Creation Of Canopy Gaps Research Articles

Changes in the vegetation of clear-fells and closed canopy stands in an English oak wood over a 30-year period

The vascular plant ground flora of clear-felled and undisturbed oak stands were studied using 10 m × 10 m temporary and permanent plots over a 30-year period in a wood in southern England. Within the undisturbed stands, the mean species richness declined from 10 to 15 species per plot in 1981 to about five in 2012. The decline was correlated with a decrease in openness of the tree and shrub layer over this period. In the clear-fells, the species richness in the first two growing seasons after felling was 1.5 to 3 times higher than in the undisturbed stands. Ruderal species and grasses in particular increased, but some ancient woodland indicators (AWI) were present, even after a decade of openness. Change in the openness of stands appears to be a strong driver of ground flora richness up to about 50% cover; in more open conditions, competition and other processes within the ground flora assemblage may come to play a greater part. Forest management has played a key role in creating canopy gaps in small ancient woods in Britain but, in future, tree deaths from disease and extreme weather events are likely to increase in importance.

Spatially Heterogeneous Nest-Clearing Behavior Coincides with Rain Event in the Leaf-Cutting Ant Atta cephalotes (L.) (Hymenoptera: Formicidae).

Leaf-cutting ants of the genus Atta construct the probably largest nests among ants and are ecosystem engineers because they alter light and nutrient availability at nest sites. Besides creating canopy gaps in the forest, workers remove all vegetation from atop their nest mounds. Here, we examined the extent and spatial distribution of this nest-clearing behavior by transplanting Licania tomentosa seedlings on Atta cephalotes (Linnaeus) nest mounds in the Atlantic forest in northeast Brazil and documented defoliation patterns by the workers. Within 9 days, workers removed around 53% of the total leaf area planted per colony. All colonies showed a synchronized start of defoliation after a rain event in the fifth night after the seedlings had been transplanted. Defoliation increased with time elapsed since transplanting and with the number of entrances surrounding each seedling. In addition, workers started defoliation on the top of the mound. In contrast, the distance to the next entrance and the size of the seedling did not affect the defoliation pattern. Defoliation was not part of the colony foraging activities but was identified as an element of nest maintenance. Possible cues triggering nest-clearing behavior and the potential link between nest-clearing activities and the control of microclimate of ant nests are discussed.

Silvicultural intensification and agroforestry systems in secondary tropical forests: a review

Secondary forests are the woody vegetation that results from the successional processes of species colonization after the original primary forest clearance. Primary forest clearances occur due to disturbances caused by natural factors such as windstorms, hurricanes, landslides, or fire. Human activities as deforestation to implement crop fields, pastures, mines, and roads can also promote the increase of secondary forests. This paper discusses the problems faced to achieve economic profitability in tropical secondary forests, giving as example the northeast region of Para state in the Brazilian Amazonia. One alternative to transform tropical secondary forests into more economically competitive land use is the intensification of silvicultural treatments aiming increased productivity of timber and non‑timber tree species. This can be implemented through Organized Disturbances to promote: a) the improvement of natural regeneration; b) enrichment planting in gaps; c) tending over naturally established seedlings and saplings of commercial species; and d) Assisted Densification of low density species. In addition to these silvicultural treatments, we propose a cyclic agroforestry system inside artificially created canopy gaps in small lands covered by secondary forests. This cyclic agroforestry system can provide crop production of grains and fruits with secondary forest conservation.

Combined effects of gap creation and deer exclusion on restoration of belowground systems of secondary woodlands: A field experiment in warm-temperate monsoon Asia

