Rates of spontaneous hybridization and hybrid recruitment in co-existing exotic and native mature larch populations

Exotic larch (Larix decidua Mill., Larix kaempferi (Lamb.) Carr. and Larix sibirica Ledeb.) and their hybrids have been introduced into Canada to test adaptation and growth for reforestation purposes. For tree breeding operations, provenance trials and seed orchards of exotic larches and their hybrids have been established in arboreta adjacent to natural forest stands of the indigenous species Larix laricina (Du Roi) K. Koch. This proximity offers an opportunity to measure spontaneous hybridization and the recruitment of hybrids. Using a combination of two chloroplast and three mitochondrial DNA markers, we estimated the rate of spontaneous hybridization taking place between native (L. laricina) and exotic (L. decidua, L. kaempferi, L. sibirica) species of larch in two arboreta. More than 5,000 seeds and 1,000 recruits from the two experimental sites were sampled and genetically identified. The occurrence of spontaneous hybridization between L. laricina and the exotics was found both in seeds and from recruits freely established near the plantations. A low hybridization rate (3 % or less) was observed among the seeds of L. laricina maternal trees. Spontaneous native-exotic hybrids were also observed to establish and survive in the environment under the current climatic conditions at a similarly low rate. However, a much higher and variable rate of establishment was observed for recruits with exotic components (up to 34.6 % at one site). These results indicate that the spontaneous hybridization and the recruitment of hybrid and exotic recruits do occur and should be taken into consideration in the management and establishment of plantations of fast-growing species with exotic components.     

Influencing factors on early vegetation restoration in burned area of Pinus pumila – Larch forest

Pinus pumila (Dall.) Regel, a rare conifer and key species in high latitude and high altitude mountains, has an important role in soil and water conservation. This evergreen shrub grows 3–6 m high in P. pumila – larch (Larix gmelini) open forest at altitudes of 800–1200 m in the Greater Kingan Mountain Range (Daxing’an Mountain). Forest fires are major natural hazards to P. pumila – larch forest. The unique ecological role of this community gives important theoretical and practical significance to research on P. pumila – larch forest restoration after fires. Literature concerning factors influencing early vegetation restoration in burned areas in this habitat is sparse. We studied these factors, especially those related to P. pumila seedling establishment. The results showed fires in P. pumila – larch forest usually resulted in severe burns. Typically almost no P. pumila survived after fires. Nearly all ground fuels were consumed. Second growth after fires exhibited low species richness. The dominant tree/shrub seedlings found after fires were birch (Betula platyphylla) and larch, with small number of P. pumila. Other shrub seedlings found were Ledum palustre, Vaccinium vitis-idaea, Betula fruticosa, and Rubus arcticus. The main herb species found were Deyeuxia langsdorffii, and Chamaenerion angustifolium. Important factors influencing early vegetation restoration after fires included seed dispersal, fire size, and site condition. Seed dispersal in birch and larch is higher than in P. pumila; more seedlings of birch and larch were found in burned areas than seedlings of P. pumila. Most seeds germinated in the first year following a fire. The extent of the burned area influences seedling distribution patterns, especially in species with limited seed dispersal ability. Birch and larch seedlings were evenly distributed in the entire burned area, while seedlings of P. pumila were found only at the fire edge. No P. pumila seedlings were found more than 50 m away from seed source trees. Site condition significantly influenced seed germination and growth in birch and larch; these seedlings only grew well in burned areas with good site conditions (shallow slopes, thick soils, etc.). They did not grow well in burned areas with poor site conditions (steep slopes, thin soils, etc.). However, P. pumila seedlings could grow well in burned areas with either good or poor site conditions. The strong vitality of P. pumila seedlings gives this species an enormous ecological advantage in soil conservation and environmental restoration and conservation. We conclude that the main factor influencing seedling establishment in P. pumila is its weak seed dispersal ability. Although the P. pumila seeds can germinate in all burned areas, natural regeneration rarely restores burned areas to the original P. pumila – larch forest. Planting seeds and/or seedlings may facilitate burned area restoration to P. pumila – larch forest. Because P. pumila seedlings grow very slowly, the restoration process may take decades.     

