Stem Exclusion Phase Research Articles

Soil seed banks provide a storage effect in post-logging regrowth forests of southeastern Australia

Despite the importance of soil seed banks in diversity maintenance, our understanding of plant response to changes in resource availability is largely limited to above-ground vegetation. We investigated how forest structure and edaphic properties influenced the above-ground vegetation and soil seed banks following logging in montane regrowth forests dominated by Eucalyptus regnans in the Central Highlands of southeastern Australia. We surveyed above-ground vegetation, soil seed banks, forest structure and soil properties across 20 harvest units, aged 16 to 20 years. A total of 80 species were identified in above-ground vegetation with 72 species in the soil seed bank, with only 34% of species co-occurring in both species’ pools. Climate, soil, topography, and light availability shaped plant composition but the relative importance of these properties varied among individual species and pools of diversity (combined, above-ground, soil seed bank, life form). Annual heat moisture index (AHMI) was the most important factor that influenced plant community composition across all species pools with individual species positively associated with AHMI. Acacia, but not Eucalyptus stem density further moderated plant response via effects on both light availability and soil nitrogen at the stem exclusion phase of stand development. Structurally mediated controls on above-ground community composition flow through to the soil seed bank via above-ground diversity effects on below-ground composition. For the combined species pool, individual species were most frequently related to Acacia stem density that on average explained 20% of model variation. Soil nutrients accounted for most of the model variation for individual species in above-ground (26%), soil seed bank (20%) and non-woody (32%) pools of diversity, with AHMI accounting for most of the model variation for woody species (18%). The differential response of individual species and community composition to gradients in resource availability across different pools of species diversity and life forms clearly demonstrates a storage effect in wet temperate forests of southeastern Australia that promotes post-disturbance recruitment and persistence. Our work suggests logging interacts with broad environmental controls to shape species diversity. The key role of Acacia stem density as a biotic filter of plant diversity provides opportunity to shape future plant diversity through management interventions such as thinning in the early stages of stand development. With native forest harvesting of these forests set to end from 2024, our findings provide important information on future patterns of plant diversity as logged forests recover over time, opportunities for management interventions in shaping plant composition, and potential impacts of future wildfire.

Spatial patterns and species coexistence in mixed Abies marocana-Cedrus atlantica forest in Talassemtane National Park

Understanding spatially-explicit interactions between co-existing tree species provides valuable information about the trade-offs between facilitation and competition driving species coexistence and forest dynamics. Here, we analyzed the fine scale spatial pattern of Moroccan fir (Abies marocana) and Cedrus atlantica (Cedrus atlantica) in a mixed forest (Talassemtane National Park, northern Morocco) and discuss its consequences on intra- and inter-specific interactions and stand dynamics. All trees were mapped and measured in ten plots to investigate uni- and bivariate spatial point patterns using pair-correlation and mark correlation functions. Tree ring data was also investigated from increment cores to assess the growth and age patterns of both species. Fir and cedar trees showed similar basal area and mean diameter distributions, while A. marocana presented a higher density and regeneration. There was an overall aggregation of conspecific and heterospecific individuals at small distances up to 2 m. Univariate analysis of tree size classes supports that juveniles of both species, as well as A. marocana adult trees, show an aggregated pattern at fine scales up to 2 m, while C. atlantica adults exhibit a random pattern. Observed spatial patterns are consistent with the hypothesis of attraction mechanisms between fir and adult trees of C. atlantica. Abies marocana recruitment mainly occurred inside the canopy of dominant trees (at distances varying from c.a 1 m under cedar adults to 3 m under adult conspecifics), while C. atlantica was the dominant regeneration in small gaps. The studied A. marocana-C. atlantica mixed forests showed a transition from an initiation phase led by C. atlantica to a subsequent stem exclusion phase, where dominant trees shade out less-competitive trees and mainly fir trees can establish in the forest understory. According to the observed spatial patterns and growth rates, enhancing C. atlantica establishment in degraded areas may be a suitable proactive and adaptive management to promote long-term conservation of endemic Abies marocana under current climate change trends.