Ground-layer vegetation in abandoned woodlands of the Northern Hemisphere is rapidly declining because of overshading and an overabundance of ungulate herbivores; these changes may be affecting belowground ecosystems. An effective countermeasure is to restore ground-layer vegetation by establishing deer exclosures immediately after creating canopy gaps. However, such measures disturb belowground processes (e.g., decelerate decomposition rates and decrease soil water content) and may cause further disintegration of the whole system. To estimate the combined effects of gap creation and exclosure installation, we established a 2×2-factorial experiment (including gap creation, exclosure installation, both methods, and a control) in warm-temperate secondary woodlands. After five years, we studied ground vegetation, litter cover, soil–water physics, abundance of litter-dwelling invertebrates, and litter decomposition in each experimental plot. A significant increase in ground vegetation was found in gap-and-exclosure plots and gap-and-deer plots, although the increase in the latter was limited to approximately half of that in the former; in contrast, no significant increase was found in the control or canopy-and-exclosure plots. The abundance of litter-dwelling invertebrates was significantly reduced by gap creation, although this effect was offset by installation of exclosures in gaps. Fine porosity and water holding capacity of topsoil were reduced by gap creation, but these negative effects were mitigated by exclosures installed in gaps. Meanwhile, coarse porosity and water permeability of topsoil were not affected by gap creation; they tended to be increased by exclosure installation both in gaps and under the canopy. Litter decomposition tended to be delayed in the gap-and-deer plot, although this effect was temporary; exclosure installation slightly increased decomposition rates, but this effect was limited. These results show the possibility of restoring belowground dynamics in abandoned forests by creating small canopy gaps and establishing ungulate exclosures. This mixture of interventions should increase habitat heterogeneity in these forests and help prevent biological homogenization while mitigating long-term negative effects of gap creation on belowground ecosystems.

Effects of forest management on ground beetle diversity in alpine beech (Fagus sylvatica L.) stands

European beech forests are of particular importance for biodiversity, although relatively little is known about how beech forest management impacts on invertebrate communities. In this paper we investigated the influence of beech forest management history [i.e. over-mature coppices (OC) and coppices in conversion to high forests (CCHF)], climatic, topographic and microhabitat characteristics on ground beetle diversity (measured as total relative abundance, species richness, Shannon diversity and abundance of the endangered endemic species Carabus olympiae) in northern Italy. The diversity of forest specialist carabids was higher in OC and in forest stands characterized by a higher mean temperature and lower relative humidity. Moreover, we detected a positive response of several diversity variables to coarse wood debris cover or volume, herb cover, and the standard deviation of tree diameter. Currently, OC seems to be a more favorable habitat for forest carabids, including C.olympiae, although succession over time can lead to a progressive homogenization of the vegetation structure, with negative consequences for the conservation of the forest carabid assemblage.Based on our results, we suggest that the traditional management of beech coppice and its conversion to high forest be modified by including practices aimed at promoting structural and microhabitat diversity such as retention of large trees, creation of canopy gaps, retention of coarse wood debris and the preservation of ‘islands’ of older trees in the managed stands.

Reversing legacy effects in the understory of an oak-dominated forest

Current forests developed under conditions different from original forests, with higher deer densities, reduced fire frequency, denser canopies, and smaller canopy gaps. These alterations have led to understories dominated by species simultaneously browse tolerant, shade tolerant, and fire sensitive leading to difficulties in the regeneration of oak species (Quercus spp.) in some areas. We evaluated how three key processes — understory fire, canopy gaps, and browsing — influenced tree species in east central West Virginia. We were particularly interested in the response of oak species because they are the dominant overstory species and of maple species (Acer spp.), black birch (Betula lenta L.), and yellow-poplar (Liriodendron tulipifera L.) as these are likely to replace the current forest. Single-process effects were significant and significant interactions among processes were numerous. In general, fire caused significant reductions of seedlings and saplings of red maple (Acer rubrum L.) and striped maple (Acer pensylvanicum L.) and increased the seedlings of black birch and yellow-poplar. Canopy gaps increased the abundance of black birch and yellow-poplar seedlings and saplings. Gaps and fire together caused an increase in the relative abundance of yellow-poplar. Excluding browsers and creating canopy gaps together nearly doubled oak sapling importance values versus either treatment alone; however, oak importance values remained low. Given the significant interactions of browse control with the other two processes, browse control should be considered when planning oak regeneration treatments such as canopy gaps or prescribed fire.

Survival and Growth of Suppressed Baldcypress Reproduction in Response to Canopy Gap Creation in a North Carolina, USA Swamp

Little is known of the ability of baldcypress (Taxodium distichum (L.) L.C. Rich.) in a suppressed sub-canopy position to respond to increases in light. We measured 3 years of diameter growth response of long-suppressed baldcypress saplings to canopy gap creation in a hydrologically altered, water tupelo-dominated backswamp of the lower Roanoke River floodplain in eastern North Carolina. Baldcypress saplings responded strongly to gap creation. Saplings showed a significant positive growth trend, with mean basal area growth in gaps expanding from 2.5 cm2 in year one, 4 times that of control saplings, to 8.5 cm2 in year three, 20 times that of control saplings. Within canopy gaps, a positive trend between growth and initial sapling diameter was evident, while such a trend among controls was non-existent. Mortality was similar between gap and control saplings after 1 year, but by year three gap sapling mortality had declined to 2 %, while control mortality increased to 16 %. It is clear that long-suppressed baldcypress, sometimes at the brink of death, are able to respond vigorously to an increase in light from canopy gaps with decreased mortality and increased growth. This has important implications for wetlands requiring restoration or baldcypress management within the framework of two-age or uneven-aged silvicultural systems.

