Atmospheric nitrogen (N) deposition has been found to significantly affect plant growth and physiological performance in terrestrial ecosystems. Many individual studies have investigated how N addition influences plant functional traits, however these investigations have usually been limited to a single species, and thereby do not allow derivation of general patterns or underlying mechanisms. We synthesized data from 56 papers and conducted a meta-analysis to assess the general responses of 15 variables related to leaf economics, gas exchange, and hydraulic traits to N addition among 61 woody plant species, primarily from temperate and subtropical regions. Results showed that under N addition, leaf area index (+10.3%), foliar N content (+7.3%), intrinsic water-use efficiency (+3.1%) and net photosynthetic rate (+16.1%) significantly increased, while specific leaf area, stomatal conductance, and transpiration rate did not change. For plant hydraulics, N addition significantly increased vessel diameter (+7.0%), hydraulic conductance in stems/shoots (+6.7%), and water potential corresponding to 50% loss of hydraulic conductivity (P50, +21.5%; i.e., P50 became less negative), while water potential in leaves (-6.7%) decreased (became more negative). N addition had little effect on vessel density, hydraulic conductance in leaves and roots, or water potential in stems/shoots. N addition had greater effects on gymnosperms than angiosperms and ammonium nitrate fertilization had larger effects than fertilization with urea, and high levels of N addition affected more traits than low levels. Our results demonstrate that N addition has coupled effects on both carbon and water dynamics of woody plants. Increased leaf N, likely fixed in photosynthetic enzymes and pigments leads to higher photosynthesis and water use efficiency, which may increase leaf growth, as reflected in LAI results. These changes appear to have downstream effects on hydraulic function through increases in vessel diameter, which leads to higher hydraulic conductance, but lower water potential and increased vulnerability to embolism. Overall, our results suggest that N addition will shift plant function along a tradeoff between C and hydraulic economies by enhancing C uptake while simultaneously increasing the risk of hydraulic dysfunction.

/pdf/responses-of-woody-plant-functional-traits-to-nitrogen-rt0nfe4thl.pdf

Responses of Woody Plant Functional Traits to Nitrogen Addition: A Meta-Analysis of Leaf Economics, Gas Exchange, and Hydraulic Traits

Summary � Cycads are the most ancient lineage of living seed plants, but the design of their leaves has received little study. We tested whether cycad leaves are governed by the same fundamental design principles previously established for ferns, conifers and angiosperms, and characterized the uniqueness of this relict lineage in foliar trait relationships. � Leaf structure, photosynthesis, hydraulics and nutrient composition were studied in 33 cycad species from nine genera and three families growing in two botanical gardens. � Cycads varied greatly in leaf structure and physiology. Similarly to other lineages, light-saturated photosynthetic rate per mass (Am) was related negatively to leaf mass per area and positively to foliar concentrations of chlorophyll, nitrogen (N), phosphorus and iron, but unlike angiosperms, leaf photosynthetic rate was not associated with leaf hydraulic conductance. Cycads had lower photosynthetic N use efficiency and higher photosynthetic performance relative to hydraulic capacity compared with other lineages. � These findings extend the relationships shown for foliar traits in angiosperms to the cycads. This functional convergence supports the modern synthetic understanding of leaf design, with common constraints operating across lineages, even as they highlight exceptional aspects of the biology of this key relict lineage.

/pdf/extending-the-generality-of-leaf-economic-design-principles-4n057nwx7x.pdf

Extending the generality of leaf economic design principles in the cycads, an ancient lineage

Five insect species including three species of weevils (Coleoptera) and two species of lycaenid butterflies (Lepidoptera) that feed exclusively on the carcinogenic and neurotoxic tissues of cycads were found to share a core set of bacterial phylotypes, including the bacterium Raoultella ornithinolytica, which has known anti-cancer and nitrogen-fixing capabilities. The other shared bacteria belong to lineages that include insect-associated and extremophilic taxa. The presence of Raoultella ornithinolytica and an unknown Enterobacteriaceae was in contrast to a set of non-cycadfeeding relatives of these insects, none of which contained this same set of shared bacterial phylotypes. Given the considerable phylogenetic distance between the cycadivorous insect species as well as the fact that shared microbiota are not found in their non-cycad-feeding relatives, our data suggest that this core set of shared bacteria are important in helping cycad feeders detoxify their poisonous host plants.

