The following are the latest papers accepted for publication in Journal of Ecology.
Resilience to chronic defoliation in a dioecious understorey tropical rain forest palm
Lopez-Toledo et al.
Accepted 4 May 2012
1. Perennial plants often endure chronic loss of leaf area due to recurrent physical damage, herbivory and, for species used as non-timber forest products, due to leaf harvesting. However, little is known about functional and demographic resilience (extent and speed of recovery) of plants subjected to varying levels of chronic defoliation.
2. We used a dioecious, understorey palm (Chamaedorea elegans) to evaluate temporal trajectories and rates of recovery of leaf functional traits and vital rates (survival, growth, and reproduction) after being subjected to experimental chronic defoliation regimes.
3. Pristine populations of mature C. elegans, categorized by gender (male and female), were subjected to five defoliation levels (0%, 33%, 50%, 66%, or 100% of newly produced leaves) every six months over a period of three years (1997–2000). To evaluate recovery from defoliation, surviving palms were monitored for three years after the cessation of the defoliation treatment (2000–2003). We recorded leaf functional traits (leaf persistence, leaf production rate, leaf size and leaf area) and annual rates of mortality, growth and reproduction.
4. Cumulative effects of chronic defoliation concomitantly reduced leaf traits, survival, growth and reproduction, and this effect was stronger in female than in male palms, independent of plant size. Recovery from defoliation was faster in males than in females but proceeded gradually overall. Survival increased first, followed by growth, while reproductive traits showed the slowest recovery rate. Recovery was independent of plant size. Notably, three years after defoliation treatment the standing leaf area and probability of reproduction had not recovered to pre-defoliation levels. Additionally, we found that the occurrence of a severe drought in the first year (2000) after defoliation ceased, led to decreased survival, growth, and reproduction and the ability of plants to recover from defoliation.
5. Synthesis. Chronic defoliation reduces fitness components of C. elegans palms differentially between genders. Recovery is gradual and is slower and less complete in females compared to males. The lower level of resilience to chronic defoliation shown by female plants may have profound consequences for the dynamics and genetic variability of populations of tropical understorey plants undergoing prolonged defoliation. Such effects may be aggravated by severe drought episodes, which are expected to increase in frequency according to global climate change predictions
Resource-based habitat associations in a neotropical liana community
Dalling et al.
Accepted 30 April 2012
1. Lianas are a conspicuous element of many tropical forests, accounting for up to 40% of woody stem density and 20% of species richness in seasonal forests. However, lianas have seldom been surveyed at sufficiently large spatial scales to allow an assessment of the importance of habitat variables in structuring liana communities.
2. We compare the association patterns of 82 liana species and an equivalent sample of tree species on the 50 ha Forest Dynamics Project plot on Barro Colorado Island (BCI), Panama, with topographic habitat variables (high and low plateau, slope, swamp and streamside), and thirteen mapped soil chemical variables. In addition, we test for liana species associations with canopy disturbance using a canopy height map of the plot generated using LiDAR.
3. For all liana species combined, densities differed among topographic habitat types in the plot, with significantly higher densities on the seasonally drier lower plateau habitat (1044 individuals ha-1) than the moister slope habitat (729 individuals ha-1). Lianas were also significantly more abundant than expected in areas with low canopy height.
4. The proportion of liana species associated with one or more topographic habitat variables (44%) was significantly lower than that for trees (66 %). Similarly, liana species were significantly less frequently associated with PC axes derived from soil chemical variables (21%) than trees (52%). The majority of liana species (63%) were significantly associated with areas of the plot with low canopy height reflecting an affinity for treefall gaps.
5. Synthesis. The habitat associations detected here suggest that liana density is associated primarily with canopy disturbance, and to a lesser extent with topography and soil chemistry. Relative to trees, few liana species were associated with local variation in topography and soil chemistry, suggesting that nutrient availability exerts only weak effects on liana community composition compared to trees. Results from this study support the contention that increases in forest disturbance rates are a driver of recently observed increases in liana abundance and biomass in neotropical forests.
Landscape context and management regime structure plant diversity in grassland communities
Schmucki et al.
Accepted 24 April 2012
1. Theoretical models show that environmental heterogeneity and dispersal are major determinants of species diversity at multiple scales, yet there are few studies from real landscapes that adequately integrate variation in the surrounding matrix. Understanding how landscape context and management influence species composition and diversity patterns across habitats and scales is an important goal in ecology with relevance for both management and conservation.
2. We used a system of 25 landscapes distributed across islands in the Baltic Sea to investigate the effect of current and historical landscape context and management on plant diversity and composition in grassland communities. Plant diversity was measured at three hierarchical scales (1 m2, habitat, landscape) in grazed fields and adjacent wood pastures to calculate α-, β-, and γ-diversity values across habitats and scales.
