Nitrogen and Carbon Mineralization from Green and Senesced Leaf Litter Differ between Cycad and Angiosperm Trees.

Plant leaf litter decomposition is directly influenced by the identity of the source plants and the leaf age. Defoliation of forests by tropical cyclones (TC) transfers copious amounts of high-quality green leaf litter to soils. We used a soil amendment approach with the incubated buried bag method to compare carbon (C) and nitrogen (N) mineralization dynamics of green and senesced leaf litter from cycad Cycas micronesica and angiosperm Morinda citrifolia trees on the island of Guam. Soil priming increased the decomposition of pre-existing organic C, and were greater for green leaf litter additions than senesced leaf litter additions. Available N content increased by day 14 and remained elevated for the entire 117-d incubation for soils amended with green M. citrifolia litter. In contrast, available N content increased above those in control soils by day 90 and above those in soils amended with senesced litter by day 117 for green C. micronesica litter. The net N mineralization rate was higher than control soils by 120% for the senesced litter treatments and 420% for the green litter treatments. The results reveal a complex but predictable interplay between TC defoliation and litter quality as defined by tree identity. We have illuminated one means by which increased frequency of intense TCs due to climate change may alter the global C and N cycles.

Metals and Metalloids Increase in Cycas micronesica Seed Gametophyte Tissue in Shaded Growth Conditions.

Exposure to environmental toxins may be partly responsible for mammal neurodegenerative disorders. Consumption of seeds from Guam's cycad tree has been linked to the disorder known as amyotrophic lateral sclerosis-parkinsonism dementia complex (ALS-PDC). The unambiguous identification of causal agents of ALS-PDC has been elusive. We have examined the levels of eight metals and metalloids in cycad seeds as a function of the ambient shade in which the plants were grown. Of these metals, the data strongly suggest that aluminum (Al) and selenium (Se) are present in washed flour prepared from southern Guam's cycad seed tissues at elevated levels, especially when the trees are grown in shade. Previous authors have speculated that Al and Se are involved in various ALS outcomes, and our results support this interpretation.

Temporal Variation of Litterfall and Nutrient Return of Serianthes nelsonii Merr. in a Tropical Karst Forest.

Trees contribute to ecosystem nutrient cycling through the amount, timing, and composition of litterfall. Understanding the nature of this contribution from endangered tree species may aid in species and habitat recovery efforts. Serianthes nelsonii is an endangered tree species from the Mariana Islands, and little is known about litterfall dynamics. The timing of leaf, fruit, and stem litterfall was determined to more fully understand the return of nutrients via litter. The total annual litterfall was 272.8 g·m−2, with 45% represented by leaves, 48% represented by stems, and 7% represented by fruits. Stem litterfall weight contrasted more from month to month than the other organs, and leaf litterfall exhibited the most even distribution throughout the year. The timing of fruit and stem litterfall was influenced by the timing of extreme wind events. Leaf litter contributed nutrients in the following order: carbon > calcium > nitrogen > potassium > magnesium > iron > phosphorus > manganese > boron > zinc > copper. Fruit and stem litter contributed nutrients in the following order: carbon > calcium > nitrogen > magnesium > potassium > phosphorus > iron > manganese > boron > zinc > copper. Based on carbon/nitrogen, the stem litter exhibited the lowest quality and leaf litter exhibited the highest quality for speed of nutrient release via decomposition. Conservationists may use this knowledge to more fully integrate S. nelsonii trees into habitat management plans.

Not that young: combining plastid phylogenomic, plate tectonic and fossil evidence indicates a Palaeogene diversification of Cycadaceae.

BACKGROUND AND AIMS: Previous molecular dating studies revealed historical mass extinctions and recent radiations of extant cycads, but debates still exist between palaeobotanists and evolutionary biologists regarding the origin and evolution of Cycadaceae. METHODS: Using whole plastomic data, we revisited the phylogeny of this family and found the Palawan endemic Cycas clade was strongly related to all lineages from Southeast Eurasia, coinciding with a plate drift event occurring in the Early Oligocene. By integrating fossil and biogeographical calibrations as well as molecular data from protein-coding genes, we established different calibration schemes and tested competing evolutionary timelines of Cycadaceae. KEY RESULTS: We found recent dispersal cannot explain the distribution of Palawan Cycas, yet the scenario including the tectonic calibration yielded a mean crown age of extant Cycadaceae of ~69-43 million years ago by different tree priors, consistent with multiple Palaeogene fossils assigned to this family. Biogeographical analyses incorporating fossil distributions revealed East Asia as the ancestral area of Cycadaceae. CONCLUSIONS: Our findings challenge the previously proposed Middle-Late Miocene diversification of cycads and an Indochina origin for Cycadaceae and highlight the importance of combining phylogenetic clades, tectonic events and fossils for rebuilding the evolutionary history of lineages that have undergone massive extinctions.

