Anthropogenic disturbance and the formation of oak savanna in central Kentucky, USA

Aim To deepen understanding of the factors that influenced the formation of oak savanna in central Kentucky, USA. Particular attention was focused on the link between historical disturbance and the formation of savanna ecosystem structure. Location Central Kentucky, USA. Methods We used dendrochronological analysis of tree‐ring samples to understand the historical growth environment of remnant savanna stems. We used release detection and branch‐establishment dates to evaluate changes in tree growth and the establishment of savanna physiognomy. We contrasted our growth chronology with reference chronologies for regional tree growth, climate and human population dynamics. Results Trees growing in Kentucky Inner Bluegrass Region (IBR) savanna remnants exhibited a period of suppression, extending from the establishment date of the tree to release events that occurred c. 1800. This release resulted in a tripling of the annual radial growth rate from levels typical of oaks suppressed under a forest canopy (< 1 mm year^?1) to levels typical of open‐grown stems (3 mm year^?1). The growth releases in savanna trees coincided with low branch establishment. Over the release period, climatic conditions remained relatively constant and growth in regional forest trees was even; however, the growth increase in savanna stems was strongly correlated with a marked increase in Euro‐American population density in the region. Main conclusions Our data suggest that trees growing in savanna remnants originated in the understorey of a closed canopy forest. We hypothesize that Euro‐American land clearing to create pasturelands released these trees from light competition and resulted in the savanna physiognomy that is apparent in remnant stands in the IBR. Although our data suggest that savanna trees originated in a forest understorey, this system structure itself may have been a result of an unprecedented lack of Native American activity in the region due to population loss associated with pandemics brought to North America by Euro‐Americans. We present a hypothetical model that links human population dynamics, land‐use activities and ecosystem structure. Our model focuses on the following three land‐use eras: Native American habitation/utilization; land abandonment; and Euro‐American land clearance. Ecological understanding of historical dynamics in other ecosystems of eastern North America may be enhanced through recognition of these eras.     

Drought-driven growth reduction in old beech (Fagus sylvatica L.) forests of the central Apennines, Italy

Productivity of old‐growth beech forests in the Mediterranean Basin was measured by average stem basal area increment (BAI) of dominant trees at two mountain sites in the Italian Apennines. Both forests could be ascribed to the old‐growth stage, but they differed markedly with regard to elevation (1000 vs. 1725 m a.s.l.), soil parent material (volcanic vs. calcareous), mean tree age (less than 200 years vs. 300 years), and stand structure (secondary old‐growth vs. primary old‐growth forest). Drought at the two sites was quantified by the self‐calibrated Palmer Moisture Anomaly Index (Z‐index), and by the self‐calibrating Palmer Drought Severity Index (PDSI) for summer (June through August) and the growing season (May through September). Dendroclimatological analyses revealed a moisture limitation of beech BAI at interannual (water availability measured by Z‐index) and decadal scales (water availability measured by PDSI). Both BAI and water availability increased from 1950 to 1970, and decreased afterwards. Trees were grouped according to their BAI trends in auxological groups (growth‐type chronologies), which confirmed that growth of most trees at both sites declined in recent decades, in agreement with increased drought. Because BAI is not expected to decrease without an external forcing, the patterns we uncovered suggest that long‐term drought stress has reduced the productivity of beech forests in the central Apennines, in agreement with similar trends identified in other Mediterranean mountains, but opposite to growth trends reported for many forests in central Europe.     

Vertical distribution of epiphytic bryophytes in an Indonesian rainforest

We studied species richness, composition and vertical distribution of epiphytic bryophytes in submontane rainforest of Central Sulawesi. Bryophytes were sampled on eight canopy trees and on eight trees in the forest understorey. Microclimate was measured at trunk bases and at crown bases. The total recorded number of 146 epiphytic bryophyte species is among the highest ever reported for tropical forests and underlines the importance of the Malesian region as a global biodiversity hotspot. Species composition differed significantly between understorey trees and canopy tree trunks on the one hand, and the forest canopy on the other. Fourty-five percent of the bryophyte species were restricted to canopy tree crowns, 12% to the understorey. Dendroid and fan-like species mainly occurred in the forest understorey whereas tufts were most species rich in the tree crowns. The findings reflect the different microclimatic regimes and substrates found in the understorey and in the forest canopy. The results indicate that assessments of the bryophyte diversity of tropical forests are inadequate when understorey trees and tree crowns are excluded.     