3. Structural equation modelling (SEM) was used to model and quantify the effects of landscape context on species diversity and spatial turnover, and Constraint Analysis of Principal coordinates (CAP) to relate variation in species composition to landscape variables.
4. Proportion of open land, spacing and grazing intensity positively affected species diversity in both habitats, while the effect of historical landscape context was only significant in open fields. Plant diversity in field pastures was mainly determined by the number of species found at a small scale, while both local species density and spatial turnover were key determinants of diversity in wood pastures.
5. Habitat proximity influenced species composition as compositional similarity was higher between adjacent field and wood pastures compared to randomly paired habitats. While increasing flow of propagules from adjacent patches can promote local coexistence, dispersal can result in spatial homogenisation.
6. Synthesis. Plant diversity in grassland communities is substantially influenced by species occurring in adjacent habitats. While the effect of landscape context and management on small-scale diversity was consistent across habitats, the effect on spatial turnover was habitat specific. Our study shows that plant diversity is structured through the interplay between local and landscape processes and highlights that plant communities in specific habitat types cannot be considered in isolation from the surrounding landscape matrix.
1. Predicting and mitigating climate change effects on ecological communities is a tremendous challenge. Little attention has been given to how endemic-rich communities on isolated patches of low-nutrient soil (e.g. serpentine) will respond to climate change.
2. To address spatial factors (the isolated nature of outcrops), we incorporate habitat patchiness into species distribution models under climate change. The degree of overlap between current and future suitable habitat does not change when patchy habitats are incorporated, probably because serpentine occurs in mountainous regions where climatically and edaphically suitable regions geographically coincide. The dispersal distances required to move to newly suitable habitat are large, however, making successful migration unlikely.
3. To address how non-spatial factors affect the climate change responses of serpentine plant communities (e.g. the impacts of nutrient limitation and stress-tolerant functional traits), we conduct a literature review. Some studies suggest that serpentine communities may be at less risk than ‘normal’ soil communities due to their stress-tolerant functional traits, but there is also evidence to the contrary.
4. Synthesis. Assessing climate change risk for the world’s diverse edaphic floras requires determining impacts on both special and “normal” soil communities. Studies are needed that use functional traits, evaluate the role of evolutionary plasticity, examine responses across spatial and temporal scales, and assess the efficacy of managed relocation efforts.
1. Although it is recognized that plant species vary in their flammability, we currently lack a mechanistic understanding of how plant traits influence fire and how litter mixtures behave in a fire. As modified fire regimes and climate change shift the species composition of communities, a mechanistic perspective is especially important in order to understand and predict fire in potentially novel plant communities. This work addresses three questions: (i) How do eight species common in Sierra Nevada mixed-conifer forest differ in their litter flammability?; (ii) What leaf traits are associated with various flammability components?; and (iii) Do individual species measurements predict multi-species combinations, or are there non-additive effects?
2. Leaf litter was collected in Sequoia and Kings Canyon National Parks, California, from eight dominant tree species in mixed-conifer forest. Controlled flammability tests were performed on reconstructed monospecific litter beds and on mixed litter beds, using litter from three species. We tested for non-additive effects in multi-species mixtures using the weighted mean of single-species measures for each flammability component as a null expectation for each mixture; departures from this null indicated non-additive effects.
3. Most flammability components fell within two major axes of variation, one relating to total heat release and another to fire intensity. The eight species differed significantly in all flammability components, with large-leaved species creating litter that burned with higher intensity.
4. Non-additive species mixture effects are common in this system. Flammability tends to be driven by the most flammable component of the mixture.
5. Synthesis. We have demonstrated positive non-additive effects in mixtures of leaf litter. The most flammable constituent species of a mixture has disproportionate effects on the fire environment faced by the entire community. This could potentially influence community assembly and alter the selective environment faced by co-occuring species.
1. Reciprocal transplant experiments designed to quantify genetic and environmental effects on phenotype are powerful tools for the study of local adaptation. For long-lived species, especially those in habitats with short growing seasons, however, the cumulative effects of many years in novel environments may be required for fitness differences and phenotypic changes to accrue.
2. We returned to two separate reciprocal transplant experiments thirty years after their initial establishment in interior Alaska to ask whether patterns of differentiation observed in the years immediately following transplant have persisted. We also asked whether earlier hypotheses about the role of plasticity in buffering against the effects of selection on foreign genotypes were supported. We censused survival and flowering in three transplant gardens created along a snowbank gradient for a dwarf shrub (Dryas octopetala) and six gardens created along a latitudinal gradient for a tussock-forming sedge (Eriophorum vaginatum). For both species, we used an analysis of variance to detect fitness advantages for plants transplanted back into their home site relative to those transplanted into foreign sites.