Succession of abandoned coppice woodlands weakens tolerance of ground-layer vegetation to ungulate herbivory: A test involving a field experiment

Ground-layer vegetation of abandoned woodlands is threatened by excessive shading and increasing pressure from ungulate herbivores. Although these issues are studied separately, they are presumed to be interactive because decreasing energy input into ground layer seems to limit herbivory tolerance in plants. To separately estimate impacts of succession, herbivory, and their interactions, I conducted a 2-by-2 factorial experiment (creating canopy gaps, setting deer exclosures, their combinations and controls) in abandoned woodlands, where ground-layer vegetation had nearly vanished from deer herbivory. Changes of plant community were monitored three times since half a year before the start of experiment. Rapid increases in foliar cover, plant height, and species richness were found in plots under gaps within half a year from the beginning of the experiment. The major cause of this rapid increase was reestablishment of light-demanding and fast-growing species, whose compensation growth exceeded consumption by deer. In contrast, no significant change was found in ground-layer vegetation under a closed canopy, where only shade-tolerant and slow-growing species existed. These results suggested that limits in light resulting from the progress of forest succession is a primary factor inhibiting recovery of ground-layer vegetation from herbivory damage. In the short term, a combination of gap creation and the removal of herbivores is necessary to restore ground-layer plants in abandoned woodlands.

Understory plant community resilience to partial harvesting in riparian buffers of central Canadian boreal forests

Partial harvesting in riparian buffers has been suggested as a forest management option to emulate natural disturbance patterns in riparian buffers. However, little is known about the effects of canopy gap creation by partial harvesting in riparian buffers on understory plant communities of boreal forests. We investigated the responses of understory species to gaps created by partial harvesting at up to 50% basal area removal in stream-side riparian buffers of boreal forests in northcentral Ontario, and compared them to nearby un-harvested buffers and riparian reference areas of undisturbed forests. Gap sizes ranged from 10 to 400m2, but most were between 25 and 125m2. Gaps created by partial harvesting measurably increased canopy openness, ground-level solar radiation, and shallow soil temperatures proportional to gap sizes, but the changes were small and only consistently higher than the range of these conditions in un-harvested riparian areas when gaps were >50m2. Understory plant abundance in gaps tended to be higher than in un-harvested sites but significant differences could not be detected. Richness and diversity were higher among medium and large gaps (>20m2) than in small gaps, but they were not higher than in un-harvested buffers or reference riparian areas. Multivariate community composition analyses could not detect significant differences between harvested and un-harvested areas. However, gaps created by partial harvesting affected foliar morphology of selected common understory plants. Leaf dry matter content was higher and specific leaf area was lower in harvested gaps than in un-harvested buffers or reference riparian areas, and both were related to gap size. Based on the unaltered understory community composition coupled with leaf morphological adaptations of selected understory plants to gaps, we conclude that the plant community is resilient to partial harvesting. Implications for riparian forest management under the emulation of natural disturbance paradigm are discussed.

Responses of insect pollinators and understory plants to silviculture in northern hardwood forests

Communities of flower flies (Diptera: Syrphidae), bees (Hymenoptera: Apoidea), and flowering plants were compared between harvested and unharvested hardwood stands in Algonquin Provincial Park, Ontario, Canada. Group-selection silviculture (where groups of trees are removed from a forested matrix, rather than single trees), increased the abundance of pollinators and flowering stems, but only after leaf-out. Wild red raspberry (Rubus strigosus) and bees benefitted most from the creation of canopy gaps. The combination of increased light, warm, bare soils, and abundant nectar-rich raspberry flowers probably created ideal habitat for soil-nesting bees, factors which are relatively absent from unharvested stands. By contrast, before leaf-out, spring ephemerals and high light-levels were universal and pollinators were even across treatments. More pollinators were caught in canopy gaps than in forested areas, and the proportion of fertilized ovules of spring beauty (Claytonia caroliniana) was higher in gaps than in the forest, suggesting that pollinators prefer foraging in gaps, even in spring. Group-selection silviculture in hardwood forests proved beneficial to native pollinating insects, at least in the short-term.

