Second-growth Northern Hardwood Forest Research Articles

Stand spatial structure outcomes of forest adaptation treatments in northern hardwood forests in North America

Spatial arrangement of trees is determined by a complex suite of factors, including disturbance history, competition, and resource availability. These spatial patterns drive adaptive capacity by influencing arrangement of growing space, neighborhood competitive relationships, and disturbance response, with irregular patterns supporting higher adaptive capacity. While spatial structure in relation to disturbance and climate change resilience has been studied in dry conifer forests and old-growth temperate forests, it has never been explored in the context of climate adaptive management in mesic, second-growth forests. To address this gap, we analyzed tree spatial patterns in second-growth northern hardwood forests under four different climate adaptation management approaches: no action; resistance or resilience to impacts of climate change; and transition to future-adapted forest types. We used spatial point statistics approaches to describe how patterns differed among the four treatments. We found that the treatments focused on future adaptation led to patterns with variable tree spacing and clumping, while those focused on perpetuating current conditions resulted in less pattern variation. This indicates that adaptation strategies that include uneven-aged regeneration methods that restore and maintain tree spatial patterns historically generated by gap dynamics can be successful in altering resource availability patterns and adaptation space in forest stands.

Growth, yield, and financial return through six decades of various management approaches in a second-growth northern hardwood forest

Evaluation of long-term studies provides information about the sustainability of forest management on a time scale valuable to forest managers. A long-term silvicultural study at the Argonne Experimental Forest in northern Wisconsin has been continuously maintained for over six decades, providing an opportunity to test differences in growth and yield between three partial cutting methods: single-tree selection, crop tree release, and diameter-limit cutting. Over 65 years, results showed that single-tree selection with low residual stocking (14 m2 ha−1 basal area) performed best in regard to financial returns, growth, and yield, with high financial returns, greater levels of ingrowth, lower mortality, and greater residual percent volumes of high quality sawtimber. Diameter growth was maximized in low to moderate (17 m2 ha−1 residual basal area) residual stocking selection. The diameter-limit cut (18 cm dbh limiting diameter) was not successful in producing high-quality trees or consistent yield. After 50 years the crop tree release treatment did increase percent of high-quality harvested volumes, but to a lesser extent than the selection treatments. Selection treatments removing more basal area provided an overall better return on investment, providing potential incentive for modification to traditionally practiced single-tree selection to higher residual basal areas.

Harvested opening size affects cohort development and failures in a second-growth northern hardwood forest

Group selection is one potential tool in contemporary, uneven-aged silviculture for mitigating increased dominance by shade-tolerant species while continuing to meet stocking goals and harvest quotas. However, recent experimental group-selection openings in northern hardwood forests on some sites in the Upper Great Lakes region have failed to meet management goals of increased tree diversity and timely natural regeneration. Few observational studies have documented the long-term stand development of nascent cohorts following group-selection prescriptions. This study revisits a long-term group-selection experiment which previously found that opening size had little effect on regeneration diversity and found that large areas devoid of saplings and dominated by understory vegetation were common more than a decade following harvest. Our goals were to reassess the state of stand development and diversity within harvested openings, as well as look for early post-harvest indicators of regeneration success or failure after 23 years. At 23 years post-harvest, we found that opening size had little effect on regeneration diversity but rather influenced patterns of cohort development. Smaller openings (6–20 m diameter) showed signs of self-thinning, while larger openings (30–46 m diameter) showed divergent developmental patterns including both self-thinning and ongoing sapling recruitment. Most areas of regeneration failures persisted through 23 years, and the largest openings (46 m diameter) were most likely to display regeneration failure, with 42% of subplots devoid of saplings but containing dense understory vegetation. The density of understory vegetation at 2 years post-harvest had no effect on year-23 sapling density. Instead, only seedling/sapling density in year 2 correlated with regeneration success after 23 years. These results suggest that in northern hardwood forests in the upper Great Lakes region, opening creation alone may not produce the desired effects of increased canopy diversity and sustained timber yields. Canopy openings >30 m diameter appear more likely to result in partial, multi-decade regeneration failures compared to smaller openings; however, our results highlight the importance of advance regeneration for preventing these failures. Thus, simultaneously achieving management goals of increased tree diversity and adequate regenerative stocking with group selection likely requires additional inputs (e.g. herbivore protection, planting, scarification, herbicides, repeated thinning, etc.) beyond the current status quo.

