect community structure and composition by lowering abundance of more preferred species over time.

Genotypic variation in physiological and growth responses of Populus tremuloides to elevated atmospheric CO2 concentration.

Physiological and biomass responses of six genotypes of Populus tremuloides Michx., grown in ambient t (357 micromol mol(-1)) or twice ambient (707 micromol mol(-1)) CO2 concentration ([CO2]) and in low-N or high-N soils, were studied in 1995 and 1996 in northern Lower Michigan, USA. There was a significant CO2 x genotype interaction in photosynthetic responses. Net CO2 assimilation (A) was significantly enhanced by elevated [CO2] for five genotypes in high-N soil and for four genotypes in low-N soil. Enhancement of A by elevated [CO2] ranged from 14 to 68%. Genotypes also differed in their biomass responses to elevated [CO2], but biomass responses were poorly correlated with A responses. There was a correlation between magnitude of A enhancement by elevated [CO2] and stomatal sensitivity to CO2. Genotypes with low stomatal sensitivity to CO2 had a significantly higher A at elevated [CO2] than at ambient [CO2], but elevated [CO2] did not affect the ratio of intercellular [CO2] to leaf surface [CO2]. Stomatal conductance and A of different genotypes responded differentially to recovery from drought stress. Photosynthetic quantum yield and light compensation point were unaffected by elevated [CO2]. We conclude that P. tremuloides genotypes will respond differentially to rising atmospheric [CO2], with the degree of response dependent on other abiotic factors, such as soil N and water availability. The observed genotypic variation in growth could result in altered genotypic representation within natural populations and could affect the composition and structure of plant communities in a higher [CO2] environment in the future.

Canopy Cover Effects on Cellulose Decomposition in Oak and Pine Stands

Cellulose mass loss was measured for four levels of canopy cover,i.e., clearcut, 25%, 75%, and uncut, in northern red oak (Quercus rubra) and red pine (Pinus resinosa) stands in northern Lower Michigan, USA. Cellulose mass loss was more rapid in the clearcut and 25% canopy cover treatments than in the 75% canopy cover and uncut treatments. After 4 month incubation of cellulose filter papers, mass loss rates averaged 75.2% in the clearcut, 56.3% in the 25% canopy cover, 46.9% in the 75% canopy cover, and 45.7% in the uncut stands. For the clearcut and the 25% canopy cover treatments, cellulose mass loss in the mineral soil layer was significantly higher than in the forest floor after 2 and 4 months of incubation, while cellulose mass loss of the uncut treatment was significantly lower in the soil layer than in the forest floor after 4 months of incubation. Cellulose mass loss was not significantly different between the oak and the pine stands (p > 0.05), but cellulose mass loss rates in other canopy cover treatments except for the clearcut were generally higher in red oak stands than in red pine stands. These results suggest that canopy manipulation increases cellulose decomposition and may stimulate nutrient cycling process in canopy removal stands.

