Minor Tree Species Research Articles

Developing a Structural Complexity Index for Oriental Beech Forests in Northern Iran

Abstract The structural complexity index (SCI) has become an important metric for forest managers to monitor ecosystem services and conservation value in a wide variety of forest types. In this study, we developed an SCI for an unmanaged mixed Fagus orientalis Lipsky forest in northern Iran, which incorporated structural information specific to mature and old-growth forests. Our results showed that we were able to develop an SCI for this forest that would assist managers to make conservation decisions in a forest where large overstory trees and small understory trees are equally important. The SCI was significantly positively correlated with the density of five minor tree species (Acer velutinum Boiss., Acer cappadocicum Gled., Tilia begoniifolia Chun & H.D. Wong, Quercus castaneifolia C.A. Mey., and Prunus avium L.) and the density of large ([50 cm < diameter at breast height {DBH} < 75 cm]) and very large (DBH > 75 cm) overstory trees. The SCI remains a highly flexible tool for forest conservation and decision making and may assist with decisions about forest management in response to climate change and shifting disturbance regimes. Study Implications: Forest managers have begun to use the structural complexity index (SCI) to assess the successful achievement of conservation management objectives. In many mature, mixed-species forests, large trees, minor species, and understory tree species are important structural components. In this study, we demonstrate that reduction of the minimum diameter used for sampling trees and careful selection of the variables used to calculate SCI results in a valuable metric for making conservation decisions. The advantage of SCI as a forest decision tool is that forest managers are able to adjust the inputs used to calculate SCI to reflect specific management objectives or monitoring goals.

Cambiamenti climatici in atto: osservazioni sugli impatti degli eventi siccitosi sulle foreste toscane

The empirical observation of the impacts of drought and heat waves that occurred between 2017 and 2022 in several forest areas of Tuscany allowed to describe various processes and effects on forest trees and shrubs. The most severe impacts were observed on the evergreen Mediterranean tall woodlands and the aged coppices (on holm oak trees), causing defoliation and even mortality. In most cases, the attempts of resprouting from dormant buds (from the base of the tree trunk or in the crown), following the drought events, were not successful. Deciduous broad-leaved trees (beech, downy oak) suffered a strong summer defoliation, especially on steep slopes with shallow soils and south-facing aspect. These species were generally able to replenish the foliage the following year. In the most unfavorable conditions, however, we observed extensive attacks of opportunistic parasites, such as the fungi of the genus Biscogniauxia, which were favored by drought and were the ultimate cause of mortality. Turkey oak showed mortality in small groups or individual trees. Heat and drought also affected several minor tree species, with defoliation, desiccation and, in the most serious causes, death of the tree. Some meso-hygrophilous species that are restricted with relict populations to specific microclimatic sites within the Mediterranean and sub-Mediterranean habitats were also impacted, with consequences on biodiversity, regeneration, and production of fruits for wildlife. Increasing drought and heat will trigger regressive dynamic processes that will lead in many sites to the transformation of holm oak forests into bushy forms of scrub, and of scrublands into sparse garrigues or even steppes. A more intense natural thinning of aged coppice stands with especially Turkey oak is also occurring, due to the combined effect of drought and intra- or inter-specific competition. The implementation of a regional monitoring system is recommended, based on the integration of past and existing experiences with new initiatives, to quantify the dynamics, extent and severity of the impacts. A crucial point is the need to adapt forestry, and especially utilization of coppice woods, to the changed ecological conditions to avoid increasing the impact on them.

Long-term forest monitoring reveals constant mortality rise in European forests.

European forests are an important source for timber production, human welfare, income, protection and biodiversity. During the last two decades, Europe has experienced a number of droughts which have been exceptional within the last 500 years, both in terms of duration and intensity. These droughts seem to leave remarkable imprints on the mortality dynamics of European forests. However, systematic observations on tree decline, with emphasis on a single species, has been scarce so far so that our understanding of mortality dynamics and drought occurrence is still limited at a continental scale. Here, we make use of the ICP Forest crown defoliation dataset, permitting us to retrospectively monitor tree mortality for all major conifers, major broadleaves, as well as a pooled dataset of minor tree species in Europe. In total, we analysed more than three million observations gathered during the last 25 years and employed a high-resolution drought index which can assess soil moisture anomaly based on a hydrological water-balance and runoff model. We found overall and species-specific increasing trends in mortality rates, accompanied by decreasing soil moisture. A generalized linear mixed model identified a previous-year soil moisture anomaly as the most important driver of mortality patterns in conifers, but the response was not uniform across the numerous analysed plots. We conclude that mortality patterns in European forests are currently reaching a concerning upward trend which could be further accelerated by global change-type droughts in the near future.

