Importance of phosphorus for groves in Korup
Except for the lack of T. bifoliolata, the grove composition at Isangele Road was similar to that on transect P in the Park and can be taken as a representative of that large area. T. bifoliolata could be found separated 100–200 m north of the Isangele Road plot, however. The addition of P over 2 yr was successful in raising soil P that was available to the trees. There were no visual signs of deficiencies of other elements in leaves. There were neither strong imbalances in N : P ratios, nor obvious side-effects in forest processes such as change in appearance of litter on the forest floor. Although the addition of P as fertilizer is a rather special and unnatural event, or even a disturbance to the ecosystem, the release of P would have been gradual over several months and years. A further point is that adding TSP fertilizer may have lowered the soil pH in an already low-pH system. The consequences of this are unknown.
Addition of P to the grove ecosystem resulted in increased concentrations of P in plant leaves but overall no increase in growth and survival of seedlings and trees. This simply means that the plants took up the extra P supplied to them. From this set of consistent results it may be concluded that P was not limiting growth and survival of tree seedlings, and in particular that of M. bisulcata, and it was not limiting the growth of small and medium-sized trees of the forest overall. The exception was the demographic study of M. bisulcata in which added P led to better survival up to c. 1 yr and then this effect was lost. Indeed the added-P treatments led to decreased survival of some transplanted species and a tendency towards lowered growth compared with the control. Unfortunately, there were no intermediate-sized M. bisulcata trees on which to specifically test the effect of added P.
What might limit and control survival and growth of seedlings could be quite different for trees. Here the argument that PAR might limit tree response to added P is less plausible because larger trees display their leaves higher in the canopy and are better illuminated. The evidence for P not limiting tree growth is stronger. Possibly P is a limiting factor when the groves are establishing after a disturbance (Newbery & Gartlan, 1996), and competition for P among saplings and small trees on these P-poor soils leads to a selection of the ectomycorrhizal species. Later in the established stands P might become less of a premium and other elements, also beneficially acquired and cycled by the then larger ectomycorrhizal trees, take on a more controlling role. Gartlan et al. (1986) demonstrated that the occurrence of ectomycorrhizal caesalps was associated with low K as well as low P soils in Korup and it may be that K now becomes a limiting factor to tree growth. The means of P cycling in mature stands dominated by the ectomycorrhizal caesalps was found to be highly efficient with a strong feedback mechanism, in that the concentration of P in leaf litter was high and decomposition was fast (Chuyong et al., 2000, 2002) plus the fact that a not insubstantial amount of P is inputted annually in incident rain (Chuyong, 1994, G. B. Chuyong, D. M. Newbery, N. C. Songwe; unpubl. data). As the forest grove establishes into a stand of mature trees the importance of leaching of K from the vegetation is likely to increase and, despite the presence of the fine-root and ectomycorrhizal mat (Newbery et al., 1997), control of K cycling on an also generally K-poor soil might suggest that K, not P, limits growth at later stages. Indications of K were only recently appreciated from throughfall and canopy leaching results (G. B. Chuyong, D. M. Newbery, N. C. Songwe; unpubl. data).
If all of the tree species in Korup were already adapted to low-P soils then perhaps they would not be expected to respond at all to added P (Tanner et al., 1998). This seems unlikely on two counts: first M. bisulcata and the other ectomycorrhizal caesalps have high tree growth rates (Zimmermann, 2000; DM Newbery et al. unpubl. data) which is not an expected characteristic of slow growing tolerant and conservative species, and second many of the tree species in Korup are widespread on a range of soil types in Cameroon suggesting no localized specialization to low nutrient soil per se, for example S. pseudocola in the present experiment, and Irvingia gabonensis (Green & Newbery, 2001a) did not respond to added P. It would seem unlikely that any species is adapted to a single factor, but to a suite of related factors, which includes inter alia limited water supply in the dry season, low external radiation and intense canopy leaching with the heavy rains in the wet season.
Fertilization trials in rain forest in a broader context
Walker & Syers (1976 ) suggested that long-term pedogenesis leads to soils poor in P and to P-limitation, especially on highly weathered oxisols in the tropics. Young soils, such as those on volcanic lavas, have been shown to be largely N-limited in the early successional stages ( Vitousek et al., 1993 ), but they become more P-limited in later stages ( Herbert & Fownes, 1995 ).
Experiments in primary tropical forests on nutrient limitation have been conducted rarely, with only one reported study in primary lowland forest by Mirmanto et al. (1999) for Bornean Indonesia. The investigations of Tanner et al. (1990, 1992) in Jamaica concerned montane forest. Tanner et al. (1998) have, however, suggested that pantropically N might be the limiting nutrient to forest growth in montane areas and P the limiting nutrient in the lowlands. As yet there are rather too few observations to test this hypothesis: the present study does not anyway support it.
