Tree mortality risk of oak due to gypsy moth

We present prediction models for estimating tree mortality resulting from gypsy moth, Lymantria dispar, defoliation in mixed oak, Quercus sp., forests. These models differ from previous work by including defoliation as a factor in the analysis. Defoliation intensity, initial tree crown condition (crown vigour), crown position, and species grouping classes were highly significant in categorical analysis of variance for mortality. Heavy defoliation intensity was shown to have a strong, consistent influence in increasing the probability of tree mortality. Classification and Regression Tree (CART) analysis, a binomial decision tree procedure, was used to develop prediction models of mortality risk for use by forest managers. The best decision tree had 65 groups that correctly classified 75% of the live trees and 76% of the dead trees. Models were run separately by defoliation class and provided correct classifications between 63 and 78% of the trees. Forest land managers can use these models to assign probabilities of death for moderate and heavy defoliation intensity levels and compare predicted mortality to mortality of undefoliated trees to determine how gypsy moth defoliation will affect their stands. The probabilities can be used to develop marking guides Lased on projected defoliation levels for implementing silvicultural treatments to minimize gypsy moth effects in forest stands prior to infestation.     

A non-invasive method for reconstructing the relative mortality rates of trees in mixed-age,mixed-species forests

The death of overstory trees drives changes throughout forest ecosystems. Knowledge of mortality rates for these larger trees provides a long-term perspective on forest development and forest health. Tree mortality rates are typically determined by repeated censusing of trees over extended time intervals. We describe a method of reconstructing relative mortality rates that does not require injuring live trees on the study plots. To determine when trees died, we used cross-dating (matching tree-ring patterns of dead and live trees), identification of sapling releases, and assessment of tree decomposition. Dead trees were identified to genus or sub-genus by microscopic examination of wood. We reconstructed live tree community structure so that mortality could be relativized on a taxonomic-group- and tree-size-specific basis by the tree densities existing when mortality occurred. We modified a forest development model to operate backwards in time and validated this model by comparing past tree stem diameters predicted by the model with past stem diameters known from tree-ring measurements. All tree-ring measurements from live trees were obtained off the study plots. We report data for all trees ≥20 cm diameter at breast height (DBH) from seven oak-hickory forest sites in Arkansas, Illinois, Indiana, and Ohio, USA. Total plot area was 6.7 ha. Mortality rates were determined for the past 20 years by genus or sub-genus, DBH, and decade. The average mortality rate was 0.60% per year. The accuracy of the reconstructed mortality rates approaches that of mortality rates obtained by repeated censusing. While this mortality reconstruction method does not yield data for individual species, and may involve some compromise in accuracy, the method offers substantial benefits: long-term mortality data may be obtained from a short-term study, mortality rates may be obtained for the past, and because this reconstruction method is non-invasive, future mortality rates can be measured on the same plots.     

小陇山锐齿栎天然林结构动态分析

[目的]为了解锐齿栎天然林群落动态变化过程。[方法]采用每木定位监测样地重复观测的方法,对甘肃小陇山林区百花林场王安沟营林区内的锐齿栎天然林进行研究,从树种组成及多样性结构、径级结构、空间结构等几方面分析了锐齿栎天然林群落的结构动态特征。[结果]表明:2次调查群落树种组成和优势树种的重要值排序变化不大,有2个稀少种退出群落,死亡林木40株,死亡率8.3%;群落乔木层的物种丰富度和树种空间多样性下降,优势树种的集中性变大,物种个体数目分配的均匀程度下降。林分径级结构由典型的反"J"型分布变化为左偏的单峰状曲线;群落的空间结构没有发生显著变化,林木分布格局仍为随机分布,中林层和上林层林木个体增加,垂直结构更趋复杂;树种隔离程度下降,建群种锐齿栎的优势度增强,膀胱果、白桦和青榨槭种群的优势度下降。[结论]锐齿栎天然林群落组成和结构变化是一个复杂和缓慢的过程,6年间仅发生了一些微小的波动。     

