Recruitment Of Tree Species Research Articles

Integrated assessment of the direct and indirect effects of resource gradients on tree species recruitment

Understanding the dynamics of tree establishment is critical to assess forests' composition, management practices, and current responses to global change. We carried out a field seedling transplant experiment to assess not only the direct effects of resources influencing recruitment of four tree species, but also their indirect and combined effects. Our analysis integrated first growing season demographic data together with estimates of mycorrhizal fungal colonization and resource availability (light, soil moisture, and soil nitrogen). Only by considering both the direct and indirect effects of resources we were able to account for most of the variability observed during seedling recruitment. Contrary to expectations, increasing light levels were not always beneficial for recruitment even in low light habitats, and soil moisture availability benefited seedling growth but not survival. In addition, mycorrhizal fungal colonization was not always favored by high light levels or by increasing soil moisture. Seedling survival for all species was lower in plots with higher arbuscular mycorrhizal fungi, while the association with ectomycorrhizal fungi varied from beneficial to detrimental. When integrating the direct, indirect, and interactive effects of resource availability and mycorrhizal fungal colonization on tree recruitment dynamics we found that species responded in a nonlinear fashion to increasing resource levels, and we also identified thresholds, i.e., shifts in the direction of the response, along the resource gradient. Our integrated assessment considerably outperformed a null model where only direct effects of resources were accounted for. These results illustrate how the combination of direct, indirect, and combined effects of driving variables better represents the complexity of the processes determining tree species recruitment than simple resource availability mechanisms.

Forest loss or management intensification? Identifying causes of mammal decline in cacao agroforests

Changes in biodiversity and ecosystem services in terrestrial systems are being driven by the synergistic interactions between the loss of native vegetation and land-use intensification. Disentangling the influence of these two processes on species persistence is an important step towards reconciling conservation and agriculture production. We investigated how forest cover and management intensification affect the conservation value of cacao agroforests for mammals in an Atlantic forest landscape in southern Bahia, the most extensive area of diverse shaded cacao plantations in Brazil. Although both factors affected the distribution of mammal species, management intensification had a stronger effect, negatively affecting a larger number of species. Frequency of domestic dogs, an indirect aspect of management intensification, negatively affected four species and lower connectivity of shading trees decreased the use of cacao agroforests by three species of conservation concern. In comparison, the reduced forest cover negatively affected two species of conservation concern. Our results suggest that controlling domestic dog populations is particularly valuable for large mammal conservation in agroforestry mosaics, since it does not affect crop productivity. Improving shade cover within agroforests, on the other hand, should be implemented carefully, maintaining acceptable yield while promoting canopy connectivity and the recruitment of tree species used by forest dependent mammals. However, the importance of both local management intensification and local forest cover are likely to be context dependent, and changes in their relative importance are likely to occur with different levels of deforestation at the landscape scale.

The effects of deer herbivory and forest type on tree recruitment vary with plant growth stage

Understanding the combined effects of land-use changes and expanding generalist herbivores on the recruitment of tree species is critical to predict forest community dynamics and for fulfilling conservation purposes. We assessed how deer herbivory and forest-type affected the diversity of seedlings and saplings of dominant tree species in a temperate forest of Eastern USA, during four consecutive years. Fenced and unfenced plots were established in hardwood and pine forests and tree seedlings and saplings identified and monitored annually. Tree recruitment patterns varied widely from year to year, particularly for seedlings. Sapling communities were richer in species, more diverse and with lower indexes of dominance than seedling communities. The diversity of seedlings and saplings was significantly affected by inter-annual variation of tree recruitment but not by deer herbivory or forest type. Herb cover was reduced for more than fourfold in unfenced hardwood plots. Results show that inter-annual variation of recruitment, herbivory and forest type can combine to shape the composition of tree seedlings and saplings. When assessing effects of deer on tree recruitment interactions between biotic (e.g. herbivory) and abiotic (e.g. forest type) factors need to be considered. The outcome of such interactions depends on seedling or sapling life stage.

