Jul 3, 1992 - The seasonal biomass and resource allocation patterns of a perennial geophytic species, Haemanthus ... Why has the geophytic life form been so successful in the ..... GRIME, J.P., CRICK, J.C. & RINCON, J.E. 1986. The ...
S.AfrJ.Bot., 1993, 59(2): 251 - 258
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Seasonal allocation of biomass and resources in the geophytic species Haemanthus pubescens subspecies pubescens in lowland coastal fynbos, South Africa C. Ruiters*, 8. McKenzie, J. Aalbers and L.M. Raitt Botany Department, Faculty of Science, University of the Western Cape, Private Bag X17, Bellville 7535, Republic of South Africa Received 3 July 1992; revised 25 November 1992
The seasonal biomass and resource allocation patterns of a perennial geophytic species, Haemanthus pubescens subspecies pubescens, were investigated. Four phenophases for the species were identified and the annual patterns of dry mass and resource allocation were identified from individuals sampled from natural populations. From the results, four general resource allocation patterns were identified: (1) mobile macronutrients (N, P and K) and Cu; (2) immobile macronutrients (Mg and Ca) and Mn; (3) soluble carbohydrates, starch, Zn and Fe; and (4) Na, all with their own distinctive allocation patterns. This study confirms distinct allocation patterns between the resources and biomass. Furthermore, it was found that environmental constraints contributed towards the differential seasonal allocation patterns of the resources and biomass. The development of an underground storage organ is regarded as an evolutionary mechanism to cope with soils of a low nutrient status, characteristic of the Cape fynbos, and as a strategy for the protection of dry matter and resources against seasonal periodic droughts and fires . Die seisoenale biomassa en reserwe toekenningspatrone van 'n meerjarige geofiet spesie, Haemanthus pubescens subspecies pubescens, is ondersoek. Vier fenofases is vir hierdie spesie ge'identifiseer en die jaarlikse patrone vir droa massa en reserwe toekennings is ge'identifiseer vanaf individue wat uit natuurlike populasies gemonster was. Uit die resultate is vier algemene reserwe toekenningspatrone ge'identifiseer vir: (1) mobiele makrovoedingselemente (N, P en K) sowel as Cu, (2) immobiele makrovoedingselemente (Mg en Ca) sowel as Mn, (3) oplosbare koolhidrate, stysel, Zn en Fe, en (4) natrium, almal met hul eie kenmerkende toekenningspatrone. Hierdie studie bevestig opvallende verskille tussen die toekenning van reserwes en die biomassa. Verder is dit ook gevind dat omgewingsbeperkings bydra tot die differensiale seisoenale toekenningspatrone van reserwes en biomassa. Om die rede word die ontwikkeling van 'n ondergrondse stoororgaan beskou as 'n meganisme om die kenmerkende lae voedingstatusgronde van die Kaapse fynbos, te hanteer. Dit word ook as 'n strategie beskou om die minerale- en droa materiaal reserwes te beskerm teen seisoenale periodieke droogtes en veldbrande. Keywords: Biomass allocation, Haemanthus pubescens subspecies pubescens, hysteranthous geophyte, resource . • To whom correspondence should be addressed.