3. For D. octopetala, the original patterns of local adaptation observed in the decade following transplant appeared even stronger after three decades, with the complete elimination of foreign ecotypes in both fellfield and snowbed environments. For E. vaginatum, differential survival of populations was not evident 13 years after transplant, but was clearly evident 17 years later. There was no evidence that plasticity was associated with increased survival of foreign populations in novel sites for either D. octopetala or E. vaginatum.
4. Synthesis. We conclude that local adaptation can be strong, but nevertheless remain undetected or underestimated in short-term experiments. Such genetically-based population differences limit the ability of plant populations to respond to a changing climate.
1. It is expected that climate warming will enhance tree growth of mountain conifer forests in cold regions. However, trees have shown unstable, age-related and site-dependent growth responses to climate throughout the past century, but information on the drivers controlling such responsiveness at the site and tree scales is lacking. We evaluated whether such changing growth responses are more influenced by site features, such as altitude, or by tree features, such as size and sapwood area.
2. We quantified the growth trends at the site and tree levels in Iberian Pinus uncinata forests using dendrochronology. Tree-ring width was converted to basal area increment (BAI) to assess the relationships between growth and site and tree variables over three time periods (1901–1994, 1901–1947, 1948–1994) using structural equation models.
3. Trees were older at higher altitudes, and the amount of sapwood decreased as trees aged. BAI trends were lower in the period 1948–1994 than in the period 1901–1947, i.e. tree growth is decelerating, despite BAI values of both periods showing the reverse pattern. Sapwood area and, to a minor extent, tree age were the main positive and negative drivers, respectively, controlling BAI during the 20th century, whereas altitude played a minor role.
4. Synthesis. Our results highlight the relevance of tree individual characteristics as the main drivers modulating growth responses to climate warming. We conclude that climate warming will have a lower effect on radial growth in slow-growing high elevation trees than in fast-growing low elevation trees, which produce a greater sapwood area. Trees may become relatively insensitive to climate as they age and reach a size-related functional threshold linked to reduced sapwood production.
1. Determining the drivers of plant demography is integral to understanding the processes that shape plant species abundances and distributions. Despite recognition that interactions among drivers have important effects on demographic processes, few demographic studies test for interactions among multiple drivers in plants.
2. We used a factorial-design experiment to study the interactive effects among three common forms of disturbance in the tropics (fire history, grazing and leaf harvest by humans) on the vital rates of Phoenix loureiri (mountain date palm) in South India. In addition, we tested for interactive effects among these disturbances, abiotic conditions and plant size. We also tested for non-consumptive effects of grazing and harvest, such as trampling, by measuring the intensities of grazing and harvest in plots open to these disturbances.
3. Intensities of leaf harvest and grazing varied with abiotic conditions and disturbance. Leaf harvest decreased with increasing grazing intensity, suggesting that the net effect of harvest on palm populations is less where it co-occurs with grazing. In areas without fire, plots with lower soil moisture had higher grazing intensities.
4. We found multiple significant main and interactive effects of disturbance on palm vital rates. Palm mortality increased with fire and grazing. Grazing and harvest reduced growth, but growth increased following fire. The negative impact of harvest on palm individuals was reduced when harvest occurred in plots with fire.
5. We found evidence of nonconsumptive effects of grazing and harvest on palm growth, likely from trampling. Studies inferring the effects of grazing by comparing grazed and ungrazed individuals within an area where grazing occurs will likely underestimate grazing effects.
6. Synthesis. Our findings reveal that Phoenix loureiri demographic rates are driven by interactive effects among multiple forms of disturbance and abiotic factors, and that the intensities of disturbance are themselves driven by interactions between other forms of disturbance and abiotic factors. These results illustrate that understanding the effects of, and interactions among, multiple drivers will be key in attempts to mitigate the effects of environmental change on plant species declines.
1. Facilitation allows some plant species to occupy environments that they are otherwise unable to inhabit, potentially leading to greater fitness, greater productivity or abundance, and range expansions. However, we know little about the costs incurred by facilitators or how variation in resource allocation by facilitators influences effects and costs.
2. Gynodioecy provides an opportunity to explore the effects of facilitation and potential costs because females and hermaphrodites differ in resource allocation patterns. We explored whether environmental stress and gender-specific facilitation influence species interactions in an alpine plant community. We investigated the degree of facilitation and correlative associated costs for the gynodioecious alpine plant, Silene acaulis, at two elevations (2317 m and 2560 m) in the northern Rocky Mountains of Montana, USA.
3. Hermaphroditic Silene individuals supported a greater number of plant species and individual plants than female Silene (respectively: hermaphrodites = 4.2 ± 0.3, females = 3.5 ± 0.2; hermaphrodites = 11.5 ± 1.0, females = 9.1 ± 1.1). Facilitative effects on species richness significantly increased with elevation (2.1 ± 1.6 species at the low site versus 3.2 ± 1.8 species at the high site), but abundance, and percent cover did not.