Aulacaspis yasumatsui Invasion Reduced Cycas micronesica Microstrobilus Size and Pollinator Brood Site Competence.

Aulacaspis yasumatsui Takagi invaded Guam in 2003, and the influence on survival and demography of the host Cycas micronesica K.D. Hill population has been well-studied. To more fully understand how A. yasumatsui has threatened the host cycad species, we determined the microstrobilus size and number of pollinators per microstrobilus from 2001 to 2021. The microstrobilus height and diameter were measured directly, and the volume was calculated. Microstrobili were 58 cm in height, 13 cm in diameter, and 4740 cm3 in volume prior to direct A. yasumatsui infestations. Microstrobili decreased in size immediately after direct infestations by A. yasumatsui, and then began to slowly increase in size until 2021. For example, the volume was 24% of pre-invasion volume in 2007, and was 57% of pre-invasion volume in 2021. Microstrobili were harvested; then, the number of pollinator pupae were counted after an incubation period. Pollinator pupae counts per microstrobilus declined to 66% of pre-invasion levels by 2007 and have remained similarly constrained through 2021. Our results revealed that A. yasumatsui damage to the host C. micronesica population is not limited to attrition of the extant plant population, but also includes a loss in male reproductive effort and the risk of coextinction of the insular pollinator.

Biotic Threats to Cycas micronesica Continue to Expand to Complicate Conservation Decisions.

Invasions of non-native species can threaten native biodiversity, and island ecosystems are ideal for studying these phenomena. In this article, first, we report on the invasive species that combine to threaten the island cycad Cycas micronesica by reviewing the history of previously reported invasions and providing an update of recent invasions. Then, we prioritize the threat status of each herbivore and the interactions among them. Plant damage was initiated in 2003─2005 by the non-native Aulacaspis yasumatsui Takagi armored scale, Erechthias sp. Meyrick leaf miner, and Luthrodes pandava Horsfield butterfly, which elicited unprecedented irruptions of the native Acalolepta marianarum Aurivillius stem borer and increased herbivory by feral pigs (Sus scrofa L.). The combined impact of these five consumers represents the greatest sustained threat to the cycad tree species. Mitigation of the damage caused by phytophagous non-native species is urgently needed to conserve this unique gymnosperm tree.

Burrowing activity of coconut rhinoceros beetle on Guam cycads.

Guam's established population of non-native coconut rhinoceros beetle (CRB, Oryctes rhinoceros L.) began creating burrows in stem apices of several cycad species in a managed garden. We conducted an island-wide survey to determine the spatial patterns of CRB burrowing of stems of in situ Cycas micronesica. We also measured starch of healthy and unhealthy coconut leaf tissue and compared this with starch of cycad stem tissue. The starch concentration of the central unexpanded leaf in healthy Philippine coconut trees was ≈90 mg·g-1, and that of unhealthy Guam coconut trees was ≈40 mg·g-1. The starch concentration of the tissue within the CRB burrow locations on C. micronesica trees was 145 mg·g-1. Burrowing of C. micronesica was restricted to female CRB adults and was found throughout the full latitudinal gradient of Guam. Our findings indicate Guam's unhealthy coconut trees are no longer nutrient-dense, and the female CRB population may have exhibited a phylogenetically distant host shift to the abundant C. micronesica plants for a starch-rich diet within the concepts of the 'ecological fitting' hypothesis. We add proximity to coconut tree habitats as a new threat to Guam's endangered C. micronesica population.

Height increment of Cycas micronesica informs conservation decisions.

Growth dynamics of pachycaulous stems of arborescent cycad plants are not well understood, and most observations have been made in cultivated garden plants. We studied Cycas micronesica plants in Guam, Tinian, and Yap to understand the influences of geography, plant size, sex, and herbivory on stem growth. We also determined the changes in demography of Guam's population after 15 years of damage by non-native insect herbivores. The height increment (HI) was similar for plants within the height range from 100 cm to more than 600 cm, so the relative growth rate declined with height. Female tree HI was 68% of male tree HI, and Yap tree HI was 87% of Guam tree HI. Chronic herbivory by non-native insect herbivores caused a mean 44% decline in HI. Plants in managed gardens grew more rapidly than plants in a wild habitat. The HI was used to estimate that Guam has experienced a complete loss of ≈70 y of demographic depth resulting from the selective mortality of small plants since 2005. When future conservation interventions successfully mitigate the ubiquitous biological threats, our HI may be useful for empirically quantifying recovery of plant health.