Emergent properties of downed woody debris in canopy gaps: A response of the soil ecosystem to manipulation of forest structure

Natural forest disturbance events can influence soil biogeochemical processes in two ways - by creating downed woody debris (DWD; fallen tree boles or branches) and by creating canopy gaps that alter forest microclimate. DWD represents a substrate for microbial growth and a persistent store of carbon and nutrients, but microbial activity is also sensitive to temperature and moisture. We studied the potential interaction of DWD and canopy gaps on soil microbial processes, and wondered if microclimatic conditions resulting from the manipulation of forest structure would be enough to inhibit production, thereby altering a critical ecosystem process. Gaps and DWD (>10 cm diameter) were added to a maturing, even-aged, second-growth northern hardwood forest (the Flambeau Experiment; N Wisconsin, USA) to enhance structural complexity and promote key ecosystem processes typically associated with late-successional forests. We investigated the influence of DWD and gaps on soil microbial community composition, extracellular enzyme activity and soil characteristics. Soils were sampled near intermediately and highly decayed DWD and 2 m away from DWD (control) in gaps and closed canopy a decade after manipulation.DWD decomposition influenced the surrounding soil differentially depending on decay class and canopy condition. Mean C- and P-potential extracellular enzyme activities (BG, BX and AP) were enhanced near highly decayed DWD in gaps. The relative abundance of bacteria (actinomycete, anaerobic, gram-negative and gram-positive) remained constant in gaps but decreased from May to August in closed canopy. In gaps, soil total exchangeable cations increased by 34.6%, available phosphorus by 152% and fungal to bacterial ratios by 23.3% but temperatures decreased by 3.42% suggesting that canopy condition continues to affect soil properties and microbial processes a decade after gap creation. These results highlight the contribution of DWD to the forest floor and the influence of decaying wood characteristics on belowground ecosystems critical to future forest productivity. Retaining or adding heterogeneously distributed DWD of varying decay status may be essential to maintain ecosystem functions associated with nutrient cycling and microbial community dynamics in managed forests.

Initial tree regeneration response to natural-disturbance-based silviculture in second-growth northern hardwood forests

Northern hardwood stands in the Great Lakes region are often managed using single-tree selection, which generally favors regeneration of shade-tolerant species, especially sugar maple (Acer sacchar...

Do biological legacies moderate the effects of forest harvesting on soil microbial community composition and soil respiration

Ecological forestry is a management approach that uses natural disturbance processes as models for designing silvicultural prescriptions that restore or sustain ecosystem biodiversity and function in actively managed forests. We evaluated how a novel ecologically-based multi-cohort silvicultural treatment affects the soil microbial community (SMC) and tested whether supplemental dead wood in the form of girdled trees alters these effects. We also tested SMC function by measuring soil CO2 flux over multiple growing seasons, and examined if these patterns were related to soil microbial groups. Our experimental harvests were conducted in second-growth northern hardwood forests in northern Wisconsin, USA. Treatments included a modified shelterwood harvest (SH), a shelterwood harvest plus dead wood supplementation (SH + CWD), and an unharvested control; here we report responses three to five years post-treatment. The SMC composition (determined using PLFA) in both harvests was significantly different from the control, a difference driven by greater bacterial abundance in the harvested areas, and particularly by gram negative bacteria in SH. Microbial community composition was not significantly different between the two harvests (SH and SH + CWD). Total soil respiration was significantly lower in SH than in the control and SH + CWD treatments, a difference most likely driven by a reduction of the autotrophic respiration component in SH treatments due to harvesting, while in the SH + CWD treatment roots from living girdled trees contributed to autotrophic soil respiration. The relationship between the SMC and soil respiration varied with treatment and season. In general, soil respiration in the unharvested controls was most significantly correlated with microbes that relate to autotrophic respiration sources, while respiration in SH + CWD was most significantly correlated with heterotrophic microbes. These results indicate that, although the SMC composition was affected by forest harvesting practices incorporating live and dead biological legacies, supplementing the number of standing dead trees through girdling and felling maintained SMC function, as measured through total soil respiration, an indicator of some important aspects of ecosystem function.

Temporal trends and sources of variation in carbon flux from coarse woody debris in experimental forest canopy openings.

Pulses of respiration from coarse woody debris (CWD) have been observed immediately following canopy disturbances, but it is unclear how long these pulses are sustained. Several factors are known to influence carbon flux rates from CWD, but few studies have evaluated more than temperature and moisture. We experimentally manipulated forest structure in a second-growth northern hardwood forest and measured CO2 flux periodically for seven growing seasons following gap creation. We present an analysis of which factors, including the composition of the wood-decay fungal community influence CO2 flux. CO2 flux from CWD was strongly and positively related to wood temperature and varied significantly between substrate types (logs vs. stumps). For five growing seasons after treatment, the CO2 flux of stumps reached rates up to seven times higher than that of logs. CO2 flux of logs did not differ significantly between canopy-gap and closed-canopy conditions in the fourth through seventh post-treatment growing seasons. By the seventh season, the seasonal carbon flux of both logs and stumps had decreased significantly from prior years. Linear mixed models indicated the variation in the wood inhabiting fungal community composition explained a significant portion of variability in the CO2 flux along with measures of substrate conditions. CO2 flux rates were inversely related to fungal diversity, with logs hosting more species but emitting less CO2 than stumps. Overall, our results suggest that the current treatment of CWD in dynamic forest carbon models may be oversimplified, thereby hampering our ability to predict realistic carbon fluxes associated with wood decomposition.