INTERACTIVE EFFECTS OF ATMOSPHERIC CO2AND SOIL-N AVAILABILITY ON FINE ROOTS OFPOPULUS TREMULOIDES

The objective of this experiment was to understand how atmospheric carbon dioxide (CO2) and soil-nitrogen (N) availability influence Populus tremuloides fine-root growth and morphology. Soil-N availability may limit the growth response of forests to elevated CO2 and interact with atmospheric CO2 to alter litter quality and ecosystem carbon (C) and N cycling. We established a CO2 × N factorial field experiment and grew six genotypes of P. tremuloides for 2.5 growing seasons in 20 large open-top chamber/root-box experimental units at the University of Michigan Biological Station in northern lower Michigan (USA). In this paper we describe an integrated examination of how atmospheric CO2 and soil-N availability influence fine-root morphology, growth, mortality, and biomass. We also studied the relationship between root biomass and total soil respiration. Over 80% of the absorbing root length of P. tremuloides was accounted for by roots <0.4 mm in diameter, and specific root length (100–250 m/g) was much greater than reports for other temperate and boreal deciduous trees. Elevated atmospheric CO2 increased the diameter and length of individual roots. In contrast, soil N had no effect on root morphology. Fine-root length production and mortality, measured with minirhizotrons, was controlled by the interaction between atmospheric CO2 and soil N. Rates of root production and mortality were significantly greater at elevated CO2 when trees grew in high-N soil, but there were no CO2 effects at low soil N. Fine-root biomass increased 137–194% in high-N compared to low-N soil, and elevated atmospheric CO2 increased fine-root biomass (52%) in high soil N, but differences in low soil N were not significant. Across all treatments, dynamic estimates of net fine-root production were highly correlated with fine-root biomass (soil cores; r = 0.975). Mean rates of soil respiration were more than double in high-N compared to low-N soil, and elevated atmospheric CO2, when compared to ambient atmospheric CO2, increased mean rates of soil respiration 19% in 1995 and 25% in 1996. Across all treatments, total root biomass was linearly related to mean rates of soil respiration (r2 = 0.96). Our results indicate that atmospheric CO2 and soil-N availability strongly interact to influence P. tremuloides fine-root morphology, growth, and C turnover. Aspen-dominated ecosystems of the future are likely to have greater productivity fueled by greater nutrient uptake due to greater root length production. Further, it appears that elevated atmospheric CO2 will result in greater C inputs to soil through greater rates of fine-root production and turnover, especially in high-fertility soils. Increased C inputs to soil result in greater rates of soil respiration. At this time, it is not clear what effects increased rates of root turnover will have on C storage in the soil.

Landscape influence on the spatial and temporal distribution of the Kirtland's warbler at the Bald Hill burn, northern Lower Michigan, U.S.A.

The Kirtland's warbler (Dendroica kirtlandii Baird) is an endangered songbird that nests in northern Lower Michigan in ecosystems dominated by young jack pine (Pinus banksiana Lamb.). We used the l...

Ecosystem Management and Public Participation: Lessons from the Field

Abstract This study explores the public participation component of the Northern Lower Michigan Ecosystem Management Project. The study was conducted to determine the perceptions of natural resource professionals about public participation and document how those perceptions change over time as a result of a nontraditional, focus group--based model of public participation. The professionals' perceptions were then compared with public perceptions about public participation processes. The results suggest that the new approaches to public participation embodied in ecosystem management are more consistent with public desires than are traditional methods, but improvements are still possible.

Genotypic variation for condensed tannin production in trembling aspen (POPULUS TREMULOIDES, salicaceae) under elevated CO2 and in high‐ and low‐fertility soil

The carbon/nutrient balance hypothesis suggests that leaf carbon to nitrogen ratios influence the synthesis of secondary compounds such as condensed tannins. We studied the effects of rising atmospheric carbon dioxide on carbon to nitrogen ratios and tannin production. Six genotypes of Populus tremuloides were grown under elevated and ambient CO(2) partial pressure and high- and low-fertility soil in field open-top chambers in northern lower Michigan, USA. During the second year of exposure, leaves were harvested three times (June, August, and September) and analyzed for condensed tannin concentration. The carbon/nutrient balance hypothesis was supported overall, with significantly greater leaf tannin concentration at high CO(2) and low soil fertility compared to ambient CO(2) and high soil fertility. However, some genotypes increased tannin concentration at elevated compared to ambient CO(2), while others showed no CO(2) response. Performance of lepidopteran leaf miner (Phyllonorycter tremuloidiella) larvae feeding on these plants varied across genotypes, CO(2), and fertility treatments. These results suggest that with rising atmospheric CO(2), plant secondary compound production may vary within species. This could have consequences for plant-herbivore and plant-microbe interactions and for the evolutionary response of this species to global climate change.

