558 S.-Afr. Tydskr. Plantk., 1988, 54(6): 558--564 Response of the fynbos shrubs Protea repens and Erica p/ukenetii to low levels of nitrogen and phosphorus applications A.J. Lamb* and E. Klaussner South African Forestry Research Institute, Jonkershoek Forestry Research Centre, Private Bag X5011, Stellenbosch, 7600 Republic of South Africa Accepted 17 June 1988 The responses of five-year-old fynbos shrubs, Protea repens (L.) L. and Erica plukenetii L., to low levels of nitrogen and phosphorus application were examined. Ammonium nitrate (2 9 N m-~, superphosphate (0,2 9 P m-2) and ammonium nitrate plus superphosphate (2 9 N m -2 + 0,2 9 Pm -2) were applied in early spring 1982. Erica plukenetii responded to the phosphorus and particularly nitrogen plus phosphorus treatment with increases in height, stem diameter, crown cross-sectional area and biomass by the end of the second growing season. These responses were lower on plots treated with nitrogen than on the control. Vegetative growth of Protea repens on the fertilized plots, particularly those treated with nitrogen and nitrogen plus phosphorus, was lower than on the control. Nitrogen addition increased shoot production in the second year while phosphorus addition increased flower production. Low levels of nutrient application are quite capable of altering morphological characters of fynbos species. Nutrient enrichment by atmospheric inputs might therefore lead to changes in species composition and could result in increased invasion of fynbos areas by alien herbaceous species. Die reaksie van vyfjaar-oue fynbos struike, Protea repens (L.) L. en Erica plukenetii L., op die toediening van klein hoeveelhede stikstof en fosfor, is ondersoek. Ammoniumnitraat (2 9 N m-2), superfosfaat (0,2 9 P m-2) en ammo­ niumnitraat -plus-superfosfaat (2 9 N m -2 + 0,2 9 P m - 2) is in die vroee lente van 1982 toegedien. Erica plukenetii het op die fosfor- en veral op die stikstof- plus-fosfortoedienings gereageer, aan die einde van die tweede groei-seisoen met 'n toename in hoogte, stamdeursnee, kroonoppervlak-deursnee en biomassa. Op die persele wat met stikstof behandelde persele, veral die wat met stikstof en stikstof-plus-fosfor behandel is, was laer as op die kontrole. Die toe­ voeging van stikstof het loot-aanwas in die tweede jaar opgestoot terwyl fosfor blomproduksie bevorder het. Lae vlakke van voedingstof-toediening is in staat om die morfologiese karaktereienskappe van fynbos spesies te verander. Voe­ voedingstof-toediening is in staat om die morfologiese karaktereienskappe van fynbos spesies te verander. Voe­ dingstotverryking deur atmosferiese invoere mag dus lei tot veranderings in spesie-samestelling en mag daartoe lei dat fynbos-gebiede in 'n groter mate deur kruidagtige indringerspesies ingeneem word. Keywords: Fynbos, nitrogen, nutrient additions, phosphorous, south-westem Cape *To whom correspondence should be addressed Introduction nutrients accumulated in standing dead biomass and litter The soils of the South African Fynbos Biome are generally (Rundel & Parsons 1980). low in mineral elements and are considered to be closely Nitrogen and phosphorus have been recognized as the allied to those of Australian heathlands (Groves 1983; Mit­ elements most likely to control rates of growth in medi­ chell et al. 1984). These oligotrophic (strongly-leached) terranean ecosystems (Read & Mitchell 1983) . Phosphorus soils are characterized by a pH of less than 6,0, total nitro­ has been shown to increase the growth of Australian heath gen less than 0,10%, total phosphorus less than 0,02% and on oligotrophic soils (Specht 1963). Californian chaparral total exchangeable cations less than 5 meq 100 g -I soil on mesotrophic soil shows a response to additions of nitro­ (Specht & Moll 1983). In contrast, the other mediterra­ gen rather than phosphorus (Hellmers et al. 1955). Fertili­ nean ecosystems (California, Chile and the Mediterranean zation with a combination of phosphorus and nitrogen also Basin) have mesotrophic (moderately-leached) soils results in increased growth in Australian heath (Specht (Specht & Moll 1983). 