Because of this paucity of data on Pisonia, and because its 

 forests have diminished significantly on coral islands this 

 century, we are presenting our data from Caroline, with 

 comparisons with Vostok and Flint, in the hopes that it might 

 inspire more research. 



One point is clear: on all three of the Southern Line 

 Islands, Pisonia grandis has recovered fast from disturbance 

 (except for total forest elimination), reaching close to its 

 maximum height and ecological maturity in 70 years or less. 

 Mature Pisonia, under optimal conditions of soil, temperature, 

 and rainfall, may attain 35 m, as on Fanning and Washington 

 (Garnett, 1983 and personal communication). However, in the 

 Southern Line Islands, canopies of similarly virgin Pisonia on 

 Vostok rarely exceed 25-30 m tall (Kepler, 1990c). 



Caroline's prime grove — 21 m tall, with circumferences 

 (at 1.5 m) to 660 cm, and bearing multiple trunks and root 

 suckers — we now know date back only to the 1920's. They 

 have thus averaged a growth rate of 0.32 m per year since, say, 

 1925 (65 years). Growth was undoubtedly fastest during the 

 first few years. 



Further evidence of fast growth rates is provided from 

 Flint Island. In 1934 only one small Pisonia was recorded 

 (St. John&Fosberg, 1937). Fosberg( personal communication) 

 recalled that only scant traces of native vegetation existed at the 

 time, virtually the entire island (324 ha) being planted with 

 Cocos. In 1990, transect surveys (Kepler, 1990b,d), coupled 

 with an analysis of aerial photographs, revealed that single to 

 multi-trunked Pisonia trees, now quite common on the windward 

 side of Flint, attained maximum heights of 30 m, having 

 circumferences at the base and at 1 .5 m of 1 ,000 cm and 200 cm, 

 respectively (Table 12). These compare favorably with one 

 large Pisonia, presumably virgin, measured on Atafu Island 

 (Tokelau) by the US Exploring Expedition in 1 840, which was 

 more than 600 cm in circumference at its base and about 12 m 

 tall (Wilkes, 1845, Vol. V, p. 9). Furthermore, indigenous 

 forests (Pisonia, Cordia, Guettarda), with canopies of 

 4 to >20 m, covered 57 ha, 28% of Flint' s vegetated area. Thus, 

 numerous Pisonias have not only established themselves since 

 the plantation was abandoned in the late 1930's but have 

 averaged approximately 0.5 m growth per year. This faster 

 growth rate than on Caroline may be due to Flint's higher 

 rainfall and greater relative humidity due to the presence of a 

 more successful coconut plantation inland: Caroline's annual 

 output of copra was 15 tons, compared to 230 tons for Flint 

 (Young, ca. 1922; Maude, 1953). 



Species Diversity in Pisonia Forests : Caroline's motus 

 harbor every stage in the development of a Pisonia forest, from 

 stately monoty pic groves to a single tree. The plant communities 

 between these extremes harbor the greatest species diversity 

 and most luxuriant growth on the atoll. The following species 

 are present (Table 2): 



Trees: Morinda citrifolia, Cordia subcordata, Cocos 

 nucifera, Pandamis tectorius, Pisonia grandis; 

 Shrubs: Tournefortia argentea; and 



Herbs: Boerhavia repens, Portulaca lutea, Laportea 

 ruderalis, Lepturus repens, Achyranthes canescens, 

 Phymatosorus scolopendria, and Ipomoea macrantha. 



The number of species within Pisonia forests ranges from 

 1 to 14 (Table 14). As Pisonia becomes more dominant, their 

 trees are taller (21 m), and species diversity is less (Table 14). 

 Here, the average number of species is 3.4. Species diversity 

 is also very low at the other extreme of Pisonia development: 

 in one young motu (Azure), only a single 6-m-tall Pisonia tree 

 is present (x = 4.0 m). The smallest islet on which we found 

 Pisonia, Azure is only 0.20 ha in area and 77 m wide (Fig. 55, 

 PI. 53); more than half of it is rubble. The width of its scrub is 

 only 38 m. Along a transect within the majestic Pisonia grove 

 ( 100% canopy cover) on Brothers (Fig. 46), we found no other 

 plant species, an extreme case of the barrenness of Pisonia 

 understory. This grove, 13mtallandextending42mfromeast 

 to west, was sharply delineated from the 6-m-high Tournefortia 

 forests on both sides and provides a striking example of 

 complete ecological succession since its Cocos plantation days 

 of the 1920's. 



The highest species diversity occurred with mixed co- 

 dominants (Tournefortia, Cordia), and Pisonia coverage 

 25-50% (Table 14). Here, the average number of species was 

 6.2 (range 3-10). Regardless of the area or width of the motu 

 on which they occurred, these mixed stands (x = 7 m tall) were 

 always shorter than pure Pisonia forest. 



Ecology : On Caroline, most plant species are established 

 early in the evolution of individual motus, increasing 

 in abundance and stature while the motus are quite small. 

 Pisonia typifies this pattern: single trees occur on 2 motus 

 whose areas are only 0.2 ha (Table 6). This suggests that 

 Pisonia is partly salt-tolerant, at least in its early growth stages. 

 In general, however, motus less than 0.7 ha on Caroline have 

 little Pisonia (Table 6). It is difficult to imagine a freshwater 

 lens on Motu Nautonga (1 ha), where an 11-m-tall Pisonia 

 forest is found (Table 1 1). Further evidence for the salt-tolerant 

 nature of Pisonia comes from Vostok, where a Pisonia 

 forest, the sole woodland, extends to the edge of the shoreline 

 rubble and herb mat. The trees, tightly pruned by wind and salt, 

 have no buffer of coastal scrub. During storms, seawater 

 reaches Vostok' s interior forest, yet this 24-ha island supports 

 one of the largest and tallest (25 m high) groves in the Pacific 

 (Clapp & Sibley, 1971b; Fosberg, 1977b and personal 

 observation). 



Many Pisonia trees were heavily infested with scale 

 insects (Coccidae) and Neuropteran larvae (Chrysopa sp.), 

 identified by Dr. Scott Miller (Bishop Museum, Honolulu, 

 Hawaii). This appears to be a natural phenomenon, as they 

 were also abundant on the virgin Pisonia forests on Vostok and 

 also on secondary Pisonias at Flint. 



Relationships Between Pisonia Forest Height and Motu 

 Dimensions : Contrary to expectations, the tallest, most mature 

 forests did not all occur on the largest motus (Table 1 1 ). The 

 three prime forests (90-100% canopy cover) are on Nake 

 (107.46 ha), Pig (7.21 ha), and Booby (0.84 ha). Trees on 

 Booby are smaller in girth than those on Nake and Pig, but their 

 height (20 m) is impressive; as far as we know, Booby was 

 never cleared. Fine forests occur on other small, undisturbed 

 motus; for example, Pisonia grew to 14 m on Raurau (3.48 ha) 

 and to 1 1 m on Kimoa ( 1 .80 ha). 



31 