Past levels of canopy closure affect the occurrence of veteran trees and flagship saproxylic beetles

Aim

Open woodlands are biologically highly diverse habitats, and veteran (i.e., old, senescent) trees are key structures supporting their biodiversity. Open canopy structure had been maintained by both natural‐ and human‐induced disturbances. In the past two centuries, suppression of such disturbances, together with forestry intensification, has turned most lowland woodlands into closed‐canopy forests. We investigated the effect of increased canopy closure on veteran trees and several threatened beetles associated with them.

Location

Floodplain woodlands along the lower Dyje and Morava rivers, Czech Republic.

Methods

We used an approach combining the study of aerial photographs with on‐ground survey of veteran trees and associated endangered beetles. The aerial images were used to obtain the information on historical (1938) and recent (2009) canopy closure in the area of 146 km², where we mapped large oaks (d.b.h. >70 cm), hollow trees and three associated beetles including the hermit beetle (Osmoderma barnabita), the great capricorn beetle (Cerambyx cerdo) and the jewel beetle Eurythyrea quercus.

Results

The presence of large oaks, hollow trees and their associated beetle species are negatively related to recent high canopy closure, and the historical level of canopy closure matters, as in nowadays closed‐canopy stands, the beetles and veteran trees are more common in places that were rather open in 1938 than in the places with closed canopy already in 1938. Moreover, the health state of veteran trees highly depends on the canopy closure.

Main conclusion

The negative effect of canopy closure on veteran trees and their endangered inhabitants is several decades delayed and may thus often go undetected. In the forests, however, large and hollow trees and their associated biodiversity are relics of the past, more open conditions. The restoration of open woodlands is therefore vital for preventing their further decline. Conservation management planning needs to take this into account wherever, veteran trees and associated biota are concerned.

     

Ring-Tailed Lemur Home Ranges Correlate with Food Abundance and Nutritional Content at a Time of Environmental Stress

In order to determine whether ring-tailed lemurs (Lemur catta) adapt their ranging and select an optimal diet at a time of food shortage, we observed two adjacent troops in Berenty Reserve, Madagascar for over 250 h. The troops, created by a recent fission, ranged through closed canopy gallery forest next to a river and open forest away from the river. We conducted the study in September–October, 2000, normally a time of seasonally low resource availability, which was intensified by damage from a previous windstorm and recent drought. To examine the impact of environmental stress, we mapped their ranging patterns, intertroop encounters, feeding patches, siesta trees, and sleeping trees. We then correlated their ranging and feeding behavior with nutritional analyses of leaves and fruit from tamarind trees located in different parts of their ranges. One of the troops, D1A, ranged farther into open forest than previously. However, the range for troop D1B and the closed canopy portion of D1A's range were located in traditional positions for historical troops D and E. Both troops ate significantly more mature leaves from the tamarind trees in the closed canopy forest, where the leaves had significantly higher nutritional content (water and protein) than that of open forest samples. They fed on tamarind fruit significantly more often in the open forest away from the river, where it was more abundant. The lemurs selected a diet that maximized leaf water and protein and ranged where fruit was most abundant but at high energetic costs for troop D1A.     

Tree Regeneration in Church Forests of Ethiopia: Effects of Microsites and Management

Tree regeneration is severely hampered in the fragmented afromontane forests of northern Ethiopia. We explored how trees regenerate in remnant forests along the gradient from open field, forest edge to closed sites and canopy gaps inside the forest. We investigated the effects of seed sowing, litter removal, and weeding on the regeneration success along this gradient. Regeneration success was investigated for four indigenous tree species, and measured in terms of seedling establishment, growth, and survival. Species performed differently according to site conditions. Within the forest, local canopy openings facilitated seed germination ( Ekebergia ), seedling growth (all species except Olea ), or survival ( Ekebergia and Olea ), suggesting that all species benefited from local high light conditions in the forest. Outside the forest, germination (all species) and growth rates ( Juniperus and Olea ) were lower in the open field, most probably due to water stress in the dry season. Outer edge conditions favored growth for three of the four species. Natural seed germination was, however, zero at any site for Juniperus and Olea and low for Ekebergia and Prunus in the open field. Soil scarification influenced germination positively, while weeding did not have a positive effect. These results suggest that simple measures may improve seedling establishment, and that, for some species, forest edges are particularly useful for growth and survival after succesful establishment. Together with erecting fences, needed to protect seedlings against grazing, seed sowing, planting seedling, and soil scarification may contribute to maintain and restore church forests in the fragmented landscapes of northern Ethiopia.     