Bi-Pinnate Compound Serianthes nelsonii Leaf-Level Plasticity Magnifies Leaflet-Level Plasticity.

Numerous leaf traits exhibit developmental plasticity in response to irradiance, an attribute that maximizes performance in the prevailing light. The use of leaflets to represent whole leaf traits of tree species with compound leaves is common in the acclimation literature. These methods ignore the potential for whole leaf plasticity to augment leaflet plasticity. We grew Serianthes nelsonii plants in incident light ranging from 6% to 100% of sunlight and quantified numerous leaflet and leaf traits to determine plasticity index (PI: (maximum-minimum)/maximum)) of each. Leaflet acclimation such as changes in length of palisade mesophyll occurred as expected. However, leaf-level morphometric traits such as rachillae insertion angle also exhibited acclimation potential. The leaf-level plastic behavior enabled acclimation approaches that simple-leaved species do not possess. We illuminate the need to look at the entire leaf when quantifying acclimation potential of tree leaves, and indicate that the historical use of leaflets to represent species with compound leaves under-estimated the acclimation potential when compared to species with simple leaves.

Artifleck: The Study of Artifactual Responses to Light Flecks with Inappropriate Leaves.

Methods in sunfleck research commonly employ the use of experimental leaves which were constructed in homogeneous light. These experimental organs may behave unnaturally when they are challenged with fluctuating light. Photosynthetic responses to heterogeneous light and leaf macronutrient relations were determined for Cycas micronesica, Glycine max, and Zea mays leaves that were grown in homogeneous shade, heterogeneous shade, or full sun. The speed of priming where one light fleck increased the photosynthesis during a subsequent light fleck was greatest for the leaves grown in heterogeneous shade. The rate of induction and the ultimate steady-state photosynthesis were greater for the leaves that were grown in heterogeneous shade versus the leaves grown in homogeneous shade. The leaf mass per area, macronutrient concentration, and macronutrient stoichiometry were also influenced by the shade treatments. The amplitude and direction in which the three developmental light treatments influenced the response variables were not universal among the three model species. The results indicate that the historical practice of using experimental leaves which were constructed under homogeneous light to study leaf responses to fluctuating light may produce artifacts that generate dubious interpretations.

Cycas micronesica Stem Carbohydrates Decline Following Leaf and Male Cone Growth Events.

The growth of synchronized leaf flushes or male cones on Cycas trees is an ephemeral event, and non-structural carbohydrates (NSCs) are likely deployed from stem and root storage tissues to support their construction. The relationships among various stem NSCs and these rapid growth events have not been studied to date. Monosaccharides, disaccharides, and starch were quantified in Cycas micronesica stem tissue prior to and immediately after the growth of leaf flushes or male cones to determine the influences on the concentration of these carbohydrates. The pre-existing leaves were removed from half of the plants to determine if the elimination of this carbon source would influence the NSC behaviors. Starch and sucrose dominated the NSC profiles, and these two NSCs declined following cone or new leaf growth. Removal of pre-existing leaves generated a greater decline in starch and sucrose for cone growth, and a greater decline in sucrose, but not starch following new leaf growth than in control trees with no leaf removal. The initial differences in starch and sucrose among cortex, vascular, and pith tissues disappeared as the concentrations declined in all three tissue categories to reach similar post-growth concentrations among the stem tissue categories. The fructose, glucose, and maltose behaviors were not consistent, and their concentrations were low such that their influence on the total NSC behaviors was minimal. These results provided indirect evidence that stem NSCs were mobilized to support ephemeral male cone and new leaf growth for this arborescent cycad. Growth of female strobili is slow and lengthy, so we did not include female trees in this study. The contributions of stem NSCs to female strobili growth remain to be studied with alternative methods.

Diel patterns of stem CO2 efflux vary among cycads, arborescent monocots, and woody eudicots and gymnosperms.

The diel patterns of stem carbon dioxide efflux (Es) were determined for cycads, monocots, and woody eudicot and gymnosperm tree species. Stem Es at a height of 30-40 cm was measured every 2 h throughout 31-h campaigns. Our range of Es was 1.5-4.0 µmol·m-2·s-1 for cycads, 1.0-3.5 µmol·m-2·s-1 for arborescent monocots, and 1.5-4.5 µmol·m-2·s-1 for woody eudicot and gymnosperm trees species. Time of day did not influence Es of cycads or monocots. In contrast, the woody stems of eudicots and gymnosperms exhibited diurnal Es that was 36% to 40% greater than nocturnal Es. The established literature based on Es of woody tree species cannot be used to estimate habitat carbon cycles in habitats which contain cycad or monocot trees. Time of day must be included for accuracy of research on Es of woody tree species. Failures to account for the spatiotemporal differences of Es may explain some of the disparity in outcomes of published stem respiration studies.