Soil Warming and Carbon Loss from a Lake States Spodosol

AbstractElevated soil temperatures may increase C loss from soils by accelerating microbial respiration and dissolved organic C leaching. We evaluated the effect of elevated soil temperatures on C losses from a forest Spodosol by incubating soil cores from surface (Oa + A + E) and subsurface (Bhs) horizons at two seasonal temperature regimes. One regime simulated the normal course of soil temperatures in northern lower Michigan, and the other simulated soil temperatures representing an amount of warming that might occur under some global warming theory calculations. We measured the amounts of CO2‐C respired and dissolved organic C leached from the soil cores during a 33‐wk period. Microbial respiration rates, after adjustment for variation in initial rates, were significantly increased by soil warming and were greater in surface than in subsurface horizons. Warming significantly increased cumulative C respired, with greater losses from surface soils (≥50 mg C g‐1 C) as compared with subsurface soils (≤25 mg C g‐1 C). Mean quantities of dissolved organic C leached, ranging from 2.3 to 3.2 mg C g‐1 C, did not differ significantly by soil horizon or temperature regime. Increased microbial respiration in surface soil horizons was the process most responsive to soil warming in the Spodosol samples we examined. Whether this is a short‐term effect that would disappear once pools of labile C are exhausted, or represents a long‐term response to soil warming, remains uncertain.

Regeneration of Northern Red Oak in Relation to Ectomycorrhizae in Oak and Pine Stands after Overstory and Understory Manipulations

Abstract Growth of northern red oak (Quercus rubra L.) seedlings in relation to colonization by indigenous ectomycorrhizal (ECM) fungi was studied in oak and pine stands in northern Lower Michigan that were subjected to overstory and understory manipulations. Two stand types (oak and pine), three blocks of each stand type, four canopy cover treatments (clearcut, 25% cover (50% cover in the first year), 75% cover and uncut), and two understory treatments (shrub removal and untreated control) were involved in the experiment. Northern red oak acorns from a common seed source were sown in May 1991 to simulate natural regeneration. Seedling growth and its relation to percent ECM were evaluated for the first two growing seasons. A significantly larger root-collar diameter of northern red oak seedlings was found in pine stands than in oak stands for the first growing season (P &lt; 0.001). However, this difference could not be explained by overall ECM colonization. Seedling growth and ECM colonization were not affected by the shrub removal treatment during the first two growing seasons. In contrast, northern red oak seedling size and weight were strongly influenced by the overstory treatment, with lower levels of canopy cover resulting in larger seedlings. Seedlings had the greatest percent ECM in the partial cover treatment (25-50%) and the lowest percent ECM in the clearcut. After accounting for the effects of canopy cover, the relationship between total biomass of northern red oak seedlings and percent ECM was positively correlated (P = 0.001) during the first growing season and negatively correlated (P = 0.038) during the second growing season. A positive relationship between root/shoot ratio and percent ECM also existed in the first year (P = 0.003) in both oak and pine stands, but only in the oak stands in the second year (P = 0.039). These results indicate that ECM promoted more root development than shoot development, particularly underpartial canopy cover (25%-50%) treatments, where the greatest percent ECM and largest root/shoot ratio were found. Moreover, our results suggest that these partial canopy cover treatments provide a favorable balance between ECM abundance and northern red oak seedling development in both oak and pine stands on intermediate quality sites, and may lead to northern red oak regeneration success on such sites. North. J. Appl. For. 15(4):182-190.

Soil Nitrogen Mineralization at Various Levels of Canopy Cover in Red Pine (Pinus resinosa) Plantations

The objective of this study was to determine the rate of nitrogen (N) mineralization in response to various levels of canopy cover in red pine (Pinus resinosa Ait.) stands. Experimental plots consisted of various levels of canopy cover,i.e., clearcut, 25% (50% during first sampling year), 75%, and uncut in red pine plantations in northern Lower Michigan, USA. Net N mineralization and nitrification in the top 15 cm of mineral soil were examined during the first two growing seasons (1991–1992) following the canopy cover manipulations, using anin situ buried bag technique. Mean net N mineralization over the course of both growing seasons (May—October) ranged from 26.9 kg ha–1 per growing season in the clearcut treatment to 13.4 kg ha–1 per growing season in the uncut stand. Net N mineralization and nitrification increased significantly in the clearcut treatment compared to the uncut treatment during the second growing season only. However, net N mineralization and nitrification did not differ significantly between the partial canopy cover treatments and the uncut stand. Increased N mineralization and nitrification in the clearcut during the second growing season may be associated with increased soil temperature and changes of organic matter quality with time since canopy removal.