Harvest-Created Canopy Gaps Increase Species and Functional Trait Diversity of the Forest Ground-Layer Community

Biodiversity conservation within managed forests depends, in part, on management practices that restore or maintain plant community diversity and function. Because many plant communities are adapted to natural disturbances, gap-based management has potential to meet this need by using the historical range of variation in canopy disturbances to guide elements of harvest design. We tested this hypothesis with a well-replicated gap size experiment in a second-growth northern hardwood forest. We evaluated plant communities within and among experimental gaps of differing size, 13 years after an initial harvest. We used a resampling approach to estimate how conventional and gap-based management affect diversity partitioning and species and trait diversity of ground-layer plants. These diversity measures highlight relevant scales and function of ground-layer plants among harvest gap sizes and scenarios. Results from our field experiment showed that, at the gap-level, increasing gap size increased functional trait diversity of plants, while species diversity was higher in gaps than uncut references and maximized in medium-sized (20‐30 m diameter) gaps. In harvest scenarios created by resampling our empirical data, we found that at the stand scale, increasing harvest intensity (larger gaps and greater proportion of forest in gaps) increased species richness and variability in initial bloom times, tolerance to shade, and number of life-forms in the plant community. Based on the measures of diversity and function used in our study, our results suggest that size of harvest-created gaps and proportion of forest in gaps can be manipulated to attain biodiversity goals but evaluating the regional species pool and seed sources (e.g., presence of invasives, rare species) will be important to maintain or restore conservation value.

Experimental Manipulation of Forest Structure: Near-Term Effects on Gap and Stand Scale C Dynamics

Canopy gaps and coarse woody debris are two forest structural features that are more abundant in old-growth forests than in second-growth, even-aged stands. These features directly influence the carbon balance of the ecosystem, yet few studies have quantified their interactive effects. We experimentally manipulated the forest structure of a second-growth northern hardwood forest in north-central Wisconsin (USA) and measured the shift of C between pools of the ecosystem components. Here, we question the longevity of the changes to the aboveground pools and address their implications for total ecosystem C (TEC) and net ecosystem production (NEP) at both the gap and stand scale. At the scale of the gap, the harvest and removal of trees significantly reduced NEP (−3.2 to −3.5 Mg C ha−1 for gaps vs 2.2 to 2.5 Mg C ha−1 for reference conditions), but did not alter heterotrophic respiration. The addition of woody debris without harvest significantly increased heterotrophic respiration, decreasing soil C storage of the gap area (−0.5 to −1.1 Mg C ha−1). The combined treatment of gap creation and woody debris addition made the gap area a significant C source to the atmosphere for the 3 years of the study (−4.9 to −5.1 Mg C ha−1). We also estimated how these structural features would affect C dynamics at a broader scale. The conversion of 10% of the stand canopy to gap conditions caused only a brief decrease in the stand NEP with the C balance returning to reference conditions by the third year following tree harvest. The woody debris additions caused an increase in both TEC and heterotrophic respiration. When combined the addition of canopy gaps and woody debris caused plots to initially become significant C sources, relative to undisturbed locations that were consistently accumulating C, with an annual NEP ranging from 2.1 to 2.8 Mg C ha−1 y−1. Understanding the effects of these structural features on forest C dynamics is highly relevant as the maturing forests of the region transition to more structurally complex forests and the demand for managing ecosystems for long-term C sequestration increases.

Spatial Patterns of Soil Surface C Flux in Experimental Canopy Gaps

To explore within-gap spatial patterns of soil surface CO2 flux, we measured instantaneous soil surface CO2 flux, soil surface temperature, and soil moisture in north–south transects across canopy gaps and in adjacent contiguous forest from April to November 2010 in a second-growth northern hardwood forest in Wisconsin, USA. Throughout the growing season, soil surface CO2 flux was higher in the northern 1/3 and northern edge of gaps compared to the central and southern portions. These patterns were driven primarily by within-gap variation in soil temperature, which was itself driven by within-gap patterns of insolation. Most locations in the northern 1/3 and northern edge of gaps had significantly higher modeled total growing season C flux (mean 725 g C m−2) compared to the contiguous forest (mean 706 g C m−2), whereas C flux in the central and southern portions of gaps (mean 555 g C m−2) was significantly lower than both the contiguous forest and the northern portions of gaps.