Vertical Strata and Stem Carbon Dioxide Efflux in Cycas Trees.

Stem respiration is influenced by the vertical location of tree stems, but the influence of vertical location on stem respiration in a representative cycad species has not been determined. We quantified the influence of vertical strata on stem carbon dioxide efflux (Es) for six arborescent Cycas L. species to characterize this component of stem respiration and ecosystem carbon cycling. The influence of strata on Es was remarkably consistent among the species, with a stable baseline flux characterizing the full mid-strata of the pachycaulous stems and an increase in Es at the lowest and highest strata. The mid-strata flux ranged from 1.8 µmol·m-2·s-1 for Cycas micronesica K.D. Hill to 3.5 µmol·m-2·s-1 for Cycas revoluta Thunb. For all species, Es increased about 30% at the lowest stratum and about 80% at the highest stratum. A significant quadratic model adequately described the Es patterns for all six species. The increase of Es at the lowest stratum was consistent with the influence of root-respired carbon dioxide entering the stem via sap flow, then contributing to Es via radial conductance to the stem surface. The substantial increase in Es at the highest stratum is likely a result of the growth and maintenance respiration of the massive cycad primary thickening meristem that constructs the unique pachycaulous cycad stem.

Stem CO2 efflux of Cycas micronesica is reduced by chronic non-native insect herbivory.

Carbon dioxide efflux (Es) from the base of Cycas micronesica K.D. Hill stems was quantified in four locations containing healthy populations and in one location with populations threatened by non-native insect herbivores in order to determine the influence of reduced plant health on Es. Minimal variation of Es occurred among the four locations with healthy plants, and Es ranged from 1.68 to 1.79 µmol·m-2·s-1. The threatened in situ populations were on the island of Guam, where recent invasions of non-native insects have caused epidemic plant mortality, and the Es was 0.59 µmol·m-2·s-1. This is the first known report of Es for any cycad species, and the values for the unique pachycaulous stem form fit in the lower half of the range of published Es for woody trees. The results illuminate the potential for using Es to screen in situ C. micronesica populations to identify the individual trees with the greatest likelihood of surviving conservation measures.

Tree conservation can be constrained by agents from conservation permitting and funding agencies.

Recent conservation actions for Serianthes nelsonii Merr. and Cycas micronesica K.D. Hill in the Mariana Islands have illuminated some negative consequences associated with ill-informed agents representing permitting and funding agencies. Several cases from the islands of Guam and Tinian are discussed as ineffective conservation examples, and these are countered with two examples of successful conservation approaches. When biologists that act as points of contact for federal permitting and funding agencies do not possess education, knowledge, and experience that is germane to federally listed species, sound science may be marginalized from the conservation agenda. When rapid turnover of federal conservation agents introduces dysfunction, discontinuities in collaborations may thwart success. When lapses in conservation contracts are allowed, short-term extemporary contracting approaches are utilized, and conservation practitioners that lack the ability to include an experimental approach to conservation actions are employed, the co-production of new knowledge to enable decision support tools for future decision-makers may be hindered.

Late successional tree species in Guam create biogeochemical niches.

The soils beneath and surrounding mature Artocarpus mariannensis, Elaeocarpus joga, and Serianthes nelsonii trees were studied in northern Guam limestone forests to determine the role of these trees in maintaining spatial heterogeneity of biogeochemistry. The soils beneath S. nelsonii were nutrient-enriched compared to soils away from S. nelsonii. The soils beneath A. mariannensis were depauperate for some nutrients or were not different from the soils away from A. mariannensis for other nutrients. The soils beneath E. joga exhibited increases in some nutrients such as nitrogen, carbon, and phosphorus, but decreases in other nutrients such as potassium and calcium when compared to the soils away from E. joga trees. These three tree species influenced spatial heterogeneity in soil nutrient status in the order A. mariannensis < E. joga < S. nelsonii and their presence added greatly to surface soil heterogeneity. Iron, manganese, and pH exhibited the least variation within the paired sites. Calcium, magnesium, potassium, and zinc exhibited the greatest variation among the paired sites. These findings indicate that continuing loss of these trees from Guam's forests will diminish the associated biogeochemical spatial heterogeneity.

Repetitive pruning of Serianthes nursery plants improves transplant quality and post-transplant survival.