Deadwood anisotropic facilitation on seedling establishment after a stand-replacing wildfire in Aosta Valley (NW Italy)

The capacity of a forest ecosystem to recover following major disturbances depends on the regeneration characteristics of the species and the environment at the time of establishment, resulting from several interacting biotic and abiotic factors. At climatically stressed sites major drivers of recruitment are the presence of a seed source and the availability of ‘safe sites’ for germination, particularly in the harsh conditions resulting after a stand-replacing wildfire. Post-fire rehabilitation and restoration treatments can produce a deep ecological impact on recruitment processes, acting on biotic legacies and altering the variety and abundance of microsites.Two common post-fire management strategies, salvage logging and the absence of any post-fire intervention, were contrasted in a burned area located in Aosta Valley (North-Western Italy) to identify main environmental variables affecting seedlings and to quantify the effect of shelter elements on tree recruitment. Our hypothesis is that post-fire burned wood management may greatly affect microhabitat suitability for seedling survival in a dry mountain Pinus sylvestris forest.Regeneration density five years after the fire was still low, even though tree species recruitment started immediately following the disturbance. Differences in species composition were found among the investigated treatments. Regeneration density and diversity were positively associated with deadwood. An exception was represented by Populus tremula, regenerating mostly vegetatively, whose behaviour differed from the other tree species. Ground cover conditions contributed to patterns of seedling occurrence. The strong spatial association of seedlings with deadwood suggests that deadwood produces microsites that enhance establishment of regeneration. The relationship between nurse deadwood elements and regeneration was found to be highly anisotropic, as a consequence of the higher protection from radiation and reduced soil moisture loss in the shady sides of the shelter element. In this context post-fire management, particularly when removing burned wood, should be implemented with an understanding of its potential to affect the capacity of the ecosystem to restore, influencing both directly and indirectly recruitment.

Recruitment of tree species in mixed selection and irregular shelterwood forest stands

Context Recruitment is an important process in forest stand dynamics, especially in uneven-aged stands. Continuous recruitment is a prerequisite for diverse, uneven-aged silvicultural systems, but patterns may vary significantly.

Experimental manipulation of seed shadows of an Afrotropical tree determines drivers of recruitment

The loss of animals in tropical forests may alter seed dispersal patterns and reduce seedling recruitment of tree species, but direct experimental evidence is scarce. We manipulated dispersal patterns of Manilkara mabokeensis, a monkey-dispersed tree, to assess the extent to which spatial distributions of seeds drive seedling recruitment. Based on the natural seed shadow, we created seed distributions with seeds deposited under the canopy ("no dispersal"), with declining density from the tree ("natural dispersal"), and at uniform densities ("good dispersal"). These distributions mimicked dispersal patterns that could occur with the extirpation of monkeys, low levels of hunting, and high rates of seed dispersal. We monitored seedling emergence and survival for 18 months and recorded the number of leaves and damage to leaves. "Good dispersal" increased seedling survival by 26%, and "no dispersal" decreased survival by 78%, relative to "natural dispersal." Using a mixed-effects survival model, we decoupled the distance and density components of the seed shadow: seedling survival depended on the seed density, but not on the distance from the tree. Although community seedling diversity tended to decrease with longer dispersal distances, we found no conclusive evidence that patterns of seed dispersal influence the diversity of the seedling community. Local seed dispersal does affect seedling recruitment and survival, with better dispersal resulting in higher seedling recruitment; hence the loss of dispersal services that comes with the reduction or extirpation of seed dispersers will decrease regeneration of some tree species.

The structure and reproduction of the virgin forest: a review of Eustace Jones (1945)

Jones (1945) was a milestone paper exploring the natural forest concept with examples from the temperate and boreal ecosystems. It has become a classic because of its use of field observation of regeneration, succession and structure to assess theories about disturbance and the dynamic properties of natural forests. His main aim was to review some of the features of the structure and reproduction of the north temperate virgin forests, and this article presents, discusses and evaluates the main features of this legendary paper. Jones had international experience of both the ecological and silvicultural research communities and combined long-term field observations with theory to develop a realistic assessment of natural forest properties that formed the basis for current understanding. He demonstrated that natural disturbance regimes could generate a variety of structures and that a stable, “climax” forest concept was often not supported by field data. He also showed that even-aged components are common in these forest ecosystems and that the recruitment of tree species proceeds irregularly even in undisturbed stands. His work has influenced subsequent development of related subjects such as disturbance theory, gap-phase dynamics and long-term vegetation changes and has left a legacy with practical relevance for nature conservation and silviculture.