Mapping temperate forest tree species using dense Sentinel-2 time series

Precise information on tree species composition is critical for forest management and conservation, but mapping tree species with satellite data over large areas is still a challenge. Since 2017, Sentinel-2A/B provide multi-spectral time series with global coverage at an unprecedented spatial and temporal resolution. This is a new opportunity for mapping tree species over large areas that has not yet been fully explored. Because of the high spatial and temporal resolution, Sentinel-2 time series improve the characterization of vegetation phenology and canopy structure, parameters that are intrinsically linked to tree species. The objective of this study was to test the utility of a Sentinel-2 time-series based approach for mapping tree species in a temperate forest region in Central Europe. Using stand-wise forest inventory data for single species stands we assess how well main and minor tree species can be mapped, and if the addition of environmental variables and spatial texture metrics improves the classification accuracy. Our time series approach utilizes all available Sentinel-2 observations and an ensemble of radial basis convolution filters to build cloud-free 5-day time series for each spectral band. The time series are then used as input features to classify seventeen tree species.Our results show the potential of Sentinel-2 time-series based classification, but they also show the challenges associated with mapping a diverse portfolio of tree species. Accuracy of the nine main species, with an area proportion greater than 0.5%, ranged between 98.9% and 66.8%, which is promising for a large area. Adding detailed environmental data and texture metrics to the spectral model only marginally increased the accuracy of a few minor tree species. Overall, the eight minor tree species with area proportions less than 0.5% were most strongly affected by classification errors. Although the absolute mapped area of minor species correlated well with the estimated reference area, the small class areas of minor species lead to high classification errors in relative terms. Mapping minor tree species is challenging for statistical reasons (i.e., class imbalance, small sample size and class variance). Using all available Sentinel-2 data allows building dense time series at high spatial resolution that are mandatory for improved tree species mapping. We were able to show that the spectral time series is the prime explanatory information, even when complementing our analyses with texture information and various environmental data. The results suggest that with the applied data harmonization approach precise regional tree species mapping is feasible.

The use of sentinel logs to assess host shifts in early beetle colonisers of deadwood under climate‐ and forestry‐induced tree species substitutions

Abstract Global change scenarios project drastic modifications in tree species range and an increase in exotic tree plantations. Subsequent tree species substitutions may alter habitat conditions for biodiversity. We measured substitutability between tree species for early deadwood colonisers, through a sentinel log approach, i.e. through the experimental exposure of paired down deadwood (DDW) pieces to native beetles in native stands. We compared two native/substitute tree species pairs: one conifer pair composed of a rapidly expanding exotic species (Douglas fir) and a declining native species (silver fir), and one deciduous pair composed of two native species, one expanding (sessile oak) and one retreating (beech) at the regional scale. The effects of expanding exotic and native trees on beetle communities were not in line with expectations. Species assemblages in Douglas fir DDW were indistinguishable from those in native silver fir DDW and did not contain fewer species. Assemblages were not more generalist on average in substitutes than in substituted trees: we did not note any decrease in species richness of functional groups to the detriment of specialist species. Moreover, species richness and abundance were higher in substitute oak than in native beech DDW, confirming that species from the regional pool were able to colonise oak, even though it is a minor tree species at the regional level. Large‐scale monitoring schemes including multi‐taxon, multi‐year and multiple native/substitute pairs would further our knowledge of the generic effects of tree species substitution on biodiversity and ecosystem functioning.

