Treefall Gaps Research Articles

Assessing Forest Level Response to the Death of a Dominant Tree within a Premontane Tropical Rainforest

Small-scale treefall gaps are among the most important forms of forest disturbance in tropical forests. These gaps expose surrounding trees to more light, promoting rapid growth of understory plants. However, the effects of such small-scale disturbances on the distribution of plant water use across tree canopy levels are less known. To address this, we explored plant transpiration response to the death of a large emergent tree, Mortoniodendron anisophyllum Standl. & Steyerm (DBH > 220 cm; height ~40 m). Three suppressed, four mid-story, and two subdominant trees were selected within a 50 × 44 m premontane tropical forest plot at the Texas A&M Soltis Center for Research and Education located in Costa Rica. We compared water use rates of the selected trees before (2015) and after (2019) the tree gap using thermal dissipation sap flow sensors. Hemispherical photography indicated a 40% increase in gap fraction as a result of changes in canopy structure after the treefall gap. Micrometeorological differences (e.g., air temperature, relative humidity, and vapor pressure deficit (VPD)) could not explain the observed trends. Rather, light penetration, as measured by sensors within the canopy, increased significantly in 2019. One year after the tree fell, the water usage of trees across all canopy levels increased modestly (15%). Moreover, average water usage by understory trees increased by 36%, possibly as a result of the treefall gap, exceeding even that of overstory trees. These observations suggest the possible reallocation of water use between overstory and understory trees in response to the emergent tree death. With increasing global temperatures and shifting rainfall patterns increasing the likelihood of tree mortality in tropical forests, there is a greater need to enhance our understanding of treefall disturbances that have the potential to redistribute resources within forests.

Low- to Moderate-Level Forest Disturbance Effects on Plant Functional Traits and Associated Soil Microbial Diversity in Western Himalaya

This study quantifies the effect of low- to moderate-intensity anthropogenic activities and natural activities affecting less than 40% of the canopy cover on major plant functional traits and associated soil microbial diversity in western Himalayan temperate forests. Plots of 0.1 ha were placed in the temperate forests along disturbance gradient and were categorized based on lopping intensity, deadwood counts, grazing, and litter removal. Plots were classified into three classes, i.e., low disturbance intensity (LDI) and moderate disturbance intensities (MDI1 and MDI2) based on lopping activities and canopy cover. The study was conducted on functional traits related to growth and survival strategies of a species in a complex forest ecosystem. Furthermore, DNA was extracted and metagenome of soil samples was performed using Illumina MiSeq platform from three disturbance classes to study the effect of disturbance and plant traits on microbial diversity. Tree basal area was found to be most significantly affected by disturbance intensity. Total density was found to be greater for the LDI site. Specific leaf area (SLA) and crown cover (CC) were the most affected traits in the moderately disturbed sites, whereas maximum plant height (HT) and seed mass (SM) were least affected by disturbance. Soil microbial diversity was found to be negatively associated with disturbance index. Microbial biomass carbon (MBC) and microbial biomass nitrogen (MBN) were found to be significantly higher in LDI sites. Disturbance level elicited variation in microbial community composition.Proteobacteriawas the most abundant phylum andPhenylobacterium,DA101, andCandidatus solibacterwere mainly abundant at the genus level. A decreasing level of disturbance due to the absence of human residences in the LDI site led to the dominance ofPhenylobacterium(27%) which reduced to 18% in MDI1 site. Shannon alpha bacterial diversity and plant species diversity (H′) were found to be greatest for MDI2 site. In forests with varying levels of management, treefall gaps due to low levels of logging intensity might have a similar effect to those of reduced lopping intensities and deadwood count (due to natural disturbances) in the study sites. The study concludes that moderate disturbance is important for promoting species diversity and species richness (SR), but species having conservative ecological strategies would be more prone to continued disturbance intensity. Therefore, low- to moderate-level disturbance in such forests can be used as a model for natural treefall gaps, and moderate-level disturbance intensity plays a powerful role in buffering ecosystem processes.

Short-lived legacies of Prunus serotina plant-soil feedbacks.

