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Tree carbon allocation is a dynamic process that depends on the tree’s environment, but we know relatively little about how biotic interactions influence these dynamics. In central Kenya, the loss of vertebrate herbivores and the savanna’s invasion by the ant Pheidole megacephala are disrupting mutualisms between the foundational tree Acacia (Vachellia) drepanolobium and its native ant defenders. Here, we piloted a 13Carbon (C) pulse-labeling method to investigate the influence of these biotic interactions on C allocation to ant partners by adult trees in situ. Trees withstood experimental conditions and took up sufficient labeled 13CO2 for 13C to be detected in various C sinks, including ant mutualists. The δ13C in ants collected shortly after labeling suggested that trees exposed to herbivores allocated relatively more newly assimilated C to native ant defenders. Our results demonstrate the viability of the pulse-labeling method and suggest that C allocation to ant partners depends on the biotic context of the tree, but further investigation with replication is needed to characterize such differences in relation to invasion and herbivore loss. 

Abstract Intensifying drought conditions across the western United States due to global climate change are altering plant–insect interactions. Specialist herbivores must find their host plants within a matrix of nonhosts, and thus often rely upon specific plant secondary chemistry for host location and oviposition cues. Climate-induced alterations to plant chemistry could thus affect female selection of larval food plants. Here, we investigated whether host-plant water limitation influenced oviposition preference in a threatened invertebrate: the monarch butterfly (Danaus plexippus). We found that females deposited more eggs on reduced-water than on well-watered narrowleaf milkweed plants (Asclepias fascicularis), but we could not attribute this change to any specific change in plant chemistry. Specialist herbivores, such as the monarch butterfly, which are tightly linked to specific plant cues, may experience shift in preferences under global-change conditions. Understanding oviposition preferences will be important to directing ongoing habitat restoration activities for this declining insect. 

Some invasive ants have worldwide distributions and impose substantial impacts on human society and native biodiversity. Yet we know little about how ants impact soil ecosystems in general, much less how soil ecosystems shift when invasive ants move in. We excavated the coarse roots of a monodominant savanna tree in invaded and uninvaded areas to test the hypothesis that the presence of invasive ants would be associated with changes in root distribution and biomass across the landscape. We found that in the presence of invasive ants, trees had a shifted distribution of lateral coarse roots, with proportionally less root biomass near the surface and far from tree stems. In addition, the density of lateral coarse-root biomass was ~ 20% lower for trees within invaded landscapes. Our results suggest that soil-nesting invasive ants can drive important changes in rooting strategy for a tree species that serves a foundational role in the biogeochemical cycles of vertisol savannas. 

Abstract Background and Aims In dryland ecosystems, conifer species are threatened by more frequent and severe droughts, which can push species beyond their physiological limits. Adequate seedling establishment will be critical for future resilience to global change. We used a common garden glasshouse experiment to determine how seedling functional trait expression and plasticity varied among seed sources in response to a gradient of water availability, focusing on a foundational dryland tree species of the western USA, Pinus monophylla. We hypothesized that the expression of growth-related seedling traits would show patterns consistent with local adaptation, given clinal variation among seed source environments. Methods We collected P. monophylla seeds from 23 sites distributed across rangewide gradients of aridity and seasonal moisture availability. A total of 3320 seedlings were propagated with four watering treatments representing progressively decreasing water availability. Above- and below-ground growth-related traits of first-year seedlings were measured. Trait values and trait plasticity, here representing the degree of variation among watering treatments, were modelled as a function of watering treatment and environmental conditions at the seed source locations (i.e. water availability, precipitation seasonality). Key Results We found that, under all treatments, seedlings from more arid climates had larger above- and below-ground biomass compared to seedlings from sites experiencing lower growing-season water limitation, even after accounting for differences in seed size. Additionally, trait plasticity in response to watering treatments was greatest for seedlings from summer-wet sites that experience periodic monsoonal rain events. Conclusions Our results show that P. monophylla seedlings respond to drought through plasticity in multiple traits, but variation in trait responses suggests that different populations are likely to respond uniquely to changes in local climate. Such trait diversity will probably influence the potential for future seedling recruitment in woodlands that are projected to experience extensive drought-related tree mortality. 

