Short‐term effects of herbicides and a prescribed fire on restoration of a shrub‐encroached pine savanna

Shrub encroachment occurring worldwide in savannas and grasslands has commonly been hypothesized to result from anthropogenically altered environments. Two disturbance‐based approaches to restoration have involved: (1) application of selective herbicides to reduce density/cover of shrubs; (2) reinstatement of natural fire regimes to generate environmental conditions favoring herbaceous species. We studied short‐term responses of native shrubs, vines, and grasses in a Louisiana pine savanna to herbicides followed by a prescribed fire and fire alone. Plots established in the summer, 2013, were hand‐sprayed in the fall with Imazapyr and Triclopyr, Triclopyr alone, or no herbicide, then prescribed burned the following spring. Numbers of species of shrubs and vines at scales of 1 and 100 m², numbers of stems and regrowth of stems produced by six common species of shrubs, and the number of flowering culms of perennial C₄ grasses were assessed postfire in 2014. Compared with fire alone, herbicides followed by fire resulted in (1) small reductions in species richness of shrubs and no effects on vines, (2) fewer stems comprising shrub genets, but similar postfire regrowth of resprouting shrub stems, and (3) fewer flowering culms of C₄ grasses. Underground storage organs of savanna shrubs and vines survived both aboveground disturbances. Thus, single applications of herbicides followed by fires reduced, but did not reverse shrub encroachment and negatively affected grasses. Because effects of herbicides overrode those of prescribed fires, these disturbances did not act synergistically, suggesting that reinstating natural fire regimes should be a priority in restoration of savannas and grasslands.     

Shrub facilitation drives tree establishment in a semiarid fog‐dependent ecosystem

Questions

The exceptional occurrence of tall rain forest patches on foggy coastal mountaintops, surrounded by extensive xerophytic shrublands, suggests an important role of plant–plant interactions in the origin and persistence of these patches in semi‐arid Chile. We asked whether facilitation by shrubs can explain the growth and survival of rain forest tree species, and whether shrub effects depend on the identity of the shrub species itself, the drought tolerance of the tree species and the position of shrubs in regard to wind direction.

Location

Open area–shrubland–forest matrix, Fray Jorge Forest National Park, Chile.

Methods

We recorded survival after 12 years of a ~3600 tree saplings plantation (originally ~30‐cm tall individuals) of Aextoxicon punctatum, Myrceugenia correifolia and Drimys winteri placed outside forests, beneath the shrub Baccharis vernalis, and in open (shrub‐free) areas. We assessed the effects of neighbouring shrubs and soil humidity on survival and growth along a gradient related to the direction of fog movement.

Results

B. vernalis had a clear facilitative effect on tree establishment and survival since, after ~12 years, saplings only survived beneath the shrub canopy. Long‐term survival strongly depended on tree species identity, drought tolerance and position along the soil moisture gradient, with higher survival of A. punctatum (>35%) and M. correifolia (>14%) at sites on wind‐ and fog‐exposed shrubland areas. Sites occupied by the shrub Aristeguietia salvia were unsuitable for trees, presumably due to drier conditions than under B. vernalis.

Conclusions

Interactions between shrubs and fog‐dependent tree species in dry areas revealed a strong, long‐lasting facilitation effect on planted tree's survival and growth. Shrubs acted as benefactors, providing sites suitable for tree growth. Sapling mortality in the shrubland interior was caused by lower soil moisture, the consequence of lower fog loads in the air and thus insufficient facilitation. While B. vernalis was a key ecosystem engineer (nurse) and intercepted fog water that dripped to trees planted underneath, drier sites with A. salvia were unsuitable for trees. Consequently, nurse effects related to water input are strongly site and species specific, with facilitation by shrubs providing a plausible explanation for the initiation of forest patches in this semi‐arid landscape.     

The resprouting response of co‐occurring temperate woody plant and grass species to elevated [CO2]: An insight into woody plant encroachment of grasslands

