Effects of anabolic and catabolic nutrients on woody plant encroachment after long-term experimental fertilization in a South African savanna

Abstract

The causes of the worldwide problem of encroachment of woody plants into grassy vegetation are elusive. The effects of soil nutrients on competition between herbaceous and woody plants in various landscapes are particularly poorly understood. A long-term experiment of 60 plots in a South African savanna, comprising annual applications of ammonium sulphate (146–1166 kg ha-1 yr-1) and superphosphate (233–466 kg ha-1 yr-1) over three decades, and subsequent passive protection over another three decades, during which indigenous trees encroached on different plots to extremely variable degrees, provided an opportunity to investigate relationships between soil properties and woody encroachment. All topsoils were analysed for pH, acidity, EC, water-dispersible clay, Na, Mg, K, Ca, P, S, C, N, NH4, NO3, B, Mn, Cu and Zn. Applications of ammonium sulphate (AS), but not superphosphate (SP), greatly constrained tree abundance relative to control plots. Differences between control plots and plots that had received maximal AS application were particularly marked (16.3 ± 5.7 versus 1.2 ± 0.8 trees per plot). Soil properties most affected by AS applications included pH (H2O) (control to maximal AS application: 6.4 ± 0.1 to 5.1 ± 0.2), pH (KCl) (5.5 ± 0.2 to 4.0 ± 0.1), acidity (0.7 ± 0.1 to 2.6 ± 0.3 cmol kg-1), acid saturation (8 ± 2 to 40 ± 5%), Mg (386 ± 25 to 143 ± 15 mg kg-1), Ca (1022 ± 180 to 322 ± 14 mg kg-1), Mn (314 ± 11 to 118 ± 9 mg kg-1), Cu (3.6 ± 0.3 to 2.3 ± 0.2 mg kg-1) and Zn (6.6 ± 0.4 to 3.7 ± 0.4 mg kg-1). Magnesium, B, Mn and Cu were identified using principal component analysis, boundary line analysis and Kruskal-Wallis rank sum tests as the nutrients most likely to be affecting tree abundance. The ratio Mn/Cu was most related to tree abundance across the experiment, supporting the hypothesis that competition between herbaceous and woody plants depends on the availability of anabolic relative to catabolic nutrients. These findings, based on more than six decades of experimentation, may have global significance for the theoretical understanding of changes in vegetation structure and thus the practical control of invasive woody plants.

The Role of Environmental Factors in Promoting and Limiting Biological Invasions in South Africa

“…However, it might be possible to generalise for alien trees invading Fynbos and Grassland. In particular the shift from a relatively short plant form (shrubs or grasses) to a tall plant form (trees) can be explained in part by the Catabolic Theory (Milewski and Mills 2010;Mills et al 2016Mills et al , 2017. The theory has three main premises: first the availability of catabolic versus anabolic nutrients has marked effects on vegetation structure; second that short plants (e.g.…”

“…The presence or absence of alien trees in fynbos and encroaching native trees in grassland environments of South Africa have been linked to a wide range of nutrients, soil properties, and soil treatments, including pH, acid saturation, Mg, Ca, Mn, Cu, Zn, B, P, and N-fertilisation (Mills and Allen 2018;Mills et al 2017). Unpublished data collected from 25 diverse sites across South African Fynbos, Grassland and Savanna sites have shown that soils in sub-sites relatively poor in boron or relatively rich in phosphorus tend to be less wooded than adjacent sub-sites.…”

Basal stem area is a better measure of woodiness than canopy cover in the savannas of the Kruger National Park

“…Basal stem diameter measurements were used to calculate basal stem area (πr 2 ) and then summed for each plot to provide a cumulative measure of total woody area (cm 2 ). The methods of soil sampling and analysis are described in Mills et al 7 . All data were analysed using R statistical software.…”

Searching for David within the Goliath of alien woody plant invasions in the Western Cape Province

“…B, Mg and Mn). [13][14][15] The reasoning underpinning the theory is that if a soil nutritional regime favours the production over the metabolism of photosynthates, then a photosynthate surplus arises, which will be used for synthesising wood. The concept of short plants outcompeting tree seedlings is not new 16 , and the Catabolic Theory has been corroborated by studies on treeless vegetation surrounded by treed vegetation in both Australia and South Africa 14,15 .…”

“…A long-term fertilisation experiment in a South African savanna has shown conclusively that changes in soil nutrient regimes can either promote or entirely constrain woody plant encroachment. 15 It is highly probable that soil nutrients exert similar strong controls on woody plants in all biomes globally, not only in South African savannas. There are 13 plant nutrients which have the potential to affect the competitive interplay between fynbos plants and their woody alien invaders.…”