Biomass Increases Go under Cover: Woody Vegetation Dynamics in South African Rangelands.

Woody biomass dynamics are an expression of ecosystem function, yet biomass estimates do not provide information on the spatial distribution of woody vegetation within the vertical vegetation subcanopy. We demonstrate the ability of airborne light detection and ranging (LiDAR) to measure aboveground biomass and subcanopy structure, as an explanatory tool to unravel vegetation dynamics in structurally heterogeneous landscapes. We sampled three communal rangelands in Bushbuckridge, South Africa, utilised by rural communities for fuelwood harvesting. Woody biomass estimates ranged between 9 Mg ha(-1) on gabbro geology sites to 27 Mg ha(-1) on granitic geology sites. Despite predictions of woodland depletion due to unsustainable fuelwood extraction in previous studies, biomass in all the communal rangelands increased between 2008 and 2012. Annual biomass productivity estimates (10-14% p.a.) were higher than previous estimates of 4% and likely a significant contributor to the previous underestimations of modelled biomass supply. We show that biomass increases are attributable to growth of vegetation <5 m in height, and that, in the high wood extraction rangeland, 79% of the changes in the vertical vegetation subcanopy are gains in the 1-3 m height class. The higher the wood extraction pressure on the rangelands, the greater the biomass increases in the low height classes within the subcanopy, likely a strong resprouting response to intensive harvesting. Yet, fuelwood shortages are still occurring, as evidenced by the losses in the tall tree height class in the high extraction rangeland. Loss of large trees and gain in subcanopy shrubs could result in a structurally simple landscape with reduced functional capacity. This research demonstrates that intensive harvesting can, paradoxically, increase biomass and this has implications for the sustainability of ecosystem service provision. The structural implications of biomass increases in communal rangelands could be misinterpreted as woodland recovery in the absence of three-dimensional, subcanopy information.

Effects of nutrients on the distribution of dry mass, nitrogen and phosphorus in seedlings of Protea repens (L.) L. (Proteaceae).

The response of seedlings of the sclerophyllous shrub, Protea repens (L.) 1., to increasing concentrations of phosphorus (P), nitrogen (N) and a mixture of all essential nutrients excluding N and P (M) was determined in potted Clovelly soil collected Irotn a lowland fynbos site at pella, south-western Cape, South Africa. Pot culture resulted in increased soil mineral nitrogen, in particular nitrate, and decreased available (resin-extractable) phosphorus concentrations compared to field soil. High amounts of N (4-64 g m2 N) and M addition resulted in seedling mortality. Plant dry mass, leaf area and phosphorus and nitrogen contents increased in response to increasing application of N, but no significant differences were found in response to M. Increasing applications of N resulted in reduced plant dry mass, leaf area and phosphorus content. These patterns of mortality and growth are interpreted as a response to an imbalance between nitrogen and phosphorus availability. These results are compared to the response of mature fynbos shrubs to fertilizer additions in the field and the responses of pot-sclerophyllous plants from other Mediterranean-type ecosystems.

Leaf specific mass confounds leaf density and thickness.

We explored the relationship between leaf specific mass (LSM) and its two components, leaf density and thickness. These were assessed on the leaves of (a) the moderately sclerophyllous tree Arbutus menziesii distributed along a natural nutrient/moisture gradient in California, (b) eight sclerophyllous shrub species on four substrates in south-western Australia, and (c) seedlings of two morphologically contrasting Hakea species grown under varying soil nutrient, moisture and light regimes in a glasshouse experiment. Leaf area, mass, LSM, density and thickness varied greatly between leaves on the same plant, different species, and with different nutrient, moisture and light regimes. In some cases, variations in LSM were due to changes in leaf density in particular or thickness or both, while in others, density and thickness varied without a net effect on LSM. At lower nutrient or moisture availabilities or at higher light irradiances, leaves tended to be smaller, with higher LSM, density and thickness. Under increased stress, the thickness (diameter) of needle leaves decreased despite an increase in LSM. We concluded that, while LSM is a useful measure of sclerophylly, its separation into leaf density and thickness may be more appropriate as they often vary independently and appear to be more responsive to environmental gradients than LSM.

Disproportionate allocation of mineral nutrients and carbon between vegetative and reproductive structures in Banksia hookeriana.

