Measuring spatial and temporal shifts in forest structure and composition in T high elevation beech forests in response to beech bark disease in Great Smoky Mountains National Park

Exotic forest pests and pathogens are among the most serious environmental threats to millions of hectares of forested land worldwide. Beech Bark Disease (BBD) is a non-native, pathogenic complex consisting of associations between scale insects and fungi. First confirmed in Great Smoky Mountains National Park (GRSM) in 1986, this complex has since threatened local high elevation beech forests, which are G-1 ranked (critically imperiled) forest communities where American beech (Fagus grandifolia Ehrh.) is a foundational tree species. In 1994, GRSM initiated the BBD Monitoring Protocol at 10 high elevation beech forest plots in the Park. The plots were sampled biennially from 1994 to 2012 and again in 2017 to investigate infestation patterns and host mortality. Permutational multivariate analysis of variance (PERMANOVA) and nonmetric multidimensional scaling (NMDS) techniques were used to investigate shifts in forest structure and composition over the 23-year study period. Species-specific changes were analyzed using repeated measures linear mixed-effects models (RMLMM). High elevation beech forest communities are changing through time with trajectories becoming more influenced by Acer and Betula species. Despite great variability in plot location, aspect, slope, and beginning species composition, time was a significant factor, explaining ~10% of the variation in relative basal area. Species-specific shifts were highly variable. While the overstory Fagus grandifolia basal area declined significantly over the 23-year study period, no significant change in total basal area was observed, indicating that the loss of these mature trees was compensated by co-occurring species. Understory sapling and woody seedling abundance of F. grandifolia increased relative to most other species in the study. The loss of the foundational species, F. grandifolia (the dominant tree species which defines high elevation beech forests), will have broad consequences for associated biota, ecosystem function, and potentially, the long-term persistence of high elevation beech forests in GRSM

Data from: Short-term, low-level nitrogen deposition dampens a trophic cascade between bears and plants

Human activities have substantially increased atmospheric nitrogen (N) deposition in ecosystems worldwide, often leading to higher plant quality for herbivores and greater herbivory. Predators frequently suppress herbivores and indirectly benefit plants via ‘trophic cascades’, and the strength of these interactions can also depend on N availability. However, the evidence for N deposition effects on cascades primarily comes from studies of high-level N deposition. Most terrestrial ecosystems currently receive elevated, but low-level N deposition, and it is unclear whether this subtle N enrichment has any effect on cascades. Here, I asked whether low-level N deposition alters a trophic cascade from black bears to plants in Colorado. In this ecological network, bears indirectly benefit plants by consuming ants and suppressing positive effects of ants on herbivores. Using a three year N enrichment experiment, I assessed changes in this cascade by measuring plant and arthropod responses to simulated N deposition, bear damage to ant nests, and the presence of mutualist herbivores and ants. I found that low-level N enrichment and bears had interacting effects on plant reproduction. In ambient N conditions, bears indirectly increased plant reproduction by causing ant nests to become inactive and suppressing positive ant effects on herbivores that were detrimental for plants. Yet, bear-induced ant nest inactivity had no effect on plant reproduction in N-enriched conditions. When N was added, ants had greater positive effects on herbivores, but herbivores had weak effects on plants, potentially because plants were more resistant to herbivores. Ultimately, the results indicate that N enrichment strengthened resource control of the community and weakened plant-herbivore interactions and the cascade from bears to plants. This study suggests that common rates of low-level N deposition are changing the strength of trophic cascades and may have already altered resource vs. consumer control of ecological community structure in many ecosystems

Plant&Insect_Data_Grinath_2018_E&E.csv

Data for plant-insect communities evaluated by Grinath (2018)

Measuring spatial and temporal shifts in forest structure and composition in T high elevation beech forests in response to beech bark disease in Great Smoky Mountains National Park

Exotic forest pests and pathogens are among the most serious environmental threats to millions of hectares of forested land worldwide. Beech Bark Disease (BBD) is a non-native, pathogenic complex consisting of associations between scale insects and fungi. First confirmed in Great Smoky Mountains National Park (GRSM) in 1986, this complex has since threatened local high elevation beech forests, which are G-1 ranked (critically imperiled) forest communities where American beech (Fagus grandifolia Ehrh.) is a foundational tree species. In 1994, GRSM initiated the BBD Monitoring Protocol at 10 high elevation beech forest plots in the Park. The plots were sampled biennially from 1994 to 2012 and again in 2017 to investigate infestation patterns and host mortality. Permutational multivariate analysis of variance (PERMANOVA) and nonmetric multidimensional scaling (NMDS) techniques were used to investigate shifts in forest structure and composition over the 23-year study period. Species-specific changes were analyzed using repeated measures linear mixed-effects models (RMLMM). High elevation beech forest communities are changing through time with trajectories becoming more influenced by Acer and Betula species. Despite great variability in plot location, aspect, slope, and beginning species composition, time was a significant factor, explaining ~10% of the variation in relative basal area. Species-specific shifts were highly variable. While the overstory Fagus grandifolia basal area declined significantly over the 23-year study period, no significant change in total basal area was observed, indicating that the loss of these mature trees was compensated by co-occurring species. Understory sapling and woody seedling abundance of F. grandifolia increased relative to most other species in the study. The loss of the foundational species, F. grandifolia (the dominant tree species which defines high elevation beech forests), will have broad consequences for associated biota, ecosystem function, and potentially, the long-term persistence of high elevation beech forests in GRSM

Effects of precipitation, nitrogen and predaceous spider manipulations on <i>Nicotiana tabacum</i> traits.