The phenomenon of woody plant thickening in grasslands has been observed globally and is likely to have widespread ecological consequences. It has been proposed that woody plant thickening is driven in part by rising atmospheric [CO₂] enhancing the resprouting ability of woody plants relative to grasses so they respond more strongly after disturbances such as herbivory and fire. The aim of this study was to examine the CO₂ effect on the resprouting ability of 16 co‐occurring temperate woody plant and grass species (eight species from each growth form). Plants were grown in a controlled glasshouse experiment under ambient (400 ppm) and elevated [CO₂] (600 ppm) for 14 weeks after which their resprouting ability was measured. Root non‐structural carbohydrate (NSC_mass) and nitrogen (N_mass) storage was used as proxies to measure the resprouting ability of woody plants while for the grasses it was measured directly. We found that both the woody plants (22% on average; P = 0.003) and grasses (20% on average; P = 0.003) produced more biomass under elevated [CO₂]. Despite the woody plants not allocating additional carbon to belowground storage under elevated [CO₂], they had significantly greater root NSC_mass (23% on average; P = 0.007) due to increased root biomass production (8% on average; P = 0.007). In contrast, root N_mass of the woody plants did not differ between CO₂ treatments (P = 0.373). Surprisingly, the resprouting ability of the grasses did not significantly differ between the CO₂ treatments (P = 0.067). These results provide evidence that the differing resprouting response of woody plants and grasses under elevated [CO₂] may be contributing to woody plant encroachment of grasslands worldwide.     

Long‐term livestock exclusion facilitates native woody plant encroachment in a sandy semiarid rangeland

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Influence of differently managed bush‐encroached sites on the large herbivore distribution in the Namibian Savannah

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Multi‐scale habitat selection of Mus minutoides in the Lowveld of Swaziland

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Lotus SHAGGY‐like kinase 1 is required to suppress nodulation in Lotus japonicus

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A fungal endophyte of a palatable grass affects preference of large herbivores

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Species‐ and site‐specific impacts of an invasive herbivore on tree survival in mixed forests

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Diving behavior in a free‐living,semi‐aquatic herbivore,the Eurasian beaver Castor fiber

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Generalist trophic ecology in a changing habitat: The case of the four‐striped mouse in a woody‐encroached savannah

Trophic generalists tolerate greater habitat change than specialists; however, few studies explore how generalist trophic ecology is affected. We established how the trophic ecology of an extreme generalist, Rhabdomys pumilio, changed in relation to a directionally changing woody‐encroached savannah in Eastern Cape, South Africa by investigating (a) foraging behaviour, (b) trophic niche and (c) feedback effects. (a) Giving‐up densities showed that R. pumilio preferred foraging in subcanopy microhabitat during the night as a result of lower thermoregulatory costs, but had similar preferences for sub‐ and intercanopy microhabitats during the day. (b) An isotope analysis revealed that the dietary composition and trophic niche occupied by R. pumilio differed among tree canopy cover levels (0%, 30% and 80%), which appeared to be related to changes in C₄ grass material and invertebrate availability. (c) Artificial seed patches suggested that R. pumilio was a potentially important postdispersal seed predator of the woody‐encroaching species, Vachellia karroo. Thus, an increase in tree canopy cover altered the trophic niche of R. pumilio by reducing foraging costs at night and providing alternative food resources in terms of availability and source. These findings demonstrate how an extreme generalist adapted to human‐induced habitat change through changes in its trophic ecology.     

Effects of plant age on performance of the tropical perennial fodder grass,Cenchrus ciliaris L. subjected to elevated ultraviolet‐B radiation

• Abiotic stress, notably high ultraviolet‐B (eUV‐B), limit growth and productivity of many crop plants, but information on response of forage grasses to eUV‐B radiation is rather limited.
• The present study was therefore conducted to increase our understanding of differential age‐related responses on growth, metabolism and fodder quality of Cenchrus ciliaris‐3108 (Buffel grass) to elevated UV‐B (eUV‐B: 7.2 kJ·m^?2·day^?1).
• Plant growth at both growth stages was notably reduced in response to eUV‐B, except for the number of nodes and tillers at vegetative and reproductive stages. At anthesis, tillering increased due to the perennial habit of this plant, but leaf senescence reduced the number of leaves per tiller. Unlike ambient UV‐B, eUV‐B at the vegetative stage resulted in diversion of photosynthate for the formation of secondary metabolites (tannins and phenolics), providing dual protection from photooxidative damage and from herbivory.
• The forage biomass as well as quality showed a marked decline under eUV‐B and relative nutritive value was reduced at both growth stages.

     

Insect oviposition preference between Epichloë‐symbiotic and Epichloë‐free grasses does not necessarily reflect larval performance