Three-dimensional light structure of an upland Quercus stand post-tornado disturbance

Light is the most common limiting factor in forest plant communities,influencing species composition,stand structure,and stand productivity in closed canopy stands.Stand vertical light structure is relatively simple under a closed canopy because most light is captured by overstory trees.However,wind disturbance events create canopy openings from local to landscape scales that increase understory light intensity and vertical light structural complexity.We studied the effects of an EF-1 tornado on horizontal and vertical(i.e.three-dimensional)light structure within a Quercus stand to determine how light structure changed with increasing disturbance severity.We used a two-tiered method to collect photosynthetic photon flux density at 4.67 m and 1.37 m above the forest floor to construct three-dimensional light structure across a canopy disturbance severity gradient to see if light intensity varied with increasing tornado damage.Results indicate that increased canopy disturbance closer to the tornado track increased light penetration and light structure heterogeneity at lower forest strata.Increased light intensity correlated with increased sapling density that was more randomly distributed across the plot and had shifted light capture higher in the stand structure.Light penetration through the overstory was most strongly correlated with decreased stem density in the two most important tree species(based on relative dominance and relative density)in the stand,Quercus alba L.(r=-0.31)and Ostrya virginiana(Mill.)K.Koch(r=-0.27,p<.01),and indicated that understory light penetration was most affected by these two species.As managers are increasingly interested in patterning silvicultural entries on natural disturbances,they must understand residual stand and light structures that occur after natural disturbance events.By providing spatial light data that quantifies light structure post-disturbance,managers can use these results to improve planning required for long-term management.The study also provides comparisons with anthropogenic disturbances to the midstory that may offer useful comparisons to natural analogs for future silvicultural consideration.     

Influences of woody encroachment and restoration thinning on overstory savanna oak tree growth rates

Midwestern savannas historically covered >10 M ha in central North America, but are now rare due to agricultural conversion and anthropogenic modifications to disturbance regimes - particularly fire suppression. Throughout this range, Midwestern savannas are characterized by scattered overstory trees; however, with fire suppression, these systems are invaded by non-savanna trees. Restoration of encroached savannas involves removal of invading trees, yet little is known about the impacts of encroachment or encroachment removal on the relict savanna overstory trees, which define these systems. Here, we use tree ring analysis to investigate savanna tree growth rates in encroached, non-encroached, and experimentally restored Midwestern oak savannas in central Iowa. We found that woody encroachment led to pronounced declines in growth rate (ring width) of relict overstory white oak (Quercus alba), relative to Q. alba trees in competition-free, open-grown conditions, or in an encroachment-free remnant woodland. To further understand effects of encroachment removal on relict Q. alba savanna trees, we conducted a large-scale restoration experiment, where encroaching trees were mechanically removed from four encroached savannas, with an additional four savannas retained as encroached controls. Restoration led to elevated tree growth rates, with these changes generally persistent through 7 years post-restoration (2003-2009). Over the course of this post-restoration study period, ring width, basal area increment, and relative basal area increased by 49%, 59%, and 55%, respectively, in trees from restored sites, relative to trees from encroached, control sites. These results suggest that woody encroachment has strong influence on overstory savanna trees, through increased competitive dynamics; however, woody encroachment removal may help to restore relict savanna tree growth rates, even after prolonged periods of encroachment (>40 years). To restore the oak savannas at our sites, and perhaps elsewhere, we advocate a three step process: (1) mechanical woody encroachment removal, (2) maintenance of the encroachment-free state through prescribed fire, and (3) promotion of a diverse understory layer, characteristic of oak savanna in our region. While promoting oak regeneration will be important for the long-term maintenance of these sites as oak savanna, relict savanna trees appear responsive to restoration and should maintain overstory conditions through the near-term.     

