Characteristics of the Psidium Cattleianum (Myrtaceae) Seed Bank in Hawaiian Lowland Wet Forests1
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Characteristics of the Psidium cattleianum (Myrtaceae) Seed Bank in Hawaiian Lowland Wet Forests1 Amanda L. Uowolo2,3 and Julie S. Denslow3 Abstract: Psidium cattleianum Sabine (strawberry guava) is one of Hawai‘i’s most disruptive alien plants. Dense stands can suppress growth and establishment of native species, support high populations of crop-damaging fruit flies, and pre- clude restoration or management of native forests. Our research investigated factors affecting persistence of P. cattleianum seeds in lowland wet forest soils. We collected soil cores from four forested sites immediately after fruit fall and 6.5 months later. We found abundant germination of P. cattleianum seeds imme- diately after fruit drop. Soil collected under mature P. cattleianum clumps yielded 761 viable seeds/m2. We found no viable seeds 6.5 months after fruit drop. We evaluated seed longevity using seed bags buried below the litter layer that we retrieved after 28, 56, 196, and 365 days. Seeds either germinated or de- teriorated rapidly after fruit drop; after 28 days, 22.3% of the buried seeds were viable and there were no viable seeds at 196 days. Predator effects were assessed using trays with a known number of seeds with and without predator exclosures. After 28 days, 37% of the seeds in the open trays were damaged by predators. The lack of a persistent seed bank likely is due to a combination of rapid, high germination rates, postdispersal seed predation, and seed mortality. We suggest that chemical or mechanical control efforts would be most efficient and effective if conducted at least 3 months after the fruiting season, when the vast majority of seeds have either germinated or died. Psidium cattleianum Sabine (Myrtaceae), elsewhere in the Pacific due to its ability to also known as strawberry guava, is considered invade even relatively undisturbed wet forests one of Hawai‘i’s most severe invasive weeds and form dense thickets (Smith 1985, Huen- (Smith 1985, Wagner et al. 1999, Motooka neke and Vitousek 1990). The fruits of P. cat- et al. 2003). Psidium cattleianum is common tleianum support high populations of fruit on all the major Hawaiian islands and flies (e.g., Dacus dorsalis Hendel) that are con- throughout the Pacific where it continues to sidered one of the most important agricul- increase in density and impact. Rapid growth tural pests in Hawai‘i (Vargas et al. 1990). rates ( J.S.D. and A.L.U., unpubl. data), The efficacy of any control method for P. cat- prolific fruit production, and high rates of tleianum is influenced by the characteristics of suckering produce dense stands quickly from its seed bank, because a persistent seed bank small populations (Huenneke and Vitousek contributes to high population growth rates 1990). Psidium cattleianum is a serious threat and reduces the likelihood of local eradica- to native forest ecosystems in Hawai‘i and tion. We investigated longevity of seeds of Psidium cattleianum in Hawaiian soils, the characteristics of the seed bank, and factors 1 Manuscript accepted 26 April 2007. affecting the duration of seeds in lowland 2 Corresponding author (e-mail: [email protected]). wet forest soils in Hawai‘i. 3 Institute of Pacific Islands Forestry, U.S. Depart- ment of Agriculture Forest Service, 60 Nowelo Street, Hilo, Hawai‘i 96720. materials and methods Species Description Pacific Science (2008), vol. 62, no. 1:129–135 Work of the U.S. Government Psidium cattleianum has invaded native Hawai- Not under copyright ian ecosystems ranging from moist to very 129 130 PACIFIC SCIENCE . January 2008 wet conditions from 15 to 1,300 m elevation an ‘o¯hi‘a (Metrosideros polymorpha Gaud.) low- ( Jacobi and Warshauer 1992, Wagner et al. land wet forest community type (Gagne´ and 1999). All forms of P. cattleianum grow as Cuddihy 1999). shrubs or small trees, typically 2–6 m tall in mesic to wet forests in Hawai‘i (Wagner et Seed Longevity al. 1999). We used seeds from the common round, yellow-fruited form (P. cattleianum f. Seed longevity of P. cattleianum in soil was as- lucidum Degener) in this study. Fruits contain sessed by tracking the fates of known num- 15–70 angular seeds (Diong 1982, Huenneke bers of seeds placed in bags and inserted and Vitousek 1990) of 2.5–5 mm in length under the litter at each of the four study sites. (Morton 1987, Wagner et al. 1999). Peak Fully ripened and intact P. cattleianum fruits fruit fall typically occurs between June and were collected from and beneath trees in Oc- December (Diong 1982, Huenneke and tober and pooled from multiple P. cattleianum Vitousek 1990). Psidium