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Foliar Nutrition of Serianthes nelsonii Seedlings as a Conservation Tool


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Conservation of the endangered Serianthes nelsonii is constrained by lack of research. Transplanted containerized plants die in the competitive in situ environment. This study determined if foliar applications of nutrient solution could replace edaphic fertilizer applications for mitigating competition for soil nutrients. Weekly sprays with 0.1× Hoagland solution were compared with weekly drenches of 0.5× Hoagland solution. Plants receiving edaphic or foliar nutrition were not different in height, and height growth was 72% above that of control plants. Similar results were obtained for stem diameter and leaf number. Leaf nutrient concentrations were not different for the two nutrition treatments, but stem nutrient concentration differences were dependent on the element. Stem copper, nitrogen, phosphorus, potassium, and zinc concentrations were not different for edaphic vs. foliar nutrition. Contrarily, stem boron, calcium, iron, magnesium, and manganese concentrations were greater in plants receiving edaphic nutrients. The results indicate nutritional needs of recently out-planted plants may be supplied directly to leaves to mitigate below-ground competition for nutrient resources.
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HORTSCIENCE 57(3):389390. 2022.
Foliar Nutrition of Serianthes nelsonii
Seedlings as a Conservation Tool
Thomas E. Marler
Western Pacic Tropical Research Center, University of Guam, Mangilao,
GU 96923
Additional index words. conservation biology, endangered plants, Guam, nutrient resorption
Abstract. Conservation of the endangered Serianthes nelsonii is constrained by lack of
research. Transplanted containerized plants die in the competitive in situ environ-
ment. This study determined if foliar applications of nutrient solution could replace
edaphic fertilizer applications for mitigating competition for soil nutrients. Weekly
sprays with 0.1× Hoagland solution were compared with weekly drenches of 0.5×
Hoagland solution. Plants receiving edaphic or foliar nutrition were not different in
height, and height growth was 72% above that of control plants. Similar results were
obtained for stem diameter and leaf number. Leaf nutrient concentrations were not
different for the two nutrition treatments, but stem nutrient concentration differences
were dependent on the element. Stem copper, nitrogen, phosphorus, potassium, and
zinc concentrations were not different for edaphic vs. foliar nutrition. Contrarily,
stem boron, calcium, iron, magnesium, and manganese concentrations were greater in
plants receiving edaphic nutrients. The results indicate nutritional needs of recently
out-planted plants may be supplied directly to leaves to mitigate below-ground com-
petition for nutrient resources.
Conservation of the endangered S. nelsonii
has suffered from a history of limited research
(Marler et al., 2021). A chronic conservation
failure has been mortality of container-grown
plants after transplanting to in situ forests.
Limited root growth at the time of transplant-
ing (Marler, 2019) and competition for resour-
ces in this biodiverse setting (Marler and
Musser, 2015) have been identied as causes
of in situ seedling mortality.
Mineral nutrients are derived from the
soil, and soil-applied fertilizers form the
foundation of horticultural mitigation of de-
ciencies. Under some conditions, these added
nutrients may become unavailable to the
managed plants due to soil characteristics or
plant competition. Foliar applications and
trunk injections of nutrients have been used
to mitigate these conditions. This study deter-
mined if foliar applications of nutrient solu-
tions to S. nelsonii seedlings could generate
growth and tissue concentrations similar to
soil-applied solutions.
The study was conducted in a conserva-
tion nursery in Angeles City, Philippines.
The seedlings were sourced from urban street
trees on Rota Island, and original provenance
was not known. The seedlings were initially
grown in tubes (5-cm diameter, 12-cm depth)
and were 27.8 ± 1.1 cm tall (mean ± SE)with
a basal diameter of 5.1 ± 0.2 mm at the initia-
tion of the study. Seedlings were bare-rooted
on 5 Nov. 2017 and planted individually in
2.6-L containers in a medium composed of
one-third loam soil and two-thirds quarried
river sand. The soil ensured a suite of nutrients
to sustain limited growth of control plants, and
the sand ensured adequate drainage in the con-
tainers. This substrate was impoverished, as
indicated by soil analyses that revealed total
nitrogen was 4.9 ± 0.2 mg·g
, available phos-
phorus was 5.9 ± 0.5 mg·kg
, and excha-
ngeable potassium was 29.6 ± 1.6 mg·kg
(mean ± SE,n=4).