Patterns of daily stem growth in different tree species in a warm-temperate forest in northern China

Radial growth in trees responds to environmental changes in various ways ranging from immediate to hysteretic responses. However, species-specific tree radial growth patterns and their responses to short-term weather changes are not fully understood. Here, the daily stem radial changes (SRCs) in four common tree species, linden (Tilia mongolica), birch (Betula dahurica), oak (Quercus wutaishanica) and larch (Larix principis-rupprechtii), were monitored with high-resolution point dendrometers during the main growing seasons in 2017–2019 on Dongling Mountain, northern China. The SRC was differentiated into tree water deficit-induced stem shrinkage (TWD) and growth-induced irreversible stem expansion (GRO) to evaluate species-specific responses to weather variables and short-term drought events. We found that the TWD and GRO of the four species were significantly different. The TWD was influenced primarily by the vapor pressure deficit (VPD), whereas the GRO was influenced primarily by precipitation (P). In linden and birch, a larger proportion of the GRO occurred at higher air temperature (T_mean) and VPD values; in contrast, the range of these changes was lower in oak and larch. With the increased durations of drought periods, oak and larch experienced large and rapid increases in TWD, whereas birch and linden showed small and slow increases. These results indicate that oak and larch would be sensitive to warmer and drier weather conditions predicted for the future, while linden and birch would have a conservative growth strategy. Our results provide further insights into the physiology of these four tree species and allow us to better predict the growth response of forest dynamics under climate change.     

Losses of seeds of temperate trees to soil fungi: effects of habitat and host ecology

Evidence suggests that impacts of fungal pathogens on tree recruitment tend to be greater in the forest understory than in openings, and that shade-tolerant trees are less vulnerable than shade-intolerant species. To investigate the role that harmful soil fungi may have in reducing regeneration of temperate trees, we applied fungicide to buried seeds of matched pairs of species differing in their relative shade tolerance and/or successional status (Acer negundo versus Acer saccharum, Prunus virginiana versus Prunus serotina, and Pinus strobus versus Tsuga canadensis), in three habitats that differed in their degree of openness (old field, forest gap, and forest understory). Our results indicated that soil fungi reduced germination of A. negundo, P. virginiana, P. serotina. and T. canadensis, and reduced viability of ungerminated seeds of P. strobus; no significant effects of fungi on seeds of A. saccharum were detected. However, we found seeds were not less likely to survive following burial in forest understory than in gaps. As well, results for only one species pair (A. negundo versus A. saccharum) were consistent with the prediction that shade-intolerant or successional species should be more susceptible to fungal attack than mature forest species. These results contrast with other studies of temperate and especially tropical forest trees.     

In situ regeneration of Pinus strobus and P. resinosa in the Great Lakes forest communities of Canada

Abstract. Two extensive forest vegetation survey datasets are explored, using ordination and classification, for evidence of in situ regeneration by Pinus strobus (Eastern white pine) and P. resinosa (Red pine). Ordination of tree species contributions to total basal area in 320 upland northern hardwood- conifer stands produced distinct stand groups for P.banksiana, P. resinosa, P. strobus and mesic hardwoods in an ascending sequence along the first axis. Quercus rubra (red oak), Q. alba (white oak) and tolerant conifer groups formed segregates from the hardwood complex along the second axis. P. strobus mixes with all other forest types, but P. resinosa is restricted to its own group. Seedlings and trees of P. strobus are more abundant than saplings, which are restricted to the pine and oak forests. Therefore, seed production, dispersal and seedling establishment seem to be less of a barrier to in situ regeneration by P. strobus than subsequent survival and growth. Canonical correspondence analysis of 170 pine-dominated stands from the Canadian Shield of Ontario, in which tree species variables are segmented into height-class pseudo- species, yielded no linear relationship between environmental features or stand structure and seedling densities of P. strobus. However, total tree basal area appears to impose an upper limit to seedling density on the forest floor. Strong correlations emerged between pine seedling density and understorey vegetation. Stand classification of the understorey vegetation, using constrained indicator species analysis, yielded distinct high and low seedling groups. Low pine seedling density was associated with abundant broadleaved shrubs, herbs and seedlings as well as feathermosses and tolerant conifers. High seedling density could not be ascribed to the presence of seedbed taxa, such as Polytrichum, but is ascribed to the absence of competition and other forms of inhibition in the understorey vegetation and down through the canopy profile. In situ regeneration of P. strobus does, therefore, occur but conditions over the forest landscape are largely restrictive.     

