Search for: All records

A fundamental assumption of functional ecology is that functional traits are related to interspecific variation in performance. However, the relationship between functional traits and performance is often weak or uncertain, especially for plants. A potential explanation for this inconsistency is that the relationship between functional traits and vital rates (e.g., growth and mortality) is dependent on local environmental conditions, which would lead to variation in trait-rate relationships across environmental gradients. In this study, we examined trait-rate relationships for six functional traits (seed mass, wood density, maximum height, leaf mass per area, leaf area, and leaf dry matter content) using long-term data on seedling growth and survival of woody plant species from eight forest sites spanning a pronounced precipitation and soil phosphorus gradient in central Panama. For all traits considered except for leaf mass per area-mortality, leaf mass per area-growth, and leaf area-mortality relationships, we found widespread variation in the strength of trait-rate relationships across sites. For some traits, trait-rate relationships showed no overall trend but displayed wide site-to-site variation. In a small subset of cases, variation in trait-rate relationships was explained by soil phosphorus availability. Our results demonstrate that environmental gradients have the potential to influence how functional traits are related to growth and mortality rates, though much variation remains to be explained. Accounting for site-to-site variation may help resolve a fundamental issue in trait-based ecology – that traits are often weakly related to performance – and improve the utility of functional traits for explaining key ecological and evolutionary processes. 

Elevational and latitudinal gradients in species diversity may be mediated by biotic interactions that cause density‐dependent effects of conspecifics on survival or growth to differ from effects of heterospecifics (i.e. conspecific density dependence), but limited evidence exists to support this. We tested the hypothesis that conspecific density dependence varies with elevation using over 40 years of data on tree survival and growth from 23 old‐growth temperate forest stands across a 1,000‐m elevation gradient. We found that conspecific‐density‐dependent effects on survival of small‐to‐intermediate‐sized focal trees were negative in lower elevation, higher diversity forest stands typically characterised by warmer temperatures and greater relative humidity. Conspecific‐density‐dependent effects on survival were less negative in higher elevation stands and ridges than in lower elevation stands and valley bottoms for small‐to‐intermediate‐sized trees, but were neutral for larger trees across elevations. Conspecific‐density‐dependent effects on growth were negative across all tree size classes and elevations. These findings reveal fundamental differences in biotic interactions that may contribute to relationships between species diversity, elevation and climate.

 

Identifying key traits that can serve as proxies for species drought resistance is crucial for predicting and mitigating the effects of climate change in diverse plant communities. Turgor loss point (π_tlp) is a recently emerged trait that has been linked to species distributions across gradients of water availability. However, a direct relationship between π_tlpand species ability to survive drought has yet to be established for woody species. Using a manipulative field experiment to quantify species drought resistance (i.e., their survival response to drought), combined with measurements of π_tlpfor 16 tree species, we show a negative relationship between π_tlpand seedling drought resistance. Using long‐term forest plot data, we also show that π_tlppredicts seedling survival responses to a severe El Niño‐related drought, although additional factors are clearly also important. Our study demonstrates that species with lower π_tlpexhibit higher survival under both experimental and natural drought. These results provide a missing cornerstone in the assessment of the traits underlying drought resistance in woody species and strengthen π_tlpas a proxy for evaluating which species will lose or win under projections of exacerbating drought regimes.

 

In tropical forests, drought and herbivory represent two potent stresses on seedlings. Climate change is expected to increase the frequency of severe droughts in many tropical forests, which may influence seedling vulnerability to herbivores if drought stress affects seedling palatability. Furthermore, contrasting selective pressures in wetter vs drier forests could mean that species well‐adapted to herbivores are less drought resistant and vice versa. In this study, we measured seedling performance and herbivory in a common garden experiment where seedlings of 15 tree species were subjected to irrigation or rainfall exclusion treatments across two dry seasons in Panama. Water manipulation had no effects on foliar herbivory during the experiment for all species combined and for 14 of the 15 focal species when analyzed separately. There was large variation among species in herbivore damage, but no relationship between the sensitivity of species to drought and the amount of herbivory they experienced. Altogether, our findings suggest that increasing drought stress is unlikely to directly alter tropical tree seedling susceptibility to herbivore attack in this forest. Additional studies are needed to determine whether drought alters tropical plant‐herbivore interactions via other mechanisms, such as through changes in herbivorous insect communities and/or increases in fitness costs of herbivory.

