Seedling and clonal recruitment of the invasive tree Psidium cattleianum: Implications for management of native Hawaiian forests
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Biological Conservation 53 (1990) 199-211 ,~L , 1 ~,,.
Seedling and Clonal Recruitment of the Invasive Tree PMdium cattleianum: Implications for Management of
Native Hawaiian Forests
Laura Foster Huenneke
Department of Biology, New Mexico State University, Box 30001, Las Cruces, New Mexico 88003, USA
Peter M. Vitousek
Biological Sciences, Stanford University, Stanford, California 94305, USA
(Received 15 June 1989; revised version received 30 November 1989; accepted 7 December 1989)
A BS TRA C T
Non-native plants present serious management problems in many preserves. Strawberry guava Psidium cattleianum (Myrtaceae), a small tree and aggressive invader of tropical areas, is rapidly spreading through many Hawaiian forests including those of the two US national parks in Hawaii. Feral pigs and non-native birds disperse Psidium seeds; pigs also create soil disturbances that may enhance the tree's spread. Our study of guava's reproductive biology focussed on its dependence on non-native animals. We found that the abundantly produced seed germinated rapidly under a wide range of conditions, without scarification. Psidium seedlings occur on the same substrates as do native seedlings, usually on undisturbed sites. Both seedlings and clonally produced suckers are common, but suckers contribute greater leaf areas. Guava's clonal growth may partially explain its success in dominating native forests. Apparently germination and establishment do not depend on animal dispersal, or on disturbances created by pigs; thus, control of the plant cannot rest entirely on control of non-native animals.
199 Biol. Conserv. 0006-3207/90/$03"50 1990 Elsevier Science Publishers Ltd, England. Printed in Great Britain
200 Laura Foster Huenneke, Peter M. Vitousek
Invasive plants can pose a significant challenge to the management of preserves or conservation areas. Invading plants (those non-native species able to establish themselves and spread in relatively undisturbed natural ecosystems) flourish at the expense of natives; they may even alter the structure and function of the invaded ecosystem (Vitousek, 1986). The resulting changes may be of conservation concern, when the management goal is to maintain natural ecosystem processes or features, or when native plant or animal species are adversely affected. Over the past few years a number of reviews have addressed the nature, mechanisms, and conse- quences of plant and animal invasions (e.g. Mooney & Drake, 1986; Kornberg & Williamson, 1986; Joenje et al., 1987). There has been growing realization that invasive plants can represent substantial threats to natural ecosystems, including those in parks or other preserve areas.
Like many other oceanic islands, the Hawaiian Islands have proven vulnerable to invasive non-native plants. Some species were introduced by the Polynesians, and thousands more in the past 200 years. Of these, 86 are considered to have become serious problems in natural ecosystems (Smith, 1985). Psidium cattleianum Sabine (Myrtaceae), the strawberry guava, is a small tree believed to have been brought to Hawaii from Brazil (Wagner et al., in press). The species has invaded Hawaiian forests on several islands, presenting management problems in national parks and other natural areas. On Maui strawberry guava has come to dominate extensive areas of previously native forest (L. Cuddihy, pers. comm.) in Haleakala National Park; on the Big Island of Hawaii, the species has been found in a variety of native ecosystems, from quite dry to very wet forests, from 100 m to 1000 m in elevation (Jacobi & Warshauer, in press). While strawberry guava forms dense thickets along roadsides and in other disturbed areas, it is also successful in seemingly undisturbed intact forest.
Psidium cattleianum produces abundant, juicy fruits (red in forma cattleianum, yellow in forma lucidum) attractive to birds and mammals. Non-native birds and feral pigs Sus scr~?[il may represent important dispersal agents for the hard-coated seeds within the fruits. Pigs may serve another role, as they create extensive areas of soil disturbance by their rooting activities within native forest; seedling establishment for weedy plant species might be favored in such disturbed sites. Potential feedback loops between guava and non-native animals, then, may present critical targets for any control of guava's further spread and domination of native forests.
The primary objective of our research on the population biology of P. cattleianum was to supply information on its reproductive behavior, and
Strawberry guava encroachment in Hawaiian forests 201
particularly its relationship with animals such as feral pigs, that might guide management and control programs. Are reproductive behavior or growth related to elevation, suggesting an upper elevational limit to the species' potential range? Is guava dependent on scarification or dispersal of its seeds by animals for successful germination? Is seedling establishment dependent on or responsive to soil disturbance? Finally, what is the relative importance of seed reproduction versus clonal behavior in guava's ability to form dense thickets and exclude native plants?