A multi-scale dendroclimatological analysis of four common species in the southern Canadian boreal forest

A comprehensive assessment of the tree growth/climate relationship was undertaken to better understand the potential impacts of climate change on the growth dynamics of four widespread and common boreal tree species, namely jack pine (Pinus banksiana), black spruce (Picea mariana), eastern larch (Larix laricina), and trembling aspen (Populus tremuloides), located at the southern limits of the Canadian boreal forest. Over intra-annual time scales, results show that precipitation is likely the main driver of stem radius change (∆R), with jack pine radius exhibiting the most consistent positive relationship. Precipitation had a stronger relationship with stem radius variation in black spruce and eastern larch during periods when volumetric water content (VWC) in the root zone was below average, pointing to the likelihood that certain species rely more heavily on available moisture in the uppermost layers of the soil column to replenish stem water, especially during extended dry periods. Warm air temperatures had an immediate negative impact on stem water content due to transpiration. This was most marked during periods of reduced moisture availability in the root zone, when trees are more susceptible to net water volume loss. During periods when moisture was not limiting, a positive relationship between lagged air temperature and ∆R was detected. Warm air temperatures may therefore play an important role in stimulating radial growth when moisture requirements are met. At annual temporal resolution, the growth/climate relationship changed over the lifetime of our study species. Over the last several decades, the relationship between precipitation and annual radial tree growth has weakened, while positive relationships between spring and summer air temperature and annual radial tree growth have emerged, likely signaling a decrease in moisture limitations, and a positive response to spring warming. Our findings reveal that boreal forest tree species may benefit from spring and summer warming over the near term, providing there is sufficient moisture to support growth. Over the long term, rates of evapotranspiration are expected to overshadow gains in moisture related to an increase in precipitation. Under these circumstances, we are likely to see reduced growth rates and an increasingly negative response of boreal tree species growth to warm air temperatures.     

Competitive effects of the invasive shrub,Lonicera maackii (Rupr.) Herder (Caprifoliaceae), on the growth and survival of native tree seedlings

Invasive plants are often associated with reduced cover of native plants, but rarely has competition between invasives and natives been assessed experimentally. The shrub Lonicera maackii, native to northeastern Asia, has invaded forests and old fields in numerous parts of eastern North America, and is associated with reduced tree seedling density in Ohio forests. A field experiment was conducted to test the effects of established L. maackii on the survival and growth of transplanted native tree species. The experiment examined above-ground competition (by removing L. maackii shoots) and below-ground competition (by trenching around transplanted seedlings). The effects of above-ground competition with L. maackii were generally more important than below-ground competition, though both were detected. Shoot treatment was the key determinant for the survival of all species except P. serotina, whereas trenching only enhanced survival for A. saccharum caged and P. serotina, and only in the shoot removal treatment. For the surviving seedlings, L. maackii shoot removal increased growth of A. saccharum seedlings protected with cages, but actually reduced the growth of unprotected Q. rubra and A. saccharum seedlings, indicating that L. maackii shoots confer some protection from deer browsing. Significant interactions between root and shoot treatment on Q. rubra growth parameters, specifically greatest growth in the shoot present & trenched treatment, is attributed to protection from deer browsing combined with release from below-ground competition. Despite this protective function of L. maackii shoots, the overall effect of this invasive shrub is increased mortality of native tree seedlings, suggesting it impacts the natural regeneration of secondary forests.     

Contrasting responses of seedling and sapling densities to livestock density in the Mongolian forest-steppe

The past and present regeneration of Siberian larch (Larix sibirica) was studied in the forest-steppe of the Mongolian Altai, an area which has experienced an increase in annual mean temperature by 2.1 °C since 1940 and is subjected to grazing by mixed herds of livestock owned by pastoral nomads. Past regeneration was reconstructed from tree rings and present regeneration was analyzed by surveying seedlings, sapling-sized trees and the occurrence of viable larch seeds in the soil seed bank. Forest regeneration occurred throughout the twentieth and the early twenty-first centuries in the forest interiors, but ceased after the late 1970s at forest edges. The density of sapling-sized larch trees decreased with livestock density linearly in the interior and exponentially at the edge. Most sapling-sized trees had visible damage from livestock browsing, which also manifested in wood-anatomical anomalies. By contrast, the densities of 1- and 2-year old seedlings increased with livestock density at the forest edge, suggesting that seedlings in this habitat benefitted from the reduction in competition intensity due to livestock grazing. This relationship also suggests that larch seedlings, in contrast to sapling-sized trees, were avoided by the livestock, as otherwise removal by the herbivores should have counteracted the promotion due to reduced competition. Near-consistency of the correlations of total livestock and goat densities with sapling and seedling densities suggests that the control of larch regeneration is primarily a function of goat density, which have tripled in the Mongolian livestock during the past 20 years for economic reasons.     