Plant-soil feedbacks (PSFs) are often involved in fundamental ecological processes such as plant succession and species coexistence. After a plant initiating PSFs dies, legacies of PSFs occurring as soil signatures that influence subsequent plants could persist for an unknown duration. Altered resource environments following plant death (especially light availability) could affect whether legacy effects manifest and persist. To evaluate PSFs and their legacies, we obtained soils from a chronosequence of Prunus serotina harvests. In a greenhouse experiment, we planted conspecific seedlings under two light levels in these soils of varying time since the influence of live Prunus serotina, and compared seed/seedling survival in soils from live trees, stumps, and surrounding forest matrix within each site and across the chronosequence. PSF legacies were measured as the difference between seedling performance in live tree and stump soils within a site. Negative PSF legacies of P. serotina were short-lived, lasting up to 0.5years after tree removal. These effects occurred under 5% but not 30% full sun. PSFs and their legacies manifested in seed/seedling survival, but not biomass. Though restricted to low light, short-lived legacies of P. serotina PSFs could have lasting impacts on plant community dynamics during post-disturbance regeneration by disfavoring P. serotina regeneration in small tree-fall gaps.

Birds associated with treefall gaps in a lowland forest in southwestern Brazilian Amazonia

ABSTRACT Natural communities are dynamic systems in time and space. The spatial distribution of plants and animals tends to coincide with the availability of resources needed for the survival and reproduction of each species. Natural treefall gaps offer a number of resources that influence the distribution of birds within the forest. We compared the understory bird assemblages of natural treefall gaps (15 sampling points) with those found in the adjacent forest (15 points) in the Humaitá Forest Reserve in southwestern Brazilian Amazonia. We used mist-nets to sample birds and obtained 700 captures of 105 species. Species richness, number of individuals, and species composition were all similar between gaps and forest, although six species presented some degree of association with the gaps, and nine with the forest. Nectarivores preferred gaps significantly over forest, whereas insectivores and frugivores were distributed equally between gaps and forest. Vegetation height and density differed between gaps and forest, and influenced the distribution of bird species in the two environments. Fruit availability was positively correlated with frugivore abundance in gaps. Overall, 33.3% of the birds associated with the treefall gaps are from lower forest canopy, while the others are understory species. We showed that the use of natural gaps by birds in a fragmented landscape of the Amazon forest contributes to the environmental heterogeneity and succession dynamics following natural events of habitat disturbance.

Advanced canopy regeneration: an unrecognized mechanism of forest dynamics.

Gaps in the forest canopy, created by treefalls, are fundamental to the process of forest dynamics (Yamamoto 2000). Tree-fall gaps are critical for the regeneration of shade-intolerant tree species while at the same time, providing opportunities for the growth and establishment of shade-tolerant tree species (Yamamoto 2000). Gap dynamics is central to understanding change in old-growth forests, and for predicting successional trends and the processes that explain the present structure and composition of tropical and temperate forest communities in the absence of large-scale disturbances (Yamamoto 2000, Muscolo et al. 2014).

Single dead trees matter: Small-scale canopy gaps increase the species richness, diversity and abundance of birds breeding in a temperate deciduous forest

Canopy gaps produced by natural disturbances are an important source of structural and functional heterogeneity within forest ecosystems. However, the effect of natural, small-scale canopy gaps, caused by the mortality of a single tree or a group of trees, on bird assemblages has not been adequately addressed in temperate deciduous forests. We studied the effects of small-scale canopy gaps on the breeding birds of old-growth deciduous forests in the Carpathians (SE Poland). Our hypothesis was that such small gaps and the adjacent closed-canopy forest would differ in bird richness, diversity and relative abundance. Breeding bird assemblages were surveyed using point counts in 30 canopy gaps (mean area 301 m2 ± 228 SD, range: 37–970 m2) and in 30 control plots situated in the adjacent closed-canopy forest. The species richness, diversity and relative abundance of birds were significantly higher in the gaps than in the control plots and were correlated with the number of large snags. Collared Flycatcher Ficedula albicollis and White-backed Woodpecker Dendrocopos leucotos occurred more frequently in the gaps than in the control plots. Eurasian Treecreeper Certhia familiaris, Blackcap Sylvia atricapilla, Song Thrush Turdus philomelos and Eurasian Nuthatch Sitta europaea were more abundant in the gaps than in the control plots. The species richness of cavity- and ground-nesters was higher and these birds were more abundant in the gaps than in the control plots. The presence/absence of a gap and the number of large snags recorded in it explained 35.4% and 52.7% of the deviance in the total number of species and the total number of individual birds, respectively. Our results indicate that even small-scale canopy gaps, in combination with the large snags recorded in them resulting from the death of a single tree or group of trees, affect bird assemblages in deciduous forests as measured by the occurrence or abundance of individual species or community indices. We suggest that treefall gaps, which are heterogeneous structures, mediate the spatial distribution of bird territories within a stand and are crucial for the biodiversity of temperate deciduous forests.