Abstract Plasticity in plant traits, including secondary metabolites, is critical to plant survival and competitiveness under stressful conditions. The ability of a plant to respond effectively to combined stressors can be impacted by crosstalk in biochemical pathways, resource availability and evolutionary history, but such responses remain underexplored. In particular, we know little about intraspecific variation in response to combined stressors or whether such variation is associated with the stress history of a given population.Here, we investigated the consequences of combined water and herbivory stress for plant traits, including relative growth rate, leaf morphology and various measures of phytochemistry, using a common garden ofAsclepias fascicularismilkweeds. To examine how plant trait means and plasticities depend on the history of environmental stress, seeds for the experiment were collected from across a gradient of aridity in the Great Basin, United States. We then conducted a factorial experiment crossing water limitation with herbivory.Plants responded to water limitation alone by increasing the evenness of UV‐absorbent secondary metabolites and to herbivory alone by increasing the richness of metabolites. However, plants that experienced combined water and herbivory stress exhibited similar phytochemical diversity to well‐watered control plants. This lack of plasticity in phytochemical diversity in plants experiencing combined stressors was associated with a reduction in relative growth rates.Leaf chemistry means and plasticities exhibited clinal variation corresponding to seed source water deficits. The total concentration of UV‐absorbent metabolites decreased with increasing water availability among seed sources, driven by higher concentrations of flavonol glycosides, which are hypothesized to act as antioxidants, among plants from drier sites. Plants sourced from drier sites exhibited higher plasticity in flavonol glycoside concentrations in response to water limitation, which increased phytochemical evenness, but simultaneous herbivory dampened plant responses to water limitation irrespective of seed source.Synthesis. These results suggest that climatic history can affect intraspecific phytochemical plasticity, which may confer tolerance to water limitation, but that co‐occurring herbivory disrupts such patterns. Global change is increasing the frequency and intensity of stress combinations, such that understanding intraspecific responses to combined stressors is critical for predicting the persistence of plant populations. 

Abstract Cooperative interactions may frequently be reinforced by “partner fidelity feedback,” in which high‐ or low‐quality partners drive positive feedbacks with high or low benefits for the host, respectively. Benefits of plant–animal mutualisms for plants have been quantified almost universally in terms of growth or reproduction, but these are only two of many sinks to which a host‐plant allocates its resources. By investigating how partners to host‐plants impact two fundamental plant resources, carbon and water, we can better characterize plant–partner fidelity and understand how plant–partner mutualisms may be modulated by resource dynamics. In Laikipia, Kenya, four ant species compete forAcacia drepanolobiumhost‐plants. These ants differ in multiple traits, from nectar consumption to host‐plant protection. Using a 5‐year ant removal experiment, we compared carbon fixation, leaf water status, and stem non‐structural carbohydrate concentrations for adult ant–plants with and without ant partners. Removal treatments showed that the ants differentially mediate tree carbon and/or water resources. All three ant species known to be aggressive against herbivores were linked to benefits for host‐plant resources, but only the two species that defend but do not prune the host,Crematogaster mimosaeandTetraponera penzigi, increased tree carbon fixation. Of these two species, only the nectivoreC. mimosaeincreased tree simple sugars.Crematogaster nigriceps, which defends the tree but also castrates flowers and prunes meristems, was linked only to lower tree water stress approximated by pre‐dawn leaf water potential. In contrast to those defensive ants,Crematogaster sjostedti, a poor defender that displaces other ants, was linked to lower tree carbon fixation. Comparing the effects of the four ant species across control trees suggests that differential ant occupancy drives substantial differences in carbon and water supply among host trees. Our results highlight that ant partners can positively or negatively impact carbon and/or water relations for their host‐plant, and we discuss the likelihood that carbon‐ and water‐related partner fidelity feedback loops occur across ant–plant mutualisms. 

Abstract Nearly every terrestrial ecosystem hosts invasive ant species, and many of those ant species construct underground nests near roots and/or tend phloem‐feeding hemipterans on plants. We have a limited understanding of how these invasive ant behaviours change photosynthesis, carbohydrate availability and growth of woody plants.We measured photosynthesis, water relations, carbohydrate concentrations and growth for screenhouse‐rearedAcacia drepanolobiumsaplings on which we had manipulated invasivePheidole megacephalaants and nativeCeroplastessp. hemipterans to determine whether and how soil nesting and hemipteran tending by ants affect plant carbon dynamics. In a field study, we also compared leaf counts of vertebrate herbivore‐excluded and ‐exposed saplings in invaded and non‐invaded savannas to examine how ant invasion and vertebrate herbivory are associated with differences in sapling photosynthetic crown size.Though hemipteran infestations are often linked to declines in plant performance, our screenhouse experiment did not find an association between hemipteran presence and differences in plant physiology. However, we did find that soil nesting byP. megacephalaaround screenhouse plants was associated with >58% lower whole‐crown photosynthesis, >31% lower pre‐dawn leaf water potential, >29% lower sucrose concentrations in woody tissues and >29% smaller leaf areas. In the field, sapling crowns were 29% smaller in invaded savannas than in non‐invaded savannas, mimicking screenhouse results.Synthesis. We demonstrate that soil nesting near roots, a common behaviour byPheidole megacephalaand other invasive ants, can directly reduce carbon fixation and storage ofAcacia drepanolobiumsaplings. This mechanism is distinct from the disruption of a native ant mutualism byP. megacephala, which causes similar large declines in carbon fixation for matureA. drepanolobiumtrees.Acacia drepanolobiumalready has extremely low natural rates of recruitment from the sapling to mature stage, and we infer that these negative effects of invasion on saplings potentially curtail recruitment and reduce population growth in invaded areas. Our results suggest that direct interactions between invasive ants and plant roots in other ecosystems may strongly influence plant carbon fixation and storage.