Differential overstory leaf flushing contributes to the formation of a patchy understory

Understory individuals were found to form patches in a 100-year-old deciduous broad-leaved forest. The closed forest canopy was uniform, and so the light conditions at various locations across the forest floor differed little after the leaf flush of the overstory. To explain the distribution pattern in the understory, a hypothesis was proposed: in spring, the forest floor is divided into patches according to the timing of leaf flush of the overstory individuals, and the light conditions are more favorable for understory plants under the crowns of trees with later-flushing leaves. In the plot, three groups of early, intermediate, and late, were recognized in the overstory concerning the timing of leaf flush. As for the start of leaf flush, a difference of 31.6 days was recognized among tree species, and for the end of leaf flush, a difference of 40.3 days. In the spring of 1998, the relative photosynthetic-photon-flux density under an intensively studiedCastanea crenata tree (late-flushing species) usually showed higher values than that under a similarly studiedAcer mono tree (early-flushing species). Analysis of the spatial-distribution pattern using Morisita’s1δ index revealed that the understory community had an aggregated distribution. In the overstory, the late- and the intermediate-flushing-species groups showed aggregated distributions, while the early-flushing-species group showed random distribution. Spatial correlation between the understory and the overstory was analyzed by using Morisita’sRδ index. The distribution of whole understory community spatially co-occurred with that of the late-flushing-species group of the overstory. In contrast, the understory community was less developed below the members of the early-flushing-species group of the overstory. We consider that the data presented here support our hypothesis, and we suggest that the growth and survival of understory individuals were promoted in the places receiving light for long periods in spring.     

SMALL-DIAMETER KOREAN PINES AND THEIR ROLES INNATURAL KOREAN PINE FOREST

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Forest stand dynamics and sudden oak death: Mortality in mixed-evergreen forests dominated by coast live oak

Sudden oak death (SOD), caused by the recently discovered non-native invasive pathogen, Phytophthora ramorum, has already killed tens of thousands of native coast live oak and tanoak trees in California. Little is known of potential short and long term impacts of this novel plant–pathogen interaction on forest structure and composition. Coast live oak (Quercus agrifolia) and bay laurel (Umbellularia californica) form mixed-evergreen forests along the northern California coast. This study measured tree mortality over a gradient of disease in three time periods. Direct measurements of current mortality were taken during 2004, representing a point-in-time estimate of present and ongoing mortality. Past stand conditions, c. 1994, were estimated using a stand reconstruction technique. Future stand conditions, c. 2014, were calculated by assuming that, given a lack of host resistance, live trees showing signs of the disease in 2004 would die. Results indicate that coast live oaks died at a rate of 4.4–5.5% year⁻¹ between 1994 and 2004 in highly impacted sites, compared with a background rate of 0.49% year⁻¹, a ten-fold increase in mortality. From 2004 to 2014, mortality rates in the same sites were 0.8–2.6% year⁻¹. Over the entire period, in highly impacted sites, a 59–70% loss of coast live oak basal area was predicted, and coast live oak decreased from 60% to 40% of total stand basal area, while bay laurel increased from 22% to 37%. Future stand structures will likely have greater proportions of bay laurel relative to coast live oak.     

Pine mistletoe (Viscum album ssp. austriacum) contributes to Scots pine (Pinus sylvestris) mortality in the Rhone valley of Switzerland

In recent years unusual high mortality of Scots pine (Pinus sylvestris) has been observed in the Swiss Rhone Valley. The exact causes, however, are not known. At a 2‐ha monitoring plot, tree mortality and crown condition have been monitored since 1996. Between 1996 and 2004, 59% of the Scots pines died, most of them following the drought periods 1996–1998 and 2003–2004, while only 15% of the deciduous trees died. Crown transparency, needle discolouration, dead branch percentage, mistletoe (Viscum album ssp. austriacum) rating, Tomicus sp. shoot feeding, male flowering effect, tree stem diameter, crown shading and social tree class assessed in 1998 were used in a logistic regression model to predict tree mortality. Crown transparency, mistletoe rating and percentage of dead branches were found significant in the model and the probability of tree mortality increased with increasing rankings of these parameters. Needle discolouration could be used to substitute ‘dead branch percentage’ as predictor. While crown transparency increased with mistletoe rating, for trees in the same transparency class, trees with medium and heavy mistletoe infection were two to four times more likely to die than trees with no or only low mistletoe infection. For the surviving trees we found that trees with mistletoes showed a significantly higher increase in transparency in the year following a drought than trees without, while in a drought year the opposite was true. At the beginning of the observations no significant differences in transparency had been found between the trees with and without mistletoe. However, by the end of the observation period trees with mistletoe had significantly higher crown transparency. We conclude that mistletoe infection can be considered as both a predisposing factor for tree death, by increasing needle loss following drought and a contributing factor by increasing water stress during drought.     