Densification and State Transition Across the Missouri Ozarks Landscape

World-wide, some biomes are densifying, or increasing in dense woody vegetation, and shifting to alternative stable states. We quantified densification and state transition between forests ecosystems in historical (ca. 1815–1850) and current (2004–2008) surveys of the Missouri Ozark Highlands, a 5-million ha landscape in southern Missouri, USA. To estimate density of historical forests, we used the Morisita plotless density estimator and applied corrections for surveyor bias. For contemporary forests, we used known densities at plots to predict continuous densities with random forests, an ensemble regression tree method. We also calculated basal area and percent stocking to determine changes in wood volume. Historical forests had densities ranging from about 75 to 320 trees/ha. Current forest densities were about 2.3 times greater and more uniform, at about 300–400 trees/ha (DBH ≥ 12.7 cm). Not all forests have increased in basal area and percent stocking because trees in contemporary forests are smaller in diameter than historical forests. Although oak species still are dominant (as defined by ≥10% composition), oak dominance is being replaced by many fire-sensitive species, of which only eastern redcedar and maples have become dominant. Densification and community changes in functional traits have produced a state transition from open oak forest ecosystems to predominantly closed eastern broadleaf forests in the Missouri Ozarks. This shift is not at equilibrium, as fire-sensitive species are continuing to increase and turnover in long-lived oaks is slow.     

Pattern and dynamics of the ground vegetation in south Swedish Carpinus betulus forests: importance of soil chemistry and management

The vegetation and environmental conditions of south Swedish hornbeam Carpinus betulus forests are described with data from 35 permanent sample plots The main floristic gradient of the ground vegetation is closely related to acid-base properties of the top soil base saturation, pH and organic matter content Other floristic differences are related to tree canopy cover and the distance of the sample plots to the Baltic coast Species richness of herbaceous plants typical of forests increases with soil pH, The number of other herbaceous species, occurring in both forests and open habitats, and of woody species is not related to pH Comparisons of vegetation data from 1983 and 1993 show relatively small compositional differences of the herbaceous forest flora The number of other herbaceous species increased considerably m those plots where canopy trees had been cut after 1983 The number of new species in managed plots increases with soil pH Species losses and gains of the herbaceous forest flora between 1983 and 1993 are generally lower as compared with other herbaceous species and woody species However, the ground cover of herbaceous forest species, especially of Oxalis acetosella and Lamium galeobdolon , was considerably lower in 1993 as compared to 1983 in both unmanaged and managed plots Possible explanations for this decrease are current soil acidification and drought during the growing season     

Drought and the interannual variability of stem growth in an aseasonal,everwet forest

Linking drought to the timing of physiological processes governing tree growth remains one limitation in forecasting climate change effects on tropical trees. Using dendrometers, we measured fine‐scale growth for 96 trees of 25 species from 2013 to 2016 in an everwet forest in Puerto Rico. Rainfall over this time span varied, including an unusual, severe El Niño drought in 2015. We assessed how growing season onset, median day, conclusion, and length varied with absolute growth rate and tree size over time. Stem growth was seasonal, beginning in February, peaking in July, and ending in November. Species growth rates varied between 0 and 8 mm/year and correlated weakly with specific leaf area, leaf phosphorus, and leaf nitrogen, and to a lesser degree with wood specific gravity and plant height. Drought and tree growth were decoupled, and drought lengthened and increased variation in growing season length. During the 2015 drought, many trees terminated growth early but did not necessarily grow less. In the year following drought, trees grew more over a shorter growing season, with many smaller trees showing a post‐drought increase in growth. We attribute the increased growth of smaller trees to release from light limitation as the canopy thinned because of the drought, and less inferred hydraulic stress than larger trees during drought. Soil type accounted for interannual and interspecific differences, with the finest Zarzal clays reducing tree growth. We conclude that drought affects the phenological timing of tree growth and favors the post‐drought growth of smaller, sub‐canopy trees in this everwet forest. Abstract in Spanish is available with online material.     