4. Silene fitness at the high elevation site (2560 m) was reduced compared to the lower site (2317 m) as measured by flower and seed production. Female flower production decreased by 40%, the number of seeds per fruit by 11.6%, and leaf size by 24%.
5. Increases in the percent cover of beneficiaries reduced the number of flowers per Silene plant, indicating a cost of facilitating other species, and this cost was slightly greater for females. However, seed size for hermaphrodites and leaf length for females increased with the percent cover of beneficiaries, suggesting a mutual benefit of harbouring other species.
6. Synthesis. Our results show that gender can affect the balance between competitive and facilitative interactions. Stronger facilitation by hermaphroditic Silene acaulis, coupled with a lower cost of harbouring beneficiaries, suggests that life history traits and related patterns of resource allocation can influence the facilitative effects of a species.
The interacting effects of clumped seed dispersal and distance- and density-dependent mortality on seedling recruitment patterns
Beckman, Neuhauser & Muller-Landau
Accepted 22 March 2012
1. Seed dispersal and natural enemies both influence spatial patterns of seedlings, which in turn influence future abiotic and biotic interactions, with consequences for plant populations, distributions, and diversity. Clumped seed deposition is common, especially for vertebrate-dispersed seeds, and has the potential to significantly affect interactions with density-responsive enemies, yet has received relatively little attention.
2. We used spatially explicit simulation models to examine how different patterns of seed dispersal and natural enemy attack structure seedling spatial patterns. We simulated clumped seed dispersal by combining a two-dimensional Student’s T dispersal kernel for expected seed rain with a negative binomial distribution for seed deposition. We based our models for seed mortality on published data reflecting differing life histories of insect seed predators and soil-borne pathogens. We varied dispersal distance, degree of clumping, type of enemy, enemy dispersal distance, and fecundity among simulations.
3. Under insect seed predation, seeds escaped predation by dispersing longer distances than insects, resulting in “Janzen-Connell” patterns in which seedling recruitment peaks at intermediate distances. When insects dispersed longer distances than seeds, higher seed densities near the tree satiated insects, resulting in “McCanny” patterns in which seed deposition, survivorship, and seedling establishment all decrease with distance from the parent tree. Total seedling establishment was lowest when insects and seeds dispersed similar distances.
4. Under pathogen attack, Janzen-Connell patterns predominated except when seedling survival was virtually zero or one everywhere, or in the case where pathogen dispersal distances exceeded seed dispersal distances, producing “Hubbell” patterns in which seed deposition and seedling establishment decrease with distance, though survivorship increases.
5. Clumped seed deposition increased the probability of seedling establishment under both insect seed predation and pathogen attack as it led to local satiation of insect seed predators and made it harder for pathogen distributions to track seeds.
6. Synthesis. Our modelling study suggests that the relative dispersal distances of seeds and natural enemies are crucial to determining establishment rates and spatial patterns of seedlings. Better characterization of the movement and natural histories of natural enemies is critical to improving our understanding of seedling distributions and plant-enemy interactions.
1. There are now approximately ten examples of plants that use volatile cues emitted by damaged neighbours to adjust their defences against herbivores. For two of these examples, preliminary evidence suggested that plants may experience net benefits from such eavesdropping, although eavesdropping was uncommon in one case and estimates of plant fitness were ambiguous in the other case.
2. In this current study, we examined the long-term consequences of exposure to cues emitted by experimentally clipped sagebrush neighbours. In this sagebrush system we have repeatedly found that sagebrush plants that have experimentally clipped neighbours experience less herbivore damage over the season than plants with unclipped control neighbours under field conditions. We followed a cohort of young sagebrush plants from emergence in 1999 for 12 years. Neighbours of half of these plants were artificially clipped every spring from 2004–2008 and survival and flowering was measured in each autumn from 1999–2011.
3. Survival of marked branches of young plants was not consistently affected by whether its neighbour was clipped. Plants near clipped neighbours produced more branches during this period than those near unclipped neighbours. There were no measurable treatment effects on plant survival over the 12 yrs. Branches near clipped neighbours produced more inflorescences than branches near unclipped neighbours.
4. Seedlings were more likely to survive to the end of their first dry season in two different years near clipped neighbours compared to unclipped neighbours.
5. Synthesis. The results suggest different effects of clipped neighbours that depend on plant age. Responding to the cues of experimental clipping may provide a slight net benefit, considering these results and other published studies, even though these cues provided little predictive value about actual risk of herbivory. Responding to reliable cues may be even more beneficial and may favour plants that eavesdrop on neighbours.
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