Mycorrhizal networks mediate overstorey-understorey competition in a temperate forest

In forests, common mycorrhizal networks (CMNs) often connect the roots of neighbouring plants. Observations of material flows between hosts connected by CMNs have given rise to the hypothesis that CMNs limit the negative effects of competition by overstorey trees on seedlings recruiting underneath them. I conducted an experiment in a temperate forest dominated by ectomycorrhizal conifers and hardwoods to isolate the effects of CMNs on the growth and survival of four tree species that co‐occur in the understorey. Ectomycorrhizal networks had strong negative effects on the survival of an arbuscular mycorrhizal species, Acer rubrum, and neutral effects on the survival of three ectomycorrhizal species, Betula allegheniensis, Pinus strobus, and Tsuga canadensis. CMNs had positive effects on the growth of at least one ectomycorrhizal species, P. strobus. Interspecific differences in juvenile responses to CMNs may influence forest community development, promoting coexistence of some tree species while limiting the success of others.     

The influence of the lower Ob River runoff on radial growth of trees

The influence of the Ob River runoff in its lower reaches on the radial growth of main forest-forming tree species-the Siberian spruce (Picea obovata Ledeb.), Siberian stone pine (Pinus sibirica Du Tour.), and larch (Larix sibirica Ledeb.)-was studied in the north of Western Siberia in various test sites both at the riverside and at a distance of 3 to 80 km from the Ob River floodplain. Differences in responses of the radial growth to air temperature in October and repeated frost damage of tree annual rings are observed in the Siberian spruce and Siberian stone pine depending on the distance from the river. The correlations of the radial growth of trees and frost damage with the effect of the Ob River runoff are discussed.     

Age and radial growth pattern of four tree species in a subtropical forest of China

Subtropical forests are usually composed of many tree species. Knowledge of the age and radial growth variation of the dominant tree species is useful for understanding forest dynamics and community structure and function. The aims of this study are to explore whether there are identifiable annual growth rings in the main tree species and to examine the growth characteristics within and among the species in Mount Gutian subtropical forest of China. The results showed that four out of eight tree species from which samples were collected had visible and cross-datable rings. There were no stable relationships between the age and diameter for these subtropical trees. Significant differences existed in radial growth rate within and among the four species, suggesting a high spatial heterogeneity in the mixed-species subtropical forest. The common pattern in age distribution of multiple species suggests a stand-wide disturbance occurring around the 1960s. It is interesting to note that the growth rate at the same age intervals was different for trees younger than 40 years of age and older than 40 years of age, suggesting a change in climate or forest structure in the two time periods. The results obtained from this study help understand the growth dynamics in other subtropical forests having these tree species.     

Growth of regenerated tree seedlings associated with microclimatic change in a mature larch plantation after harvesting

In order to further develop methods of sustainable forest management, we evaluated the effects of logging practices during the winter on microclimatic factors and growth of four seral deciduous broad-leaved tree seedlings regenerated in a larch plantation in northern Japan. We found that winter logging practices drastically changed microclimatic factors, especially in terms of light intensity and the vertical distribution pattern of CO₂ concentration. Harvesting overstory larch trees increased photosynthetic photon flux (PPF), which may enhance the photosynthesis of understory plants. We examined the undergrowth for tree seedlings of the following species: two late successional species, Prunus ssiori and Carpinus cordata; one gap phase species, Magnolia hyporeuca; and one mid-late successional species, Quercus mongolica var. crispula. All of the four studied tree species responded well to the practices of winter logging after the second year of harvesting overstoried larch trees. The current shoot diameter and current shoot length increased significantly. Therefore, we conclude that winter logging practices should improve shoot growth of deciduous broad-leaved tree seedlings regenerated in a larch plantation through the change in environmental factors that accompanies larch harvest.     