Increased reproductive trait diversity, evolutionary history and distinctiveness during the succession of tropical forest

AbstractQuestionsDo old‐growth forests retain more evolutionary history and more reproductive trait diversity along the successional gradient? How similar are the phylogenetic branches and reproductive attributes at each phase? In which succession stages are distinct lineages of trees and shrubs present? Do distinct lineages have distinct traits? Is there a turnover of species, lineages and traits of trees and shrubs along the successional gradient?LocationAtlantic Forest, Reserva Natural Guaricica (25°19′ S, 45°42′ W) and Reserva Natural das Águas (25°21′ S, 48°46′ W), Antonina, Paraná, southern Brazil.MethodsWe assessed the variation of alpha and beta taxonomic, functional and phylogenetic diversities of tree and shrub communities along a successional gradient (phase 1, stand initiation phase, from 0 to 9 years old; phase 2, stem exclusion phase, 10–25 years old; and phase 3, understorey re‐initiation phase, 25–200 years old). We also evaluated the occurrence of distinct lineages and traits related to seed dispersal and pollination.ResultsIn addition to the expected change in species composition, lineages and trait states and values were replaced along the chronosequence, with stronger variation for trees. Old‐growth forests harbor distinct lineages and traits and a greater amount of evolutionary history and functional diversity of reproductive traits. We also found that distinct lineages were those that performed distinct functions.ConclusionsOur study showed that as the succession proceeds, turnover of lineages and traits occurred, with the understorey re‐initiation stage harboring the largest taxonomic, phylogenetic and functional diversity. In addition, understories of the re‐initiation stage are important for the occurrence of distinct lineages, such as magnoliids, and of distinct traits, such as large fruits and seeds, which highlights the importance of conservation of old‐growth forests, and its preservation can prevent the loss of distinct evolutionary histories and functions.

Short-term effects of variable-density thinning on regeneration in hardwood-dominated temperate rainforests

Second-growth temperate forests usually have simpler composition and structure than comparable old-growth stands. We evaluated the application of variable density thinning (VDT) as a way to increase old-growth attributes, specifically tree species composition and density in two types of second-growth forests: a mixed evergreen-dominated forest (SE, stem exclusion phase) and a Nothofagus dombeyi-dominated forest (UR, understory reinitiation stage) in southern Chile (40°S Lat.). Six one-ha plots were established in each forest. We applied VDT in three plots in each forest, which included the following sub-treatments: a thinned matrix (M), large and small patches (LP and SP), and large and small reserves (LR and SR). We also established three one-ha plots in an old-growth forest (OG), which served as a reference for overall stand structural variables and the tree regeneration layer. OG had significantly more total regeneration and shade-tolerant regeneration, than the untreated SE and UR. After VDT, canopy openness was significantly greater in SE than in UR within the treated portions of the plots (M, LP and SP). The similarities in stand characteristics between untreated and pre-harvest treated plots in each second-growth forest suggest that harvesting was responsible for the lower densities observed in thinned plots. Regeneration patterns two growing seasons after harvests were reflective of the interaction between forest type and sub-treatments. While both second-growth forests showed increases in regeneration densities, none of these increases was significant, although proportionally and numerically they were greater in the SE forest. The sub-treatments showed differences only for shade-intolerant and shade-tolerant species in both forests. These differences in general illustrated a preference of shade-intolerant species for patches (only in the smaller height class in both forests), and of shade-tolerant species for the matrix or the reserves in all height classes from >50 cm through saplings (only in the SE forest). Overall, tree regeneration has been more responsive to VDT in the forest currently in an earlier successional stage, suggesting that second-growth forests in more advanced stages may need to be treated more intensively (larger patches, lower residual densities). Correspondingly, VDT protocols must be developed to account for the interaction between harvest disturbances, developmental stage, and silvics of the component species.

An Evaluation of Stand Age as a Factor of Mastication Efficiency and Effectiveness in the Central Sierra Nevada, California