Interactions of temperature and moisture with respiration from coarse woody debris in experimental forest canopy gaps

Coarse woody debris (CWD) and canopy gaps are thought to be important structural components that influence forest ecosystem processes, yet few studies have assessed whether these features interact to influence decay dynamics, and we know of none that does so under controlled experimental canopy gap conditions. By manipulating forest structure to create canopy openings, we tested whether canopy gaps, through their influence on microclimate, affect the respiration rate of CWD in a mature second-growth northern hardwood forest of Wisconsin, USA. We sought to determine the relative contribution of moisture or temperature as predictors of respiration, and to provide estimates of the annual C emission from CWD in these treatments. Differences in the surface temperature of CWD were apparent immediately following gap treatments, with debris in gaps staying significantly warmer than beneath undisturbed canopy. Coarse woody debris moisture content differed more between gaps and undisturbed canopy in the second post-treatment year due to higher insolation and temperature in gaps relative to undisturbed canopy. The gap treatment increased respiration rates and altered the relationship with CWD temperature and moisture in complex ways. The impacts of gap creation on C fluxes from CWD depended on the range of temperature and moisture being examined. The interaction of the temperature and moisture of the debris in combination with the decay class explained 60% of the observed variation in CWD respiration beneath undisturbed canopies. In gaps, only 23% of the variation of flux could be explained by environmental variables. Annual C fluxes were 128.2 g C kg −1 year −1 in canopy openings and 108.5 g C kg −1 year −1 in undisturbed canopy locations.

Effects of selective tree harvests on aboveground biomass and net primary productivity of a second-growth northern hardwood forest

Restoring structural features of old-growth forests, such as increased canopy gap sizes and coarse woody debris, is a common management goal for second-growth, even-aged stands. We experimentally manipulated forest structure by creating variable-size canopy gaps in a second-growth northern hardwood forest in north-central Wisconsin following two growing seasons of pre-treatment monitoring. The objectives of this study were to quantify the influence of canopy gaps of different sizes (50–380 m2) on aboveground biomass and productivity of each vegetation stratum two growing seasons following treatment. Two years after treatment, ground layer biomass in canopy openings increased significantly relative to surrounding undisturbed transition zones. The response of ground layer biomass was greatest in the large versus the medium and small gaps. Sapling aboveground net primary productivity was significantly greater in undisturbed transition zones than within gaps across gap sizes following the second post-treatment growing season. Annual stem diameter increment was greatest for trees along gap borders and was correlated with crown class, percentage of crown perimeter exposed, gap area, and shade tolerance. Total aboveground net primary productivity was significantly lower in the gap addition plots the first year but by the second post-treatment growing season no longer differed from that in the control plots.

Patterns of plant community structure within and among primary and second-growth northern hardwood forest stands

Forest scientists advocate the use of natural disturbance-based forest management for restoring the characteristics of old-growth forests to younger second-growth northern hardwood stands. However, prescriptions rely upon studies that have (1) not spanned the full range of conditions and species assemblages, and (2) focused primarily on contrasting old-growth and mature second-growth stands at a single scale. To examine how the legacy of historical logging activities influences forest structure and function, we compared and contrasted patterns of plant community structure within and among second-growth and primary stands on the north shore of Lake Superior in Minnesota, USA — near the current range limits of the dominant species, sugar maple ( Acer saccharum). We expected second-growth stands to be in younger developmental stages, and structurally less heterogeneous both within and among stands. Furthermore, we expected those differences to be associated with patterns of plant community composition and diversity. Three of the four primary stands and one of the eight second-growth stands were in the old-growth stage of development. Yellow birch ( Betula alleghaniensis) and conifers as a group ( Thuja occidentalis, Picea glauca and Abies balsamea) were more abundant, and yellow birch was more variable, within primary stands than second-growth stands. The volume and heterogeneity of coarse woody debris in intermediate decay- and size classes was also greater within and among primary stands relative to second-growth stands. While mean subplot richness of overstory tree species was greater in primary stands, mean quadrat richness, and rates of species accumulation for forest herbs as well as total herbaceous cover, and graminoid cover were greater in second-growth stands. Furthermore, total basal area (BA), the BA of conifer species, the density of yellow birch trees, understory vegetation and light transmittance were more variable among second-growth stands. At the multivariate level, primary stands were distinguished from second-growth stands not by differences in stand structure, but by a greater abundance of yellow birch and conifer species in the canopy, which was also related to O-horizon depth and understory plant species composition and structure. Differences in community structure between primary and second-growth stands may have resulted from the original cutover as well as high-grade logging, which together may have disrupted the mechanisms that maintain populations of important co-dominant tree species and associated understory plant communities in northern hardwood stands.

Nitrogen Storage and Cycling in Old- and Second-Growth Northern Hardwood Forests

Nitrogen Storage and Cycling in Old- and Second-Growth Northern Hardwood Forests