Seedbed and moisture availability determine safe sites for early Thuja occidentalis (Cupressaceae) regeneration

Regeneration of many late-successional tree species depends on specialized safe sites. The primary objective was to investigate the roles of seedbed and moisture retention as dimensions of safe sites for the early regeneration of drought-sensitive northern white cedar (Thuja occidentalis). We hypothesized that rates of germination, survival, and growth of T. occidentalis are unlikely to differ among seedbed types under conditions of abundant water, but that differences are likely to emerge as water becomes more limited. In a 67-d greenhouse experiment, cedar seeds were sown on logs, leaf litter, and soil of cedar and paper birch (Betula papyrifera) canopy origin. Seedbeds were subjected to three water treatments. Among the water treatments, highest germination rates occurred within the high water treatment, although germination on cedar litter was comparable to that of the low water treatment. Higher germination and survival rates occurred on decayed logs than other natural seedbeds for medium (P = 0.001) and low (P < 0.0001) water treatments. Germination on birch logs occurred at higher rates than on cedar logs within the low water treatment (P = 0.04). Seedling growth for the medium water treatment was lower on leaf litter than any other type of seedbed (P < 0.01). Results generally demonstrated that the interplay between seedbed and moisture retention is a component of safe sites for T. occidentalis regeneration.     

Feathermoss seedbeds facilitate black spruce seedling recruitment in the forest–tundra ecotone (Labrador,Canada)

As climate warms, conifers are expected to expand their ranges into alpine tundra where ecological factors such as seedbed availability, and post‐dispersal seed and seedling predation may control local recruitment. Seedbed composition may influence microhabitat, nutrients, physical structure, and predation level and, therefore, affect the success of conifer recruitment, thereby providing the template for future expansion. In the boreal forest, seedbed–seedling competition dominates such that seedbed removal increases black spruce recruitment. In the harsher climate of the Mealy Mountains boreal forest–tundra ecotone (Labrador, Canada) the Stress gradient hypothesis (SGH) predicts that facilitation may dominate seedbed–seedling interactions. This study investigated potential mechanisms of seedbed facilitation (temperature, water, nutrients, physical protection) in three conifer seedbeds (Pleurozium schreberi, Cladonia spp., bare soil) and examined whether seed predation and/or seedling herbivory varied among seedbeds over three years. Seed emergence was low overall (< 10% on all treatments), but highest on Pleurozium (6.3%), followed by bare ground (4.6%) and Cladonia (0.3%). Facilitation was observed between Pleurozium and black spruce as seedling height increase (31%) and survival (55%) were highest; herbivory, seed predation and overwinter mortality were lowest compared to both Cladonia and bare ground seedbeds. Unlike in the closed canopy boreal forest, seedlings recruited poorly on bare soil as seedling height increase and survival were 20.5% and 26%. Temperature and water availability were similar across seedbeds, while nutrient availability was higher on Pleurozium. The physical structure of Pleurozium likely protects first to third‐year seedlings from temperature extremes and predators. As climate warms and seed availability increases, Pleurozium may facilitate black spruce recruitment and treeline expansion.     

Gap formation, microsite variation and the conifer seedling occurrence in a subalpine old-growth forest, central Japan

To evaluate the effects of canopy gaps and forest floor microsites (soil, fallen logs, root-mounds, buttresses and stumps) on regeneration of subalpine forests, the gap regeneration and seedling occurrence of conifers (Abies mariesii, Abies veitchii, Picea jezoensis var. hondoensis and Tsuga diversifolia) were studied in two stands of a subalpine old-growth forest, central Japan. The percentage of gap area to total surveyed area was 11.2–11.3% in the stands. Gap regeneration was not common for P. jezoensis var. hondoensis and T. diversifolia. In contrast, gap regeneration by advanced regeneration was relatively common for Abies. Seedling occurrence of P. jezoensis var. hondoensis and T. diversifolia was restricted on elevated surfaces such as stumps and root-mounds, while Abies seedlings could occur on soil as well as on elevated surfaces. Rotten stumps were the most favorable microsites for conifer seedling occurrence, which covered small area in the forest floor. Although canopy gaps were not always favorable for seedling occurrence, all conifer seedlings were larger under canopy gaps than under closed canopy. Canopy gaps and forest floor microsites clearly affected seedling occurrence and growth of conifers. This suggests that regeneration of conifers is related to the difference of growth advantage under canopy gaps and favorable microsites for seedling occurrence.     