Canopy gaps and the natural regeneration of Matang mangroves

Canopy opening or gap creation by lightning strike is an important form of exogenous disturbance in the mangroves of Matang in Malaysia. The almost circular shaped canopy gaps consist of a group of standing dead trees which decay simultaneously with the growth of new trees. This characteristic is chiefly different with terrestrial forests where canopy gaps are normally created by tree-falls. Lightning strike disturbance in mangroves has created a means for natural regeneration whereby dormant seedlings growing underneath the canopies have had the opportunity to progress and ultimately replacing the canopies of the dead trees. Historical and recent aerial photographs of the Virgin Jungle Reserve of Pulau Kecil in Matang were analysed to measure gap turnover rate and to calculate canopy turnover. The average (±SD) canopy turnover is 25.5±6.9years. This finding shows that the continuous dynamics of gap creation and closure have sustainably maintained the condition of the forest. The finding also supports the hypothesis that this disturbance prevents mangroves from reaching more senescent stage, thus, canopy gap creations are justified to be the key driver in the natural regeneration of the tropical Rhizophora-dominated mangroves. Curiously compared, the average turnover is almost similar to the existing 30-year rotation cycle system adopted by the Forestry Department for the management of forest compartments in Matang.

Nothofagus dombeyi regeneration in declining Austrocedrus chilensis forests: Effects of overstory mortality and climatic events

This research examines the regeneration dynamics of Nothofagus dombeyi and Austrocedrus chilensis in A. chilensis-dominated forests growing near the eastern limit of N. dombeyi where precipitation is limiting. In these forests the widespread decline and mortality of overstory A. chilensis trees, known as ‘mal del ciprés’ (cypress sickness), generates large canopy gaps in which new individuals establish. Our objective was to study the population dynamics of N. dombeyi and A. chilensis in these forests to investigate the influences of overstory tree death and climatic variation on establishment. We sampled 6 symptomatic A. chilensis stands and used dendrochronological techniques to reconstruct basal area development and regeneration establishment over time. Bivariate event analysis was performed to examine the temporal relationships between tree establishment and mortality events and climatic variation. Overstory A. chilensis trees established as post-fire cohorts, with subsequent establishment of A. chilensis and N. dombeyi during the past 50–60 years. Regeneration in the past two decades was primarily N. dombeyi. The establishment of both A. chilensis and N. dombeyi was synchronous with overstory tree mortality events, but it was more consistent among stands and prolonged for N. dombeyi. Establishment of A. chilensis was not associated with climatic events but N. dombeyi establishment was synchronous with droughts, possibly related to climate-driven mortality creating canopy gaps or reducing competition within gaps. We have demonstrated that N. dombeyi has the ability to establish in post-fire A. chilensis-dominated forests resulting in mixed-species, uneven-aged forests. The ongoing increase in the abundance of N. dombeyi relative to A. chilensis represents a shift in composition and increased complexity in stand structure driven by ‘mal del ciprés’ and climatic variation.

A general integrative framework for modelling woody biomass production and carbon sequestration rates in forests

Summary1. Forests are an important, yet poorly understood, component of the global carbon cycle. We develop a general integrative framework for modelling the influences of stand age, environmental conditions, climate change and disturbance on woody biomass production and carbon sequestration. We use this framework to explore drivers of carbon cycling in New Zealand mountain beech forests, using a 30‐year sequence of data from 246 permanent inventory plots.2. A series of disturbance events (wind, snow storms, earthquakes and beetle outbreaks) had major effects on carbon fluxes: by killing large trees, they removed significant quantities of carbon from the woody biomass pool, and by creating canopy gaps, they reduced the crown area index (CAI) of stands (i.e. canopy area per unit ground area) and woody biomass production. A patch‐dynamics model, which we parameterized using permanent plot data, predicts that episodic disturbance events can create long‐term (c. 100‐year) oscillations in carbon stocks at the regional scale.3. Productivity declined with stand age, as shown in many other studies, but the effect was hard to detect because of canopy disturbance. Individual trees can increase productivity by adjusting the positioning, nutrient content and angle of leaves within canopies. We show that such optimization is most effective when trees are large and suggest it reduces the impact of water and nutrient limitation in old stands.4. We found no evidence that forests were responding to changing climatic conditions, although strong altitudinal trends in biomass production indicate that global warming could alter carbon fluxes in future.5. Synthesis.Our study emphasizes the critical role of disturbance in driving forest carbon fluxes. Losses of biomass arising from tree death (particularly in older stands) exceeded gains arising from growth for most of the 30‐year study, moving 0.3 Mg C ha−1 year−1from biomass to detritus and atmospheric pools. Large‐scale disturbance events are prevalent in many forests world‐wide, and these events are likely to be a driving factor in determining forest carbon sequestration patterns over the next century.