Effect of canopy gap size on soil respiration in a mixed broadleaved-Korean pine forest: Evidence from biotic and abiotic factors

Understanding the effects of forest structure, the frequency and size of canopy gaps is pivotal for precisely modelling forest soil carbon budgets. We aimed to determine the differences in soil respiration between closed canopy areas and canopy gaps with various sizes and how soil respiration is affected by biotic and abiotic factors, e.g., soil nutrients, soil microbial biomass and community composition, enzyme activities and fine root biomass. Soil respiration and these environmental factors were measured in eleven tree fall gaps (separated into small, medium and large with sizes ranging from 20 m2 to 450 m2) and three control plots in a mixed broadleaved-Korean pine forest during the growing season. Our results showed that soil respiration was similar between the control and small canopy gaps, while it was 17–49% lower in the medium and large canopy gaps compared to that of the control. There was a significant parabolic relationship between soil respiration and soil moisture, and the optimum moisture was approximately 40%. Fine root biomass and soil enzyme activities showed positive effects on soil respiration across different levels of canopy gaps. Furthermore, soil respiration was significantly declined with increasing soil microbial diversity (OTUs of bacteria). Our findings demonstrate that canopy gap has significant impact on soil CO2 emission in a temperate mixed forest, however, further studies concerning other forest ecosystem types and larger scale are warranted.

Competition for light and persistence of rare light-demanding species within tree-fall gaps in a moist tropical forest.

Current evidence suggests that tree-fall gaps can influence forest structure and dynamics by enabling certain species guilds to persist over the long term. Here we assessed the development of local size hierarchies and asymmetric competition for light in tree-fall gaps, and the role played by these two processes for the persistence of rare light-demanding species in the Barro Colorado Island Forest Dynamics Plot (Panama). We performed spatial point pattern analysis, considering both the spatial locations (x,y) and the diameter at breast height (DBH) of all the woody plant recruits from the 1985 and 2000 censuses located in tree-fall gaps, and followed their fate up to the 1990-2010 and 2005-2010 censuses, respectively. For these two recruit cohorts, we found that, from the initial census until 5-10yr later, close neighbors presented a larger DBH than the mean DBH of all individuals within gaps, which points to a positive growth response of recruits to the increased light levels in the gap centers. However, close neighbors of the 1985 cohort also showed larger than expected DBH differences that disappeared in subsequent censuses, indicating an enhancement of size differences between neighbors and the mortality of the smaller individuals. Finally, for both recruit cohorts, we found that 10-15yr after gap formation, surviving individuals of rare light-demanding species had a negative impact on survival of neighboring individuals of other species. Our results indicate that gaps favor the persistence of rare light-demanding species through the development of local size hierarchies and asymmetric competition for light. The strength of this process, however, apparently depends upon gap size and the role played by the woody plants already existing at the time of gap formation in early colonization. Moreover, our findings suggest that in this forest, gaps may enhance colonization of plant species typical of nearby dry tropical areas, and that, over the coming decades, similar processes could strongly modify the structure and dynamics of moist tropical forests in the region.