Abstract in Spanish is available with online material.

 

Forest conversion and habitat loss are major threats to biological diversity. Forest regeneration can mitigate the negative effects of old‐growth forest loss on species diversity, but less is known about the extent to which forest loss reduces genetic diversity in remnant populations and whether secondary forests play a role in the maintenance of genetic diversity. We quantified genetic diversity in a tropical hummingbird‐pollinated understorey herb,Heliconia tortuosa, across a landscape mosaic of primary and secondary forest regrowth. Using microsatellite genotypes from >850 adult and juvenile plants within 33 forest patches and extensive bird surveys, we examined the effect of contemporary and historical landscape features including forest age (primary vs. secondary forest), stand isolation and pollinator assemblages on genetic diversity and levels of inbreeding inH. tortuosa. We found that inbreeding was up to three times higher in secondary forest, and this effect was amplified with reductions in primary forest in the surrounding landscape through reduced observed heterozygosity in isolated fragments. Inbreeding in forest patches was negatively correlated with the local frequency of specialist long‐distance foraging traplining hummingbirds. Traplining hummingbirds therefore appear to facilitate mating among unrelated plants—an inference we tested using empirically parameterized simulations. Higher levels of inbreeding inH. tortuosaare therefore associated with reduced functional diversity of hummingbirds in secondary forests and forest patches isolated from primary forests. Our findings suggest a cryptic consequence of primary forest loss and secondary forest regeneration through the disruption of mutualistic interactions resulting in the erosion of genetic diversity in a common understorey plant.

 

As extreme climate events are predicted to become more frequent because of global climate change, understanding their impacts on natural systems is crucial. Tropical forests are vulnerable to droughts associated with extreme El Niño events. However, little is known about how tropical seedling communities respond to El Niño–related droughts, even though patterns of seedling survival shape future forest structure and diversity. Using long‐term data from eight tropical moist forests spanning a rainfall gradient in central Panama, we show that community‐wide seedling mortality increased by 11% during the extreme 2015–16 El Niño, with mortality increasing most in drought‐sensitive species and in wetter forests. These results indicate that severe El Niño–related droughts influence understory dynamics in tropical forests, with effects varying both within and across sites. Our findings suggest that predicted increases in the frequency of extreme El Niño events will alter tropical plant communities through their effects on early life stages.

 

Deforestation can impact the quality of pollen received by target plants (i.e. delivery of incompatible pollen, self‐pollen or pollen from closely related individuals). Such reductions in plant mating quality may be direct, when deforestation reduces plant population size and the availability of pollen donors, or indirect, when decreased mating quality results, for example, from shifts in the composition of the pollinator community. As most flowering plants depend on animal pollinators for reproduction, there is a need to understand the direct and indirect links between deforestation, pollinator community composition and plant mating quality.

We quantified the direct, pollen donor‐mediated and indirect, pollinator‐mediated effects of deforestation on mating quality inHeliconia tortuosa, a tropical herb pollinated by low‐ and high‐mobility hummingbirds. We used a confirmatory path analysis to test the hypothesis that deforestation (amount of forest cover and forest patch size) influenced mating quality (haplotype diversity of pollen pools, outcrossing and biparental inbreeding) directly and indirectly through functional shifts in the composition of pollinator communities (proportion of high‐mobility hummingbirds).

We found that deforestation triggered functional shifts in the composition of pollinator communities, as the proportion of high‐mobility hummingbirds increased significantly with the amount of forest cover and forest patch size. The composition of the pollinator community affected mating quality, as the haplotype diversity of pollen pools increased significantly with the proportion of high‐mobility hummingbirds, while biparental inbreeding decreased significantly. Although we did not detect any significant direct, pollen donor‐mediated effects of deforestation on mating quality, reductions in the amount of forest cover and forest patch size resulted in functional shifts that filtered out high‐mobility hummingbirds from the pollinator community, thereby reducing mating quality indirectly.

Synthesis. Deforestation primarily influenced plant mating quality through a cascading effect mediated by functional shifts in the composition of the pollinator community. Our results indicate that plant mating quality strongly depends on the composition of local pollinator communities. Functional shifts that filter out highly mobile and effective pollinators may reduce the transfer of genetically diverse pollen loads from unrelated plants. Such shifts may have pronounced effects on plant population dynamics and disrupt genetic connectivity.