Introduction Why has the geophytic life form been so successful in the mediterranean-climate biome of the south-western Cape? There are more species and a greater proportion represented by this life form than in similar biomes across the world (Goldblatt 1978). This study is a contribution to the understanding of this diversity by considering the resource allocation patterns within a geophyte. It is concerned with Haemanthus pubescens subspecies pubescens, a hysteranthous bulbous geophyte with a perennial storage organ, which aestivates during spring and summer. Resource allocation is generally described as the connection between fitness and the pattern of allocation of some crucial substances and implies the differential movement of materials to and from various organs (Abrahamson 1979; Fitter & Setters 1988; Lovett Doust 1989). It is well documented that nutrient concentrations and allocation patterns vary greatly between plant organs, and at different growth stages (cf. Abrahamson & Caswell 1982; Gross et al. 1983; Hume & Cavers 1983; Whigham 1984; Fitter & Setters 1988; Nault & Gagnon 1988), and that the allocation of nutrients and biomass do not necessarily show similar
responses to environmental conditions (cf. Van Andel & Vera 1977; Doust 1980). The use of biomass allocation (the proportion of total biomass stored in each organ) was originally introduced as a means to study resource allocation (Harper & Ogden 1970). Thompson & Stewart (1981) questioned the use of carbon as the primary limiting resource in plants. They suggested that nutrient content also be investigated because the dry matter content in numerous plants, in particular geophytes, represents the summation of several years of growth (sensu Gross et al. 1983). Studies of resource allocation for geophytes in Cape fynbos have been neglected, although conceptual frameworks do exist for the investigation of biomass and resource allocation patterns (Hickman 1977; Platt & Weis 1977; Dafni et al. 1981a; 1981b; Abrahamson & Caswell 1982; Pate & Dixon 1982; Gross et al. 1983; Hume & Cavers 1983; Whigham 1984; Pate & Dell 1984; Fitter & Setters 1988). This study examined the allocation of mineral elements and non-structural carbohydrates in relation to biomass allocation under field conditions. The bulbous habit of this species allows for the carry-over of substantial amounts of nutrients and food reserves from one growing
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season to the next. Furthennore, an accumulation of storage materials is a prerequisite for flowering in hysteranthous geophytes (Rees 1972; Dafni et al. 1981b). The key objective of this study was to investigate the differential allocation of biomass and nutrient resources to the constituent plant parts and to describe how resources were manipulated from season to season. Allocation patterns were examined only for flowering plants, and this paper discusses biomass allocation, nutrient concentrations and allocation patterns within and between plant parts over a year. Study site The site was located near Lynedoch (34°65'S and 18°46'E) where high bulb densities were found in open areas and only a few individuals occurred under dense, closed canopies. The site is an unconsolidated marine, aeolian sandy habitat in lowland coastal fynbos which has been invaded by large numbers of the alien Acacia saligna. Methods Collection of specimens Randomized blocks were demarcated in three large, wellprotected populations of H. pubescens subsp. pubescens. Each block was divided into 16 randomized quadrats of 12 m2 each. Sampling was done monthly using a random permutation method (Moses & Oakford 1963) for the selection of quadrats. Fifteen to twenty 'large-sized' flowering plants (> 10 years old) (cf. Ruiters et al. in press a) were excavated each month for chemical analyses from the chosen quadrats. Individuals were marked by means of steel rods to aid in their location during the dormant phase. All excavated plants were brought to the laboratory and carefully cleaned. Chemical analyses Plants were weighed, separated into constituent parts, and then dried at 60°C to a constant mass. The dry plant material was ground with a Wiley Intermediate Mill to pass through a forty-mesh sieve. Chemical analyses were performed on five replicates for each population. In cases where insufficient plant material was available, ego reproductive structures and roots, the material for each plant part from different plants was pooled and five replicates were then taken for analysis. Concentrations of macro- and micronutrients were determined on sulphuric-peroxide digests with a Pye Unicam SP9 atomic absorption spectrophotometer (Allen et al. 1986). Phosphorus was determined in the same digests as the phosphomolybdate blue-complex (Allen et al. 1986). Soluble carbohydrates and starch were determined colorimetrically with a Varian Techtron model 635 double-beam spectrophotometer according to the anthrone reagent and perchloric acid methods, respectively (Allen et al. 1974). Nitrogen was determined by Micro-Kjeldahl analysis (Allen et al. 1986). Biomass and resource allocation Two distinct measures are generally considered for describing production and resource storage: (i) Allocation in a storage organ can be characterized by concentration, providing the allocation ratios remain the same or constant during