Rapid colonization and turnover of birds in a tropical forest treefall gap

Rapid colonization and turnover of birds in a tropical forest treefall gap

Composition and fractal features of soil micro-aggregates in microsites of different types of treefall gaps

Through field survey and laboratory analysis, we examined the composition and fractal features of soil micro-aggregates in different types of treefall gaps and microsites (pit bottom and mound top) in broad-leaved Korean pine forest and spruce-fir-Korean pine forest. Results showed that the contents of soil microaggregates under the classes of 0.25-2 mm and 0.05-0.25 mm were higher in both forest types, ranging from 25.7% to 50.7% and from 27.0% to 42.8%, respectively, and that of <0.002 mm was the lowest, ranging from 4.4% to 8.9%. In the pit bottom and mound top of gaps, soil bulk density was higher in both forest types. Soil nutrient content in mound top was higher than that in pit bottom and was higher in broad-leaved Korean pine forest than spruce-fir-Korean pine forest. Soil microaggregates of <0.002 mm had no correlation with soil physical and chemical properties, whereas that of 0.25-2 mm and 0.002-0.02 mm had significantly positive and negative correlation with soil non-capillary porosity, total porosity, aeration porosity, organic matter, total phosphorus, total nitrogen and organic carbon, respectively. On the whole, soil fractal dimension (D) and the proportion of characteristic soil micro-aggregates (PCM) in broad-leaved Korean pine forest were larger than those in spruce-fir-Korean pine forest, and the ratio of soil microaggregates diameter (RMD) in mound top and pit bottom was increased in two forest types. Soil D and PCM had no significant correlation with soil physical and chemical properties, while RMD was negatively correlated with capillary porosity, total porosity, soil bulk density and aeration porosity. In two forest types, the formation of mound and pit microsites could decrease the larger size micro-aggregates and the stability of soil micro-aggregate, increase soil D and PCM, and signifi-cantly increase RMD. RMD could be used as a quantitative index of soil physical and chemical properties in pit and mound microsites of forest.

Survival and Growth of Brazil-Nut Seedlings in Tree-Fall Gaps and Forest Understory

Abstract This study aimed to evaluate the influence of canopy openness on the survival and growth of Bertholletia excelsa seedlings, through experimental plantings in tree-fall gaps and forest understory in Madre de Dios, Peru. Sixty-four seedlings were planted within the experimental design, with eight repetitions each with four seedlings in two treatments of canopy openness. Basal diameter, height, and incidences of seedlings mortality and regrowth were evaluated for twelve months. Survivorship, and the annual increase in height and diameter were significantly higher in gaps than in the understory (p < 0.05). Regression analysis showed a direct and highly significant relation between canopy openness and diameter increment (r2 = 0.61), and between canopy openness and height increment (r2 = 0.34). Due to the greater performance and high survival rate of Brazil-nut seedlings in the tree-fall gaps, we recommend enrichment planting in the clearings of natural forests in Madre de Dios.

From habitat use to social behavior: natural history of a voiceless poison frog, Dendrobates tinctorius.

Descriptive studies of natural history have always been a source of knowledge on which experimental work and scientific progress rely. Poison frogs are a well-studied group of small Neotropical frogs with diverse parental behaviors, distinct calls, and bright colors that warn predators about their toxicity; and a showcase of advances in fundamental biology through natural history observations. The dyeing poison frog, Dendrobates tinctorius, is emblematic of the Guianas region, widespread in the pet trade, and increasingly popular in research. This species shows several unusual behaviors, such as the lack of advertisement calls and the aggregation around tree-fall gaps, which remain poorly described and understood. Here, we summarize our observations from a natural population of D. tinctorius in French Guiana collected over various field trips between 2009 and 2017; our aim is to provide groundwork for future fundamental and applied research spanning parental care, animal dispersal, disease spread, habitat use in relation to color patterns, and intra-specific communication, to name a few. We report sex differences in habitat use and the striking invasion of tree-fall gaps; describe their courtship and aggressive behaviors; document egg development and tadpole transport; and discuss how the knowledge generated by this study could set the grounds for further research on the behavior, ecology, and conservation of this species.