Diversity and spatio-temporal dynamics of dead wood in a temperate near-natural beech forest (<Emphasis Type="Italic">Fagus sylvatica</Emphasis>)

The diversity, spatial patterns and temporal dynamics of dead wood were examined within the near-natural beech forests (Fagus sylvatica) of Serrahn (North-eastern Germany). Data were collected in an 8 ha sample plot and in two permanent plots (0.36 and 0.25 ha) that had been established at the end of the 1960s. The mean volume of dead wood was 94 m³ ha⁻¹, amounting to 14% of the total volume of all trees. The dead wood displayed a large variation in dead wood type, tree size and decay class. Standing dead wood accounted for about one-third of the total dead wood volume. The densities of standing dead trees were about 10% of the densities of the living trees over a wide range of diameters. The overall spatial distribution of dead trees exhibited a random pattern. Among the different dead wood types, standing entire dead trees and uprooted trees deviated from this pattern by displaying a significantly aggregated pattern. In the permanent plots a high mortality of overstorey trees was observed (1.3% year⁻¹) and the average amount of dead wood increased greatly from 2.9 to 111.6 m³ ha⁻¹ over the 35-year observation period. The near-natural beech forests of Serrahn have experienced a long period of low human interference. Nevertheless, our results suggest that the structure and dynamics of dead wood are strongly affected by the last major disturbance events that took place at the end of the Middle Ages. Information about the forest history is therefore a basic requirement when interpreting the results obtained in near-natural forests.     

红花尔基沙地樟子松天然林枯立木特征分析

[目的]了解沙地樟子松天然纯林中枯立木的数量及空间结构特征,探究枯立木形成的原因,为樟子松林的保护和经营提供依据。[方法]在沙地樟子松天然纯林中设置2块1 hm~2的大样地,用全站仪对样地中所有胸径大于5 cm的立木进行定位并进行全面调查;对调查样地的基本特征,枯立木的数量特征及径级分布进行了分析,提出了用于表达林分中枯立木微环境的活立木比的概念,并采用林分空间结构参数一元分布和二元分布分析方法,对枯立木与其最近4株相邻木的关系进行分析。[结果]2块不同密度的樟子松天然纯林下更新幼苗和枯立木数量相差较大,密度较小(样地1)的样地更新幼苗和枯立木较少,而密度较大的样地(样地2)中枯立木达到200棵,林下更新幼苗数量达到15 280株·hm~(-2);樟子松天然纯林样地内枯立木主要以小径级木为主,胸径集中在11 cm以下;样地1枯立木径级连续分布,幅度较窄;样地2中的枯立木径级幅度较宽,但在20 22 cm缺刻,有2株大于23 cm的枯立木;2块样地中枯立木的分布格局均为随机分布,样地1中枯立木周围的4株相邻立木大多为活立木,且胸径较枯立木大;样地2中,只有一半的枯立木周围的最近4株立木为活立木,且有三分之一以上的枯立木胸径不是最小的,枯立木有连续分布的现象。2块样地中枯立木的角尺度-大小比数二元分布特征的差异不明显,而角尺度-活立木比二元分布特征和大小比数-活立木比二元分布特征差异明显,样地1中枯立木的最近4株随机分布于其周围的相邻木为活立木且胸径大于枯立木的比例明显高于样地2,而枯立木最近4株随机分布于其周围的相邻木有枯立木的比例明显小于样地2。[结论]樟子松天然纯林枯立木以小径级林木为主,枯立木的数量与林分密度相关,林木竞争是林木死亡的主要原因,密度过大也会产生病虫害,因此,对天然樟子松纯林要进行适度经营,保持合理密度。     