We compared above-ground allocation patterns in mature shrubs of Banksia hookeriana from three 13-year-old populations, growing on nutrient-impoverished sands to determine whether C (dry mass) could be a substitute for mineral nutrients (N, P, K, Ca, Mg and NA). The percentage of reproductive structures to total above-ground growth (reproductive effort; RE) was integrated over nine successive reproductive cycles. Only 0.5% of above-ground dry mass was allocated to seeds compared with 31% to total RE. Allocations of N (24%) and P (48%) to seeds, and N (44%) and P (65%) to RE were much higher. Allocations of K, Ca, Mg and Na to seeds (<1-3%), and RE (21-35%) were closer to that of dry mass. Relative allocation (RA) is defined as the proportion of a nutrient element allocated to a structure relative to its dry mass. RA of P to seeds was 91 and N was 44, but for K, Ca, Mg and Na ranged from only 6 for K to<1 for Na. Thus P, and to a lesser extent N, provide a much more sensitive measure of the relative cost of reproduction than C in this nutrient-limited system.

A test for lottery recruitment among four Banksia species based on their demography and biological attributes.

A stage-by-stage protocol for identifying simple or biased lottery, or non-lottery, patterns of seedling recruitment is outlined. For a simple (weighted) lottery to apply, the proportion of total individuals accounted for by one species at one stage of recruitment plotted against the proportion accounted for at a previous stage over a wide range of recruitment conditions should obey a linear regression with a=0 and b=1. For a biased lottery to hold, the regression is significant but a≠0 and/or b≠1. Demorgraphic, size and water relations data were collected over 3 years for four co-occurring Banksia species following two contrasting experimental fires. The first summer was exceptionally wet and the second was exceptionally hot and dry. Seedlings still alive by the 3rd year relative to the fire-killed parent plants conformed to a biased lottery in the case of B. speciosa and B. baxteri, while B. coccinea and B. pulchella had no mathematical structure (mean of the proportional ratios ≠ 1). Intervening stages, beginning with seed release, showed transient deterministic or simple lottery patterns in some cases, but the overriding trend was for biased lotteries. B. speciosa dominated the responses, with greater seed release than expected, fewer initial seedlings, lottery survival of 1st year seedlings and greater survival of 2nd year seedlings, when compared with the previous stages. Large seeds and subsequent high growth rates enabled B. speciosa to exploit soil water preferentially during the severe summer drought. The trend for B. speciosa to replace other species may be cancelled by stochastic processes not operating in this particular study.

Population fragmentation may reduce fertility to zero in Banksia goodii - a demonstration of the Allee effect.

All individuals of all known populations of Banksia goodii were assessed for seed production. Small populations produced no or only a few seeds per unit canopy area. Effects of population size on seed production per unit area and seed production per plant were present over the whole range of population sizes, indicating that even in large populations seed production may still not be at its maximum. Resource differences could not explain this disproportionate decrease in seed production with decline in population size, because there were no differences in soil properties and understorey or overstorey cover between the small and large populations. Although plants in small and large populations were similar in size, seed production per plant was much lower in small populations. This was not because plants in small populations produced fewer cones but because the fraction of these cones that was fertile was much lower. Five of the nine smallest populations (<200 m2) produced no fertile cones over the last 10 years. The number of seeds per fertile cone did not depend on population size. The results are discussed in relation to pollination biology.

Conceptualizing the human use of wild edible herbs for conservation in South African communal areas.

The importance of wild edible herbaceous species to resource poor households in most rural economies within savannas has been little studied. This is because most of the herbs grow in impoverished species communities and lands, often referred to as 'marginal lands'. The aim of this paper is to conceptualize how the economics of wild edible herbs to households can be used to add value to total livelihoods and conservation within traditional communal areas of South Africa. Analysis of the economics of the consumption of wild edible herbs in Thorndale (Bushbuckridge district) of the Limpopo province is presented. The majority of households consumed wild edible herbs, averaging 15.4 kg dried weight per household per year and valued at $167 per household. The herbs were mostly harvested from uncultivated areas of farms, and rangelands. There was little correlation between household characteristics and the dependence on wild herbs for food. The local people noted a decline in the availability of the species, although not much is known about attempts to cultivate them. The only reasons attributed to the decline were nutrient poor soils and insufficient rains. With this background, developing a local strategy to sustain the species through cultivation by households was found to be feasible. A multiple-use system for the herbs, their improvement and value addition towards commercialization and increased household usage may result in wider acceptance and subsequent cultivation. Species diversity will be enhanced whilst conserving the land on which they grow. This multiple use system may include species roles in soil and water conservation.