Conifer seedling recruitment in a southeastern Canadian boreal forest: the importance of substrate

Abstract. In order to explain conifer species recruitment in Canada's southeastern boreal forest, we characterized conifer regeneration microsites and determined how these microsites vary in abundance during succession. Microsite abundance was evaluated in deciduous, mixed and coniferous stands along a 234-yr postfire chronosequence. Conifers were most often found in relatively well-illuminated microsites, devoid of litter, especially broad-leaf litter, and with a reduced cover of lower vegetation (< 50 cm tall). Although associated with moss-rich forest floor substrates, Abies balsamea was the most ubiquitously distributed species. Picea glauca and especially Thuja occidentalis seedlings were frequently found on rotten logs. Light measurements did not show differences among seedling species nor between stand types. The percentage cover of broad-leaf litter decreased significantly during succession. Also, rotten logs covered with moss occupied a significantly larger area in the mid-successionnal stands than in early successional deciduous or late successional coniferous stands. The results suggest that the presence of specific forest floor substrate types is a factor explaining low conifer recruitment under deciduous stands, conifer codominance in the mid-successional stage, and delayed Thuja recolonization after fire. Results also suggest that some facilitation mechanism is responsible for the observed directional succession.     

Tree regeneration after bamboo die-back in Chinese Abies-Betula forests

Abstract. Gaps created by disturbance in the forest canopy are important sites for tree regeneration from seed but plants already established in gaps may slow gap-filling. This study deals with consequences of bamboo die-back for tree regeneration and the dynamics of Abies-Betula forests in southwest China. Bamboo dominates the forest understory impeding tree regeneration when in its vegetative phase. Populations of tree seedlings were sampled in 1984–85 and 1990 in two sets of permanent plots where bamboo had died back in 1983. Both Abies and Betula density increased after bamboo die-back, Betula more so than Abies, especially in gaps. Before bamboo die-back, seedlings were established on raised surfaces such as logs but afterwards seedlings became common on the forest floor. This reduced the intensity of clumping of seedling populations between 1984 and 1990. A tree by tree replacement model predicts an increase in Abies and a decrease in Betula after bamboo die-back. Life histories of tree species, gap characteristics, and the bamboo growth cycle (mature/die-back/building) interact to promote fluctuating dominance of Abies and Betula in old-growth forests.     

Seedling establishment in relation to microhabitat variation in a windthrow gap in a boreal Pinus sylvestris forest

Abstract. The characteristics of microhabitats of established Pinus sylvestris and Betula seedlings were studied in a small windthrow gap in a mature P. sylvestris-dominated forest in the Petkeljärvi National Park in eastern Finland. Seedlings were strongly clustered in disturbed microhabitats, particularly uprooting pits and mounds, formed by tree falls. They covered 3% of the 0.3.ha study area consisting of the gap and some of the forest edge. Although Betula occurred only as scattered individuals in the dominant canopy layer of the forest, it accounted for 30% of the seedlings found in the study area. Betula regeneration was almost completely restricted to pits and mounds, where 91% of the seedlings were found. Uprooting spots were also the most important regeneration microhabitats for Pinus, where 60% of the seedlings grew, even though the seedlings were found in other substrates as well, particularly on sufficiently decomposed coarse wood. Undisturbed field- and bottom-layer vegetation had effectively hindered tree seedling establishment, which emphasises the role of soil disturbance for regeneration. While the establishment of seedlings was found to be clearly determined by the availability of favourable regeneration microhabitats, the early growth of seedlings was affected by a complex interaction of environmental variables, including the type of microhabitat, radiation environment and interferences caused by competing seedlings and adjacent trees. In the most important regeneration microhabitats, i.e. in uprooting pits and on mounds, the distributions of the local elevations of Pinus and Betula seedlings were different. Pinus seedlings occurred closer to ground level, i.e. on the fringes of pits and lower on mounds, while Betula seedlings grew deeper in pits and higher on mounds. The position of the Betula seedlings indicate that they may have a competitive advantage over Pinus seedlings in the dense seedling groups occurring in uprooting spots. We suggest that this initial difference in Pinus and Betula establishment may affect the subsequent within-gap tree species succession and can, in part, explain the general occurrence of Betula in conifer-dominated boreal forests.     