Causes of arrested succession in coastal dune forest

Dispersal or recruitment limitation may arrest succession after disturbance. In north-eastern South Africa the Acacia karroo successional pathway is used to facilitate coastal forest recovery after strip-mining. However, although A. karroo establishes naturally, it forms monospecific stands, arresting forest succession for decades. This casts doubt on the efficacy of this restoration pathway. We investigated the causes of arrested succession. The seed and seedling banks of A. karroo stands and of forest at Cape Vidal, and three A. karroo stands (7–27 years old) on rehabilitated strip-mined dunes at nearby Richards Bay were examined. The establishment and growth of seedlings at Cape Vidal were also considered. The seed bank was larger and more diverse in forest, but the seedling bank was larger in Acacia stands. At Richards Bay, the size of the seed bank increased and the seedling bank decreased with Acacia stand age. Excluding mammalian herbivores in Acacia stands at Cape Vidal resulted in greater species richness and survival of naturally established seedlings, as well as two experimentally planted species. Neither seed dispersal nor seedling establishment limited recruitment of tree species in Acacia stands. Herbivory arrested forest succession by causing the differential mortality of seedlings. In contrast, at Richards Bay where there were few mammalian herbivores, the advanced regeneration in A. karroo stands converged on the diversity of nearby forests 29 years after restoration. Controlling herbivore access and seeding Acacia stands with forest species are site-specific options for preventing arrested succession when using the A. karroo successional pathway.

Enhanced Tree Recruitment Following Logging in Two Forest Reserves in Ghana

ABSTRACTWe tested if logging enhances the recruitment of tree species by comparing young tree density in areas disturbed by logging with areas unaffected by logging 6 mo after logging in two Forest Reserves in Ghana. Analysis of 46 commercial timber species showed significantly higher postlogging recruitment in felling gaps and skid trails than in areas unaffected by logging. New seedlings substantially exceeded those lost due to logging operations. Species richness was significantly greater in areas disturbed by logging.

Seedling survival in a northern temperate forest understory is increased by elevated atmospheric carbon dioxide and atmospheric nitrogen deposition

AbstractWe tested the main and interactive effects of elevated carbon dioxide concentration ([CO2]), nitrogen (N), and light availability on leaf photosynthesis, and plant growth and survival in understory seedlings grown in an N‐limited northern hardwood forest. For two growing seasons, we exposed six species of tree seedlings (Betula papyrifera, Populus tremuloides, Acer saccharum, Fagus grandifolia, Pinus strobus, and Prunus serotina) to a factorial combination of atmospheric CO2 (ambient, and elevated CO2 at 658 μmol CO2 mol−1) and N deposition (ambient and ambient +30 kg N ha−1 yr−1) in open‐top chambers placed in an understory light gradient. Elevated CO2 exposure significantly increased apparent quantum efficiency of electron transport by 41% (P<0.0001), light‐limited photosynthesis by 47% (P<0.0001), and light‐saturated photosynthesis by 60% (P<0.003) compared with seedlings grown in ambient [CO2]. Experimental N deposition significantly increased light‐limited photosynthesis as light availability increased (P<0.037). Species differed in the magnitude of light‐saturated photosynthetic response to elevated N and light treatments (P<0.016). Elevated CO2 exposure and high N availability did not affect seedling growth; however, growth increased slightly with light availability (R2=0.26, P<0.0001). Experimental N deposition significantly increased average survival of all species by 48% (P<0.012). However, seedling survival was greatest (85%) under conditions of both high [CO2] and N deposition (P<0.009). Path analysis determined that the greatest predictor for seedling survival in the understory was total biomass (R2=0.39, P<0.001), and that carboxylation capacity (Vcmax) was a better predictor for seedling growth and survival than maximum photosynthetic rate (Amax). Our results suggest that increasing [CO2] and N deposition from fossil fuel combustion could alter understory tree species recruitment dynamics through changes in seedling survival, and this has the potential to alter future forest species composition.