Response of aspen stands to forest tent caterpillar defoliation and subsequent overstory mortality in northeastern Ontario,Canada

Overstory mortality, understory tree recruitment, and vegetation development were assessed in trembling aspen (Populus tremuloides Michx.) stands following two recent episodes of forest tent caterpillar defoliation (Malacosoma disstria Hbn.) in northeastern Ontario. The results suggest that poplar (aspen and balsam poplar (Populus balsamifera L.)) mortality increased with consecutive years of insect defoliation occurring from the mid-1980s to mid-2000s and the proportion of poplars in the overstory, but decreased with improved pre-defoliation tree vigour (DBH increment). The first outbreak, which lasted from the mid-1980s to early 1990s, was more severe in terms of insect defoliation and contributed more to poplar mortality and decline. The decline began in the late 1990s and peaked in early 2000s. Poplar regeneration and understory shrubs responded rapidly to foliage loss to insect defoliation and mortality of overstory poplars. The regenerated poplars were able to maintain their growth under developing shrubs and residual overstory canopy and numbers were sufficient to compensate for the poplar trees lost to insect infestation. The defoliation-induced overstory decline will accelerate the transition of aspen stands to conifer dominance through enhanced conifer recruitment and growth, and reduced hardwood overstory in aspen-dominated stands, while hardwood dominance will persist in pure aspen stands. From a timber supply perspective, the decline caused by forest tent caterpillar defoliation could delay the availability of aspen stands for harvesting by 40–50 years.     

Diachronic analysis of individual-tree mortality in a Norway spruce stand in the eastern Italian Alps

? Understanding tree mortality processes across time requires long term studies. Spatiotemporal patterns of mortality in a 200 years-old mono-layered Norway spruce stand were evaluated to determine what factors affected individual-tree mortality.

? We performed an analysis on two surveys (1993 and 2005) in a 1-ha permanent plot in the Paneveggio forest (Eastern Italian Alps). Tree diameter and age distribution between surveys were compared. We examined spatial patterns of living and dead trees before 1993, in 1993 and in 2005 using univariate and bivariate Ripley’s K(d) function, and a kernel estimator of local crowding. A logistic model was used to assess the effects of diameter, age, recent growth and competitive pressure on tree mortality.

? Spatial pattern analysis indicated mortality was associated to tree neighbourhood (neighbour effect at 2–5 m). An increment of regularization of tree spatial pattern occurred due to density-dependent mortality. Logistic regression showed tree diameter and recent growth were determinant on mortality risk during the monitoring period.

? Even if the stand is relatively aged, mortality dynamics are those typical of stem exclusion stage. Mortality was related to competitive dynamics, and small suppressed trees with slow growth rate had higher probability to die.

     

Assessing the potential for ash canopy tree replacement via current regeneration following emerald ash borer-caused mortality on southeastern Michigan landscapes

The emerald ash borer (EAB) has killed millions of ash trees in Michigan and at least fourteen other states since its first detection near its introduction point in metropolitan Detroit in 2002. Despite overstory ash mortality near 100% in many areas, ash seedling and saplings remain unaffected and provide the potential for ash re-establishment into the canopy of deciduous forests of the region. We examined the potential for ash re-establishment in areas of heavy mortality by measuring the status and change of ash regeneration at 45 sites across southeastern Lower Michigan in 2007 and 2009. Ash regeneration was found to be abundant in all forest types, particularly in the smallest height classes, though it was more abundant in ash species of upland forests compared to those of lowland forests. New seedlings 1-2 years old were less common than other regeneration size classes, and declined substantially between 2007 and 2009, suggesting a depletion of the ash seed bank in these forests. Regeneration density was not explained well by the presence of overstory ash prior to EAB introduction, suggesting that competitive interaction with other tree species in higher canopy strata is an important driver of ash regeneration density. Regeneration is sufficient to replenish overstory ash to pre-EAB levels in upland forests, though ecological changes caused by overstory ash mortality in lowland areas may affect local hydrology in a way that reduces opportunities for tree regeneration. Despite abundant ash regeneration, it remains unclear whether ash will recover to the overstory of forests in southeastern Michigan because of uncertainties in the future dynamics of EAB in the region.     