State of fallen logs and the occurrence of conifer seedlings and saplings in boreal and subalpine old-growth forests in Japan

Fallen logs on the forest floor play an important role in tree seedling establishment in boreal and subalpine old-growth forests. We examined the abundance and state (degree of decay, moss coverage on their surfaces) of fallen logs on the forest floor and compared the occurrence of conifer seedlings and saplings on soil and fallen logs in old-growth evergreen boreal (Taisetsu) and subalpine (Yatsugatake) coniferous forest stands in Japan. The forest floors of both stands were covered mostly by soil, and fallen logs covered only a small proportion (4.5% in Taisetsu and 2.5% in Yatsugatake) of the forest floor. In both stands, no seedlings were found on freshly fallen logs. Both moss coverage and the occurrence of seedlings or saplings on fallen logs increased with decay in both stands. In Taisetsu, the occurrence of all conifer seedlings and saplings was, in general, restricted to fallen logs. In contrast, in Yatsugatake, Picea and Tsuga seedlings occurred much more frequently on fallen logs than on soil, whereas Abies seedlings and saplings became established on both soil and fallen logs. Seedlings were taller on decayed fallen logs, but taller saplings were rare on fallen logs in Yatsugatake. Results indicate that species characteristics, the condition of the fallen logs and environmental factors determine the preponderance of seedling and sapling establishment on fallen logs. The condition of fallen logs varies with the degree of decay, and fallen logs that function as seedling establishment sites on the forest floor are thought to be limited by time and space.     

Feedbacks between canopy composition and seedling regeneration in mixed conifer broad‐leaved forests

To address the role of canopy‐seedling feedbacks in the structure and dynamics of mixed conifer broad‐leaved forests in the eastern US, we monitored seedling regeneration patterns and environmental conditions in the understorey of stands dominated by either hemlock (Tsuga canadensis) or red oak (Quercus rubra) for three years. Hemlock seedlings were favoured over other species’ seedlings in hemlock stands (a true positive feedback), due to a combination of high seed inputs, high seedling emergence and relatively high seedling survival during the growing season, which allowed hemlock to remain dominant under its own canopy. Red oak stands favoured a suite of mid‐successional broad‐leaved species over hemlock. A more even age structure of broad‐leaved species in red oak stands revealed that high seedling survival in such stands were driving this feedback. Canopy‐mediated variations in both understorey light availability (1.5% for hemlock vs 3.5% for red oak) and soil pH (3.9 for hemlock vs 4.4 for red oak) were found to be the primary correlates of stand‐level differences in seedling regeneration dynamics. In mixed temperate forests in the eastern US, canopy‐seedling feedbacks could act to slow successional trajectories and contribute to the maintenance of a stable landscape structure over many generations.     

Stand structure and regeneration in a Kamchatka mixed boreal forest

Abstract. A 1‐ha plot was established in a Betula platyphylla‐Picea ajanensis mixed boreal forest in the central Kamchatka peninsula in Russia to investigate stand structure and regeneration. This forest was relatively sparse; total density and stand basal area were 1071/ha and 25.8 m²/ha, respectively, for trees > 2.0 cm in trunk diameter at breast height (DBH). 25% of Betula regenerated by sprouting, and its frequency distribution of DBH had a reverse J‐shaped pattern. In contrast, Picea had a bimodal distribution. The growth rates of both species were high, reaching 20 m in ca. 120 yr. The two species had clumped distributions, especially for saplings. Betula saplings were not distributed in canopy gaps. Small Picea saplings were distributed irrespective of the presence/absence of gaps, while larger saplings aggregated in gaps. At the examined spatial scales (6.25–400 m²) the spatial distribution of Betula saplings was positively correlated with living Betula canopy trees and negatively with dead Picea canopy trees. This suggests that Betula saplings regenerated under the crowns of Betula canopy trees and did not invade the gaps created by Picea canopy trees. The spatial distribution of Picea saplings was negatively correlated with living and dead Betula canopy trees and positively with dead Picea canopy trees. Most small Picea seedlings were distributed under the crowns of Picea trees but not under the crowns of Betula trees or in gaps. This suggests that Picea seedlings establish under the crowns of Picea canopy trees and can grow to large sizes after the death of overhead Picea canopy trees. Evidence of competitive exclusion between the two species was not found. At a 20 m × 20 m scale both skewness and the coefficient of variation of DBH frequency distribution of Picea decreased with an increase in total basal area of Picea while those of Betula were unchanged irrespective of the increase in total basal area of Betula. This indicates that the size structure of Picea is more variable with stand development than that of Betula on a small scale. This study suggests that Betula regenerates continuously by sprouting and Picea regenerates discontinuously after gap formation and that the species do not exclude each other.     