Understanding costs and effectiveness of mechanical fuels treatments is essential for efficiently influencing fire behavior at the landscape scale. While the capability of mastication to mitigate the potential of high severity wildfire in mature stands has been studied, the variables that determine its efficiency across age classes have received far less attention. The objective of this study was to examine how stand age influenced the efficiency and effectiveness of mastication. Mastication was performed on stand ages and structures representative of those commonly observed in the mixed conifer forest of the Sierra Nevada. Machine data from spatial and motion sensors were used to assess machine efficiency, and fire modeling was used to evaluate treatment effectiveness. The stands masticated were 10, 33, and 100 years old. Machine operation time per hectare was shown to vary with stand age, with the 10 and 100-year-old stands less costly to treat than the 33-year-old stand. However, potential for high severity wildfire immediately following treatments was reduced to a much greater degree in the 33-year-old stand, whereas it increased for the 10-year-old stand and was unaffected in the 100-year-old stand. This study's findings suggest that masticating developing stands (i.e. those entering the stem exclusion phase), while relatively costly, may also provide managers with a big “bang for their buck” in reducing fire severity across the landscape. To assess longer-term effectiveness, it will be important to track predicted fire severity as masticated fuels decompose and as tree growth responds to the reduction in density following the treatments in stands of various ages.

Climate extremes during high competition contribute to mortality in unmanaged self-thinning Norway spruce stands in Bulgaria

Climatic variability often is thought to be most important for ecosystem development at ecotones, while competition is thought to be most important farther from ecotones, where neighboring plants compete for scarce resources. However, climatic variability may also modulate consequences of competition, especially under recent and future climate change. Norway spruce (Picea abies) forests are among the most important ecosystems in the mountain regions of Europe and provide various ecosystem services. Many of these forests are currently in a self-thinning, stem-exclusion phase. Understanding processes governing forest dynamics during this phase is necessary for understanding future forest structure and processes as well as effects of climatic variability on ongoing forest development. We studied growth and mortality patterns in unmanaged 100–150years-old Norway spruce forests that originated after stand-replacing disturbances in the Parangalitsa Reserve in Bulgaria. We collected data on forest structure and tree ring samples from 648 live and dead trees (DBH >4cm) to analyze onset, pattern and duration of mortality, as well as contributing factors.We found that climate extremes acted together with competition to cause sharp growth declines lasting from a few years to several decades and, in some cases, eventually led to death. The majority of dead trees had one to several consecutive growth declines, most of which initiated in response to extreme summer droughts during periods of high within-stand competition (after trees were 40–50years old). Our tree-ring analysis revealed that some suppressed trees that died were more drought-sensitive than living trees. Other climate extremes such as unusually cold winters or summers also contributed to sharp growth reductions in some cases. Trees that died had significantly lower initial radial growth, which suggests that in the absence of external disturbances, the outcome of mortality in the stem-exclusion stage may be pre-determined from factors that determine initial growth rates. Spatial distribution data showed that there was no significant aggregation of dead and live trees and that in almost all cases, neither live nor dead trees were clustered.Our findings contribute to understanding mortality processes in self-thinning subalpine Norway spruce forests in Europe and show that under climate change scenarios that include more frequent future droughts, even forests in which competition is thought to be the main driver of dynamics, may experience higher rates of mortality.

Monitoring Forest Recovery Following Wildfire and Harvest in Boreal Forests Using Satellite Imagery

In the managed boreal forest, harvesting has become a disturbance as important as fire. To assess whether forest recovery following both types of disturbance is similar, we compared post-disturbance revegetation rates of forests in 22 fire events and 14 harvested agglomerations (harvested areas over 5–10 years in the same vicinity) in the western boreal forest of Quebec. Pre-disturbance conditions were first compared in terms of vegetation cover types and surficial deposit types using an ordination technique. Post-disturbance changes over 30 years in land cover types were characterized by vectors of succession in an ordination. Four post-disturbance stages were identified from the 48 land thematic classes in the Landsat images: “S0” stand initiation phase; “S1” early regeneration phase; “S2” stem exclusion phase; and “S3” the coniferous forest. Analyses suggest that fire occurs in both productive and unproductive forests, which is not the case for harvesting. Revegetation rates (i.e., rapidity with which forest cover is re-established) appeared to be more advanced in harvested agglomerations when compared with entire fire events. However, when considering only the productive forest fraction of each fire, the revegetation rates are comparable between the fire events and the harvested agglomerations. The S0 is practically absent from harvested agglomerations, which is not the case in the fire events. The difference in revegetation rates between the two disturbance types could therefore be attributed mostly to the fact that fire also occurs in unproductive forest, a factor that has to be taken into account in such comparisons.