The plants were sorted into three treat-
ments. Control plants received no nutritional
applications. Plants in the foliar treatment
received weekly sprays of 0.1 × Hoagland
solution. Every fully expanded leaf was
sprayed using the standard horticultural pro-
tocol of application until initial runoff. Plants
in the soil-applied treatment received weekly
drenches of 0.5 × Hoagland solution. Each
container received 200 mL of solution. Daily
irrigation using deep well water was provided
with care to refrain from wetting the leaf sur-
faces. Initial plant height and basal stem
diameter were measured. The plants were
grown under 50% shadecloth and rainfall
protection until 15 Jan. 2018.
Ending height, basal stem diameter, and
leaf number were measured. The terminal 15
cm of each stem was pruned and separated
into stem and leaf tissues. All tissue was
washed four times in reverse osmosis water
to remove all nutrients adhering to the organ
surfaces. Nutrient concentrations were quan-
tied using previously described methods
(Marler, 2021). The tissue was dried at 75 C
for 24 h and milled to pass through 20-mesh
screen. Total carbon and nitrogen were deter-
mined by dry combustion (FLASH EA1112
CHN Analyzer; Thermo Fisher, Waltham,
MA). Samples were digested by a microwave
system with nitric acid and peroxide, then
boron, calcium, copper, iron, magnesium, man-
ganese, phosphorus, potassium, and zinc were
quantied by inductively coupled plasma opti-
cal emission spectroscopy (Spectro Genesis;
SPECTRO Analytical Instruments, Kleve,
Each response variable was subjected to
one-way analysis of variance (Proc GLM;
SAS Institute, Cary, NC). Growth was calcu-
lated as the difference in height and stem
diameter from the initial to the nal measure-
ments. Tukeys honestly signicant differ-
ence was used for pairwise comparisons for
signicant response variables.
Growth was similar for plants receiving the
two nutrient application treatments (Fig. 1).
The increase in height was 72% above, increase
in diameter was 29% above, and ending leaf
number was 64% above that of control plants.
Leaf nutrient concentrations were not dif-
ferent between the two nutrition treatments,
but one or both treatments exhibited leaf con-
centrations greater than control plants for eight
of the 11 nutrients (Table 1). The exceptions
were boron, carbon, and copper.
Stem nutrient concentration differences
were heterogeneous among the nutrients
(Table 1). Stem boron, copper, nitrogen, phos-
phorus, potassium, and zinc concentrations
were not different for edaphic vs. foliar nutri-
tion. In contrast, stem calcium, iron, magne-
sium, and manganese concentrations were
greater in plants receiving edaphic applica-
tions than plants receiving foliar applications.
As expected, most of the nutrients in stems of
the plants receiving edaphic applications were
greater than stems of control plants. In addi-
tion, eight of the 11 nutrients exhibited greater
stem concentrations for the foliar application
treatments than for the control treatment.
The results indicate nutritional needs of
recently out-planted S. nelsonii plants may be
supplied as sprays to aboveground organs to
mitigate the below-ground competition for
nutrient resources. Phosphorus and potassium
limit in situ S. nelsonii productivity more than
other nutrients based on leaf stoichiometry
(Marler, 2021). Plants receiving the foliar appli-
cations exhibited leaves with 56% greater phos-
phorus and 35% more potassium than control
plants. More importantly, plants receiving foliar
applications exhibited stems with 75% more
phosphorus and 45% more potassium than con-
trol plants, indicating efcient mobilization of
foliar-applied nutrients into the stem tissue.
Nitrogen, phosphorus, potassium, and zinc
are resorbed in senescing S. nelsonii leaves
more efciently than other nutrients (Marler,
2021). Although the control plants in this study
exhibited leaf mortality, no leaf mortality
Received for publication 18 Nov. 2021. Accepted
for publication 17 Dec. 2021.
Published online 28 January 2022.
T.E.M. is the corresponding author. E-mail:
This is an open access article distributed under the
CC BY-NC-ND license (https://creativecommons.
occurred in the nutrient application treatments.
The substantial increase in stem concentrations
of these nutrients deserves further study, as the
results indicate translocation from leaf to stem
occurred in the absence of leaf senescence. The
observation that leaf longevity increased in
plants receiving either nutrient treatment also
deserves further study as one approach for
increasing plant productivity.