REGENERATION OF NORTHERN RED OAK: POSITIVE AND NEGATIVE EFFECTS OF COMPETITOR REMOVAL

The decreased ability of northern red oak (Quercus rubra) to regenerate throughout its range in the eastern United States has important ecological and economic implications. We studied regeneration of northern red oak in oak and pine stands on moderately productive sites in northern Lower Michigan. Our objectives were (1) to investigate the hypothesis that regeneration of northern red oak is more successful in pine than in oak stands and (2) to test whether removal of potential overstory and understory competitors increases regeneration success on moderately productive sites. Northern red oak acorns and 2-yr-old nursery seedlings were planted in spring 1991 in three natural oak stands and three red pine (Pinus resinosa) plantations on comparable, moderately productive sites. Each stand contained four canopy cover treatments: clearcut, 25% cover (50% the first year), 75% cover, and uncut. Each canopy cover treatment contained four understory treatments: herb-layer removal, shrub-layer removal, litter removal, and control. Seedling survival, performance, and damage due to deer and late spring frosts were quantified along with vegetation characteristics, light, soil moisture, air temperature, soil temperature, and precipitation during the 1991 and 1992 growing seasons. Survival of seedlings was significantly lower in the pine than in the oak stands; the pine stands had a higher incidence of white-tailed deer browsing and lower levels of belowground resources than the oak stands. Canopy cover treatments produced a gradient from high stress (low light and soil moisture) and low disturbance (slight browsing and frost damage) in uncut plots to low stress (high light and soil moisture) and high disturbance (high browsing and frost damage) in clear-cut plots. Understory vegetation development and soil temperature were low in uncut plots, high in clear-cut plots, and intermediate in plots with partial overstory removal. Despite poor growth, the mortality of seedlings planted in uncut plots was low. In contrast, mortality was high and surviving seedlings exhibited good growth in clearcuts. Understory treatment effects were slight compared with overstory treatment effects. Results on our sites (1) do not support the hypothesis that regeneration success of northern red oak may be greater in pine than in oak stands and (2) indicate that the positive effects of removing potential competitors on seedling growth and physiological status may be compromised by simultaneous negative effects of browsing and frost damage.

Regeneration of Northern Red Oak: Positive and Negative Effects of Competitor Removal

The decreased ability of northern red oak (Quercus rubra) to regenerate throughout its range in the eastern United States has important ecological and economic implications. We studied regeneration of northern red oak in oak and pine stands on moderately productive sites in northern Lower Michigan. Our objectives were (1) to investigate the hypothesis that regeneration of northern red oak is more successful in pine than in oak stands and (2) to test whether removal of potential overstory and understory competitors increases regeneration success on moderately productive sites. Northern red oak acorns and 2-yr-old nursery seedlings were planted in spring 1991 in three natural oak stands and three red pine (Pinus resinosa) plantations on comparable, moderately productive sites. Each stand contained four canopy cover treatments: clearcut, 25% cover (50% the first year), 75% cover, and uncut. Each canopy cover treatment contained four understory treatments: herb-layer removal, shrub-layer removal, litter removal, and control. Seedling survival, performance, and damage due to deer and late spring frosts were quantified along with vegetation characteristics, light, soil moisture, air temperature, soil temperature, and precipitation during the 1991 and 1992 growing seasons. Survival of seedlings was significantly lower in the pine than in the oak stands; the pine stands had a higher incidence of white-tailed deer browsing and lower levels of belowground resources than the oak stands. Canopy cover treatments produced a gradient from high stress (low light and soil moisture) and low disturbance (slight browsing and frost damage) in uncut plots to low stress (high light and soil moisture) and high disturbance (high browsing and frost damage) in clear-cut plots. Understory vegetation development and soil temperature were low in uncut plots, high in clear-cut plots, and intermediate in plots with partial overstory removal. Despite poor growth, the mortality of seedlings planted in uncut plots was low. In contrast, mortality was high and surviving seedlings exhibited good growth in clearcuts. Understory treatment effects were slight compared with overstory treatment effects. Results on our sites (1) do not support the hypothesis that regeneration success of northern red oak may be greater in pine than in oak stands and (2) indicate that the positive effects of removing potential competitors on seedling growth and physiological status may be compromised by simultaneous negative effects of browsing and frost damage.