Tree recruitment and growth on 20-year-old, unreclaimed surface mined lands in West Virginia

Surface mining in West Virginia has disturbed large tracts of forested land. In the 1930s to 1960s, reclamation generally involved replanting of trees on mined sites, but revegetation practices gradually evolved into seeding of forages for erosion control as federal and state laws were enacted in the 1970s. Reforestation of mined lands has recently become an important issue and both federal and state regulatory agencies are returning to forestry as a preferred post-mining land use. This study evaluated tree species recruitment and growth on three, 20-year-old, surface mined areas that were not transplanted with trees. Three transects of 150 m at each site extended across three positions: (1) the flat top after coal removal; (2) the outslope down from the flat top where soil and weathered rock materials had been pushed down the hill; (3) the undisturbed forest. Tree canopy cover and herbaceous cover were determined at 1.5 m intervals along the transect line, and tree density and diameter at breast height were measured for each species located in a plot 2.3 m wide along the line. Soil samples were taken to a depth of 25 cm at 15 m intervals along transects. Soils in all positions generally had loam textures, and soil pH ranged from 4.6 to 6.0 on undisturbed and outslope positions to 6.1 to 6.6 on flat tops. Soils were noticeably thinner and denser on flat tops than in other positions. Undisturbed forests averaged 85% canopy cover and were dominated by tuliptree (Liriodendron tulipifera L.), red maple (Acer rubrum L.) and sugar maple (Acer saccharum Marsh.), with minor contributions from red oak (Quercus rubrum L.), black gum (Nyssa sylvatica Marsh.), sourwood (Oxydendrum arboreum (L) DC), sassafras (Sassafras variifolium (Nutt. Nees) and hickory (Carya spp.). Outslopes had primarily red maple, black birch, tuliptree, sourwood and black locust (Robinia pseudo-acacia L.). Flat areas were dominated by herbaceous cover with red maple and black locust being the dominant trees. Areas seeded with herbaceous plants showed low numbers and cover by trees, while areas not seeded with herbaceous plants were almost as heavily covered by trees as undisturbed areas. New guidelines are being developed to aid survival and growth of mid- to late-successional trees on newly reclaimed sites. These include less compaction of soil materials and seeding less aggressive forage species where trees will be planted.

A long‐term study of neighbour‐regulated demography during a decline in forest species diversity

AbstractQuestion:Did disturbance, no density‐dependence of the dominant species, and negative neighbourhood interactions on rare species affect tree demography during a decline in species diversity associated with the increase of Acer saccharum from 1939–2001?Hypotheses:1. The rise in dominance of A. saccharum was because of its advantage in disturbances and lack of density‐dependence of its demography. 2. Rare species were not favoured by disturbances, including those from Dutch elm disease, and demonstrated negative neighbourhood interactions with A. saccharum.Location:Brownfield Woods in Illinois, USA.Methods:Historical maps of trees (≥ 7.6 cm DBH) from 1939, 1951, 1988, and 2001 in 16 quadrats (48 m × 68 m) were used to compare demography of eight tree species. Effects of disturbances, density‐dependence, and neighbourhood interactions on mortality and recruitment of tree species within a 6‐m radius of individual target trees were studied.Results:A. saccharum demonstrated a demographic advantage over rare species. It had lower mortality and higher recruitment rates. Disturbances facilitated recruitment of A. saccharum, but did not enhance rare species. Density‐dependence of both mortality and recruitment of A. saccharum occurred, but population projection models indicated that ecological conditions became more favourable for A. saccharum in the past 62 years. Furthermore, negative neighbourhood interactions were detected between rare species and A. saccharum. The increase in neighbouring A. saccharum significantly increased mortality and reduced recruitment of the rare species.Conclusions:The general disturbance regime, enhanced by Dutch elm disease, in Brownfield Woods facilitated the rise of dominance of A. saccharum. Meanwhile, rare species declined as a result of their disadvantage in disturbance and negative neighbourhood interactions with A. saccharum.