The mechanistic details of these plant beha-
viors may be more fully understood with more
rened methods; for example, use of solution
applications that ensure homogeneous total
nutrient applications between the two treat-
ments. Moreover, endorsement of this conserva-
tion protocol requires repeating the methods in
situ. One caveat of my methods is that rainfall
was excluded from the experimental plants. This
is not feasible in a forest setting. Rainfall may
affect the results in two ways. First, tissue con-
centrations may be less than reported here if
nutrients are washed off by rainfall before
absorption into the laminae. Second, tissue con-
centrations may be more than reported here if
some surface nutrients are washed into the soil
where they may be absorbed by the roots. More-
over, plant responses to horticultural manipula-
tions may be affected by con- and interspecic
competition, so the efcacy of this newly
described management protocol in situ is not
known until it is repeated in a competitive forest.
This study adds to a growing body of
adaptive management lessons that inform
conservation decisions for this endangered
endemic tree species (Marler et al., 2021).
For example, treatments that increase relative
root growth in a container nursery appear to
be mandatory to improve posttransplant sur-
vival, and two protocols have been identied
to achieve this goal (Marler, 2019; Marler
and Callaway, 2021). As a late successional
species, studies indicate shade is mandatory
for germination and growth of seedlings and
saplings (Marler et al., 2015). Grafting scions
on congeneric rootstocks may be used to mit-
igate the constrained seed supply (Marler,
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nursery plants improves transplant quality and
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stoichiometry, and resorption in Guamscoastal
karst forests. Diversity (Basel) 13:545, https://
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)mc( thgieh ni htworG
Growth in stem diameter (mm)
Leaf number
Fig. 1. Size of Serianthes nelsonii seedlings as inuenced by nutrition treatments. Blue = control; green = edaphic nutrition; yellow = foliar nutrition.
(A) Increase in stem height; (B) increase in stem diameter; (C)nal leaf number. Bars with same letters are not different. Mean ± SE,n=8.
Table 1. Foliar and stem nutrient concentrations of Serianthes nelsonii seedlings as inuenced by
edaphic versus foliar nutrition. Mean ± SE,n=8.
Nutrient Control Edaphic nutrition Foliar nutrition F
Boron 33.1 ± 1.6 a
35.1 ± 1.9 a 35.7 ± 1.7 a 0.726 0.496
Calcium 10.6 ± 0.5 b 12.1 ± 0.6 ab 12.8 ± 0.8 a 4.997 0.017
Carbon 447 ± 3 a 452 ± 2 a 450 ± 2 a 1.474 0.252
Copper 2.5 ± 0.3 a 2.9 ± 0.3 a 3.1 ± 0.3 a 1.209 0.318
Iron 38.8 ± 2.8 b 59.1 ± 4.2 a 57.1 ± 5.3 a 7.772 0.003
Magnesium 2.1 ± 0.1 b 2.9 ± 0.1 a 2.8 ± 0.1 a 19.557 <0.001
Manganese 24.1 ± 1.9 b 32.4 ± 3.4 a 33.9 ± 3.5 a 3.547 0.047
Nitrogen 15.1 ± 1.0 b 22.5 ± 1.5 a 21.2 ± 1.2 a 9.976 <0.001
Phosphorus 1.6 ± 0.1 b 2.6 ± 0.1 a 2.5 ± 0.1 a 30.798 <0.001
Potassium 11.2 ± 0.4 b 14.8 ± 0.5 a 15.1 ± 0.6 a 21.904 <0.001
Zinc 20.4 ± 1.9 b 36.6 ± 2.5 a 36.4 ± 2.8 a 14.847 <0.001
Boron 9.7 ± 0.3 b
11.7 ± 0.7 a 10.4 ± 0.5 ab 10.265 <0.001
Calcium 15.1 ± 1.1 a 14.7 ± 0.3 a 11.7 ± 0.2 b 8.480 0.002
Carbon 426 ± 1.8 a 427 ± 2 a 433 ± 3 a 3.369 0.058
Copper 3.1 ± 0.3 a 4.0 ± 0.8 a 3.9 ± 0.3 a 1.452 0.257
Iron 42.8 ± 2.5 a 44.5 ± 3.5 a 29.4 ± 1.2 b 3.986 0.034
Magnesium 2.4 ± 0.1 b 3.1 ± 0.2 a 2.5 ± 0.1 b 11.089 <0.001
Manganese 5.2 ± 0.3 b 6.8 ± 0.4 a 5.1 ± 0.4 b 6.345 0.007
Nitrogen 5.7 ± 0.6 b 8.8 ± 1.0 a 8.7 ± 0.4 a 6.725 0.006
Phosphorus 1.6 ± 0.2 b 2.7 ± 0.1 a 2.8 ± 0.1 a 21.821 <0.001
Potassium 10.4 ± 0.1 b 14.9 ± 0.8 a 15.1 ± 0.7 a 18.672 <0.001
Zinc 24.1 ± 1.4 b 37.5 ± 4.3 a 34.5 ± 2.7 a 5.481 0.012
Means within a row not followed by the same letter are signicantly different at P#0.05.