Stand characteristics of mixed oriental beech (Fagus orientalis Lipsky) stands in the stem exclusion phase, northern Iran

The structure of forest stands changes through developmental phases. This study is carried out in the unmanaged, oriental beech (Fagus orientalis Lipsky) stands in the north of Iran. The aim of this research was to quantify structural characteristics of stands in the stem exclusion phase using common structural indices, which include mingling, tree–tree distance, stem diameter, and tree height differentiation. According to our measurements from three stands, naturally regenerated stands tend to be mixed in species composition have slightly heterogeneous diameter distributions and uniform tree height. The average distance between trees was 3.3 m. Stocking volume of the stands had an average of 540 m3 ha−1 and 412 stem ha−1. Dead wood volume was 24 m3 ha−1, and as a standing volume, the most frequent species in dead wood pool was oriental beech (F. orientalis) (48 %). The common form of dead trees was snag (41 %). The mean value of mingling and tree-to-tree interval indices revealed that beech was mixed intensively with hornbeam and appears to be a more successful competitor for space and light compared with hornbeam; moreover, we found relatively high evidence of inter-species competition in this phase. A better understanding of stand characteristics in the stem exclusion phase as a critical part of the natural dynamics of forest ecosystems could facilitate predictions about the future changes within the stand.

Spatial Distribution and Volume of Dead Wood in Unmanaged Caspian Beech (Fagus orientalis) Forests from Northern Iran

Unmanaged forests are remnants of natural ecosystems that provide a basis for close-to-nature silvicultural research and applications. These forests have high amounts of dead wood, and although this material is being increasingly studied, the diversity of dead wood in terms of different diameters, decay stages, and spatial distribution patterns is as important as its volume for understanding forest dynamics. Here, we study natural forests in northern Iran to investigate the spatial distribution, decay stages, and volume of dead wood in unmanaged temperate forests at different developmental stages. Three stem-mapped sampling plots (100 m × 100 m) were established in uneven-aged stands dominated by Caspian beech (Fagus orientalis Lispsky). The total dead wood ranged from 37 to 119 m2 ha−1. Our results imply a spatial distribution shift from aggregation to randomness for dead trees in Caspian beech forest succession. We detected significant spatial interactions (attraction) between living and dead trees at short to medium spatial scales (1–20 m) in the plot with the earlier successional stage, suggesting that intra-specific competition is a prevailing force causing tree mortality at the stem-exclusion phase. By contrast, as trees become dominant with the mortality of other trees, the random tree-mortality pattern prevails. The spatial distribution and volume of dead wood may serve as a management target in near-to-natural Caspian beech forest. On the basis of our results, conservation-oriented management strategies should take into account the increasing amount of dead wood, particularly of large diameter in a late stage of decay.

Natural development and regeneration of a Central European montane spruce forest

Montane Norway spruce forests of Central Europe have a very long tradition of use for timber production; however, recently there has been increasing concern for their role in maintaining biological diversity. This concern, coupled with recent severe windstorms that led to wide-spread bark beetle outbreaks, has brought the management of montane spruce forests to the forefront of public policy discussions in Central Europe. In order to shed light on the natural development and current structure of mature montane spruce forests, we established four 0.25 ha research plots in a semi-natural montane spruce forest in the Šumava Mountains (The Bohemian Forest), Czech Republic. We mapped all trees, extracted increment cores for age and growth-pattern analyses, and inventoried all current tree regeneration, including the substrates on which it was found. Stands were characterized by uni-modal tree diameter distributions and high basal areas (56.6 m 2 ha −1 on average), indicating a natural transition from the stem exclusion phase towards the understory reinitiation phase. The stands showed largely single-cohort recruitment age structures, however, with recruitment spanning seven decades. Our analyses suggest that this cohort existed as advance regeneration prior to major disturbances in the late 1800s, which included post-bark beetle salvage logging. Spatial pattern analyses of living and dead stems combined, showed an increase in uniformity of living trees, pointing to the role of natural density-dependent mortality. However, past growth patterns and historical documentation suggest that low intensity canopy disturbances (wind and snow) also caused mortality and diversified canopy structure. Because the stands developed naturally over the past 120+ years and thus escaped thinning operations, high volumes of coarse woody debris (94 m 3 ha −1) and snag densities (546 stems ha −1) have accrued. Advance spruce regeneration was quite abundant and existed primarily on deadwood substrates, even though these occupied only a small percent of stand area. Because of salvage logging in the late 1880s, these stands do not qualify, according to the traditional paradigm, as natural spruce forests. As a result, they are recently subject to active management practices including salvage logging that remove dead and dying trees. Given the importance of deadwood for forest regeneration and recovery from disturbance, as demonstrated in this study, we argue that dead wood removal may limit future natural regeneration in these stands. Thus, the purported benefits of removing dead and dying trees from semi-natural forests must be carefully weighed against the potential detrimental impacts on natural spruce forest regeneration and biodiversity.