Information on the best methods for producing endangered Serianthes nelsonii plants for use in species recovery projects is lacking. Plants of this species behave similarly in a nursery setting to plants of the congeneric Serianthes grandiflora and Serianthes kanehirae. Container-grown plants of these two species were repeatedly pruned in the nursery to determine if a more favorable root:shoot ratio would result, then to determine if performance after out-planting would benefit from the enhanced root growth. Repetitive pruning increased absolute root dry weight 43% to 61% and root:shoot ratio 55% to 56% above that of control plants that were not pruned. One year after out-planting, the pruned plants were greater in shoot dry weight and plant height than the control plants. More importantly, control plants exhibited 70% to 80% mortality, but the pruned plants exhibited 100% survival. This new knowledge from two surrogate species indicated that repetitive pruning of S. nelsonii plants in conservation nurseries would greatly increase the quality of the resulting transplants and therefore nurseries may add pruning as a prescribed production protocol to improve species recovery success.

Thigmomorphogenesis and biomechanical responses of shade-grown Serianthes nelsonii plants to stem flexure.

The influences of stem flexure on shade-grown Serianthes nelsonii Merr. stem growth and strength were determined in a container nursery setting. Treated stems were bent 90° two times daily for a 14 wk nursery production period. Plant height, internode length, and slenderness were decreased by stem flexure when compared with control plants that received no flexure. Two force-displacement tests revealed stem strength was increased by the flexure treatment. Control plants exhibited undesirable lean of the main stem, and 1 hr of wind stress further increased the angle of lean. Treated plants were close to orthotropic and the wind stress did not change the stem lean. Results indicate stem flexure is a reliable method for increasing the quality of shade-grown S. nelsonii plants and some form of mechanical stimulation should be added to nursery production protocols for the species.

Temperature and Imbibition Influence Serianthes Seed Germination Behavior.

The direct role of physical dormancy in delaying germination of Serianthes grandiflora Bentham, Serianthes kanehirae Fosberg, and Serianthes nelsonii Merrill seeds has not been adequately studied, nor has the role of temperature on germination behaviors. Imbibition testing indicated seeds with scarified testa absorbed water for the duration of a 24 h imbibition period, but seeds with an intact testa stopped absorbing water after 1 h. The behavior of S. nelsonii seeds most closely matched those of S. kanehirae, with the pattern of water absorption for S. grandiflora seeds deviating from that for the other species. Scarified seeds germinated readily, with initial germination occurring by 50 h for S. nelsonii and 90 hr for the other species, and maximum germination of 80% to 90% occurring by 60 h for S. nelsonii and 100 h for the other species. Predicted optimum temperature based on a fitted quadratic model was 26 °C for S. nelsonii, 23 °C for S. grandiflora, and 22 °C for S. kanehirae. Seed respiration increased within 3 h of imbibition for scarified seeds and continued to increase in a linear pattern. The linear slope was greatest for S. nelsonii, intermediate for S. grandiflora, and least for S. kanehirae, but ultimate respiration was greatest for S. kanehirae seeds. Seed respiration was so limited for un-scarified seeds that the instrument was unable to quantify any carbon dioxide efflux. Physical dormancy in seeds of these Serianthes species is a powerful trait that spreads out the timing of seedling emergence in natural settings and controls imbibition and germination speed in managed nurseries.

Rhyzobius lophanthae Behavior is Influenced by Cycad Plant Age, Providing Odor Samples in a Y-tube Olfactometer.

The scale predator Rhyzobius lophanthae Blaisdell was introduced to Guam and Rota to control invasive Aulacaspis yasumatsui Takagi armored scale infestations on the native Cycas micronesica K.D. Hill populations. The predator demonstrated a clear preference for A. yasumatsui infesting adult plants, resulting in 100% seedling mortality due to the lack of a biocontrol of the scale on seedlings. A Y-tube olfactometer was employed to determine if scale-infested seedling leaves were less attractive to R. lophanthae than scale-infested mature tree leaves. Five paired combinations of seedling versus mature tree leaves were used. The R. lophanthae adults navigated toward scale-infested and un-infested leaves of adults and seedlings when paired with an empty chamber. However, a clear preference for adult leaves occurred when paired with seedling leaves. The results were unambiguous in charcoal-filtered air, intermediate in unfiltered air from an open laboratory, and most ambiguous when conducted with unfiltered in-situ air. The number of predators that did not make a choice was greatest for in-situ air and least for charcoal-filtered air. These results indicated that the substrate used in olfactometry influenced the results, and interpretations of charcoal-filtered air assays should be made with caution. Volatile chemical cues are involved in R. lophanthae preferring A. yasumatsui located on C. micronesica adults when infested adult and seedling leaves are present.