Effects of forest dieback on wood decay,saproxylic communities,and spruce seedling regeneration on coarse woody debris

Picea is one of the most dominant conifer genera in the Northern Hemisphere and includes species which require coarse woody debris (CWD) as a seedbed for regeneration. To understand the future of forest distribution under global climate change, it is important to investigate regeneration mechanisms in Picea forests on the borders of its distribution. In the present study, we evaluated the biotic factors affecting the establishment of Picea jezoensis var. hondoensis seedlings on CWD in one of its southernmost populations in central Japan, where there is dieback of Picea forest. Amplicon sequencing of the fungal ITS1 region of rDNA obtained from wood samples showed that forest dieback increased the frequency of brown rot fungi in CWD. The frequency of brown-rotted wood, in which wood holocellulose is decayed, increased with dieback intensity. The domination of brown-rotted wood in dieback forests was negatively associated with bryophyte cover which was positively associated with Picea seedling density. Forest dieback itself also had other strong negative effects on bryophytes. Thus, linkages between dead wood and spruce seedlings via bryophytes had collapsed after the dieback event, which may partly be a reason that the spruce forest shifted to and is staying as open grassland.     

Regeneration and coexistence of two Abies species dominating subalpine forests in central Japan

Summary The mechanism of coexistence of the dominant firs Abies veitchii and A. mariesii is described in relation to regeneration patterns for climax subalpine forests of the northern Yatsugatake Mountains, central Honshu, Japan. Two mature stand types, pure conifer stands of Abies spp., and mixed stands of Abies spp. and hardwoods (mainly the birch Betula ermanii), are distinguished. Pure stands are likely to show simultaneous decay, followed by evenaged regeneration of stand-floor seedlings (<20 cm tall), Rapidly growing A. veitchii dominates over A. mariesii in this type of regeneration, which is occasionally invaded by light-demanding Betula. In constrast, mixed stands degenerate rather slowly, followed by the regeneration of Abies from the bank of suppressed saplings (>20 cm tall), which persist only in mixed stands. The more shade-tolerant A. mariesii is supeior in this type of regeneration, while Betula does not succeed, and mixed stands change to pure stands with time. The fact that two patterns of Abies regeneration occur in a certain ratio in the forest is what enables the two Abies species to coexist. A simple dynamical system model supports this conclusion.     

Legacy microsite effect on the survival of bitterbrush outplantings after prescribed fire: capitalizing on spatial variability to improve restoration

Restoration of shrubs in arid and semi‐arid rangelands is hampered by low success rates. Planting shrub seedlings is a method used to improve success in these rangelands; however, it is expensive and labor intensive. The efficiency of shrub restoration could be improved by identifying microsites where shrub survival is greater. Bitterbrush (Purshia tridentata Pursh DC) is an important shrub to wildlife that has declined because of conifer encroachment, excessive defoliation, wildfires, and low recruitment. We investigated planting bitterbrush seedlings in western juniper (Juniperus occidentalis ssp. occidentalis Hook) encroached shrublands after prescribed fire was used to control trees. Bitterbrush seedlings were planted in under (canopy) and between (interspace) former juniper canopies at five blocks and evaluated for three growing seasons. Bitterbrush survival was greater than 50% in the former canopy, but only 5% in the interspace microsite by the third growing season. Growth of bitterbrush was also greater in the former canopy compared with the interspace, potentially due to markedly less perennial vegetation in this microsite. Exotic annual grasses and annual forbs became prevalent in the former canopy in the second and third growing season, suggesting that soil resource availability was greater in this microsite. These results suggest that restoration success will vary by specific locations within a burned landscape and that this variability can be used to improve restoration efficiency. In this situation, bitterbrush restoration can be improved by planting seedlings in former canopy compared with interspace microsites.     