Sancho,the oldest known Iberian shrub

Cliffs are refuges for old trees and shrubs. In the Mediterranean Basin most dendroclimatic reconstructions have focused on high-elevation forests where tree radial growth is constrained by low temperatures in addition to drought stress. Old shrubs may provide longer ring-width series of hydroclimate proxies in low-elevation, drought-prone Mediterranean ecosystems where old trees are rare. To fill this research gap we investigated the maximum age and climate sensitivity of young, old, and recently dead Phoenician junipers (Juniperus phoenicea L.), growing on calcareous cliffs and nearby plains, in the Guara Natural Park (northeast Spain). The oldest living juniper was ¹⁴C-dated to be 927 years old, and it was named “Sancho” after Don Quixote’s squire. Based on ring counts, the maximum age was 655 years. The difference in age estimates between the ¹⁴C-dates and ring counts was 39 years indicating that ring counts underestimate age. This was due to missing and wedging rings making the cross-dating of old junipers unfeasible. Cool and wet conditions from May to July enhanced radial growth of young junipers. Old shrubs have a high dendroecological potential in Mediterranean sites where their growth is constrained by warm-dry conditions during the growing-season. Further techniques combining dendrochronological and wiggle-match ¹⁴C dating may allow reconstructing long-term hydroclimate in low-elevation Mediterranean areas.     

Ice‐age persistence and genetic isolation of the disjunct distribution of larch in Alaska

Larix laricina (eastern larch, tamarack) is a transcontinental North American conifer with a prominent disjunction in the Yukon isolating the Alaskan distribution from the rest of its range. We investigate whether in situ persistence during the last glacial maximum (LGM) or long‐distance postglacial migration from south of the ice sheets resulted in the modern‐day Alaskan distribution. We analyzed variation in three chloroplast DNA regions of 840 trees from a total of 69 populations (24 new sampling sites situated on both sides of the Yukon range disjunction pooled with 45 populations from a published source) and conducted ensemble species distribution modeling (SDM) throughout Canada and United States to hindcast the potential range of L. laricina during the LGM. We uncovered the genetic signature of a long‐term isolation of larch populations in Alaska, identifying three endemic chlorotypes and low levels of genetic diversity. Range‐wide analysis across North America revealed the presence of a distinct Alaskan lineage. Postglacial gene flow across the Yukon divide was unidirectional, from Alaska toward previously glaciated Canadian regions, and with no evidence of immigration into Alaska. Hindcast SDM indicates one of the broadest areas of past climate suitability for L. laricina existed in central Alaska, suggesting possible in situ persistence of larch in Alaska during the LGM. Our results provide the first unambiguous evidence for the long‐term isolation of L. laricina in Alaska that extends beyond the last glacial period and into the present interglacial period. The lack of gene flow into Alaska along with the overall probability of larch occurrence in Alaska being currently lower than during the LGM suggests that modern‐day Alaskan larch populations are isolated climate relicts of broader glacial distributions, and so are particularly vulnerable to current warming trends.     

Ectomycorrhizal fungal community of naturally regenerated European larch (Larix decidua) seedlings

An investigation was undertaken to assess the community structure of ectomycorrhizal (ECM) fungi on naturally regenerating European larch (Larix decidua Mill.) seedlings grown under forest conditions. The sites examined were in two managed monoculture larch forests, differentiated by soil chemistry and mature tree density. Morphological and molecular analyses revealed a total of 22 fungal taxa. From detected ECM fungal taxa, 13 were noted at Site I and 13 at Site II. Only four taxa were found in both sites (Russula ochroleuca, Thelephora terrestris, Lactarius tabidus and Paxillus involutus). The most abundant species at Site I (lower mineral concentration, high tree density) was Hydnotrya tulasnei (25.7?%), followed by Pseudotomentella tristis, Tomentella sublilacina and Russula puelaris. At Site II (higher mineral concentration, low tree density) the dominant fungal symbiont of larch seedlings was clearly Wilcoxina mikolae, which accounted for 74?% of mycorrhizal tips. The less abundant species comprised T. terrestris, L. tabidus, Xerocomus pruinatus and R. ochroleuca. The analysis of similarity (ANOSIM) and non-metric multidimensional scaling (NMDS) ordination clearly separated the ECM fungal assemblages in the two sites tested. Because our study sites were differentiated by many factors, it is not easy to distinguish one factor in particular to explain the differences observed between the ECM communities at Sites I and II. The results obtained significantly increase our knowledge about the diversity of the ECM fungi hosted by L. decidua. The large number of ECM fungi detected was the first observation showing these fungi as symbiotic partners of European larch.     

Elevated CO2 enhances the growth of hybrid larch F1 (Larix gmelinii var. japonica × L. kaempferi) seedlings and changes its biomass allocation

Key message

Elevated CO ₂ enhances the photosynthesis and growth of hybrid larch F ₁ seedlings. However, elevated CO ₂ -induced change of tree shape may have risk to the other environmental stresses.