Small-scale variation of vegetation in a mixed forest understorey is partly controlled by the effect of overstory composition on litter accumulation

We investigated how richness and composition of vascular plant species in the understory of a mixed hardwood forest stand varied with respect to the abundance and composition of the overstory. The stand is in central Spain and represents the southernmost range of distribution of several tree and herbaceous species in Europe. Understory species were identified in 46 quadrats (0.25 m²) where variables litter depth and light availability were measured. In addition, we estimated tree density, basal area, and percent basal area by tree species within 6-m-radius areas around each plot. Species richness and composition were studied using path analysis and scale-dependent geostatistical methods, respectively. We found that the relative abundance of certain trees species in the overstory was more important than total overstory abundance in explaining understory species richness. Richness decreased as soil litter depth increased, and soil litter increased as the relative proportion of Fagus sylvatica in the overstory increased, which accounted for a negative, indirect effect of Fagus sylvatica on richness. Regarding understory species composition, we found that some species distributed preferentially below certain tree species. For example, Melica uniflora was most frequent below Fagus sylvatica and Quercus petraea while the increasing proportion of Q. pyrenaica in the overstory favored the presence of Cruciata glabra, Arenaria montana, Prunus avium, Conopodium bourgaei, Holcus mollis, Stellaria media and Galium aparine in the understory. Overall, these results emphasize the importance of individual tree species in controlling the assemblage and richness of understory species in mixed stands. We conclude that soil litter accumulation is one way through which overstory composition shapes the understory community.     

Development of tree hollows in pedunculate oak (Quercus robur)

Many invertebrates, birds and mammals are dependent on hollow trees. For landscape planning that aims at persistence of species inhabiting hollow trees it is crucial to understand the development of such trees. In this study we constructed an individual-based simulation model to predict diameter distribution and formation of hollows in oak tree populations. Based on tree ring data from individual trees, we estimated the ages when hollow formation commences for pedunculate oak (Quercus robur) in southeast Sweden. At ages of about 200–300 years, 50% of the trees had hollows. Among trees <100 years old, less than 1% had hollows, while all >400-year-old trees had hollows. Hollows formed at earlier ages in fast-growing trees than in slow-growing trees, which may be because hollows are formed when big branches shed, and branches are thicker on fast-growing trees in comparison to slow-growing trees of the same age. The simulation model was evaluated by predicting the frequency of presence of hollows in relation to tree size in seven oak stands in the study area. The evaluation suggested that future studies should focus on tree mortality at different conditions. Tree ring methods on individual trees are useful in studies on development of hollow trees as they allow analysis of the variability in time for hollow formation among trees.     

Retention of wind-felled trees and the risk of consequential tree mortality by the European spruce bark beetle Ips typographus in Finland

Abstract

Autumn storms felled about 7 million m³ of forest in southern Finland in 2001. Windthrow area and timber characteristics, as well as numbers of standing spruce trees attacked and killed by Ips typographus, were recorded in 61 Norway spruce [Picea abies (L.) Karst.]-dominated windthrow areas. Generalized linear models were used to identify significant variables predicting the risk for consequential tree mortality by I. typographus. None of the windthrow areas with fewer than 20 wind-felled spruce trees (WFS) (n=28) and only half of the areas with 20 or more WFS (n=33) harboured trees killed by I. typographus during the years 2003–2005. The quantity and diameter of WFS and the basal area of recently dead standing spruce trees correlated positively with the risk of tree deaths. This study indicates that in Finland, at endemic I. typographus population levels, it is safe to leave fewer than 20 WFS in managed forests. Retention of even larger quantities of trees does not seem to evoke significant numbers of consequential tree deaths by I. typographus in managed forests. However, in stands where the natural mortality of spruce trees is high, the risks of consequential tree deaths after wind disturbance will also be higher.     