Variation in plant communities is likely to modulate the feeding and oviposition behavior of herbivorous insects, and plant‐associated microbes are largely ignored in this context. Here, we take into account that insects feeding on grasses commonly encounter systemic and vertically transmitted (via seeds) fungal Epichloë endophytes, which are regarded as defensive grass mutualists. Defensive mutualism is primarily attributable to alkaloids of fungal origin. To study the effects of Epichloë on insect behavior and performance, we selected wild tall fescue (Festuca arundinacea) and red fescue (Festuca rubra) as grass–endophyte models. The plants used either harbored the systemic endophyte (E+) or were endophyte‐free (E?). As a model herbivore, we selected the Coenonympha hero butterfly feeding on grasses as larvae. We examined both oviposition and feeding preferences of the herbivore as well as larval performance in relation to the presence of Epichloë endophytes in the plants. Our findings did not clearly support the female's oviposition preference to reflect the performance of her offspring. First, the preference responses depended greatly on the grass–endophyte symbiotum. In F. arundinacea, C. hero females preferred E+ individuals in oviposition‐choice tests, whereas in F. rubra, the endophytes may decrease exploitation, as both C. hero adults and larvae preferred E? grasses. Second, the endophytes had no effect on larval performance. Overall, F. arundinacea was an inferior host for C. hero larvae. However, the attraction of C. hero females to E+ may not be maladaptive if these plants constitute a favorable oviposition substrate for reasons other than the plants' nutritional quality. For example, rougher surface of E+ plant may physically facilitate the attachment of eggs, or the plants offer greater protection from natural enemies. Our results highlight the importance of considering the preference of herbivorous insects in studies involving the endophyte‐symbiotic grasses as host plants.     

Sap‐feeding insects on forest trees along latitudinal gradients in northern Europe: a climate‐driven patterns

Knowledge of the latitudinal patterns in biotic interactions, and especially in herbivory, is crucial for understanding the mechanisms that govern ecosystem functioning and for predicting their responses to climate change. We used sap‐feeding insects as a model group to test the hypotheses that the strength of plant–herbivore interactions in boreal forests decreases with latitude and that this latitudinal pattern is driven primarily by midsummer temperatures. We used a replicated sampling design and quantitatively collected and identified all sap‐feeding insects from four species of forest trees along five latitudinal gradients (750–1300 km in length, ten sites in each gradient) in northern Europe (59 to 70°N and 10 to 60°E) during 2008–2011. Similar decreases in diversity of sap‐feeding insects with latitude were observed in all gradients during all study years. The sap‐feeder load (i.e. insect biomass per unit of foliar biomass) decreased with latitude in typical summers, but increased in an exceptionally hot summer and was independent of latitude during a warm summer. Analysis of combined data from all sites and years revealed dome‐shaped relationships between the loads of sap‐feeders and midsummer temperatures, peaking at 17 °C in Picea abies, at 19.5 °C in Pinus sylvestris and Betula pubescens and at 22 °C in B. pendula. From these relationships, we predict that the losses of forest trees to sap‐feeders will increase by 0–45% of the current level in southern boreal forests and by 65–210% in subarctic forests with a 1 °C increase in summer temperatures. The observed relationships between temperatures and the loads of sap‐feeders differ between the coniferous and deciduous tree species. We conclude that climate warming will not only increase plant losses to sap‐feeding insects, especially in subarctic forests, but can also alter plant‐plant interactions, thereby affecting both the productivity and the structure of future forest ecosystems.     

Interannual climate variability mediates changes in carbon and nitrogen pools caused by annual grass invasion in a semiarid shrubland

Exotic plant invasions alter ecosystem properties and threaten ecosystem functions globally. Interannual climate variability (ICV) influences both plant community composition (PCC) and soil properties, and interactions between ICV and PCC may influence nitrogen (N) and carbon (C) pools. We asked how ICV and non-native annual grass invasion covary to influence soil and plant N and C in a semiarid shrubland undergoing widespread ecosystem transformation due to invasions and altered fire regimes. We sampled four progressive stages of annual grass invasion at 20 sites across a large (25,000 km²) landscape for plant community composition, plant tissue N and C, and soil total N and C in 2013 and 2016, which followed 2 years of dry and wet conditions, respectively. Multivariate analyses and ANOVAs showed that in invasion stages where native shrub and perennial grass and forb communities were replaced by annual grass-dominated communities, the ecosystem lost more soil N and C in wet years. Path analysis showed that high water availability led to higher herbaceous cover in all invasion stages. In stages with native shrubs and perennial grasses, higher perennial grass cover was associated with increased soil C and N, while in annual-dominated stages, higher annual grass cover was associated with losses of soil C and N. Also, soil total C and C:N ratios were more homogeneous in annual-dominated invasion stages as indicated by within-site standard deviations. Loss of native shrubs and perennial grasses and forbs coupled with annual grass invasion may lead to long-term declines in soil N and C and hamper restoration efforts. Restoration strategies that use innovative techniques and novel species to address increasing temperatures and ICV and emphasize maintaining plant community structure—shrubs, grasses, and forbs—will allow sagebrush ecosystems to maintain C sequestration, soil fertility, and soil heterogeneity.