Effects of overstory and understory vegetation on the understory light environment in mixed boreal forests

Abstract. The percentage of above-canopy Photosynthetic Photon Flux Density (%PPFD) was measured at 0, 50 and 100 cm above the forest floor and above the main understory vegetation in stands of (1) pure Betula papyrifera (White birch), (2) pure Populus tremuloides (Trembling aspen), (3) mixed broad-leaf-conifer, (4) shade-tolerant conifer and (5) pure Pinus banksiana (Jack pine) occurring on both clay and till soil types. %PPFD was measured instantaneously under overcast sky conditions (nine locations within each of 29 stands) and continuously for a full day under clear sky conditions (five locations within each of eight stands). The percentage cover of the understory layer was estimated at the same locations as light measurements. Mean %PPFD varied from 2% at the forest floor under Populus forests to 15% above the understory vegetation cover under Betula forests. Percent PPFD above the understory vegetation cover was significantly higher under shade intolerant tree species such as Populus, Betula and Pinus than under shade tolerant conifers. No significant differences were found in %PPFD above the understory vegetation cover under similar tree species between clay and till soil types. The coefficient of variation in %PPFD measured in the nine locations within each stand was significantly lower under deciduous dominated forests (mean of 19%) than under coniferous dominated forests (mean of 40%). %PPFD measured at the forest floor was positively correlated with %PPFD measured above the understory vegetation and negatively correlated with cumulative total percent cover of the understory vegetation (R² = 0.852). The proportion of sunflecks above 250 and 500 mmol m^–2 s^–1 was much lower and %PPFD in shade much higher under Populus and Betula forests than under the other forests. Differences in the mean, variability and nature of the light environment found among forest and soil types are discussed in relation to their possible influences on tree succession.     

Tree replacement patterns in subalpine Abies-Betula forests,Wolong Natural Reserve,China

Vegetation in canopy gaps of two old-growth Abies-Betula forest stands, one with bamboo the other without, was measured. The structure of gap vegetation at each site was used to derive tree replacement probabilities. Transition probabilities indicate different tree replacement trends in forests with bamboo compared to those without. Projected compositions show Betula to be the most abundant species in bamboo stands while Abies remains most abundant where bamboo was absent. A dense bamboo sward seems to reduce the probability of Abies filling gaps by inhibiting establishment and growth of seedlings. Bamboo preempts space after canopy gap formation by increasing shoot production which reduces opportunities for establishment and growth of other woody species. Differences in dispersal ability and longevity of Abies and Betula appear to be important factors contributing to their coexistence forests with a small canopy gap disturbance regime.     

Exposure to herbivores increases seedling growth and survival of American chestnut (Castanea dentata) through decreased interspecific competition in canopy gaps

The successful development of early stages of blight‐resistant hybrid stock has increased hopes for restoration of American chestnut (Castanea dentata) to eastern North American forests. However, these forests have undergone substantial ecological change in the century since the functional extirpation of American chestnut, and it remains unknown to what extent American chestnut will be able to recolonize contemporary forests. In particular, high densities of white‐tailed deer (Odocoileus virginianus) and competition with mesophytic tree species such as maple (Acer) may impede chestnut regeneration, much as they affect oak (Quercus). We used a split‐plot analysis of variance (ANOVA) design to examine the effects of canopy gaps and herbivory on survival and growth of third generation backcrossed (BC₃) hybrid chestnut seedlings over two growing seasons in central Indiana, U.S.A. Only 4 of 588 (0.7%) seedlings in closed‐canopy plots survived to the end of the study, as opposed to 264 of 589 (45%) seedlings in gap plots. Within the gap treatment, fencing was associated with reduced chestnut survival as well as reduced herbivory and increased cover of non‐chestnut vegetation. Our results indicate that herbivory may indirectly benefit chestnut regeneration by suppressing competition. However, this beneficial effect is likely context‐dependent and additional work is needed to establish the conditions under which it occurs.     

Conifer Regeneration Problems in Boreal and Temperate Forests with Ericaceous Understory: Role of Disturbance,Seedbed Limitation,and Keytsone Species Change

