Accounting for carbon sequestration differences between sample points

According to Marais et al. (2009) sequestration rates vary according to climate, plant density, herbivory intensity and soil type. Because rainfall is the major factor accounting for CS differentials it is discussed in detail, followed by comments on other factors that are likely to contribute to differences. In conclusion, the selection of a study site for approximating spekboom's CS potential in BLK PNR is justified.

4.1.4.1 Rainfall variation as main difference

Average rainfall figures at case study sites were obtained from the sources reporting the case studies and from Schulze & Maharaj (2006) (see Table 4.2).

Table 4.2 Mean annual rainfall of selected study sites

Case study Rainfall (mm)

Fish River Reserve 450

Kudu Reserve 400-450

Kirkwood 250-350

Krompoort 250-350

Baviaanskloof Nature Reserve 250

Bosch Luys Kloof Private Nature Reserve 230

Compiled from: Schulze & Maharaj (2006); Mills & Cowling (2006); Powell (2009)

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BLK PNR rainfall matches that of Kirkwood, Krompoort and the Baviaanskloof Nature Reserve approximately, but is closest to the latter site, thus allowing some claim to equivalency for BLK PNR CS potential vis-à-vis these three case studies. Powell (2009) ascribes the difference in total organic carbon (TOC) of spekboom between the Fish River Reserve and the Baviaanskloof Nature Reserve spekboom (see Table 4.1) thickets to a higher annual rainfall.

Ecologists readily assume a positive linear relationship between biomass volume and rainfall (lower in deserts and higher in forests) (Woodward 1987). It is assumed that low water availability in warm, semi-arid landscapes limits accumulation of biomass because water demand increases with increased biomass. While true, exceptions to this rule occur in semi-arid and arid lands where water is not the primary limiting factor to CS (Mills, O’connor et al. 2005). Physiological decoupling from water limitation can also occur where Crassulacean acid metabolism (CAM) systems allow highly efficient use of water and thus relatively high productivity and biomass in areas with very low rainfall (Guralnick & Ting 1986; Maxwell, Griffiths & Young 1994; Mills & Cowling 2006). Lechmere-Oertel (2003) and Lechmere-Oertel et al. (2008) note that increasing aridity favours the percentage cover of spekboom which is a copious litter producer. To what can one attribute this anomaly, this deviation from the standard assumption that carbon accumulation is positively correlated with a rainfall gradient?

An explanation for the phenomenon is that spekboom cyclically seasonally shifts its photosynthetic pathway from CAM during summer to carbon fixation of 3-phosphoglycerate (C3) in the winter and spring (Guralnick & Ting 1986). Most plants experience their main vegetative period using the C3 photosynthetic pathway when rainfall is plentiful, as is the case with spekboom during winter (when most rainfalls if in a winter-rainfall area). However, during the drier months when most plant growth slows or stops, spekboom switches its photosynthetic pathway to CAM, continuing in the vegetative stage. By being in the vegetative stage all year round, spekboom's high leaf and litter production, hence its high carbon storage potential, is accounted for. During winter and spring the rapid response of spekboom to rewatering and its lower water loss, favour additional carbon gain as a C3 plant (Guralnick & Ting 1986). So, does a rainfall regime tending toward more winter rain as one moves westward point to the more accurate appraisal of CS values for BLK PNR? Figure 4.2 displays the rainfall regimes of the studied sites. The graphs show that the two western-most locations (BLK PNR and Baviaanskloof Nature Reserve) have the following features: 1) they are the most arid; 2) they have the lowest summer peaks, i.e. tend toward an all-year rainfall pattern with more gentle summer peaks; and 3) rain falls more often in winter when it is more effective. This suggests a relationship between rainfall regime and CS potential for spekboom,

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Source: Calculated from Schulze & Maharaj (2006) data Figure 4.2 Median monthly rainfall for study sites

especially given its ability to switch its storage mechanism. Because BLK PNR’s rainfall regime is very similar to that of Baviaanskloof the latter’s CS characteristics seem more applicable to adopt for the BLK PNR, than those of the case studies further east. An arid environment encourages the switch from C3 to CAM which increases carbon accumulation. More frequent rainfall (low water stress) encourages spekboom to switch back to C3 and lower carbon accumulation. Therefore, most carbon accumulation occurs in areas where the CAM pathway is used, while at the same time sufficient rainfall occurs to maximize carbon accumulation. Guralnick & Ting (1986) have noted that after seven-and-a-half-months of drought spekboom completely eliminates exogenous CO2 uptake, therefore even if increasing aridity favours spekboom coverage, there is a limit to the level of aridity it can withstand. Yet, Borland et al. (2009) showed that CAM plants can survive several years without rainfall. They also concluded that the highest values of daily net CO2 uptake reported for CAM species exceed those of nearly all productive C3 and C4 crops and occurred under rain-fed as well as dry conditions when moderate day and night temperatures prevailed.