Woody debris in treefall gaps shelters palatable plant species from deer browsing, in an old-growth temperate forest

A major issue in the management of New Zealand’s remaining native forests is the extent to which the presence of introduced deer is compatible with national conservation and ecosystem restoration goals. Deer greatly reduce the abundance of their preferred plant species in forest understories, even when present at low densities. Recent work in other temperate regions has found that coarse woody debris resulting from treefalls shelters regeneration of species that are heavily browsed elsewhere in the forest. We asked if a similar effect occurs in lowland New Zealand forest dominated by Beilschmiedia tawa, comparing sapling abundance of preferred tree species in various microenvironments in two old-growth forests <30 km apart growing in near-identical physical environments—one inhabited by introduced fallow deer (Dama dama), the other from which ungulates and other introduced mammals were excluded c. 13 years earlier. Species composition of sapling communities growing within fallen tree crowns and on woody debris in treefall gaps did not differ significantly between the two forests; in contrast, the species composition of sapling communities elsewhere in treefall gaps and on understorey plots differed strongly between sites with and without deer. Saplings of species preferred by deer were equally abundant within fallen tree crowns in the two forests, demonstrating the effectiveness of tree crowns in sheltering palatable species from deer browsing; saplings growing on coarse woody debris in treefall gaps were also of similar abundance in the two forests. However, abundance of preferred species’ saplings on understorey plots was 10-fold lower at sites with deer, and their abundance in parts of treefall gaps unprotected by woody debris was 4-fold lower. Saplings of most other species did not differ significantly in abundance between sites with and without deer, in either understorey or treefall gap environments. Results suggest that in old-growth forest dominated by B. tawa, coarse woody debris associated with treefall gaps enables persistence of woody species that deer largely remove from the rest of the forest mosaic. Useful follow-up research could include modelling of the long-term impact of this effect on forest composition, and determination of the extent of its occurrence in other forest types.

Herbivore and detritivore effects on rainforest plant production are altered by disturbance.

Consumer effects on rainforest primary production are often considered negligible because herbivores and macrodetritivores usually consume a small fraction of annual plant and litter production, even though consumers are known to have effects on plant production and composition in nontropical systems. Disturbances, such as treefall gaps, however, often increase resources to understory food webs, thereby increasing herbivory and feeding rates of detritivores. This increase in consumption could lead to more prominent ecosystem‐level effects of consumers after disturbances, such as storms that cause light gaps. We determined how the effects of invertebrate herbivores (walking sticks) and detritivores (litter snails) on understory plant growth may be altered by disturbances in a Puerto Rican rainforest using an enclosure experiment. Consumers had significant effects on plant growth, but only in light gaps. Specifically, herbivores increased plant growth by 60%, and there was a trend for detritivores to reduce plant growth. Additionally, plant biomass tended to be 50% higher with both consumers in combination, suggesting that herbivores may mediate the effects of detritivores by altering the resources available to detritivore food webs. This study demonstrates that disturbance alters the effects of rainforest consumers, and, furthermore, that consumer activity has the potential to change rainforest successional processes.

Habitat use and seed removal by invasive rats (Rattus rattus) in disturbed and undisturbed rain forest, Puerto Rico

AbstractDespite frequent occurrences of invasive rats (Rattus spp.) on islands, their known effects on forests are limited. Where invasive rats have been studied, they generally have significant negative impacts on native plants, birds, and other animals. This study aimed to determine invasive rat distribution and effects on native plant populations via short‐term seed removal trials in tropical rain forest habitats in the Luquillo Experimental Forest, Puerto Rico. To address the first objective, we used tracking tunnels (inked and baited cards inside tunnels enabling animal visitors’ footprints to be identified) placed on the ground and in the lower canopy within disturbed (treefall gaps, hurricane plots, stream edges) and undisturbed (continuous forest) habitats. We found that rats are present in all habitats tested. Secondly, we compared seed removal of four native tree species (Guarea guidonia, Buchenavia capitata, Tetragastris balsamifera, and Prestoea acuminata) between vertebrate‐excluded and free‐access treatments in the same disturbed and undisturbed habitats. Trail cameras were used to identify animals responsible for seed contact and removal. Black rats (Rattus rattus) were responsible for 65.1% of the interactions with seeds, of which 28.6% were confirmed seed removals. Two plant species had significantly more seeds removed in disturbed (gaps) than undisturbed forest. Prestoea acuminata had the lowest seed removal (9% in 10 days), whereas all other species had &gt;30% removal. Black rats are likely influencing fates of seeds on the forest floor, and possibly forest community composition, through dispersal or predation. Further understanding of rat–plant interactions may be useful for formulating conservation strategies.