Conifer regeneration failure in the presence of dense ericaceous cover resulting from the removal of canopy trees by forest harvesting observed in boreal and temperate forest has been attributed to allelopathy, competition, and soil nutrient imbalance. Ecosystem-level alleopathic effect has been argued as a cause for conifer regeneration failure by citing examples from a species-poor boreal forest in northern Sweden with ground vegetation dominated by crowberry (Empetrum hermaphroditum, Ericales) and New Zealand dairy pastures invaded by nodding or musk thistle (Carduus nutans). This article aims to explain the phenomenon of vegetation shift from conifer forest to ericaceous heath by extending the argument of ecosystem-level impact of ericaceous plants and linking the disturbance-mediated regeneration strategies of the dominant conifer species and the understory ericaceous species with the quality of seedbed substrate that influence the direction of secondary succession. It has been argued that fire severity plays a pivotal role in controlling seedbed quality and the regeneration mechanisms of conifers, which in turn determines the direction of post-disturbance succession. The post-fire-dominated ericaceous plants and their habitat-modifying effects have been explained from the point of view of keystone species concept and their role as ecosystem engineers. In the absence of high severity natural fires the canopy keystone species (conifer) fails to regenerate successfully mainly due to limitation of favorable seedbed. On the other hand, the understory ericaceous plants regenerate vigorously by vegetative methods from the belowground components that survived the fire. Forest harvesting by clearcutting or selective cutting also create similar vigorous vegetative regrowth of ericaceous plants, but conifer regeneration suffers from the lack of a suitable seedbed. Thus in the absence of successful conifer regeneration, the vigorously growing understory ericaceous plants become the new keystone species. The new keystone ericaceous species bring about a significant long-term habitat change by rapid accumulation of plyphenol-rich humus. Ericaceous phenolic compounds have been found to inhibit seed germination and seedling growth of conifers. By forming protein-phenol complexes they cause a further reduction of available nitrogen of the already nutrient-stressed habitat. A low pH condition in the presence of phenolic compounds causes the leaching of metallic ions and forms hard iron pans that impair soil water movement. The phenolic allelochemicals of ericaceous humus are also inhibitory to many conifer ectomycorrhizae. On the other hand, ericaceous plants perpetuate in the community by their stress-tolerating strategies as well as their ability to acquire nutrients through ericoid mycorrhizae. Three mechanisms working at the ecosystem level can be suggested as the cause of vegetation shift from forest to ericaceous heath. These are (1) the absence of high severity natural fire and the limitation of suitable conifer seedbed in the presence of thick humus, (2) increased competition resulting from the rapid vegetative regeneration of understory ericaceous plants after forest canopy opening by harvesting or nonsevere fire, and (3) habitat degradation by phenolic allelochemicals of ericaceous plants causing a soil nutrient imbalance and iron pan formation. Thus, a shift in keystone species from conifer to ericaceous plant in the post-disturbance habitat may induce a retrogressive succession due to ecosystem-level engineering effects of the new keystone species. Vegetation management in conifer-ericaceous communities depends on land management objectives. If the objective is to produce timber and other forest products then the control of ericaceous plants and site preparation is necessary after forest harvesting. Ploughing and liming followed by conifer planting and repeated N fertilization has been applied successfully to promote afforestation of Calluna heathlands in Britain. However, such practice has not been proven successful in the reforestation of Kalmia-dominated sites in eastern Canada. If, on the other hand, the land management objective is to maintain heathlands for herbivore production or conservation of cultural landscape, as in the case of certain Calluna-dominated heathland in Western Europe, then moderately hot prescribed burning is useful as a management tool.     

Spatial dynamics of regeneration in a conifer/broad‐leaved forest in northern Japan

Abstract. This study deals with stand dynamics over a 6‐yr period in a conifer/broad‐leaved mixed forest in Hokkaido, northern Japan. The annual rates of gap formation and recovery were 81.3 m²/ha and 66.7 m²/ha, respectively and turnover time of the canopy was 125 yr. The recruitment processes of the component species in this cool‐temperate forest were governed by different canopy types: gap, canopy edge and closed canopy. Magnolia obovata regenerated in canopy edges, and Acer mono and Prunus ssiori regenerated in canopy edges and gaps. The results suggested that the mosaic structure made up of closed canopy, canopy edge and gap was related to various regeneration niches. Abies sachalinensis had high mortality rates, initiating gap expansion. The transition probabilities from closed canopy or canopy edge to gap for deciduous broad‐leaved trees were lower than for A. sachalinensis, which implies that the difference in degeneration patterns of conifer and broad‐leaved canopies contributes to the heterogeneity of spatial structure in the mixed forests. Spatial dynamics were determined by a combination of gap expansion by A. sachalinensis (neighbour‐dependent disturbance) and gap formation by deciduous broad‐leaved trees (random disturbance).     

