Constraint on woody cover in relation to nutrient content of soils in western southern Africa

Abstract

The influence of soil nutrients on woody plants is poorly understood. Are trees – fire and other disturbance factors being equal – generally promoted by nutrient‐rich or by nutrient‐poor soils? To determine the edaphic parameters controlling woody cover, we sampled soils and summed the extent of the crowns of trees and tall shrubs on 364 plots at 20 sites in Namibia and adjacent South Africa, ranging from desert lichen‐fields to caesalpiniaceous woodland with associated mean annual rainfall of 11 mm to 535 mm. Our analysis included the macro‐nutrients N, P, Mg, K and Ca and the trace elements Mn, Fe, Cu and Zn. A boundary line analysis showed that woody cover was densest, with the greatest large‐scale heterogeneity, at intermediate nutrient contents, but consistently constrained at extreme nutrient richness as well as poverty. If aridity exerted the ultimate constraint at extreme nutrient richness, no such correlation with climate apparently applies at extreme nutrient poverty, where our graphs show an ‘oligotrophic decline’. Notwithstanding the importance of water, we suggest that extreme nutrient richness and poverty both favour grasses over tree seedlings. This is because catabolic dystrophy – a regime in which the supply of catabolic nutrients shortfalls their demand – is unlikely in environments where nutrient richness allows catabolic rates to match anabolic rates or where nutrient poverty constrains anabolic rates. We also reason that surpluses of photosynthate resulting from dystrophy can be allocated to lignin and that the potential for woody growth thus corresponds to soils of intermediate nutrient content. This explains why woody cover is consistently but not homogeneously densest in nutritionally intermediate plots in our dataset. Hence, the abundance of woody plants in various biomes may be determined partly by soil nutrient content, particularly of Cu, Zn, and other elements indispensible for catabolism.

Why was the Highveld treeless? Looking laterally to the Pampas for global edaphic principles beyond biogeographical accidents

“…The local survival of species such as Vachellia karroo and Olea europaea among swards intermediate in composition between 'sweetveld' and 'sourveld' might correlate with soils moderate in certain nutrient elements. However, our preliminary tests (Mills et al, 2013a(Mills et al, , 2013b indicate that it is not the absolute concentrations as much as the relative demand and supply of catabolic nutrients that allowed the relaxation of competitive pressures from grasses on these woody plants. We note that both rock outcrops and drainage lines would tend to be washed by rain, losing crucial nutrients such as Cu deposited in aerosol.…”

“…The details of this common denominator require considerably more investigation than the studies conducted to date (i.e. Mills et al, 2012Mills et al, , 2013aMills et al, , 2013b. Suggested lines of investigation include the following: i) the relatively nutrient-rich soils in the west of the Highveld ('sweetveld') generate a sufficient supply of catabolic nutrients during periods of rapid photosynthesis; ii) the relatively nutrient-poor soils in the east of the Highveld ('sourveld') potentially constrain photosynthesis, thereby reducing demand for catabolic nutrients; iii) the crucial competitive interaction between tree seedlings and grasses may occur -via mycorrhizal interactions -in the pedoderm (Mills and Fey, 2004), i.e.…”

“…We have published several papers supporting aspects of this new theory on nutritional determination of the competitive balance between trees and grasses (Mills et al, 2012(Mills et al, , 2013a(Mills et al, , 2013b. However, the edaphic data required are not generally available in previous analyses of soil composition.…”

“…A new explanation for the overwhelming prevalence of grasses is that herbaceous plants are capable of competitive exclusion of trees on sites where supply of catabolic nutrients meets the demand created by the photosynthetic potential of the local environment (Mills et al, 2006;Milewski and Mills, 2010;Mills et al, 2012Mills et al, , 2013aMills et al, , 2013b. This competition is predicted to occur mainly underground (Casper and Jackson, 1997), among roots and mycorrhizae, and is relevant for any terrestrial ecosystem where there is competition between woody and non-woody plants.…”

Determinants of growth of the flammable grass, Triodia scariosa: Consequences for fuel dynamics under climate change in the Mediterranean region of South Eastern Australia

“…The patterns of growth and flammability in response to moisture will differ fundamentally between these different life forms (Walker 1981;O'Connor et al 2011). Soil fertility may also influence the balance between herbaceous and woody plants (Carrera et al 2000;Lehmann et al 2011;Mills et al 2012;Gibson et al 2014a) and corresponding effects on fire regimes (Gibson et al 2014b).…”