Cycad-feeding insects share a core gut microbiome

Cycads are an ancient group of tropical gymnosperms that are toxic to most animals - including humans - though the larvae of many moths and butterflies (order: Lepidoptera) feed on cycads with apparent immunity. These insects belong to distinct lineages with varying degrees of specialisation and diverse feeding ecologies, presenting numerous opportunities for comparative studies of chemically mediated eco-evolutionary dynamics. This review presents the first evolutionary evaluation of cycad-feeding among Lepidoptera along with a comprehensive review of their ecology. Our analysis suggests that multiple lineages have independently colonised cycads from angiosperm hosts, yet only a few clades appear to have radiated following their transitions to cycads. Defensive traits are likely important for diversification, as many cycad specialists are warningly coloured and sequester cycad toxins. The butterfly family Lycaenidae appears to be particularly predisposed to cycad-feeding and several cycadivorous lycaenids are warningly coloured and chemically defended. Cycad-herbivore interactions provide a promising but underutilised study system for investigating plant-insect coevolution, convergent and divergent adaptations, and the multi-trophic significance of defensive traits; therefore the review ends by suggesting specific research gaps that would be fruitfully addressed in Lepidoptera and other cycad-feeding insects.

Ecology and evolution of cycad-feeding Lepidoptera.

Establishing feeding sites is critical for the survival of neonate Lepidoptera larvae. Rapid foliar quality changes during leaf expansion create a narrow window of opportunity for establishment of ...

Defining the window of opportunity for feeding initiation by second-instar spruce budworm larvae1

 Premise of the study: Slow-growing understory cycads invest heavily in defenses to protect the few leaves they produce annually. The Neotropical cycad Zamia stevensonii has chemical and mechanical barriers against insect herbivores. Mechanical barriers, such as leaf toughness, can be established only after the leaf has expanded. Therefore, chemical defenses may be important during leaf expansion. How changes in leaf traits affect the feeding activity of cycad specialist insects is unknown. We investigated leaf defenses and incidence of specialist herbivores on Z. stevensonii during the fi rst year after leaf fl ush.  Methods: Herbivore incidence, leaf production, and leaf traits that might affect herbivory—including leaf age, lamina thickness, resistance-to-fracture, work-to-fracture, trichome density, and chlorophyll, water, and toxic azoxyglycoside (AZG) content—were measured throughout leaf development. Principal component analysis and generalized linear models identifi ed characteristics that may explain herbivore incidence.  Key results: Synchronized leaf development in Z. stevensonii is characterized by quick leaf expansion and delayed greening. Specialist herbivores feed on leaves between 10 and 100 d after fl ush and damage ~37% of all leafl ets produced. Young leaves are protected by AZGs, but these defenses rapidly decrease as leaves expand. Leaves older than 100 d are protected by toughness.  Conclusions: Because AZG concentrations drop before leaves become suffi ciently tough, there is a vulnerable period during which leaves are susceptible to herbivory by specialist insects. This slow-growing gymnosperm invests heavily in constitutive defenses against highly specialized herbivores, underlining the convergence in defensive syndromes by major plant lineages.

/pdf/leaf-traits-and-herbivory-levels-in-a-tropical-gymnosperm-3rx3h2sr1v.pdf

Leaf traits and herbivory levels in a tropical gymnosperm, Zamia stevensonii (Zamiaceae).

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Leaf traits and herbivory levels in a tropical gymnosperm, Zamia stevensonii (Zamiaceae).