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the period of study, and (ii) allocation at the whole-plant level is best measured by the pool size of stored reserves as a fraction of total growth. For the determination of the proportional allocation patterns, the resource concentrations were first determined as the amount present in dry mass of the constituent plant parts (Allen et al. 1986). Thereafter resource allocation was assessed as the product of the dry mass and nutrient concentrations of the constituent plant parts (Hickman & Pitelka 1975; Abrahamson & Caswell 1982). The allocation and partitioning of biomass, P, K, Ca, Mg, Na, N, Zn, Cu, Fe, Mn, soluble carbohydrates and starch to the different plant structures were examined over a one-year period. Plants selected randomly for analysis could vary considerably in size and this could influence the interpretation of biomass and resource allocation patterns. The data for this study have been analysed in three ways: (a) Biomass weights were used to exemplify temporal changes in biomass allocation to the constituent plant parts; (b) mineral and non-structural carbohydrate concentrations were used to demonstrate temporal changes and absolute concentrations in the constituent plant parts and to relate such differences to various functions in the plant; and (c) the resource contents of the various organs were expressed as a percentage of the total plant nutrient content. The latter is tenned proportional allocation and gives a 'scale-free estimate' (Fitter & Setters 1988). By using both the nutrient concentrations and proportional allocation estimates, it is possible to indicate the routes for material movements. Statistical analYSis The biomass data was log(x + 1) transformed prior to the application of ANOVA (Zar 1984). The a posteriori LSD test for the biomass allocation and the Duncan multiple range test for the nutrient concentrations were used following the ANOVA (Montgomery 1991). Nutrients with similar concentration patterns were identified for each of the four phenophases (Ruiters et al. in press a; Table 1) and significant levels were then detennined for each group (multiplepaired t-tests; Zar 1984). Results Biomass The leaf-bases accounted for the largest proportion (70.3 90.7%) of the total plant biomass throughout the year (Figure 1). However, the entire bulb (leaf-bases and stemstock) attained and maintained over 80% of the total plant biomass. Water accounted for 82.2 - 88.3% of the bulb's fresh weight throughout the year. Leaves began development in March and accounted for 1.0 - 15.8% of the total plant biomass between March and August. Inflorescence development began in September but significant biomass allocation was only evident from January to March. Relatively small amounts of biomass were allocated to fruits between March and April. The roots represented only a small fraction of the total plant biomass, contributing between 0.6 and 2.4% with a peak in July. Nutrient concentrations The concentrations of the mobile macronutrients (N, P and K) were relatively high in the leaves, reproductive structures
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Table 1 Nutrient concentration patterns in the plant parts of Haemanthus pubescens subspecies pubescens for the different phenophases for a calendar year Phenophase
Nutrient patterns
Nutrients
I.
Donnant period with marked below-ground inflorescence deveIopment (Jan. - Feb.)
R ;;.Ns RS ;;.Ns St >* LB R >**St >*RS;;.Ns LB RS ~s LB >*St >**R LB ;;.Ns St >** RS ;;.Ns R
N, K, P, Mg, Cu Na, Mn, Zn Sol. CHO, Fe Starch, Ca
II.
Reproductive period (March - April)
L >**St >*R >**RS >**LB R >**RS ~s St ~s LB >*L St ;;.Ns LB >** RS ;;.Ns R ;;.Ns L
N,P,K Na, Cu, Fe, Mn, Mg, Zn Starch, Ca, Sol. CHO
III.
Vegetative period (May - August)
L >** R ~s St ;;.Ns L L >**R ~s LB >**St LB ;;.Ns St >* L ;;.Ns R LB ;;.Ns L ;;.Ns R >* St
N,P,K Ca, Na, Mg, Zn Sol. CHO, Starch Fe, Mn, Cll
St ~s LB >**R R ;;.Ns St ;;.Ns LB LB ;;.Ns St >** R
N,Zn Ca, Na, Mg, Fe, Mn, Cu, P, K Sol. CHO, Starch
IV. Donnant period (Sept. - Dec.)
Symbols: **p < 0.01; *p < O. 05; NSNot significant (P >0. 05) LB = leaf-bases, St = stem-stock, L = leaves, RS = reproductive structures, R Sol. CHO = soluble carbohydrates
and roots for all the respective phenophases. Temporal differences between the plant components for these macronutrients were particularly striking (Table 1 and Appendix 1). Nitrogen concentrations were particularly high in the reproductive structures during the flowering period and, unlike the leaves, did not decline significantly as the inflorescence matured. The concentrations of the immobile nutrients, Ca and Mg, were relatively constant in all plant parts for the respective phenophases. Sodium, however, which was concentrated in the leaves and roots (Table 1 and Appendix 1), had significantly higher concentrations in the leaves during the vegetative phase. Micronutrient concentrations were more variable than the macronutrient concentrations (Table 1 and Appendix 1). A
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