Our studies of P. cattleianum in and near Hawaii Volcanoes National Park began in June 1987. Stem densities, size structure and phenology were observed in thickets along an elevational gradient outside park boundaries. The five stations range from 150m to 760m in elevation, on state forest reserve land west of Hilo (along the Stainback Highway), where strawberry guava dominates much of the roadsides. Scattered individuals of native woody plants (chiefly ohi'a, Metrosideros polymorpha, and tree ferns of the genus Cibotium) occurred in the guava thickets. Other non-native plants (shrubs, vines and grasses) were common. Numbers and diameters of strawberry guava and other woody stems were recorded for three 5 m x 5 m plots within the thicket interior at each station. Twenty-five interior and 25 thicket-edge stems were tagged at each station and monitored monthly for phenological stage (ripe fruit, unripe fruit, flowering, in bud, new leaf flush, green vegetative).
Seed germination physiology was investigated in laboratory trials. Fruits were collected in November 1987 from the canopy and forest floor of the 5 stands along the elevational gradient. The hard-coated seeds were removed from the fruit pulp and allowed to air-dry; numbers of seeds per fruit were counted for 15 fruits from each station. Seed mass (air-dry weight) was measured for 4 sets of 10 seeds each for each station.
After storage for a month at room temperature, seeds were germinated under various conditions (in light and in dark, 22C; in dark, 18C, 37C and 22C after 2 or 14 days pre-chilling treatment, after scarification with sandpaper, and with a drought period of one week imposed after 2 weeks of moisture, with and without prior scarification). Seeds were germinated on
202 Laura Foster Huenneke, Peter M. Vitousek
filter paper in covered Petri dishes, wetted with deionized water, in sets of 25 seeds per dish (4 replicate dishes per trial).
Germination experiments were also carried out in the field. In November 1987 cleaned seed was set out in groups of 10 on bryophyte mats, on cleared soil surface, and on the litter layer, at the 460 m station (10 groups on each substrate). Several intact ripe fruits were also placed on moss-covered substrates and tagged. In addition, 1-m 2 quadrats were cleared of all leaf litter (5 at the 460 m site, 5 at another site (Campbell) adjacent to the park boundary at 1200m) and the soil disturbed to simulate pig damage. All locations were revisited in late March 1988 and the number and condition of germinants noted. Seedling densities, and locations of seedlings on particular microsites such as bryophyte mats or decomposing wood, were noted in one population in the park (Thurston, l190m), at Campbell, and at the 460m Stainback station. In February 1988, one 25 cm x 25 cm 10 cm deep block of soil, and another block of equal size from a dense mat of bryophytes, were collected at the 460 m Stainback site, where fruit production had been very heavy the previous November. These blocks were removed to the laboratory and all guava seeds (or fragments of seeds) separated from them. Seeds were examined under a dissecting microscope and separated into intact, fragmented and germinated-but-dead categories (germinated seeds had pores in the seed coat corresponding to those displayed by germinated seeds in the laboratory germination trials, and often retained a portion of dead seedling tissue as well).
The relative importance of seedling vs sucker or vegetative recruitment was estimated by sampling understory guava recruits (10-50cm height) at Thurston, Campbell, and three of the Stainback stations (150 m, 460 m and 762m). Recruits were pulled from the soil; true seedlings were readily distinguished by their branched root systems, while suckers were attached to horizontal roots or rhizomes by unbranched connections. Stems were cut at the level of the soil surface, height measured, and number and total surface area of leaves recorded. Regressions of leaf area against recruit height were used to compare leaf area production of seedlings and suckers.
Demographic data (recruitment, stem growth and mortality) were collected for 3 populations within the park--Thurston (1190 m, where P. cattleianum is invading as scattered stems beneath a closed canopy of native trees and tree ferns); Upper Kalapana (830 m, a denser stand of guava in a wet ohi'a-
Strawberry guava encroachment in Hawaiian forests 203
tree fern forest); and Boundary (1260 m, in drier open ohi'a-koa forest east of the Kipuka Puaulu trail loop, adjacent to the eastern park boundary). Plot boundaries were flagged (40 m x 40 m at Thurston; 30 m 30 m at the other sites), and all guava stems within the plots tagged and measured (diameter at breast height). Each population was revisited at least once by June 1988, and all stems relocated and their diameters remeasured. Survivorship and growth rates were estimated for size classes of stems in each site.