A long-term study of neighbour-regulated demography during a decline in forest species diversity

Question: Did disturbance, no density-dependence of the dominant species, and negative neighbourhood interactions on rare species affect tree demography during a decline in species diversity associated with the increase of Acer saccharum from 1939-2001? Hypotheses: 1. The rise in dominance of A. saccharum was because of its advantage in disturbances and lack of densitydependence of its demography. 2. Rare species were not favoured by disturbances, including those from Dutch elm disease, and demonstrated negative neighbourhood interactions with A. saccharum. Location: Brownfield Woods in Illinois, USA. Methods: Historical maps of trees (≥ 7.6 cm DBH) from 1939, 1951, 1988, and 2001 in 16 quadrats (48 m · 68 m) were used to compare demography of eight tree species. Effects of disturbances, density-dependence, and neighbourhood interactions on mortality and recruitment of tree species within a 6-m radius of individual target trees were studied. Results: A. saccharum demonstrated a demographic advantage over rare species. It had lower mortality and higher recruitment rates. Disturbances facilitated recruitment of A. saccharum, but did not enhance rare species. Density-dependence of both mortality and recruitment of A. saccharum occurred, but population projection models indicated that ecological conditions became more favourable for A. saccharum in the past 62 years. Furthermore, negative neighbourhood interactions were detected between rare species and A. saccharum. The increase in neighbouring A. saccharum significantly increased mortality and reduced recruitment of the rare species. Conclusions: The general disturbance regime, enhanced by Dutch elm disease, in Brownfield Woods facilitated the rise of dominance of A. saccharum. Meanwhile, rare species declined as a result of their disadvantage in disturbance and negative neighbourhood interactions with A. saccharum.

Changes in light below the canopy of a Jamaican montane rainforest after a hurricane

ABSTRACTHurricane Gilbert caused disturbance to Jamaican montane rainforests in 1988. This study provides a detailed characterization of landscape-level changes in light below the canopy of these forests after the hurricane. Hemispherical photographs were taken below the forest canopy at four sites at permanent points 1 m above the ground between 7 and 33 mo after the hurricane. For each photograph photosynthetically active radiation (PAR) was computed. PAR declined exponentially in all sites during the period of measurement. During the first 24 mo after the hurricane, PAR beneath the canopy was significantly greater in sites that had been defoliated during the hurricane than in sites where few trees had been defoliated. By 28 mo after the hurricane there was no significant difference in PAR beneath the canopy among the four sites. By 33 mo after the hurricane canopy recovery was nearly complete and PAR was only slightly higher than measurements made before the hurricane. Our results were compared with studies of changes in light environment resulting from treefall gaps and under deciduous canopies. PAR during the first 18 mo after the hurricane was similar to that recorded in small canopy gaps in other forests. Widespread defoliation caused by hurricanes can thus increase PAR beneath the canopy over large areas and consequently opportunities may arise for widespread recruitment of tree species in response to increased light levels.

Disturbance and Tree Species Coexistence in Tropical Riparian Forest Fragments

Patterns of past fire incursions and tree falls were documented in a tropical riparian forest. Fire-scarred trees were concentrated on gentler slopes near the savanna:forest boundary, while tree fall sites were of highest frequency on convex upper valley slopes. Most tree falls were in a down-slope direction while the remainder were concentrated in directions that suggested an origin in extreme winds of atypical orientation. Tree species response to these two forms of disturbance was evaluated by assuming that the zones identified were inherently more disturbanceprone, and comparing the tree populations within and beyond the areas identified. Fire incursions had a larger measurable effect in augmenting species richness: 19% of species evaluated were significantly more abundant in burned zones, while only 8% of those evaluated were more abundant in tree fall zones. This difference is tentatively attributed to fire having a larger proximal effect on tree species recruitment as well as a spatially more persistent pattern of occurrence. Observations made at sites of recent fire incursion indicated that this caused little tree death or canopy opening but eliminated seedlings, saplings, litter and root mats: it thus created new seedbed conditions without the high light levels that would promote herbaceous establishment. Both disturbances play an augmentative, rather than exclusive, role in promoting species coexistence in these forests: they provide a varied micro-environment for seedling establishment but their patchy distribution ensures that disturbance-sensitive species can persist elsewhere in the forest patch.