Notes on habitat relationships of ungulates in Yankari Game Reserve, Nigeria

Yankari Game Reserve in northeastern Nigeria consists largely of savanna woodland with trees on the better soils growing to 15 m and with spreading crowns. On shallow and stony soils the tree height is generally less and the canopy is discontinuous. The Gaji River riparian zone supports a wide variety of vegetation types ranging from evergreen, closed canopy forest to sedge meadows and patches of open grassland.
Elephant ( Loxodonta africana ) range backwards and forwards along the riparian strip, feeding on perennial grasses and a variety of browse material and utilizing closed canopy forest patches for shade cover. The major movement patterns of other important herbivore species are perpendicular to the riparian strip. Areas used intensively are: waterbuck ( Kobus defassa )–open savanna woodland immediately behind the riparian strip: Western hartebeest ( Alcelaphus buselaphus major )– open grassy habitat in relatively poor woodland at middle distances from the river; Roan antelope ( Hippotragus equinus )–patches of well-developed and infrequently burned woodland, often at major distances from the river. Buffalo ( Syncerus caffer brachyceros ) during the dry season ranged between the riparian grassland areas and the more open sections of nearby savanna woodland, but travelled out to distant sections of the reserve after rainwater pools had formed.
A major problem in management was the development of a burning policy that would maintain an appropriate balance between perennial and annual grasses and the shade providing trees.     

Ecological succession following forest decline in a xeric oak forest of south-central United States

Aims The loss of canopy trees associated with forest decline can greatly influence the species composition and structure of a forest and have major impacts on the ecosystem. We studied the changes in forest composition and structure 1 and 5 years following nearly total canopy mortality on several hundreds of hectares of xeric oak forests in south-central United States. Because the forests were within an ecotonal vegetation type composed of a mosaic of forest, savanna and grassland, we sought to learn whether forest decline areas would recover to forest or change to more open savanna and grassland conditions in the landscape pattern of vegetation. Because low intensity fire shaped the vegetation type, we sought to learn whether fire would keep the decline areas open.Methods The study was conducted in a xeric oak forest in east-central Oklahoma, USA. Randomly located vegetation and regeneration surveys were conducted in decline and non-decline stands 1 and 5 years following nearly total canopy mortality. Diameter at breast height (DBH), regeneration and sprout origin were recorded for all woody species.Important findings The major canopy species post oak (Quercus stellata Wangenh.), blackjack oak (Quercus marilandica Muenchh.) and black hickory (Carya texana Buckl.) suffered 85–92% mortality; however, minor canopy components experienced limited mortality. Mortality affected all size classes of canopy trees except those below 5cm breast height diameter. There was abundant regeneration of all species and fire seemed to maintain a high level of sprouting. Decline appeared to decrease the relative importance of stump sprouting and increase other types including root sprouts. Decline areas had abundant true seedlings, with stem origin from a root with the same diameter as the stem, which is very unusual for xeric oak forests. Regeneration height in decline areas was twice that of non-decline forests. Our findings suggest that forest decline may lead to: (i) reduced oak dominance and species change in the canopy, (ii) change in reproduction type to increase success of true seedlings and maintain genetic diversity of oaks.     