... In situ S. nelsonii trees are primarily limited by phosphorus (P) and secondarily limited by potassium (K) [15]. To my knowledge, only one study has been published regarding the use of horticultural fertilization methods for S. nelsonii plants [16]. In this study, foliar sprays of dilute nutrient solutions were employed to successfully supply leaves with essential plant nutrients. ...
... This prediction was shown to be true by quantifying carbon dioxide efflux and nitrogen release from litter incubated in native soil [23]. A complete nutrient solution may be sprayed on S. nelsonii foliage to improve plant nutrition [16]. The current study expands our understanding of this subject by revealing the utilitarian benefits of P-only fertilizers for mitigating P deficiency. ...
Full-text available
The genus Serianthes has not received adequate research attention, leaving large gaps in the knowledge required to inform conservation decisions. For example, nutrient management protocols are not understood due to lack of research. Serianthes grandiflora, Serianthes kanehirae, and Serianthes nelsonii plants were grown in container culture to determine the influence of increasing edaphic nitrogen (N), phosphorus (P), or potassium (K) content on stem growth and leaf nutrient relations. Addition of N alone increased leaf N, stimulated stem height and diameter growth, increased leaf number, and reduced leaf tissue concentrations of most nutrients including P and K. Addition of K alone increased leaf K, did not influence stem growth, did not reduce N or P concentration, but caused substantial changes in leaf tissue stoichiometry. Addition of P alone increased leaf P, did not influence stem growth, did not reduce leaf N or K concentration, and exerted minimal influence on concentrations and stoichiometry of other nutrients. The results indicate that single element P fertilization may be used to mitigate P deficiencies of Serianthes saplings without the risk of causing substantial nutrient imbalances. This knowledge may be used to inform nutrient management decisions in Serianthes conservation projects.
... However, to orient the selection of which trees to use, there is a need expand knowledge about the potential of native trees in producing litter and contributing nutrients to the ecosystem. Several recent reports have included leaf nutrient traits for S. nelsonii [18][19][20], but nothing has been published concerning the seasonal variations of fresh or senesced leaves and other organs. Therefore, a better understanding of how nutrient content changes throughout the year is needed to appreciate the ecosystem services provided by S. nelsonii and factor this information into habitat management plans. ...
Full-text available
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.
... The ability to use native microbiota to partly mitigate this deficiency should be pursued in conservation planning. Application of fertilizer to foliage as a dilute aerosol [30] and use of P-only granular fertilizer [31] may also be factored into Serianthes nutrient management protocols. Third, the use of mefenoxam fungicide has been shown to increase the longevity of in situ S. nelsonii seedlings, indicating that soil-borne pathogens from the rhizosphere may result in negative biotic PSF [32]. ...
Full-text available
Soils from the rhizosphere of perennial plants accumulate microorganisms that influence the growth of other plants. This microorganism biodiversity may be exploited by using these soils as an inoculum in new planting sites. Soils collected from the rhizosphere of mature Serianthes trees were subjected to treatments designed to reduce or increase microorganism populations, then were used in a series of five studies to grow Serianthes plants in container culture. Serianthes kanehirae and Serianthes grandiflora stem growth was 14–19% greater, leaf nitrogen was 40─46% greater, leaf phosphorus was 50─86% greater, and leaf potassium was 28─43% greater when grown in soils from Serianthes rhizosphere than in soils away from a Serianthes tree. Treating the Serianthes rhizosphere soils with sterilization or propiconazole fungicide reduced stem growth of S. grandiflora, S. kanehirae, and Serianthes nelsonii plants by 16─47% below that of untreated soils. The sterilization and fungicide treatments also consistently reduced phosphorus (48─50%) and potassium (12─21%) content of leaves when compared with untreated rhizosphere soil. Adding Rhizophagus irregularis inoculum to the sterilized soil reversed the reduction in S. grandiflora stem growth and leaf phosphorus content. These findings indicate that soils from the Serianthes rhizosphere contain beneficial microorganisms for Serianthes plant growth and leaf nutritional status and exploiting these soils as an inoculum for new planting sites may provide a net-positive influence on post-transplant growth and survival.
