A study of the growth and distribution of colophospermum mopane (kirk ex benth.) Kirk Ex J. Leon: The interaction of nitrogen, phosphorus and soil moisture stress

Abstract

Eight sites representative of the full diversity of Colophospermum mopane woodland were chosen in Rhodesia, Botswana and South Africa. Mopane performance, as measured by S diameter at breast height times density, over six height classes, was highly correlated (69 per cent of the variation accounted for) with total N and NaHCO₃‐extractable P in the surface soil (0–10 cm).

Mopane seedlings grown in pots responded to increased N and P supply, though, not significantly, on two soils with over 20 per cent water at 0,33 bars matric suction. Fluctuations in the available water supply from 100 to 0 per cent had no effect on yield or evapotranspiration rate on these soils. On a third soil, with only seven per cent water at 0,33 bars suction, yield decreased significantly (P < 0,01) with increased N and decreasing available water supply. These effects were primarily due to increased soil osmotic suction. Mopane growth was reduced at osmotic suctions less than 3 bars. At higher suctions (6–8 bars) internal osmotic adjustment appeared to enable the plant to continue growing at a reduced rate.

These results are discussed in terms of mopane's distribution, and its apparent competitive advantage when induced to grow in “'coppice” habit.

Uittreksel

Agt lokaliteite, verteenwoordigend van die volledige groeivormdiversiteite van Colophospermum mopane, is in Rhodesië, Botswana en Suid Afrika gekies. Biomassaevaluasies van mopane, bereken as S van stamdeursnee op borshoogte teenoor standdigtheid, vir ses verskillende hoogtegroepe, het ‘n hoë korrelasie getoon met totale N en NaHCO₃‐ekstraheerbare P in die boonste 10 cm grand. Nege‐en‐sestig persent van die variasie kon hierdeur verklaar word.

Alhoewel mopane saailinge in potte gereageer het op ‘n toename in N en P, was dit nie betekenisvol vir twee grondsoorte wat meer as 20 persent water teen 0,33 bar grondwaterspanning gehad het nie. Variasies in beskikbare water van 0 tot 0, 33 persent, het geen invloed op opbrengs of evapotranspirasietempo van die gronde gehad nie. Op ‘n derde grondtipe met slegs sewe persent waterinhoud by 0,33 bar grondwaterspanning, het die opbrengs betekenisvoi afgeneem (P < 0,01) met ‘n vehoging in N en ‘n afname in water.

Hierdie verskynsels was hoofsaaklik te wyte aan die verhoogde grond‐osmotiese spanning. Met grond‐osmotiese spanning meer as drie bar is die groei van mopane gestrem. By hoër spannings (6–8 bar) kan inwendige osmotiese aanpassing die plant waarskynlik in staat stel om aan te hou groei. maar teen ‘n her tempo.

Hierdie resultate word bespreek in terme van mopane se verspreiding en die oënskyntike kompeterende voor‐deel wat dit het wanneer dit genoodsaak word om in boskasievorm te groei.