1963; Heddle & Specht 1975) and in chaparral (McMaster The relatively high proportion of available forms of N et at. 1982). and of other nutrients in ash is generally considered to be Previous fertilizer experiments, especially those of the major factor promoting rapid re-growth following fire Specht (1963) and Siddiqi et at. (1976) involved high levels in mediterranean ecosystems (Rundel et al. 1983). Nitro­ of fertilizer application. Specht (1963) added 140 g m -2 su­ gen and phosphorus estimated as likely to be released dur­ perphosphate, 75 g m -2 sodium nitrate, 1,57 g m -2 copper ing a fire in 21-year-old mountain fynbos at 10nkershoek sulphate and 1,57 g m -2 zinc sulphate over a 3-year period. amounted to 15,8 and 0,64 g m-2 respectively (van Wilgen Siddiqi et al. (1976) doubled the amounts of phosphorus, & Ie Maitre 1981), but the proportion returned to the soil magnesium and potassium available immediately after a was not quantified. A fire in a coastal fynbos ecosystem in­ fire. creased the total N concentration by 6,6 g N m -2 at the soil This paper investigates the growth response of Protea surface (Stock & Lewis 1986a). Although no significant repens (L.) L and Erica plukenetii L. to low levels of nitro­ change in total phosphorus levels was found, resin-extract­ gen and phosphorus additions, the responses to high levels able phosphorus did increase significantly (Brown & Mit­ having clearly been demonstrated elsewhere (Specht 1963; chell 1986). Such increases in nutrient availability are, how­ Siddiqi et al. 1976). Protea repens and Erica plukenetii were ever, shortlived. Plant growth rates decline steadily as post­ specifically chosen for investigation as both are widely dis­ fire age of the vegetation increases (Rundel & Parsons tributed in the fynbos of the south-western Cape (Rourke 1979). This decline is correlated with increased amounts of 1980; Baker & Oliver 1976). As fynbos ecosystems are S. Afr. 1. Bot., 1988,54(6) 559 subject to low levels of nutrient enrichment by atmospheric The mature vegetation of the study area comprises a Tall inputs (Stock & Lewis 1986b; Brown et al. 1984; van Wyk Mid-dense Proteoid Shrubland with a Mid-dense Restioid 1984) and to a lesser extent by the application of fire retar­ and Ericoid Shrubland understory (Campbell et al. 1981). dants and fertilizer drift, these inputs may represent an Protea rep ens and Erica plukenetii were the dominant important input of nutrients to natural ecosystems on species on the study site which was last burnt in 1977, with nutrient-poor soils, could alter fynbos growth responses mean heights of 0,86 and 0,36 m respectively. Numerous over time and could also lead to more rapid invasion of hemicryptophytes and some geophytes and herbs were also fynbos areas by alien herbaceous species. present in the understory. Study area Methods Sosyskloof (34° OO'S, 18° 57'E) is situated in the Jonkers­ Fertilizer treatments hoek State Forest, 15 km from Stellenbosch in the south­ Twelve 5-m x 5-m plots, separated by 5-m strips to prevent western Cape Province, South Africa. The soil is derived overlapping of fertilizer effects, were laid out in July 1982. from coarse-grained porphyritic granite of the Pre-Cape Four fertilizer treatments replicated three times, were System (Fry 1987). Total nitrogen and phosphorus levels of randomly assigned to the plots. The treatments were: no the Sosyskloof soils, as determined by the Saasveld Fores­ fertilizer (control), phosphorous only (P), nitrogen only try Research Centre soil laboratories, were 0,11% and (N), and nitrogen plus phosphorus (N+P). 0,012%, respectively. During the period 12 to 18 August 1982, 3,572 g m -2 of The climate of the Jonkershoek Valley is mediterranean, ammonium nitrate (1 g m -2 N) and 1,905 g m -2 of super­ Koppen's (1931) humid-mesothermal (type Csb) with a dry phosphate (0,2 g m -2 P) dissolved in 0,5 I m -2 of water, were summer and the average temperature of the warmest applied evenly over the soil surface. An additional},01 m-2 month below 22°e. Annual precipitation in Sosyskloof of water was applied immediately following the fertilizer averaged 1 426 mm from 1965 to 1984. Total precipitation application to wash any fertilizer solution that may have for the study periods April 1982 to March 1983 and April landed on plant foliage, onto the soil surface. An equal 1983 to March 1984 were 1 359 mm and 1 415 mm, re­ quantity of water (1,5 I m -2) was applied to the control spectively. plots. During the period 18 to 19 October, a further 3,572 g A relatively homogeneous site of 0,15 ha was chosen for m -2 of ammonium nitrate (1 g m -2 N) was applied to the N the fertilizer experiment. The site is situated at 500 m alti­ and N + P plots in the same manner. The ammonium nitrate tude and has an average slope of 17° with a SE aspect. was applied during two periods to prevent rapid leaching. 0,5 10 0,03 9 0.4 r+ 8 7 ~ 0,02 E .s T 0.3 Q; 6 Q3 E co I '6 1: en E 5 Q; Q) I U5 0,2 4 0,01 3 0,1 2 561 241 877 138 561 241 877 138 561 241 877 138 b a b a a b c d a a b c c P N N+P c P N N+P c P N N+P Figure 1 Mean heights, stem diameters and crown areas of Erica plukenetii shrubs at the end of the 1983 growing season after various fertilizer treatments at the beginning of the 1982 growing season.