Thinning improves growth and resilience after severe droughts in Quercus subpyrenaica coppice forests in the Spanish Pre-Pyrenees

During the past years, growth and productivity of different oak species have been constrained by water shortage in seasonally dry regions such as the Mediterranean Basin. Thinning could improve oak radial growth in these drought-prone regions through the reduction of tree competition for soil water in summer. However, we still lack adequate, long-term assessments on how lasting are thinning treatments effects and to what extent they contribute to oak growth recovery after drought. Here we aim: (i) to study the radial growth sensitivity to drought of Quercus subpyrenaica in the Spanish Pre-Pyrenees, and (ii) to verify if thinning represents a suitable option to enhance growth resistance to drought and post-drought growth recovery. We analysed basal area increment (BAI) trends in the period 1960–2020 of formerly coppiced oak stands thinned in 1984 and compared them with unthinned plots and also with coexisting Scots pine (Pinus sylvestris) growing in thinned plots. We used the Standardized Precipitation Evapotranspiration Index (SPEI) to estimate the severity of droughts and we also assessed climate-growth relationships. Oaks in thinned plots showed higher BAI (369 mm²) than those in unthinned plots (221 mm²). Growth rates remained higher in thinned than in unthinned plots also under severe drought stress. A severe summer drought in 1986 caused abrupt BAI reductions in both oaks (- 40.5%) and pines (- 40.1%). The positive effect of thinning on growth lasted for over 20 years and slightly declined as canopies closed. In the thinned plots, trees with smaller diameter showed the greatest growth release. Oaks in unthinned plots and Scots pine were more sensitive to short-term droughts in terms of growth reduction than oaks in thinned plots, while long term droughts have similar effects on oaks from both thinned and unthinned plots. Oaks were resilient to drought, showing recovery periods lasting from 1 to 2 years in both thinned and unthinned plots. However, intense and prolonged droughts could strongly reverse the expected growth enhancement of thinned plots, and a greater frequency of droughts would limit coppice growth and productivity thus lengthening the rotation periods.     

Reaching the canopy on the ground: incidence of infection and host-use by mistletoes (Loranthaceae and Viscaceae) on trees felled for timber in Amazonian rainforests

There is a profound absence of knowledge of infestation prevalence and host-use by mistletoes of mature South American tropical rainforests. In this study, we fill this gap using information gathered from felled trees at a logging concession area in Amazonian Brazil. We sampled individuals of 18 tree species, which occurred in two forest physiognomies; open forest with canopy interrupted by palm trees and closed, denser forest, with emergent trees. We hypothesized that infection incidence would be higher in open than in closed forest, irrespective of the mistletoe species involved. In addition, we expected that mistletoe parasitism would be higher on host species that were more abundant, taller, deciduous, and had less dense wood. We sampled 870 individual trees in both sites combined. All but one host species was infected by at least one species of mistletoe. We found 13 mistletoe species/morphospecies, Loranthaceae (7) and Viscaceae (6), parasitizing very different hosts. Mistletoe infection incidence was higher in the closed forest (10.3%) than in the open forest (5.4%). In the closed forest, host height influenced incidence positively, while deciduousness had a negative influence. Our results show that mistletoes are common in the canopy of pristine tropical forests and, contrary to expectations, that infection incidence was higher in the closed forest. The positive relation between infection incidence and host height in this forest type suggests that emergent trees have higher chances of being infected than individuals of correspondent species in the lower forest layers.     

Regional ecosystem structure and function: ecological insights from remote sensing of tropical forests

Ecological studies in tropical forests have long been plagued by difficulties associated with sampling the crowns of large canopy trees and large inaccessible regions, such as the Amazon basin. Recent advances in remote sensing have overcome some of these obstacles, enabling progress towards tackling difficult ecological problems. Breakthroughs have helped transform the dialog between ecology and remote sensing, generating new regional perspectives on key environmental gradients and species assemblages with ecologically relevant measures such as canopy nutrient and moisture content, crown area, leaf-level drought responses, woody tissue and surface litter abundance, phenological patterns, and land-cover transitions. Issues that we address here include forest response to altered precipitation regimes, regional disturbance and land-use patterns, invasive species and landscape carbon balance.     