Abstract

The hybrid larch F₁ (Larix gmelinii var. japonica × L. kaempferi) is one of the most promising species for timber production as well as absorption of atmospheric CO₂. To assess the ability of this species in the future high CO₂ environment, we investigated the growth and photosynthetic response of hybrid larch F₁ seedlings to elevated CO₂ concentration. Three-year-old seedlings of hybrid larch F₁ were grown on fertile brown forest soil or infertile volcanic ash soil, and exposed to 500 μmol mol^?1 CO₂ in a free-air CO₂ enrichment system located in northern Japan for two growing seasons. Regardless of soil type, the exposure to elevated CO₂ did not affect photosynthetic traits in the first and second growing seasons; a higher net photosynthetic rate was maintained under elevated CO₂. Growth of the seedlings under elevated CO₂ was greater than that under ambient CO₂. We found that elevated CO₂ induced a change in the shape of seedlings: small roots, slender-shaped stems and long-shoots. These results suggest that elevated CO₂ stimulates the growth of hybrid larch F₁, although the change in tree shape may increase the risk of other stresses, such as strong winds, heavy snow, and nutrient deficiency.     

Increased Summer Temperatures Reduce the Growth and Regeneration of Larix sibirica in Southern Boreal Forests of Eastern Kazakhstan

The larch forests at the southern limit of the Siberian boreal forest in Central Asia have repeatedly experienced strong recent growth declines attributed to decreasing summer precipitation in the course of climate warming. Here, we present evidence from the southernmost Larix sibirica forests in eastern Kazakhstan that these declines are primarily caused by a decrease in effective moisture due to increasing summer temperatures, despite constant annual, and summer precipitation. Tree-ring chronologies (>800 trees) showed a reduction by 50–80% in mean ring width and an increase in the frequency of missing rings since the 1970s. Climate-response analysis revealed a stronger (negative) effect of summer temperature (in particular of the previous year’s June and July temperature) on radial growth than summer precipitation (positive effect). It is assumed that a rise in the atmospheric vapor pressure deficit, which typically increases with temperature, is negatively affecting tree water status and radial growth, either directly or indirectly through reduced soil moisture. Larch rejuvenation ceased in the 1950s, which is partly explained by increasing topsoil desiccation in a warmer climate and a high drought susceptibility of larch germination, as was demonstrated by a germination experiment with variable soil moisture levels. The lack of regeneration and the reduced annual stem increment suggest that sustainable forest management aiming at timber harvesting is no longer feasible in these southern boreal forests. Progressive climate warming is likely to cause a future northward shift of the southern limit of the boreal forest.     

Gradual expansion of moisture sensitive Abies spectabilis forest in the Trans-Himalayan zone of central Nepal associated with climate change

Population structure and tree recruitment dynamics in the natural treeline ecotone of high mountains are strong indicators of vegetation responses to climate. Here, we examined recruitment dynamics of Abies spectabilis across the treeline ecotone (3439–3638 m asl) of Chimang Lekh of Annapurna Conservation Area in the Trans-Himalayan zone of central Nepal. Dendrochronological techniques were used to establish stand age structure by ring counts of adults, and by terminal bud scar count for seedlings and saplings. The results showed abundant seedling recruitment, higher regenerative inertia and colonization with a consistent range shift of the A. spectabilis treeline. The upward expansion of this sub-alpine treeline was found to be driven by a strong dependence of seedling recruitment and radial growth on snowmelt and precipitation as temperatures rise. The radial growth of A. spectabilis at the alpine timberline ecotone (ATE) and closed timberline forest (CTF) showed sensitivity to spring season (March–May) climate. Tree ring indices of CTF showed a strong positive correlation with spring and annual precipitation, and a significant negative correlation with spring and annual temperature, however, moisture sensitivity was less strong at ATE than CTF.     

Does Prunus serotina act as an aggressive invader in areas with a low propagule pressure?

Since most studies on Prunus serotina in Western Europe focused on heavily invaded areas, we wondered whether P. serotina also acts as an aggressive invader in areas with a low propagule pressure. Based on long-term data for the Liedekerke forest reserve, we found that connectivity to seed sources and light availability were the major drivers of P. serotina presence: long-distance dispersal events and ‘windows of opportunity’ seem to direct P. serotina colonization. In the studied forest, P. serotina could not be considered an aggressive invader since its spread slowed down rather quickly and did not hamper the establishment of native tree species. Furthermore, understory P. serotina showed low growth and seed production, while the high Rubus cover hampered germination and establishment. Nonetheless, calamities opening up the canopy layer in the few areas with high P. serotina sapling density might alter the course of the invasion process.