More research is needed into this switching point, the conditions under which CS is maximized, and the rate of carbon accumulation by spekboom in conjunction with precipitation. It seems justifiable to assume that spekboom in the relatively arid BLK PNR may offer higher CS yields than intuitively expected.

4.1.4.2 Other factors causing CS differences

According to Mills & Cowling (2010) soil samples, planting density and soil quality all affect the rate of CS in spekboom occurrences. These factors are discussed in turn.

0 10 20 30 40 50 60 70

Fish River Reserve Kudu Rerserve Kirkwood Krompoort Baviaanskloof Bosch Luyskloof

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i. Concerning differences in soil sampling and analysis techniques, Mills & Cowling (2010) ascribe discrepancies between values to: 1) not including rock volume in some calculations, 2) differentials in soil nutrient status between sites, 3) differences in soil sampling depth and 4) not factoring in soil bulk density. Differences in the method of analysis possibly contribute to the recorded differences in soil carbon stocks. For example, (i) soil bulk density in Baviaanskloof was relatively low (0.76-0.08 g/cm3_{, 0-3 cm deep in intact thicket} under canopy) suggesting that by calculating bulk density from texture, as done in Kirkwood (range of 1.21–1.35 g/cm3) soil carbon stocks were overestimated; (ii) stone volumes in Baviaanskloof Nature Reserve were relatively high (e.g. 22% at 0-25 cm deep under canopy in intact thicket) and were not accounted for in the Kirkwood study; and (iii) the Kirkwood study sampled to a depth of 30 cm in contrast to the 25 cm of the Baviaanskloof study. Notwithstanding the above issues and the large amount of soil carbon in the Kirkwood case, the soil carbon stocks recorded in Baviaanskloof Nature Reserve, Krompoort and the Fish River Reserve are exceptionally large relative to other semi-arid regions (Mills, O’connor et al. 2005). Slower sequestration in the Kudu Reserve was ascribed to browsing by black rhinoceros and other herbivores, shallower soil and greater stone volumes (Mills & Cowling 2006).

ii. Mills & Cowling (2006: 3) aver that “Planting density [own emphasis] and spekboom genotype appeared to affect sequestration at Krompoort. Closely-packed spekboom planting may create positive feedback through increased infiltration of rainwater.” While there are areas of degraded spekboom on BLK PNR, the intact state of dense and tall (up to 2 m) spekboom stands (illustrated in Figure 4.3) supports an argument that different planting densities should not be a factor in assigning CS values to occurrences of spekboom in BLK PNR.

iii. Furthermore, regarding soil quality Mills & Cowling (2010: 98) have observed that: (a) “nutrients play a role in stabilizing organic carbon in the soil; (b) soil organic matter strongly influences nutrient holding capacity; and (c) other factors, such as site productivity, govern both soil nutrient and carbon stocksˮ. However, Mills et al. (2011) investigated the relationship between spekboom cover and soil properties and concluded that in the thicket biome, landscapes are dominated by spekboom veld or Spekboom Thicket (Vlok, Euston- Brown & Cowling 2003) across an exceptionally wide range of climatic and soil conditions. These range between approximately 200-800 mm mean annual rainfall regimes, nutrient- rich, alkaline, shale-derived soils as well as nutrient-poor, acidic sandstone-derived soils, all

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Figure 4.3 Areas densely vegetated with spekboom in BLK PNR

suggesting that spekboom tolerates a wide range of soil conditions and is unlikely to be directly constrained by soil properties.

If rainfall and rainfall regime are governing factors in carbon accumulation, spekboom in BLK PNR (230mm rain a year) (South African Rain Atlas 2006) should possess similar carbon

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accumulation potential to Baviaanskloof's spekboom. The Baviaanskloof reserve is also a suitable benchmark since soil sampling and analysis techniques employed there precluded overestimation of carbon stocks and spekboom there appears similar in physical characteristics (see in Figure 4.4).

Source: Powell (2009: 35) Figure 4.4 Spekboom in Baviaanskloof Nature Reserve

It therefore seems justifiable to adopt the CS potential values derived experimentally at Baviaanskloof for use in the CS equivalency for BLK PNR in the next section.