Phenological observations in the 5 Stainback thickets confirmed that reproductive activity is more frequent for stems on the edge of thickets than for those in the interior. At least a few stems were flowering or fruiting at all elevations in most months, although the peak of reproductive activity occurred in June to October (Fig. 1). January through March was the season of lowest reproductive behavior. Thickets at 460 and 610 m elevation had consistently high rates of reproduction relative to the other stations, but even the lowest and the highest thickets experienced substantial rates of reproductive activity over the course of the study. All fruits observed on the Stainback gradient were yellow-skinned, except at the 150 m station where both red-fruiting and yellow-fruiting forms were present.
Leaf flushing or vegetative growth was similarly seasonal, with the peak in June through August. Frequencies of leaf flushing were roughly equal on the
0) c~ 60
a~ 0 Aug Oct Dec Feb Apr Jun Aug Oct
Fig. 1. Phenology of reproductive behavior in stems located at thicket edges at five points on an elevational gradient. Points represent percentage of stems bearing either flowers or fruits. The five elevations studied are: O, 150m; Q, 300m; A, 460m; A , 610m; , 762m.
204 Laura Foster Huenneke, Peter M. Vitousek
thicket edges and in thicket interiors, with 25-55% of stems flushing during the peak season, and 0 -12% flushing during the rest of the year. There were no conspicuous differences among elevational stations in frequency of leaf flush.
Stem densities were high at all elevations, ranging f rom 3 to 9 stems m 2. There were significant differences between elevations in total density (ANOVA, F4.14 = 8"00, p < 0"005), with highest densities at intermediate elevations (Table 1). Tota l "s tem area increased gradual ly (but not significantly) with increasing elevation up to the highest station, which had significantly lower stem area (Table 1). A large percentage of stems at the highest elevation were recent recruits less than 1 m in height (Table 1); there were significant differences among stations in the relative abundance of small stems, but only the 762 m station differed from all the others (ANOVA, F4 ,14 = 6"80, p < 0"007).
Laboratory germination trials
Germinat ion rates were high (60 80%) under most condit ions (Fig. 2), but high temperatures resulted in seed morta l i ty . Scarif ied seeds had significantly lower germinat ion than any other treatments. Chill ing prior to germinat ion had surprisingly little effect on eventual germination.
There were significant differences between elevational stations in laboratory germinat ion rates (Table 1; ANOVA, F5,~8 = 7"29, p < 0-001). Germinat ion was not correlated with elew~tion or seed weights. Seed numbers per fruit were highest for the intermediate elevations (Table 1).
Germinat ion in the field was markedly lower than that observed in the laboratory. None of the seeds placed on leaf litter or on cleared soil
TABLE 1 Stand, Seed, and Fruit Characteristics of Strawberry Guava Populations at Five Elevations
Elevation No. of stems % stems Sum steel area Seed mass Seed no. Germination (m) m 2 50- lO0 elgl (crn 2 m -2) (mg) per j?uit (%)
150 5"5 a 18"2 a 35.4 a 21"8 a 25"3 a 85-9 a 300 4.5 ab 16.2 a 44.6 a 15.0 b 59"6 bc 42"0 b 460 8"7 c 18"5 a 61"9 a 12"5 c 70"2 c 76"0 a 610 6"9 ac 14"5 a 66"6 a 15"9 bd 41"4 d 38"0 b 762 2'9 b 35'1 b 8"8 b 17"4 d 42"3 abd 40.5 b
Different letters within a column indicate significant differences between elevations.
Strawberry guava encroachment in Hawaiian forests 205
o .o f ._= III
' t t ~
0 I i I ~" I l I I I I
LT DK 18 37 P2 P14S18 S D SO
Fig. 2. Percentage of seeds germinating within 16 weeks under various laboratory germination conditions. Plotted are means and 95% confidence intervals for four replicate sets of 25 seeds each. LT, light, 22C; DK, dark, 22C; 18, dark, 18C; 37, dark, 37C; P2, prechilling for 2 days prior to DK; P14, prechilling for 14 days; S18, scarification with sandpaper, prior to 18; S, scarification with sandpaper, prior to DK; D, DK with one week of drought imposed after one week of watering, then watering resumed; SD, scarification
followed by D.
germinated; enough litter had shifted or moved that several of these seed plots were difficult to relocate. Most seeds placed on bryophytes could be relocated, intact but non-germinated; only 1 of over 180 seeds on bryophytes had germinated by March. Each of the fruits placed on moss mats had produced germinants (from 2 to 10 seedlings per fruit).