Giant Sequoia Regeneration in Groves Exposed to Wildfire and Retention Harvest

Both wildland fire and mechanical harvest have been proposed to achieve ecological restoration goals in giant sequoia (Sequoiadendron giganteum [Lindl.] Buchholz) groves of the southern Sierra Nevada, but their effectiveness on giant sequoia regeneration has received little attention. In the summer of 2010, we examined giant sequoia regeneration in four groves subjected to: 1) moderate- to high-severity wildfire in 1987 (Case Mountain, Redwood Mountain groves), 2) low-severity wildfire in 2008 (Black Mountain grove), 3) retention harvest (removal of all trees except large-diameter giant sequoia) followed by prescribed burning in the mid-1980s (Black Mountain, Bearskin groves), and 4) nearby unburned and unharvested (control) stands in all groves. Density of giant sequoia regeneration was greater in the moderate- and high-severity wildfire stands than control stands, but there was no difference in giant sequoia regeneration between low-severity burned and control stands. Stands thinned by retention harvest and prescribed burning had greater giant sequoia regeneration than control stands. Across all control and low- to moderate-severity wildfire stands, giant sequoia regeneration was positively associated with canopy gaps. In wildfire and retention harvest stands, giant sequoia regeneration was positively associated with distance to gap edge, direct and indirect solar radiation, and soil moisture. Our results corroborate previous studies in finding that giant sequoia regeneration benefits from fire. Both wildfire and prescribed fire (preceded by harvest or not) can serve to promote giant sequoia regeneration, providing that fire intensity is sufficient to create canopy gaps, increase understory light, and remove surface litter.

Habitat relationships of reptiles in pine beetle disturbed forests of Alabama, U.S.A. with guidelines for a modified drift-fence sampling method

Abstract Understanding vertebrate habitat relationships is important to promote management strategies for the longterm conservation of many species. Using a modified drift fence method, we sampled reptiles and compared habitat variables within the William B. Bankhead National Forest (BNF) in Alabama, U.S.A from April 2005 to June 2006. We captured 226 individual reptiles representing 19 species during 564 total trap nights. We used canonical correspondence analysis to examine habitat associations for the reptiles sampled and we detected a distinct habitat gradient ranging from sites with greater litter depth and percent canopy cover to more open sites with greater woody, herbaceous, and coarse woody debris (CWD) coverage, and CWD volume. Little brown skinks Scincella lateralis and eastern worm snakes Carphophis a. amoenus were associated with sites with greater litter depth and canopy cover, whereas eastern fence lizards Sceloporus undulatus, copperheads Agkistrodon contortrix, and gray ratsnakes Pantherophis spiloides were associated with sites possessing greater CWD coverage and volume. We found that disturbances due to the southern pine beetle Dendroctonus frontalis were likely important for influencing reptile distributions through the creation of canopy gaps and fallen coarse woody debris. Compared to other studies, our modified drift-fence trap technique was successful for sampling larger snake species (66 snakes in 564 trap nights). We have also provided detailed schematics for constructing drift fence array and box traps used in this study.