cattleianum seeds are groups across ‘O¯ la‘a Forest Reserve. Seeds readily spread by humans and feral pigs (Sus were immediately cleaned, air dried, and scrofa) (Diong 1982, Jacobi and Warshauer stored for 1 week in an air-conditioned envi- 1992) as well as by a variety of birds (Me- ronment. Percentage seed viability was as- deiros 2004). sessed on a subsample of the pooled seeds (10 replicates of 50 seeds each) 1 week after the seeds were collected and on the same day Study Area the seed bags were placed in the field. At each Study sites were established within the Upper study site, one set of four seed bags was Waia¯kea Forest Reserve (19 35 0 N, 155 12 0 placed under each of five groups of large P. W ), the Pu‘u Maka‘ala Natural Area Reserve cattleianum trees. Each sealed bag (6 by 7.5 (19 34 0 N, 155 12 0 W ), the ‘O¯ la‘a Forest cm, 86-mesh silkscreen fabric) contained 20 Reserve (19 27 0 N, 155 11 0 W ), and the Ka- seeds and 5 g of sieved, autoclaved soil. The hauale‘a Natural Area Reserve (19 26 0 N, bags were placed beneath the litter layer 155 100 W ) on the island of Hawai‘i. All (generally 2.5–5 cm deep) in contact with sites are on windward Hawai‘i Island at ap- the soil surface and were placed in the field proximately 900 m elevation. Estimated an- during the fruiting season in late October. nual rainfall is 3,000–4,000 mm at ‘O¯ la‘a and One seed bag from each of the five sets of Kahauale‘a and 4,000–5,000 mm at Upper bags placed at each site was retrieved after Waia¯kea and Pu‘u Maka‘ala (Giambelluca et 28, 56, 196, and 365 days. When the buried al. 1986). Projected mean annual temperature bags were retrieved, the seeds were evaluated based on adiabatic lapse rates is 17–17.5C and seeds that had germinated or decayed for the elevation range of the four study sites were separated from those that were intact. (Giambelluca and Schroeder 1998). All of the Intact seeds were tested for viability by plant- study sites are on relatively young tholeiitic ing in sterile growing medium in a greenhouse basalt lava flows that formed 200–1,500 yr under irrigation and approximately 53% sun. B.P. (Wolfe and Morris 1996). The soils at Germination of seeds was monitored over a Upper Waia¯kea and Pu‘u Maka‘ala are deep 16-week period. and well drained (Typic Udifolists, U.S. De- The buried bags provided information partment of Agriculture Natural Resource on changes in the numbers of viable, persis- Conservation Service [USDA-NRCS], un- tent seeds and seed decay and senescence publ. data) over weathered ‘a‘a¯ lava. The soils over time. We modeled the decline in num- at Kahauale‘a (Lithic Hapludands, USDA- bers of viable seeds as an exponential decay NRCS, unpubl. data) and ‘O¯ la‘a (Lithic Udi- function (SigmaPlot 8.02 [SPSS Inc. 1986– folists, USDA-NRCS, unpubl. data) are very 2001]): shallow to shallow, moderately well-drained y ¼ y eÀkt soils formed in volcanic ash deposited over 0 pa¯hoehoe lava bedrock. The forests are classi- in which y is the number of the original seeds fied as native lowland wet forests with remaining and persisting as viable seeds in the Psidium cattleianum Seed Bank . Uowolo and Denslow 131 soil at time t, y0 is the number of seeds at trays with the seeds, and the trays were lined time 0, t is the elapsed time in days, and k is with 2 mm nylon mesh to prevent washout. the decay constant. The mean residence time Trays were placed in the field in late Novem- (MRT) is calculated: ber at the end of the fruiting season at all study sites. After 28 days, the trays were col- MRT ¼ 1/k lected, seeds and litter material were sepa- The seed bags excluded mammalian and many rated, and seeds were counted and evaluated invertebrate predators from access to the for evidence of predation. We assumed that seeds but did not prevent infestation by soil secondary seed dispersal did not occur and, fungi; therefore, k and mean residence time by extension, that missing seeds had been represent longevity of the seeds in the soil in consumed and not dispersed as viable seeds. the absence of predators. Seeds with evidence of animal activity (i.e., teeth marks, seed fragmentation) were con- sidered damaged and were assumed to be Seed Bank nonviable as well. Data from one subse- We collected four soil cores (500 cm3,to5 quently overturned tray were excluded. cm deep) from beneath each of five clusters of mature P. cattleianum trees from each of Data Analysis and Synthesis the four study sites on each collection date. We selected P. cattleianum tree clusters with We used the data from the seed longevity and canopies exposed to full sun to maximize the seed predation experiments to develop a pro- probability of sampling where trees had gen- jection of the fate of P.