Seedling size and woody competition most important predictors of growth following free-to-grow assessments in four boreal forest plantations

Improvements to forest management decisions require accurate and quantifiable information. We examined the effects of various classes of competitors on crop tree growth in the context of free-to-grow standards using regression analysis. We found that seedling size accounted for most of the variation in height and volume growth of jack pine (Pinus banksiana Lamb.) and black spruce (Picea mariana [Mill.] BSP) plantations. Including herbaceous and woody competition as explanatory variables explained the additional variation on crop tree growth significantly. In the plantation initiation phase (years 2 to 6), the presence of herbaceous competitors generally reduced conifer growth but in the first part of the stem-exclusion phase (years 7 to 12) increased their growth. In all four boreal plantations in this study, woody competitors reduced conifer growth in both the initiation and stem-exclusion phases. These results have relevance to forest managers who develop and/or use free-to-grow surveys. Key words: vegetation management, silviculture, effectiveness monitoring, forest management, regeneration success, competition effect

Pourquoi et où les arbres adultes meurent dans une jeune forêt tempérée? Le rôle du voisinage

• The density and identity of tree neighbourhood is a key factor to explain tree mortality in forests, especially during the stem exclusion phase. • To understand this process, we built a logistic model for mortality in a spatially explicit context, including tree and neighbourhood predictors. Additionally, we used this model to build mortality risk frequency distributions. Finally, we tested this model against a random mortality model to predict the spatial pattern of the forest. • Annual mortality rate was high for pedunculate oak (Quercus robur, 6.99%), moderate for birch (Betula celtiberica, 2.19%) and Pyrenean oak (Q. pyrenaica, 1.58%) and low for beech (Fagus sylvatica, 0.26%). Mortality risk models for pedunculate oak and birch included stem diameter, tree height, canopy position and neighbourhood. Mortality was affected by the specific nature of the neighbourhood showing a clear competitive hierarchy: beech > pedunculate oak > birch. Models based on random mortality and logistic regression model were able to predict the spatial pattern of survivors although logistic regression predictions were more accurate. • Our study highlights how simple models such as the random mortality one may obscure much more complex spatial interactions.

Development of Tree and Understory Vegetation in Young Douglas-Fir Plantations in Western Oregon

Abstract The prevalence of young even-aged Douglas-fir (Pseudotsuga menziesii) stands in Oregon and Washington has led to concerns about a landscape being dominated by stands in the stem exclusion phase. In this context, our study documented the development of two aspects important for the diversity of plant and wildlife habitat: tree characteristics and understory vegetation. Using a chronosequence approach, we measured conditions in 39 plantations ranging from 6 to 20 years. Results confirmed intuitive trends, but the quantification indicated that some of these trends develop earlier than commonly assumed. Tree growth in young stands was positively related to stand density, but this trend reversed fairly early. Crown characteristics were influenced very early by stand density, indicating that maintaining a long-lived crown in typical plantations can only be accomplished by lowering stand density through precommercial thinning. Understory herb cover was reduced throughout time, while shrub cover increased. Species compositions were quite complex, with an initial strong presence of invader species and later dominance of species usually associated with mature forests; however, there were many exceptions and early successional species were still present after 20 years. The study showed that this early stage is very complex, that the dynamics vary for different characteristics, and that a finer resolution of the stand initiation stage may be warranted for plantations.