The contribution of the Coweeta Hydrologic Laboratory to developing an understanding of long-term (1934–2008) changes in managed and unmanaged forests

Long-term records from USDA Forest Service Experimental Forests and Ranges (EF&Rs) are exceptionally valuable scientific resources and common ground for research in natural resource management. Coweeta Hydrologic Laboratory, Southern Appalachian Mountains in western North Carolina, is one of 82 EF&Rs located throughout the United States and Puerto Rico. Since its establishment in 1934, the wealth and breadth of scientific knowledge gained from Coweeta Hydrologic Laboratory research has provided both public and private land managers information on forest land management and has added to the knowledge base of natural resource science. We described the early watershed research at Coweeta and used long-term measurements and inventories (from 1934 to 2008) to: (1) explore the influences of large-scale disturbances and vegetation responses on ecosystem processes and (2) assess the long-term and short-term impacts of an exotic, invasive species on a southern Appalachian deciduous forest. We focused on changes in vegetation patterns influenced by natural and managed disturbances and then described the linkages between long-term vegetation measurements and water yield and quality responses. For natural disturbances, we used a network of over 900 permanent vegetation plots established in reference watersheds and unmanaged areas; first measured in 1934 and a subset re-measured in 1969–1972, 1988–1993, and 2003–2008. For the managed disturbances, clearcuts and species conversion experiments, vegetation was measured in permanent plots within treated watersheds before and after treatment. By the time of the 1934 survey, the forest had only 10 years to recover from logging before chestnut blight ( Cryphonectria parasitica) induced mortality of virtually all remaining Castanea dentata trees. With the loss of C. dentata as the dominant species, Acer rubrum and Quercus montana became the dominant species and Liriodendron tulipifera and Tsuga canadensis increased in coves and along riparian corridors. T. canadensis is currently threatened by another invasive species, hemlock woolly adelgid (HWA, Adelges tsugae), with 33% tree mortality between 2003 and 2008. Smaller-scale disturbances, such as drought and windthrow, have created canopy gaps and the disturbance agent, gap size, and species-specific demography (dispersal, survival, growth, and mortality) affected colonization and recruitment. In general, the composition of species recruiting into canopy gaps was a reflection of the vegetation already in place at the time of disturbance, because regeneration occurred primarily through saplings or sprouts. In managed forests, where gaps were much larger (9–59 ha), early successional species recruit and become established and the long-term vegetation patterns are different than in unmanaged forests. The vegetation data provide a description of changes in forest structure and composition through time and space, and they have been used to develop process-based models to scale-up to the catchment level. Long-term climatic, hydrologic, biogeochemical, and vegetation databases coupled with process-based ecohydrology and ecophysiology models are essential to understanding broader- and more complex environmental issues such as climate change, carbon cycling, atmospheric deposition, and water supply and quality.

Temperate forest herbs are adapted to high air humidity — evidence from climate chamber and humidity manipulation experiments in the field

How growth and morphology of wild plants are controlled by the water vapor saturation deficit of the air (vapor pressure deficit, VPD) is not sufficiently understood. We tested the hypothesis that VPD acts on temperate woodland herbs independently from soil moisture by exposing two species ( Mercurialis perennis L. and Stachys sylvatica Torr.) to variable VPD levels in climate chambers and in open-top chambers on the forest floor. A decrease in air humidity from 85% to 40% in the climate chamber experiment, which simulates a microclimate change after canopy gap creation, resulted in a 40% decrease in biomass production of both species when grown in hydroponic culture. This result is supported by the more realistic open-top chamber experiment, which showed a productivity decrease by approximately 25% when plants were continuously exposed to an atmosphere with 15% lower air humidity than ambient. Elevated VPD levels reduced biomass production through either a reduced leaf expansion rate or a lower number of formed leaf buds. We conclude that many woodland herbs require sufficiently high air humidity for optimal growth; permanently decreased air humidity, as may occur in a future drier climate, after gap creation, or after clear-cutting of the forest, may threaten the vitality and survival of woodland herbs.

Initial Responses of Quercus serrata Seedlings and Forest Understory to Experimental Gap Treatments

Pinus thunbergii plantations in Pohang-si, Gyeongsangbuk-do, Korea, are of low ecological quality, with arrested succession and a high proportion of ruderal species. To improve the quality of the habitat, we created canopy gaps () and monitored changes in abiotic (light availability, canopy openness) and biotic (survival and growth of seedlings and understory communities) variables in 2007 and 2008 in plots that had received one of five types of treatment: cutting of canopy trees and removal of the understory (CU), cutting of canopy trees only (C), girdling of canopy trees and removal of the understory (GU), girdling of canopy trees (G) or control. Each treatment was applied to three replicate plots. Abiotic variables did not significantly differ among treatments. Survival rates of target species were slightly lower in the CU, G and control conditions. Based on logistic regression analysis, the only significant growth factor affecting survival was height growth. Positive effects of seedling height and leaf area growth on survival were also detected, but did not reach statistical significance. In treatment G, gradual improvement of overstory conditions and mitigation of competition by limitation of disturbance to the understory community were likely to have promoted seedling growth. There were no significant effects of gap treatments on changes in species abundance (cover and richness) and composition of understory between the study years. This result implies that the small gaps created in our study may be below the threshold size to affect understory growth. However, the results of this study are based on a short-term investigation of only two years. Long-term research is strongly recommended to clarify the effects of gap treatment on plant communities in afforested areas.