Changes in woody vegetation abundance and diversity after natural disturbances causing different levels of mortality

Questions: How does woody vegetation abundance and diversity differ after natural disturbances causing different levels of mortality? Location: Abies balsamea–Betula papyrifera boreal mixed‐wood stands of southeast Quebec, Canada. Methods: Woody vegetation abundance and diversity were quantified and compared among three disturbance‐caused mortality classes, canopy gap, moderate‐severity disturbances, and catastrophic fire, using redundancy analysis, a constrained linear ordination technique, and diversity indices. Results: Substantial changes in canopy tree species abundance and diversity only occurred after catastrophic fire. Shade‐tolerant, late‐successional conifer species remained dominant after canopy gap and moderate‐severity disturbances, whereas shade‐intolerant, early‐successional colonizers dominated canopy tree regeneration after catastrophic fire. Density and diversity of mid‐tolerant and shade‐intolerant understory tree and shrub species increased as the impact of disturbance increased. Highest species richness estimates were observed after catastrophic fire, with several species establishing exclusively under these conditions. Relative abundance of canopy tree regeneration was most similar after canopy gap and moderate‐severity disturbances. For the sub‐canopy tree and shrub community, relative species abundances were most similar after moderate‐severity disturbances and catastrophic fire. Vegetation responses to moderate‐severity disturbances thus had commonalities with both extremes of the disturbance‐caused mortality gradient, but for different regeneration layers. Conclusions: Current spatio‐temporal parameters of natural disturbances causing varying degrees of mortality promote the development of a complex, multi‐cohort forest condition throughout the landscape. The projected increase in time intervals between catastrophic fires may lead to reduced diversity within the system.     

Regeneration dynamics of Tsuga canadensis in the southern Appalachian Mountains, USA

Tsuga canadensis is often considered a foundation species because of its influence on the biological and ecological processes of upland forest systems. With hemlock woolly adelgid (Adelges tsugae; HWA)-induced mortality of T. canadensis in southern Appalachian upland forests, there is a crucial need to understand the regeneration dynamics of this tree species. This research was conducted within mature T. canadensis forests of Great Smoky Mountains National Park (GSMNP) prior to widespread HWA infestation. Tsuga canadensis regeneration abundance is lower than has been reported in more northern locales, but varies widely among sites in the southern Appalachian Mountains. Ordination results suggest landscape-scale patterns of regeneration abundance that are influenced by gradients in understory Rhododendron maximum cover, soil sand and nutrient content. Regression models reinforce the observed gradients in regeneration abundance. Sites with abundant T. canadensis regeneration have less R. maximum cover, soil sand and litter depth, but more soil nutrient availability. Tsuga canadensis seedlings and saplings tend to be spatially aggregated at the 14 × 21 m plot scale. Sites with spatially aggregated seedlings and saplings have greater canopy openness and more species with gap affinity. This research elucidates associations between successful T. canadensis regeneration and site factors such as soil nutrient availability and canopy dynamics. More forest surveys are required to understand the conditions necessary for successful T. canadensis regeneration. Land managers involved in forest restoration efforts may have to utilize appropriate silvicultural techniques conjoined with R. maximum removal operations to regenerate T. canadensis in southern Appalachian upland forests.     

The role of protective understorey in the regeneration system of a heavily browsed woodland

The paper tests an apparent relationship between young understorey of Ilex aquifolium and the presence of seedlings of the canopy species Fagus sylvatica and Quercus species, in heavily browsed woodland in the New Forest, southern England. A small block of woodland was searched and mapped, and the information collected subject to graphical and statistical analyses.Regeneration, though of low frequency, is present and shows non-random patterns of distribution. Older seedlings appear to be located in or near areas of understorey, or near fallen branches, logs, etc. Principal components analysis of woodland characteristics suggests six main habitat types are available to seedlings, and predictions are made about likely regeneration patterns associated with these habitats, according to a model based on the hypothesis of a protective understorey and heavy browsing pressure. Chi-square tests of actual seedling occurrences versus habitat types demonstrate marked associations of regeneration with non-trampled areas, including young Ilex understorey but also other forms of protective barrier. The association is more marked for larger seedlings.The implications for a wider understanding of regeneration processes and community dynamics in heavily browsed forest are discussed. It is concluded that regeneration theories appropriate to such forests are required, since the dominant gap-phase theories are not successful in predicting regeneration patterns when browsing is a major factor.