Background and Research Aims Conservationists aiming to establish seed orchards or restoration plantings benefit from methods that reduce tree juvenility. Previous studies suggest that seedling and sapling mortality is the major limitation to natural population recovery of the critically endangered Serianthes nelsonii. To enable effective restoration actions new methods are required to reduce sapling mortality. Methods Ex situ experiments were used to determine if trellised, shaded growth conditions would reduce juvenility of Serianthes saplings. Serianthes kanehirae was used as a surrogate for S. nelsonii. Stem growth was trained horizontally to a wire trellis in shaded tunnels of differing lengths (0, 3, 6, 9 or 12 m). Time to reach the end of each tunnel, and subsequent vertical stem growth in full sun were recorded until flower production commenced. Results The shortest S. kanehirae juvenile phase occurred under 12 m tunnels, with first flower production within a year (351 ± 10 days, mean ± SE). The juvenile phase increased in duration as tunnel length shortened. The control plants did not produce flowers during the study. Conclusion Horizontal trellised support of shaded stems was effective in reducing juvenile phase duration of Serianthes saplings. Implications for Conservation Serianthes seed orchards may use shaded trellises to produce seeds on young plants to rapidly boost species recovery efforts. The production of seeds in managed conservation gardens may improve recovery efforts by reducing the need to remove seeds from in situ communities. Therefore, this simple protocol may prove beneficial for both in situ and ex situ conservation of all federally listed woody plant species in the region.
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Greater knowledge concerning the interspecific diversity of the plant leaf ionome is required to effectively understand the spatiotemporal dynamics of biogeochemistry, but Micronesia has been ignored in this literature. The objectives of this study were to quantify the leaf ionome, resorption efficiency, and stoichiometry of leaves from 25 plant species representing Guam’s coastal karst forests. Carbon and nitrogen were quantified by dry combustion, and other minerals and metals were quantified by spectrometry. Nitrogen and calcium concentrations in Guam’s green leaves exceeded the published global means, but manganese and copper concentrations were less than the global means. The remainder of the elements were within the expected ranges. Nutrient resorption rates exhibited a decreasing order of potassium > phosphorus > nitrogen > zinc > copper. The term “accretion efficiency” is introduced to describe the accumulation of an element throughout leaf aging and senescence, and calcium and iron exhibited substantial accretion efficiency in this study. Stoichiometry relations indicated that Guam’s karst forest is most limited by phosphorus and then secondarily limited by nitrogen, although several individual taxa exhibited co-limitation by potassium. Five of the species are officially listed on extinction threat lists. Of these, the Malvaceae tree Heriteria longipetiolata exhibited leaf traits depicting the most recalcitrant litter characteristics, and the Fabaceae tree Serianthes nelsonii exhibited leaf traits depicting the most labile litter characteristics. The contributions of these two tree species to spatiotemporal diversity in biogeochemistry appear to be profound, indicating species recovery efforts are of paramount importance for maintaining ecosystem function and soil heterotroph biodiversity in northern Guam.
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Mixtures of species in natural or agricultural systems can increase the performance of individuals or groups relative to monocultures, often through facilitative mechanisms. Mechanisms include root communication by which plants can interrogate the identity of adjacent plants and respond negatively or positively. Alternatively, mixtures of species can ameliorate the harmful effects of soil biota that are pronounced in monocultures, thereby improving plant productivity. Limited investments into roots by shade-grown Serianthes plants in nurseries have been correlated with reduced survival after transplantation to forested habitats. We used companion container cultures in two studies to determine if heterospecific neighbor, or “stranger” roots could experimentally increase the root growth of Serianthes grandiflora plants used as surrogates for the critically endangered Serianthes nelsonii. In one study, native sympatric eudicot and pteridophyte companions increased relative root growth and conspecific companions decreased root growth in comparison to control plants that were grown with no companions. In a second study, the phylogeny of companion plants elicited different root growth responses following the order of congeneric < eudicot = monocot < gymnosperm < pteridophyte. We propose the use of stranger roots that are experimentally maintained in production containers as a passive protocol to improve relative and absolute root growth, leading to improved post-transplant growth and survival of container-grown Serianthes plants.