Role reversal in the stand dynamics of an angiosperm–conifer forest: Colonising angiosperms precede a shade-tolerant conifer in Afrotemperate forest

In mixed angiosperm–conifer forests worldwide, infrequent landscape-level catastrophic disturbances create a mosaic of persistent and different aged forest stands in the landscape with varying levels of dominance by the conifer component. In the ‘temporal stand replacement model’ (TSRM), disturbance creates conditions favouring a colonising cohort that is replaced by a suite of relatively shade-tolerant canopy species, which establish following the synchronous senescence of the pioneer canopy. In most southern hemisphere mixed angiosperm–conifer forests, with the exception of those in southern Africa, the establishing cohort is usually a large and very long-lived (550–650 years) conifer that is gradually replaced by angiosperms. As an explanation of the apparent dominance of the conifer Podocarpus latifolius, we examine the efficacy of the TSRM in mixed Afrotemperate forests where the establishing cohort is not a conifer. Forest succession in Afrotemperate forests was deterministic with the successive replacement of species determined first by their establishment success in shaded environments, and second, by their relative longevity. Several angiosperm species that were common canopy dominants established a pioneer cohort but were gradually replaced by P. latifolius, a shade-tolerant species. Continuous regeneration beneath the angiosperm canopy by P. latifolius eliminates synchronous canopy senescence, a key feature of the TSRM, as a mechanism driving the temporal replacement of canopy species. Senescing angiosperms created canopy gaps that were colonised by grasses and ferns, which suppressed canopy tree regeneration. In contrast, with continuous regeneration beneath the shaded canopy, P. latifolius gains a critical advantage over angiosperms at gap formation. Thus, in the absence of fairly large-scale natural disturbances, conifers come to dominate Afrotemperate forests. Commensurate with the latter, conifers in Podocarpus-forest were dated to approximately 320 years, more than 100 years older than the oldest P. latifolius in angiosperm-dominated forest. Tree life-history differences (shade tolerance, longevity) and the time since disturbance drive successional change from an angiosperm-dominated system to a stage dominated by P. latifolius. In general, the TSRM is a plausible explanation for the observed canopy tree structure and dynamics in mixed Afrotemperate forests. South African Afrotemperate forest is unusual among other southern hemisphere mixed angiosperm–conifer forests in that a suite of angiosperm canopy species, rather than a single conifer species, forms the colonising cohort.

Avian Response to Microclimate in Canopy Gaps in a Bottomland Hardwood Forest

Microclimate may infl uence use of early successional habitat by birds. We assessed the relationships between avian habitat use and microclimate (tempera- ture, light intensity, and relative humidity) in experimentally created canopy gaps in a bottomland hardwood forest on the Savannah River Site, SC. Gaps were 2- to 3-year-old group-selection timber harvest openings of three sizes (0.13, 0.26, 0.50 ha). Our study was conducted from spring through fall, encompassing four bird-use periods (spring migration, breeding, post-breeding, and fall migration), in 2002 and 2003. We used mist netting and simultaneously recorded microclimate variables to determine the infl uence of microclimate on bird habitat use. Microclimate was strongly affected by net location within canopy gaps in both years. Temperature generally was higher on the west side of gaps, light intensity was greater in gap centers, and relative humidity was higher on the east side of gaps. However, we found few relationships between bird captures and the microclimate variables. Bird captures were inversely correlated with temperature during the breeding and post- breeding periods in 2002 and positively correlated with temperature during spring 2003. Captures were high where humidity was high during post-breeding 2002, and captures were low where humidity was high during spring 2003. We conclude that variations in the local microclimate had minor infl uence on avian habitat use within gaps. Instead, habitat selection in relatively mild regions like the southeastern US is based primarily on vegetation structure, while other factors, including microclimate, are less important.