Could gaps and diverse topography compensate for habitat deficiency by the forest-dwelling bird Hazel Grouse (Tetrastes bonasia)?

The Hazel Grouse ( Tetrastes bonasia) is considered an indicator of the naturalness and heterogeneity of the forest environment. This species occurs in various woods and forages on a variety of plants. The aim of this study was to verify the hypothesis that, in Norway spruce-dominated forest stands, the Hazel Grouse compensates for habitat poverty using non-forest areas and harsh topography, which deliver food resources and shelter for the birds. We compared 14 environmental and topographic variables in forest patches occupied and unoccupied by the Hazel Grouse in the Tatra Mountains (South Poland). Multivariate modelling showed that crucial factors for Hazel Grouse occurrence in spruce-dominated high-mountain forests were southern or western exposures, the presence of streams or glades and a multilayer forest structure with lower plant species richness in the undergrowth layer. Moreover, this species avoided clearcuts and depended either on wild sites (with a high share of deadwood and treefall gaps in mature stands at higher altitudes) or juvenile stands. The results of our study confirmed that gaps in forests and diverse topography could compensate the Hazel Grouse for habitat deficiency. These findings may be important for effective protection of this species in high-mountain spruce-only dominated forests.

Primates create seedling growth hotspots through pattern of dung deposition

AbstractPrimates play important roles in tropical forests through seed dispersal and herbivory. They comprise a large part of the biomass of forest communities and tend to have clumped patterns of defecations (i.e. at favoured food trees or sleeping sites). Therefore, they may also play important roles in accelerating ecosystem nutrient cycling. Here we conduct a controlled growth experiment to quantify the effect of the addition of primate dung on the growth of both light‐demanding and shade‐tolerant seedlings over 1 year in Kibale National Park, Uganda. A mixed model analysis revealed that light‐demanding species were affected by the natural dung treatment and plants with small initial size had accelerated growth, whereas there was no effect on plant growth for shade‐tolerant species. The long‐term implications of increased dung deposition on the local tree community are unclear as shade‐tolerant species may only show an effect over the long‐term and light‐demanding species may only be able to take advantage of the increased growth if subsequently exposed to high light conditions, such as a treefall gap.

Influence of body size, topography, food availability and tree-fall gaps on space use by yellow-footed tortoises (Chelonoidis denticulatus) in Central Amazonia.

Habitat selection and extension of the area used by a given species may vary during different phases of its life and are often determined by the distribution and availability of resources throughout the landscape, such as food, breeding sites, and shelters. In this study, we assessed the influence of body size on the areas used by 21 individuals of the yellow-footed tortoises (Chelonoidis denticulatus) from January to June 2017 in a dense rain forest area in Central Amazonia. We also investigated whether individuals selected different ranges of terrain slope, elevation, areas with high food availability, or areas with treefall gaps that could be used for shelter or thermoregulation. We monitored tortoise movements using thread-bobbins, and sampled terrain characteristics, availability of potential food resources and forest gaps along the routes used by the tortoises. We also measured the same variables in plots distributed systematically throughout the study area to evaluate resource availability. Tortoises used an average area of 1.56 ha (SD = 1.51, min = 0.03, max = 6.44). The size of the area used was positively associated with the individual body size, but did not vary between sexes. Small individuals selected higher and flatter areas where the availability of fallen flowers was higher, whereas the area used by larger individuals did not differ from the natural availability of topographic traits and food in the region. Although tortoises did not select areas according to availability of tree-fall gaps, most larger tortoises were found sheltered under fallen trees (85%). Conversely, small individuals were mainly found hidden under litter (66%). Body size determined the patterns of landscape use by tortoises; larger individuals were mainly generalists, but small individuals occupied high and flat areas. The yellow-footed tortoise is endangered by hunting, illegal collection for the pet trade, habitat destruction and effects of climate change. Size-related differences in habitat selection should be taken into account in species-distribution models for the identification of suitable areas for reintroduction and the development of management plans in protected areas.