<p>Contrasts for the main effects are highlighted with colors corresponding with experimental treatments. <i>P</i>-values are from GLMs and significant differences (<i>P</i> ≤ 0.05) are indicated with stars, while marginally significant trends (<i>P</i> ≤ 0.10) are indicated by stars within parentheses. All results are shown as boxplots.</p

Interactive effects of precipitation and nitrogen enrichment on multi-trophic dynamics in plant-arthropod communities

<div><p>Patterns of precipitation and nitrogen (N) deposition are changing in ecosystems worldwide. Simultaneous increases in precipitation and N deposition can relieve co-limiting soil resource conditions for plants and result in synergistic plant responses, which may affect animals and plant responses to higher trophic levels. However, the potential for synergistic effects of precipitation and N deposition on animals and plant responses to herbivores and predators (via trophic cascades) is unclear. We investigated the influence of precipitation and N enrichment on ecological dynamics across three trophic levels, hypothesizing that herbivores and plants would exhibit synergistic responses to the combined influence of precipitation, N amendments and predators. To test this, we conducted a field experiment with arthropods on two model plant species, <i>Nicotiana tabacum</i> and <i>Nicotiana rustica</i>. First, we characterized the plant-arthropod assemblages, finding that <i>N</i>. <i>tabacum</i> hosted greater abundances of caterpillars, while <i>N</i>. <i>rustica</i> hosted more sap-sucking herbivores. Next, we evaluated the effects of rainwater, soil N, and predatory spider manipulations for both plant-arthropod assemblages. On <i>N</i>. <i>tabacum</i>, water and N availability had an interactive effect on caterpillars, where caterpillars were most abundant with rainwater additions and least abundant when both rainwater and N were added. For <i>N</i>. <i>rustica</i>, foliar chemistry had a synergistic response to all three experimental factors. Compared to spider-absent conditions, leaf N concentration increased and C/N decreased when spiders were present, but this response only occurred under high water and N availability. Spiders indirectly altered plant chemistry via a facilitative effect of spiders on sap-sucking herbivores, potentially due to intra-guild predation, and a positive effect of sap-suckers on foliar N concentration. Our study suggests that predictions of the ecological impacts of altered precipitation and N deposition may need to account for the effects of resource co-limitation on dynamics across trophic levels.</p></div

Interactive effects of precipitation and nitrogen enrichment on multi-trophic dynamics in plant-arthropod communities - Fig 1

<p><b>Diagram of interactions between soil resources, plants, herbivores and predators (A), and expected patterns of super-additive response when herbivores occur on high quality host plants (B).</b> In the diagram, consumer-resource interactions are indicated by reciprocal solid arrows, with the type of interaction provided between the arrows and the sign of the effects provided near each arrowhead. Soil resources and organisms can have indirect effects across this chain of interactions, called cascades (dashed arrows). Cascades occur from resources to consumers (resource cascades) and from consumers to resources (consumer cascades). The present study focuses on cascades from soil resources to herbivores and from predators to plants. In (B), we mapped our predictions for super-additive changes in plant and herbivore responses to greater precipitation and nitrogen deposition when plants are high in quality for herbivores. The bottom panel in (B) shows the effects of resources on plants and herbivores, while the top panel depicts indirect predator effects on plants. Predator effects on herbivores (not shown) are expected to be the opposite of predator effects on plants. These hypothetical results depict positive super-additive responses; negative super-additive responses would show the reverse pattern, with relatively large responses at low resource levels and with single resource additions and very small responses when both resources are added.</p

Path diagrams for the consumer cascade multi-group SEM analysis.

<p>Spider treatment (exogenous variable) is shown as a yellow box and has direct effects (single-headed paths) on herbivores, which in turn have direct effects on plant traits. Direct effects of foliar C and N concentrations on foliar C/N are also included to assess their relative contributions to this ratio. The dependent (endogenous) variables are depicted in white boxes; double-headed paths are unresolved correlations. Solid paths are positive and dashed paths are negative, and the width of all paths is scaled by standardized effect size (paths < 0.025 are weighted as 0.025 to remain visible). Significant (<i>P</i> ≤ 0.05) and marginally significant (<i>P</i> ≤ 0.10) paths are shown in color (red: positive, blue: negative) and indicated by stars and parentheses around stars, respectively. Both models had adequate fit to the data (<i>N</i>. <i>tabacum</i>: <i>n</i> = 75, χ² = 9.84, d.f. = 9, <i>P</i> = 0.364; <i>N</i>. <i>tabacum</i>: <i>n</i> = 74, χ² = 3.97, d.f. = 9, <i>P</i> = 0.914).</p

Effects of precipitation, nitrogen and predaceous spider manipulations on <i>Nicotiana tabacum</i> herbivores.

<p>Contrasts for main effects are highlighted with colors corresponding to treatments, except when a higher-order statistical interaction occurred. When this was the case, shading within the graph is used to indicate the two treatment groups that were significantly different from each other (Tukey post hoc: <i>P</i> ≤ 0.05). <i>P</i>-values are from GLMs and significant results (<i>P</i> ≤ 0.05) are indicated with stars.</p

Effects of precipitation, nitrogen and predaceous spider manipulations on <i>Nicotiana rustica</i> herbivores.

<p>Contrasts for main effects are highlighted with colors corresponding to treatments, except when a higher-order statistical interaction occurred. When this was the case, shading within the graph is used to indicate the two treatment groups that were different from each other (Tukey post hoc: <i>P</i> ≤ 0.10). <i>P</i>-values are from GLMs and significant results (<i>P</i> ≤ 0.05) are indicated with stars, while marginally significant trends (<i>P</i> ≤ 0.10) are indicated by stars within parentheses.</p