The Imprint of Land-use History: Patterns of Carbon and Nitrogen in Downed Woody Debris at the Harvard Forest

Few data sets have characterized carbon (C) and nitrogen (N) pools in woody debris at sites where other aspects of C and N cycling are studied and histories of land use and disturbance are well documented. We quantified pools of mass, C, and N in fine and coarse woody debris (CWD) in two contrasting stands: a 73-year-old red pine plantation on abandoned agricultural land and a naturally regenerated deciduous forest that has experienced several disturbances in the past 150 years. Masses of downed woody debris amounted to 40.0 Mg ha−1 in the coniferous stand and 26.9 Mg ha−1 in the deciduous forest (20.4 and 13.8 Mg C ha−1, respectively). Concentrations of N were higher and C:N ratios were lower in the deciduous forest compared to the coniferous. Pools of N amounted to 146 kg N ha−1 in the coniferous stand and 155 kg N ha−1 in the deciduous forest; both are larger than previously published pools of N in woody debris of temperate forests. Woody detritus buried in O horizons was minimal in these forests, contrary to previous findings in forests of New England. Differences in the patterns of mass, C, and N in size and decay classes of woody debris were related to stand histories. In the naturally regenerated deciduous forest, detritus was distributed across all size categories, and most CWD mass and N was present in the most advanced decay stages. In the coniferous plantation, nearly all of the CWD mass was present in the smallest size class (less than 25 cm diameter), and a recognizable cohort of decayed stems was evident from the stem-exclusion phase of this even-aged stand. These results indicate that heterogeneities in site histories should be explicitly included when biogeochemical process models are used to scale C and N stocks in woody debris to landscapes and regions.

Overstory and understory competition affect underplanted eastern white pine

The importance of canopy and understory competition on white pine ( Pinus strobus L.) seedlings was evaluated in stands with shelterwood treatments. We sampled 20 stands in north-central Minnesota that were underplanted with white pine 3 to 10 years before sampling. We grouped the stands with various canopy and understory treatments into 6 stand types based on residual canopy composition. The effect of the shelterwood treatments, i.e., residual basal area and understory cover on white pine seedling growth varied significantly among the stand types. Lowering basal area had a greater positive impact on seedling growth when shade tolerant softwood species, especially balsam fir ( Abies balsamea (L.) Mill.), were present in the canopy. Where shade tolerant softwood species were absent, the white pine seedling growth increased only slightly or not at all in the presence of lowered residual basal area. We hypothesize that the exclusion of the understory by a shade tolerant midstory, indicated by the presence of balsam fir, prior to shelterwood treatment created favorable conditions for white pine in the understory following the shelterwood treatment. The results suggest that shelterwood treatments on mesic, more productive hardwood sites should be linked to stand development stages where the understory is suppressed, e.g., following development of a shade tolerant midstory or during the stem exclusion phase. This method should complement present shelterwood prescriptions for drier, low quality hardwood sites. This research indicates the importance of evaluating both vertical structure and site quality prior to designing white pine shelterwood treatments.

Disturbance and the Population Dynamics of Liriodendron Tulipifera: Simulations with a Spatial Model of Forest Succession

I Population dynamics of Liriodendron tulipifera throughout the successional sere were investigated with a spatial simulator of forest community dynamics. The simulations were complemented by data from Appalachian cove forests at different stages of development. Gap size was a key consideration in simulation of Liriodendron recruitment; seed regeneration of this shade-intolerant species was unlikely except in large gaps (> 0.04 ha). 2 In simulations of long-term forest development (800 years) without exogenous disturbance, Liriodendron was a dominant species in early succession (50-150 years), but it was absent or present at low levels in stands > 500 years of age. The simulated basal area levels of Liriodendron approximated actual levels in cove forests having a disturbance regime characterized by a very low frequency of severe, large-scale events. 3 The idea that canopy gap size distributions change over the course of forest development was supported by the simulations. Given that intolerant species require large gaps for successful establishment, it was clear that temporal patterns of large gap formation affected the dynamics of such species. Intolerants were unable to regenerate during the stem exclusion phase, but they could regenerate during the subsequent understorey reinitiation and old-growth phases having large gaps. 4 Certain simulated disturbances imposed on old forests elevated Liriodendron basal area. Components of the disturbance regime included patch size, return interval, severity and spatial dispersion. The largest increases in Liriodendron basal area resulted from disturbance patches > 0.04 ha. Gaps, 0.1 ha in size, formed at intervals < 100 years by the mortality of c. 25% of the canopy trees in a stand, were sufficient to maintain the high levels of Liriodendron observed in some primeval forests of eastern North America.