Possible comparisons between Korup and other old tropical primary forest sites are limited. The detailed studies in Hawaii, on young montane volcanic substrates, showing early woodland succession are not quite so relevant: Tanner et al. (1998) give a useful overview of fertilizer trials and tree growth and nutrients on these soils, and highlight large positive growth responses to added N. In Jamaican montane forest, Tanner et al. (1990) compared a plot with added N (150 kg ha−1 yr−1) and one with added P (50 kg ha−1 yr−1) with two controls, the fertilizer being applied annually between 1983 and 1986. Adding N did not increase foliar N concentrations in four common tree species but it decreased P concentration in the two species that had higher P concentrations at the start. Trunk growth rate (trees ≥ 25 cm gbh) was 2-fold higher in the added-N plot compared with the controls. Adding P did not raise leaf P concentrations in the two already P-rich species but it did double it in the other two P-poor species. Added P increased trunk growth by 50%. Nevertheless, the effects took 2–3 yr to begin to show. Tanner et al. (1990) concluded that the forest was probably generally N-limited but that some species were P-limited in growth. On this basis the 4.7-yr interval for tree growth measurements in the present study at Korup should have been adequate.
In Venezuelan montane forest, Tanner et al. (1992) applied N, P, N + P fertilization in comparison with a control. There were five replicate plots per treatment. Levels of N and P were 225 and 75 kg ha−1 yr−1, respectively, in yrs 1 and 2, returning to the Jamaican levels in yr 3 (1986–88). Growth was recorded for 104 trees, but only in the control and N + P treatment plots. After 28 months, trunk growth with added-‘N + P’ was c. 2-fold that in the control, and further remeasurements showed that this difference was held until 1992 (E. V. J. Tanner; pers. comm.). From this it is unclear whether either P or N singly were limiting growth, and even then significant differences took 3–4 yr to appear. Adding N, P or N + P did not affect the leaf litter N concentrations, but after 2.5 yr the added P and added N + P treatments led to slight insignificant increases in the concentration of litter P. Thus whilst added N + P was associated with better growth, changes in N and P leaf concentrations were not that obvious. No estimate of retranslocation was made. Tanner et al. (1992) suggest that the N and P may have gone elsewhere in the tree besides into leaves, but there does remain the possibility that much of it was held in the soil or was lost from it by leaching. No confirming tests of soil available concentrations were made (unlike the present study). The longer-term data showed that after 6 yr, whilst added-N did lead to more litter fall, the concentrations of N and P in this litter did not change. In the context of the present discussion it can be conjectured that N per se may not have led to increased tree growth but its indirect effect on the soil altered one or more other controlling factors that allowed a growth increase.
The one other lowland study to date (besides the present one) by Mirmanto et al. (1999) in lowland dipterocarp forest in central Borneo, Indonesia, used a randomized block design with N, P and N + P applied, with a control, to four plots within each of four blocks. The levels of N and P used between 1994 and 1997 were 225 and 75 kg ha−1 yr−1, respectively, as in Tanner et al. (1992). All three treatments led to a 30% increase in litter fall over the control, although these data were only recorded early in the trial (1994–95) and may have been a temporary ‘reaction response’. In this 1 yr, leaf P, but not N, leaf litter concentrations increased for all treatments compared with the control. Trunk growth did not increase significantly with fertilization. From this study it is not possible therefore to conclude whether N or P limited tree growth or leaf production (when litter fall is taken as a surrogate) in the longer term (cf. Tanner et al., 1990; Tanner et al., 1992), but the lack of effect is tentative support for the conclusion of the present work in Korup.
The interpretation of fertilization experiments in vegetation requires some caution. The idea is often that introducing a simple treatment will have a simply interpretable response on a 1 : 1 cause-effect basis. Forests, however, are complex, long-lived and highly interconnected ecosystems in which short-term adjustment responses and time lags are to be expected. Adding large quantities of fertilizer to such a presumably near-equilibrium system (in terms of nutrient cycling) is tantamount to a major disturbance. Furthermore, adding a single nutrient can have a multitude of side-effects which lead, perhaps in the short term, to contrary, ameliorated or even opposite effects than expected.
In conclusion, P supply does not appear to limit the establishment and growth of seedlings, nor the growth of trees, of the Korup tree species as tested under current conditions. Phosphorus-limitation is not the reason why the present seedling-sapling recruitment of M. bisulcata remains so poor at this site. Whether P is important at an earlier successional stage of the proposed cyclic-mosaic process, that is after a strong disturbance event, is not yet known. Different limiting nutrients may control tree growth at different stages.