Airborne laser scanning for tree diameter distribution modelling: a comparison of different modelling alternatives in a tropical single-species plantation

This study examines the potential of airborne laser scanning (ALS) to predict diameter distributions in an even-aged plantation of Eucalyptus urograndis in Brazil. The single-species plantation conditions allow different modelling alternatives to be compared without the presence of minor tree species or an understory layer affecting the results. Three modelling alternatives based on the two-parametric Weibull function form; parameter prediction, parameter recovery and distribution matching were tested with a k-nearest neighbour prediction (k-nn) method. We also evaluated a parameter prediction alternative, in which the Weibull distribution was predicted using field attributes. The results showed that ALS information can predict diameter distributions with an error margin of slightly more than 10 per cent of the RMSE of the mean of the third power of diameter, and with error index values between 50 and 60. The degree of accuracy was only slightly improved when the Weibull distribution was predicted using field attributes. According to the accuracy metrics, the differences between modelling alternatives were minor but parameter recovery and k-nn seem to be the most favourable ALS-based prediction methods. To conclude, the results showed a strong relationship between ALS information and diameter distributions in a tropical single-species plantation and we discuss how these results could be applied in other types of forests.

Effects of Drought and Rewetting on Growth and Gas Exchange of Minor European Broadleaved Tree Species

Widespread and economically important European tree species such as Norway spruce, Scots pine, and European beech are projected to be negatively affected by the increasing intensity and frequency of dry and hot conditions in a future climate. Hence, there is an increasing need to investigate the suitability of presumably more drought tolerant species to ensure future ecological stability, biodiversity, and productivity of forests. Based on their distribution patterns and climatic envelopes, the rare, minor broadleaved tree species Sorbus torminalis ((L.) CRANTZ), S. domestica (L.), Acer campestre (L.), and A. platanoides (L.) are assumed to be drought tolerant, however, there is only limited experimental basis to support that notion. This study aimed at quantifying growth and gas exchange of seedlings of these species during drought conditions, and their capacity to recover following drought. For that purpose, they were compared to the common companion species Quercus petraea ((MATTUSCHKA) LIEBL.) and Fagus sylvatica (L.). Here, potted seedlings of these species were exposed to water limitation followed by rewetting cycles in a greenhouse experiment. Photosynthesis and transpiration rates, stomatal conductance as well as root and shoot growth rates indicated a high drought resistance of A. campestre and A. platanoides. Sorbus domestica showed a marked ability to recover after drought stress. Therefore, we conclude that these minor tree species have the potential to enrich forests on drought-prone sites. Results from this pot experiment need to be complemented by field studies, in which the drought response of the species is not influenced by restrictions to root development.

Diversity and conservation of wood-inhabiting polypores and other aphyllophoraceous fungi in Malaysia

We reviewed the ecological characteristics of wood-inhabiting fungi in Malaysia in relation to the major threats to these fungi; we also examined the forest uses that would help to conserve them. Although wood-inhabiting fungi do not show high host specificity or preference in many tropical areas, several are specific to Dipterocarpaceae trees in Malaysia and some species may preferably inhabit other minor tree species. Tree size and decomposition stage are also important determinants of which fungi are present. Among the polypores described by E. J. H. Corner, 41 and 26 species have been recorded only from Malaysian lowland rainforest and montane forest, respectively. Evidence suggests that both of these forest types in Malaysia are home to unique fungal communities. More than 30 polypore species described by Corner are known only from their type localities. These species are likely rare and may be restricted to old-growth forests. Logging decreases species richness of wood-inhabiting fungi because the number of natural treefalls is decreased in logged forests, and the decline of old-growth forests is a major threat to conservation of Malaysian fungi. However, species richness of wood-inhabiting fungi is relatively high in old secondary forests and forests undergoing reduced-impact logging. The diversity of wood-inhabiting fungi is extremely low in oil palm plantations, whereas several species inhabit rubber-tree and acacia plantations. Preservation of old-growth forest is essential for conserving rare wood-inhabiting fungi in Malaysia, but old secondary forests, reduced-impact logged forests, and matured tree plantations may have some importance for the conservation of some species.

Early regeneration of Fraxinus rhynchophylla in the understorey of Larix kaempferi stands in response to thinning