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The literature covering the biology, ecology, horticulture, and conservation of the critically endangered tree Serianthes nelsonii Merr. was reviewed. The roots, stems, and leaves of this charismatic legume tree revealed highly plastic traits and responded positively to horticultural manipulations to improve the quality of container-grown transplants. Pre-sowing seed treatments of seed coat scarification and 1 h of imbibition generated 85% to 90% germination at a temperature optimum of 26 °C. Adventitious root formation on air layers and successful unions on approach grafts were 100%. Seedling and sapling growth was maximum under 25% to 50% sunlight transmission, limited irrigation to ensure adequate root zone aeration, repetitive stem tip pruning to increase root:shoot quotient, and thigmic stress to retain an orthotropic orientation of stems. In situ regeneration on Guam was substantial but recruitment from seedling to sapling was nil. High quality leaf litter chemistry enabled rapid decomposition, and soils beneath the tree exhibited unique chemical traits that increased ecosystem health by creating spatial heterogeneity. The greatest unanswered questions focus on plant mortality. Research is needed to determine the reasons for the mortality of in situ seedlings, mortality within transplantation projects on Guam, and the mortality of 60% of the mature in situ tree population during the 26-year implementation of the national recovery plan. Horticultural researchers are ideally positioned to answer these urgent questions.
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The natural distribution of the critically endangered tree Serianthes nelsonii (Håyun lågu) is restricted to 33 known individuals located on the islands of Rota and Guam, Mariana Islands. Major risks of extinction are a limited range, a minimal number of individuals, and a lack of recruitment. The potential for grafting and air-layering propagation was evaluated to reveal new approaches, by which species recovery efforts may be achieved. Root formation occurred on 100% of the air-layered S. nelsonii stems. Using Serianthes kanehirae as rootstock, graft success was 100% for approach graft and 25% for traditional veneer graft techniques. These results have shown that asexual reproduction is highly successful for this woody legume species and is available to address the restrictions on species recovery caused by limited seed availability. These propagation strategies open up new prospects for conserving contemporary genetic diversity and reversing the failures in S. nelsonii recovery efforts, since the 1994 recovery plan was published.
Full-text available
Potential agents of seedling mortality for Guam's only known Serianthes nelsonii (Håyun lågu) tree were determined from the in situ crop of emerging seedlings. We used insecticides to mitigate exotic arthropod pressure, fertilizer to mitigate nutrient deficiency, fungicide to mitigate root pathogen pressure, and supplemental lighting to mitigate low light stress. The fungicide treatment was the only treatment that elicited a substantial increase in seedling longevity. Past reports based on general observations point to insect and ungulate herbivory as likely agents of seedling mortality. Our results identify root pathogens as an additional cause of mortality at the study site. Results may be useful for designing more complex experiments to improve species recovery efforts.
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Seedling emergence and growth traits of three rare and threatened tree species in the Mariana Islands were studied within a range of incident light levels and up to 9 months of seed storage. Seedling emergence percentage and velocity were maximized in moderate shade for Elaeocarpus joga Merr., deep shade for Serianthes nelsonii Merr., and full sunlight for Tabernaemontana rotensis (Kaneh.) P.T. Li. Seedling height was increased by shade for E. joga and S. nelsonii. Height of T. rotensis seedlings was not influenced by incident light from 25% to 100%. Nine months of seed storage at ambient temperature did not influence emergence percentage of E. joga or S. nelsonii seeds. In contrast, seeds of T. rotensis began declining in seedling emergence percentage between 2 and 3 months of storage, and seedling emergence was nil by 4 months. This study represents the first experimental approach to determining the influences of light and storage on seed and seedling behavior for any rare and threatened taxa from the Mariana Islands. Our findings that revealed highly contrasting responses among the species provide a valuable start to building the knowledge base needed to respond to formal recovery or conservation plans by defining horticultural protocols for managing a conservation nursery.
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.