The influence of rainfall, competition and predation on seed production, germination and establishment of an encroaching Acacia in an arid Namibian savanna
Highlights
► We studied a population of Acacia mellifera shrubs in an arid savanna over 10 years. ► Seed production was highly correlated with rainfall and tree size. ► Trees only produced seeds en masse in exceptionally high rainfall years. ► Seeds germinate only in seasons when seeds are produced. ► Consequently establishment requires protracted periods of high rainfall events.
Introduction
The phenomenon of bush thickening (also referred to as shrub or bush encroachment), that results in the shift from open grass dominated rangelands to thicket dominated rangelands, particularly in savannas, has received a considerable amount of scientific attention, yet consensus has not been achieved regarding a generally accepted theory. For details on some of these theories see Bond and Midgley (2000), Higgins et al. (2000), Joubert et al. (2008), Knoop and Walker (1985), Meyer et al. (2007), Polley (1997), Roques et al. (2001), Scholes and Archer (1997), Smit (2004) and van der Waal et al. (2009). The diversity of explanations for bush thickening might partly be due to the fact that different species having varying phenologies in varying climates and soils, and thus underlying mechanisms of thickening may differ (Joubert et al., 2008). Whilst these theoretical arguments are not necessarily mutually exclusive there is need for more cohesion and consensus, since bush thickening has serious economic (De Klerk, 2004) and ecological implications (Blaum et al., 2009; Sirami et al., 2009).
To achieve more applicable generality to the theory of thickening, it is essential to better understand the demography of different thickening species by investigating aspects of their phenology, and how climate, competition, fire and browsing affect these (i.e. determine key recruitment bottlenecks). Actual non-manipulated occurrences of recruitment events can often only be detected through longer term field monitoring that includes phenological measurements. Yet such studies are considered less valuable than other approaches and are cited infrequently in the literature. Relatively little attention has been applied to recruitment bottlenecks in situ, especially on how life histories or phenologies of the different implicated species might influence these bottlenecks. This is particularly true in savannas. Recruitment studies tend to centre on recruitment as a whole, rather than looking at the different processes within, that is seed production, germination and emergence and seedling establishment. The processes from seed predation to establishment of a non-thickening species, Acacia erioloba in Botswana were investigated by Barnes (2001), while Seymour (2008) included rainfall as an explored driver of recruitment of A. erioloba, but only on sapling banks. Seed production studies in savannas have focussed overwhelmingly on the role of seed predation in diminishing Acacia seed banks, both pre-dispersal (Barnes, 2001; Ernst et al., 1990; Hoffman et al., 1989; Miller, 1994) and post-dispersal (Miller, 1995; Walters et al., 2005).
In their extensive review of the demography of the African Acacias, Midgley and Bond (2001) do not mention the possibility that rainfall might limit seed production, but focus attention on the effects of pollination on seed set and seed predation. Very few studies have investigated seed production in savanna species (for example Donaldson, 1967; Ernst et al., 1989; Mduma et al., 2007; Smit and Rethman, 1998; Tolsma et al., 1987; Walters and Milton, 2003). Chidumayo (1997) and Tolsma et al. (1987) attributed much of the observed variation in seed production to nutrient limitations. Other studies have focussed on germination and establishment alone (Brown and Archer, 1999; Chidumayo, 2008; Kraaij and Ward, 2006; Ward and Esler, 2010). Although mention is made of seed production varying with rainfall in some of the above studies, only Donaldson (1967), in a preliminary study of Acacia mellifera demography in South Africa, and Chidumayo (1997), studying two species in miombo woodland in Zambia, placed some emphasis on how temporal rainfall variation might affect seed production. An implicit assumption from all these studies is thus that seed production, prior to seed predation, is not limiting.
Seedling survival has been studied in more detail in savanna woody species (Barnes, 2001; Seymour, 2008) but these studies have also tended to be short term. In situ studies of recruitment in arid rangelands show varying responses. Harrington (1991) and Joubert et al. (2008) demonstrated episodic recruitment of Dodonaea attenuata in a semi-arid woodland in eastern Australia and A. mellifera in an arid savanna in central Namibia, respectively. Watson et al. (1997) demonstrated both episodic peaks in, and continuous recruitment, of two shrub species in arid rangelands in western Australia, whilst Brown and Archer (1999) demonstrated continuous recruitment of Prosopis glandulosa seedlings in semi-arid grasslands in Texas, USA. Seymour (2008) suggested episodic recruitment from seed but a continuously recruiting sapling bank for A. erioloba in an arid South African savanna. This range of observed and reported responses in arid rangelands clearly shows that the process of thickening does not follow a uniform pattern, and lends support to the suggestion that the understanding of which parts of recruitment are the actual bottlenecks could be valuable.
We investigated seed production, predation, seedling recruitment and establishment of a major encroaching species, Acacia mellifera (Vahl) Benth. subsp. detinens (Burch.) Brenan, in arid savannas of Namibia over a nine year period (late 1998–early 2007) to better understand bottlenecks within the recruitment process. We asked the following questions: (i) How does the production of viable seed vary with rainfall and tree size, (ii) when does seed germination occur, (iii) how long-lived is the seed bank of A. mellifera, and (iv) how do competitive interactions with established trees influence recruitment of A. mellifera seedlings?
Section snippets
Study area
The survey was conducted on the 8450 ha Farm “Krumhuk” (S 22.73426°; E 17.08775°), 20 km south of Windhoek, Namibia, in the Highland Savanna Vegetation Biome (Giess, 1971), more recently described as Highland Shrubland (Mendelsohn et al., 2002). The Highland Savanna has been exposed to commercial ranching since the late nineteenth century. The terrain is highly broken and undulating, at altitudes of 1350–2400 m above sea level and with slopes that are covered by lithic leptosols. These soils
Methods
Plots (20 m × 20 m; n = 9) were selected on north facing footslopes to represent bush thickets of varying degrees of density, cover, and tree height class distribution. Woody species composition and the density and canopy cover (%) of different height classes of A. mellifera subsp. detinens were recorded in each plot. Farm rainfall records were used to determine annual rainfall, which is typically recorded from 1 August to 31 July. So the rainfall measured from 1 August 1997 to 31 July 1998 was
Seed production
Among trees >2 m height, the percentage bearing fruit (Fig. 1) and the average number of pods per tree (Fig. 2) was correlated with rainfall of the preceding rainy season. No and virtually no pods were produced in the low rainfall seasons of 1998 and 1999. Also, pod production did not show any lag effect. In other words, pod production was not correlated with the rainfall of the season just prior to the preceding one. With the exceptional rains of 1999/2000 there was a flourish of pod
Discussion
The study identifies viable seed production as a likely demographic bottleneck in A. mellifera, and shows seed production to be positively correlated with rainfall. Donaldson (1967) also noted that A. mellifera fruited profusely following a season of above average rain in the Molopo area, South Africa but that only 2% of trees fruited in drier years. As far as we know this is the most extreme case of masting reported for a woody species in arid savannas, although the concept of masting is well
Acknowledgements
Polytechnic of Namibia provided funds for the research. Dr. F.P. Graz provided assistance with the initial lay out of the plots. Numerous student assistants assisted with field work. Ulf Voigts and Ralph Ahlensdorf of Farm Krumhuk kindly allowed field work on the farm. Two anonymous reviewers greatly enhanced the quality of the article.
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