The effects of thinning were investigated on naturally regenerated Fraxinus rhynchophylla Hance (Korean ash) grown in the understorey of a Larix kaempferi (Lamb.) Carr. plantation, established on a site that previously supported old growth hardwood forests. The regeneration characteristics were evaluated after thinning of the plantation (about 60 per cent of relative density was removed). In the naturally regenerated and mixed stands, Quercus mongolica Fisch. was the most abundant tree species in the overstorey, but the understorey of the thinned plantations was dominated by F. rhynchophylla with six other minor tree species. Fraxinus rhynchophylla was regenerated by four propagation types including natural seedling, stump sprout, root sprout and layering. Before thinning, the difference of average height growth for each regeneration type was insignificant. One year after thinning, the stump sprouts showed the fastest growth among the regeneration types. Height growth of advance regeneration was slow in the year following thinning but then increased rapidly surpassing the other regeneration types 5 years after thinning. Thinning had an effect on the composition of regenerated trees also by seedling, stump sprout, root sprout and layering in the understorey. Advance regenerations occupied the upper understorey at 5 years after thinning. A large number of stump sprouts emerged in the plots 1 year after thinning, while at 5 years after thinning the occupation of stump sprouts decreased. Root sprouts and layers faded away. Considering the early growth and the composition of understorey F. rhynchophylla, its advance regeneration has a high potential to develop into overstorey trees at this site.

Forest disturbance by ice storms in Quercus forests of the southern Appalachian Mountains, USA

:In 1994, two ice storms (major freezing rain events) disturbed forests on east- and southeast-facing slopes of the southern Appalachian Mountains, Virginia, USA. This study investigates impacts of the disturbance on Quercus forests, the predominant vegetation type in the Appalachians. Forests were sampled at six sites on two neighbouring mountains during 1997 and 1998. The two storms killed a significant number of trees, reducing basal area and density by 28.7% and 29.2%, respectively, in the transects sampled for this study. The storms also removed an estimated 43.7% of the canopy cover. Uprooting of trees disturbed 3.5% of the soil surface, on average. The ice storms eliminated some minor tree species from the sampled transects, reducing species richness; however, evenness increased. Both abiotic and biotic factors (mountain, slope, tree size, and species) influenced patterns of tree damage and mortality. Differences among species in susceptibility to ice storm damage seem particularly important for forest dynamics. The dominant Quercus prinus, along with Acer rubrum and Carya species, exhibited low mortality, whereas Pinus rigida had exceptionally high mortality. More generally, mortality was low in shade-tolerant and moderately shade-tolerant species, and high in those intolerant of shade. Periodic ice storms may contribute to the successional replacement of Pinus rigida and other susceptible species by less susceptible species.

Variation in the herb species response and the humus quality across a 200‐year chronosequence of beech and oak plantations in Belgium

The present study aimed at exploring the long‐term impact of pure and mixed beech Fagus sylvatica and oak Quercus robur stands on the forest floor by documenting changes in the herb species’ behaviour and in humus index across a 200‐yr chronosequence of forest stands. The research was conducted in central Belgium, in a 4383 ha beech‐dominated forest. Analyses were carried out in stands which are replicated, of the same age, managed in the same way, and growing on the same soil type with the same land‐use history. The results of this study indicate that stand aging is an important determinant of herb species occurrence in the studied area. Most of the species studied show a different response to stand age in pure compared to mixed stands. Our results clearly show a decrease of the humus quality with age in pure stands (beech as well as oak). On the other hand, we found that mixing beech and oak maintained or improved the humus status along the chronosequence according to the proportion of each tree. So the addition of some oak to the beech made it possible to keep a constant quality of the humus. We found that, even if the understory tree species is very scarce, it may be sufficient to maintain the humus status on the long term. In the present study, a cover of 1% oak in a beech stand was sufficient to show an effect of the minor species on these soils. This pattern contrasts with the widespread idea that substantial effects of the minor tree species on soils might not develop if the ratio of major/minor species is low.

Comparison of structure and biodiversity in the Rajhenav virgin forest remnant and managed forest in the Dinaric region of Slovenia

AbstractComparisons are made between a virgin forest remnant (primeval forest) and a lightly managed (near‐to‐nature) forest with regard to horizontal forest structures, the structure of forest stands, and the diversity of plant and bird species. In the virgin forest remnant the proportion of canopy gaps is smaller, there are no stands in the developmental phase of a pole stand (10 < cm d.b.h. < 30 cm), and both the growing stock and the proportion of less vigorous trees are considerably greater. In addition, there is a higher percentage of dead trees, a smaller proportion of minor tree species and a considerably lower diversity of plants. The diversity of bird species is similar for each forest type, but rare bird species are confined to the virgin forest remnant. The biological differences between the two types of forest have led to changes in the physical site conditions. The results of the research are valuable in assessing the consequences of near‐to‐nature forest management.