Seed Rain and Disturbance Impact Recruitment of Invasive Plants in Upland Forest

AbstractA critical question in invasion biology involves the relative importance of propagule rain and community invasibility. For plant invasions, invasibility is often related to disturbance, but few studies of forest invaders have simultaneously investigated both canopy and ground-level disturbance. We investigated the relative importance of seed rain, canopy disturbance, and soil disturbance in a mature forest in Maryland on the recruitment of four invasive species: wine raspberry (Rubus phoenicolasiusMaxim.), Japanese barberry (Berberis thunbergiiDC), multiflora rose (Rosa multifloraThunb.), and Japanese stiltgrass [Microstegium vimineum(Trin.) A. Camus]. Using complete censuses of a 9-ha plot at two points in time (2011–12 and 2014), we mapped new recruits, and related their locations to canopy and soil disturbance, as well as to a seed rain index based on locations of reproducing plants and seed-dispersal kernels.We found that propagule rain, as measured by the seed rain index, was a significant predictor of recruitment forB. thunbergii,R. phoenicolasius, andM. vimineum. ForR. multiflora, seed sources were not located, precluding assessment of propagule rain, but recruitment was linked to canopy disturbance, as was recruitment ofM. vimineum. However, because reproduction ofR. phoenicolasiusand, in some years, ofB. thunbergiiis higher in treefall gaps, these gaps experience higher propagule rain, with the result that recruitment is indirectly associated with these gaps. Ground-layer disturbance was an important predictor of recruitment only forB. thunbergii. Our findings reveal that the importance of propagule rain is the most consistent driver of recruitment, but canopy or ground-layer disturbance promotes recruitment of some invasive plant species.

Gradient (elevation) vs. disturbance (agriculture) effects on primary Cloud forest in Ecuador: seed predation, seed pathogens, germination

BackgroundGradients and disturbances affect plant communities. Cloud forests exist on elevational gradients and are often disturbed by being cleared for agriculture, so they present a unique opportunity for the investigation of how gradients and disturbances affect their structure, function and dynamics. The focus of the current study was on seed predation, seed pathogens and seed germination: all key factors in tree recruitment and regeneration.MethodsThree hundred seeds of three species—Solanum stenophyllum Bitter (Solanaceae: bird-dispersed), Palicourea amethystina (Ruiz & Pav.) DC. (Rubiaceae: bird-dispersed) and Clusia flaviflora Engl. (Clusiaceae: mammal-dispersed)—were studied at primary (1°) cloud forest at Guandera Reserve. After 2 weeks in the field, losses due to seed predation or pathogens were counted and the remaining seeds were tested for germination. These results were then compared with similar data collected and published previously from a 1° cloud forest and a secondary (2°) cloud forest, both at Maquipucuna Reserve, Ecuador.ResultsIn both closed-canopy forest and tree-fall gaps at Guandera Reserve, S. stenophyllum Dunal seeds suffered the greatest losses to predators, P. amethystina seeds had the greatest germination and C. flaviflora seeds had the greatest losses to pathogens. Comparison with data from Maquipucuna Reserve showed the following: (1) Solanum sp. suffered the greatest losses for seeds lost to seed predators in general but Cecropia sp. and Ficus sp. also had high losses in Maquipucuna 1° cloud forest; (2) for seeds lost to pathogens, species that lost the most seeds were unique to each study site: Clusia flaviflora seeds at Guandera 1° cloud forest, Cecropia sp. seeds at Maquipucuna 1° cloud forest and Piper aduncum L. seeds at Maquipucuna 2° cloud forest; (3) for seeds that germinated the most, species were again unique to the study site: Palicourea amethystina seeds at Guandera 1° cloud forest, Otoba gordoniifolia (A. DC.) A.H. Gentry seeds at Maquipucuna 1° cloud forest and Solanum ovalifolium Dunal seeds at Maquipucuna 2° cloud forest; and (4) in general, forest types differed significantly for both seed predation and seed pathogens. Within the 1° cloud forest at Maquipucuna, there was a significant difference among tree seed species for pathogens and a significant difference among the tree seed species for germination, and within the 2° cloud forest at Maquipucuna, there was a significant difference among tree seed species for pathogens.ConclusionsAs elevation increases in 1° cloud forests, the proportion of seed that germinates remain largely constant, but the major seed loss shifts from being due to predators to being due to pathogens. Conversion to agriculture also leads to seeds mainly lost to predators, but individual species loss levels depended on what crop had been planted previously.