Tree mortality and habitat shifts in the regeneration trajectory underneath canopy of an old-growth subalpine forest

To better understand tree regeneration trajectories and the resultant coexistence of Abies with co-dominants, Picea jezoensis var. hondoensis, Tsuga diversifolia and Betula ermanii, in an old-growth subalpine forest, we investigated spatial mortality patterns during the regeneration of Abies mariesii and A. veitchii, which are abundant in the understory reflecting their shade tolerance. Regeneration of these Abies spp. from shaded understory to canopy status is affected by other canopy co-dominants. Snags of understory Abies spp. were common, suggesting that the primary mortality agent is suppression by the overstory. Although live, small Abies trees in the understory were positively associated with a Picea canopy, the long-term survival was reduced among Abies trees close to the canopy, suggesting that shading by large Picea in the overstory negatively affects understory Abies plants. The existence of shade-intolerant canopy co-dominants such as Picea and also Tsuga, which are larger and longer lived than the shade-tolerant Abies, may play an important role in preventing the Abies spp. from competitively displacing these other tree species, which are much rarer in the understory, though common in the canopy. Moreover, in spite of the fact that Betula canopies fostered recruitment and growth of Abies saplings, Abies showed no association with Betula canopy and their survival at later-stage was rather reduced near or beneath Betula canopies at the subsequent understory small tree stage. Based on spatially significant events related to tree death, this study detected such “habitat shifts” in the trajectory of tree regeneration. Accordingly, it can be concluded that careful consideration of the regeneration habitat is required for a fuller understanding of ecological processes in spatially complex old-growth forest systems.     

Availability of standing trees for large cavity-nesting birds in the eastern boreal forest of Québec,Canada

Large cavity-nesting birds depend on large-diameter trees for suitable nest sites. The increased spatial extent of commercial timber harvesting is modifying forest structure across the land base and may thus compromise the availability of large trees at the landscape scale. In this study, our objectives were to (1) characterize the availability of large living and dead trees in old-growth stands dominated by different tree species and surficial deposits that encompass the range of natural cover types of eastern Québec's boreal forest; (2) analyze the distribution of trees among decay-classes; and (3) compare the availability of large trees in unharvested, remnant, and harvested stands for the entire range of decay-classes. A total of 116 line transects were distributed across unharvested forests, remnant linear forests, and cutblocks in cutover areas. Unharvested forest stands (black spruce [Picea mariana], balsam fir [Abies balsamea]–black spruce, balsam fir–white spruce [Picea glauca] and balsam fir) reflected a gradient of balsam fir dominance. The remnant forests selected were isolated for 5–15 years. Analyses were performed at two diameter cut-off values. Trees with DBH ≥20 cm were considered for availability of total trees whereas trees with DBH ≥30 cm were considered for availability of large trees. Forest stands comprised high proportions of standing dead trees (33% of all stems, 8% were large dead stems). Availability of total and large standing trees increased with the dominance of balsam fir in stands. Forest stands located on thick surficial deposits showed higher densities of large dead trees for every stand type suggesting a higher productivity on those sites. Availability of stems according to decay-classes showed a dome-shaped distribution with higher densities of snags in intermediate decay stages. However, for large stems, black spruce stands showed a significantly lower availability that was consistent across all decay-classes. In linear remnant forests, pure balsam fir stands were absent. Remnant stands thus showed a much lower availability in large trees when compared with unharvested balsam fir stands. Clearcuts had the lowest densities of dead trees across sampled stands. Current even-aged management practices clearly affect availability and recruitment of large trees, therefore forest-dwelling wildlife relying on these structures for breeding is likely to be affected by large-scale harvesting in coniferous boreal forests.