山西忻州地区1900-2012年典型森林的健康历史

健康的森林对维持其区域生态系统服务起着至关重要的作用,了解树木的生长历史对评估气候变化背景下森林的健康状况具有促进作用。选择山西高原中北部忻州地区保存较好的4个森林为研究对象,利用树木年轮学方法分析其生长变化特征,获得了该地区森林健康的时间和空间特征。结果表明：该地区森林在过去1个世纪中,存在3次不健康事件（1910-1940、1970-1987和1990-2012年）且在空间上表现出明显的同步性,不同时期的不健康事件持续时间和强度不尽相同,1930s的不健康事件持续时间最长也最为显着。树木径向生长与气象观测资料的相关分析显示,该地区森林生长主要与当年5-6月份温度呈负相关,与7月份降水和5-9月PDSI指数显着正相关。生长季持续的高温或降水减少造成的极端干旱事件是不健康事件空间一致性的主要驱动力,各采样点树种以及林分组成差异是影响时间特征不一致的可能原因。研究结果提供的森林健康历史数据对评估极端气候条件对森林健康生长的影响及制定合理的森林保护措施具有积极的现实意义。     

Composition and Dynamics of Functional Groups of Trees During Tropical Forest Succession in Northeastern Costa Rica

We compared the functional type composition of trees ≥10 cm dbh in eight secondary forest monitoring plots with logged and unlogged mature forest plots in lowland wet forests of Northeastern Costa Rica. Five plant functional types were delimited based on diameter growth rates and canopy height of 293 tree species. Mature forests had significantly higher relative abundance of understory trees and slow-growing canopy/emergent trees, but lower relative abundance of fast-growing canopy/emergent trees than secondary forests. Fast-growing subcanopy and canopy trees reached peak densities early in succession. Density of fast-growing canopy/emergent trees increased during the first 20 yr of succession, whereas basal area continued to increase beyond 40 yr. We also assigned canopy tree species to one of three colonization groups, based on the presence of seedlings, saplings, and trees in four secondary forest plots. Among 93 species evaluated, 68 percent were classified as regenerating pioneers (both trees and regeneration present), whereas only 6 percent were classified as nonregenerating pioneers (trees only) and 26 percent as forest colonizers (regeneration only). Slow-growing trees composed 72 percent of the seedling and sapling regeneration for forest colonizers, whereas fast-growing trees composed 63 percent of the seedlings and saplings of regenerating pioneers. Tree stature and growth rates capture much of the functional variation that appears to drive successional dynamics. Results further suggest strong linkages between functional types defined based on adult height and growth rates of large trees and abundance of seedling and sapling regeneration during secondary succession.
Abstract in Spanish is available at http://www.blackwell-synergy.com/loi/btp     

Drought-induced mortality affects understory vegetation: release after death

In recent decades, severe droughts have become an important cause of canopy disturbance in forests, and have shown potential to cause rapid and pronounced vegetation shifts. Under dead canopy, undamaged understory could influence the nature of resource limitation for seedling growth and survival, limiting forest regeneration. We assessed the release response of understory vegetation after a severe drought event in temperate forests of northern Patagonia. Growth trends of dominant tree saplings, and changes in vegetation biodiversity and cover were compared between drought-dead and unaffected canopy. Nothofagus dombeyi undergo growth release after the climatic event in affected forests, and the response was evidenced immediately after the disturbance. For Austrocedrus chilensis, the growth release response was less evident, due mainly to a difference in age structure. In the understory the release response was barely discernable for some components. There was a tendency towards higher cover of the shrub layer in the understory of drought-affected forests, and an important presence of the exotic shrub Rosa rubiginosa. However, the clearest biotic response following drought mortality was the release in growth of understory dominant tree component. Those results strongly suggest that the environment under drought-dead canopy, and the die-off in woody sapling cohorts in a self-thinning process, could favor crown expansion and growth release of understory species that could help predict future forest trajectories in the context of the influence of climatic extreme events.     

Factors limiting the distribution of deciduous broadleaved trees in warm-temperate mountainous riparian forests

We examined differences in the responses of deciduous and evergreen broadleaved trees to fluvial disturbance and light environment near a river-facing forest edge in order to identify the factors limiting the distribution of deciduous trees, which are important components of warm-temperate mountainous riparian forests. Deciduous trees tended to be distributed on lower terraces of less than 2-m relative elevation from the water level, which corresponded with sites suffering from strong bank erosion due to high flood frequency. On the other hand, evergreen trees showed an opposite trend of distribution, indicating that high flood frequency associated with strong erosion-dominated soil disturbances might be a constraint for their establishment and/or survival. Furthermore, crowns of deciduous trees tended to be in the canopy layer at the forest edge, with fewer individuals observed beneath the canopy of evergreen trees, even at the forest edge. In contrast, evergreen trees were concentrated in the understory of the forest interior. These observations suggested that the light environment is the predominant factor affecting the establishment and survival of deciduous trees in warm-temperate mountainous riparian forests; however, the river-facing forest edge beneath the evergreen canopy is not a suitable habitat for deciduous trees. We concluded that erosion-dominated soil disturbance on lower terraces provides suitable habitats for deciduous trees by facilitating their reaching the canopy layer by limiting the establishment and/or survival of evergreen trees.

     

Impact of invasion of Acer platanoides on canopy structure and understory seedling growth in a hardwood forest in North America

Invasion by exotic plant species is known to affect native communities and ecosystems, but the mechanisms of the impacts are much less understood. In a field study, we examined the effects of a tree invader, Acer platanoides (Norway maple, NM), on canopy structure and seedling growth in the understory of a North American deciduous forest. The experimental site contains a monospecific patch of A. platanoides and a mixed patch of A. platanoides with its native congener, A. rubrum (red maple, RM). In the study, we examined canopy characteristics of three types of trees in the forests, i.e., RM trees in the mixed forest, NM trees in the mixed forest, and NM trees in its monospecific patch. Height growth and biomass production of RM and NM seedlings under intact canopies and newly created gaps of the three types of trees were followed for two growing seasons. We found that removal of half of the canopy from focal trees increased canopy openness and light transmission to the forest floor, but to a greater extent under NM trees than under RM trees. Seedlings of these two Acer species varied greatly in biomass production under canopies of the same type of trees and in their responses to canopy opening. For example, seedlings of the exotic NM grown under the native RM trees in the mixed forests increased biomass production by 102.4% compared to NM seedlings grown under conspecific trees. The native RM seedlings grown under NM trees, however, reduced biomass production by 23.5% compared to those grown under conspecific trees. It was also observed that RM was much more responsive in biomass production to canopy opening than NM. For instance, total seedling biomass increased by 632.2% in RM, but by only 134.6% in NM in response to the newly created gaps. In addition, we found that NM seedlings allocated a greater portion of biomass below-ground as canopy openness increased, whereas the same trend was not observed in RM seedlings. Our results thus demonstrated that invasion of NM significantly altered canopy structure and community dynamics in the hardwood forest. Because the exotic NM seedlings are able to grow well under the native RM trees, but not vice versa, NM will likely expand its distribution in the forests and make it an ever increasingly serious tree invader in its non-native habitats, including North America.     

Overstorey–Understorey Interactions Intensify After Drought-Induced Forest Die-Off: Long-Term Effects for Forest Structure and Composition

Severe drought events increasingly affect forests worldwide, but little is known about their long-term effects at the ecosystem level. Competition between trees and herbs (‘overstorey–understorey competition’) for soil water can reduce tree growth and regeneration success and may thereby alter forest structure and composition. However, these effects are typically ignored in modelling studies. To test the long-term impact of water competition by the herbaceous understorey on forest dynamics, we incorporated this process in the dynamic forest landscape model LandClim. Simulations were performed both with and without understorey under current and future climate scenarios (RCP4.5 and RCP8.5) in a drought-prone inner-Alpine valley in Switzerland. Under current climate, herbaceous understorey reduced tree regeneration biomass by up to 51%, particularly in drought-prone landscape positions (i.e., south-facing, low-elevation slopes), where it also caused a shift in forest composition towards drought-tolerant tree species (for example, Quercus pubescens). For adult trees, the understorey had a minor effect on growth. Under future climate change scenarios, increasing drought frequency and intensity resulted in large-scale mortality of canopy trees, which intensified the competitive interaction between the understorey and tree regeneration. At the driest landscape positions, a complete exclusion of tree regeneration and a shift towards an open, savannah-like vegetation occurred. Overall, our results demonstrate that water competition by the herbaceous understorey can cause long-lasting legacy effects on forest structure and composition across drought-prone landscapes, by affecting the vulnerable recruitment phase. Ignoring herbaceous vegetation may thus lead to a strong underestimation of future drought impacts on forests.