Parallel A: Climate Change

The MISR-HR satellite sensor enabled us to construct an 18-year record of eight-daily fraction absorbed photosynthetic radiation (FAPAR) at 275 m resolution over the entire grassland domain. The seasonally-accumulated FAPAR is a good proxy for net primary production (NPP), a hypothesis we tested using independent data. We then selected 60 sites across the grassland rainfall range, in South Africa chosen to be in un-degraded condition, to establish a relationship between accumulated rainfall and seasonally-accumulated FAPAR. The relationship is strongly linear (r2=0.805, p< 0.001) until around 700 mm, at which point it begins to level off. There is a strong association between soil types and rainfall, but the soil type itself has little effect on accumulated FAPAR. As found elsewhere, the within-site relationships to rainfall has a lower slope (rain use efficiency) than the between-site slope, an observation we attribute to biogeochemical constraints. The NPP in a given year has a strong relationship to NPP in the previous year, particularly below 600 mm (r2 = 0.35-0.4), and even a relationship to two years previously (r2~0.2). There are clear functional differences between the semi-arid and moist grasslands.

Drought can be defined in general terms as the 50 percent shortfall of rainfall over a period of three months. Drought leads to reduced rangeland productivity, which will ultimately lead to reduced livestock and wildlife production. The objective of this study was to determine the impact of drought, simulated with four different rainfall interception (RI) levels on grass annual net primary productivity (ANPP). The study was conducted at the University of Pretoria experimental farm, Hatfield, Pretoria. There were twenty 7 X 7m study plots on a natural grassland. The treatments consisted of 0, 15, 30 and 60 % RI, each replicated five times. All the herbage in the plots was cut to ground level at the beginning of the experiment on the 1st of November 2016. A fixed 1 m2 quadrat was marked at the centre of each plot and the biomass within the quadrats was harvested at the end of the year. Grasses were further separated into different species and the species were categorised into ecological statuses (increasers and decreasers). The ANPP under 60 % RI was four times lower than that of the control plots (5482 kg ha-1). Although there was no significant difference in the grass ANPP between the 0% (4582 kg ha-1), 15% (4100 kg ha-1) and 30% RI (3760 kg ha-1), the herbage yield declined as the drought intensity increased. Forbs’ ANPP was significantly lower (P< 0.05) under 15% RI (30.5 kg ha-1). Digitaria eriantha, Themeda triandra and Heteropogon contortus had a significantly lower ANPP (P< 0.05) under 60% RI. However, D. eriantha attained a significantly higher (P< 0.05) ANPP under 15% RI (3338 kg ha-1). Decreasers produced a significantly higher ANPP (P< 0.05) at the 0% RI (3432 kg ha-1) while increasers were significantly higher (P< 0.05) under 30% RI (2622 kg ha-1) than 60% RI (283 kg ha-1). Increasers used water more efficiently than decreasers. The results showed that drought has a significant negative effect on the ANPP. Herbage yield generally decreased with the increase in intensity of drought, however, the relative contribution of forbs increased with the increase in drought intensity while the opposite was true for grasses. This implies that drought (60% RI) can have severe effects on rangeland productivity, which can reduce livestock production.

Land management systems may affect the amount of plant and soil carbon-nitrogen sequestrated in different seasons. This study investigated the differences in plant and soil carbon-nitrogen sequestration and C-N isotopes between three land management systems in summer and winter seasons. Three 250 m2 belt transects were randomly laid out in each of the three camps per land management system. For herbaceous plant sampling, forages were harvested from five (0.25 m2) quadrats laid out in each belt transect, bulked and samples oven-dried at 65 oC. For woody plant sampling, leaves and twigs of a dominant plant species (Colophospermum mopane) were harvested from five plants per belt transect, bulked and oven-dried at 65 oC. Three soil samples were collected per belt transect to a depth of 20 cm using a soil auger and a soil core sampler to determine soil bulk density. The total vegetation results showed that total organic carbon (TOC) (kg ha-1) in plants (herbaceous and woody plants) was greater (P < 0.001) in the game reserve, followed by the ranch and lowest in the communal area, with a similar trend observed in both seasons. The total vegetation total nitrogen (TN) (kg ha-1) sequestered in summer was greater (P < 0.001) in the game reserve and lowest in the communal area, while in winter the game reserve and ranch had similar results and greater (P < 0.001) TN (kg ha-1) than the communal areas. The amount of vegetation TOC and TN (kg ha-1) was greater in summer than winter season. Soil TOC (kg ha-1) was greater (P < 0.001) in the game reserve than the ranch and communal land management systems. The TN (kg ha-1) in soils was unaffected (P > 0.05) by land management systems. The herbaceous δ13C and δ15N levels were greatly (P < 0.05) affected by land management systems in different seasons. In summer, the game reserve and ranch land management systems recorded greater herbaceous (P < 0.001) δ13C and lower δ15N levels than the communal area, but in winter, the game reserve had a greater (P < 0.001) herbaceous δ13C level than the ranch, though both the game reserve and ranch had similar (P > 0.001) δ15N values. The woody foliar component had markedly greater (P < 0.001) δ13C levels in summer in both the communal and ranch land management systems than the game reserve, but in winter the ranch had greater (P < 0.001) δ13C level than the game reserve and communal areas. Woody foliar samples harvested from the ranch showed variation (P < 0.05) in δ15N between seasons, being greater (P < 0.001) in summer than winter. Soil δ13C level was slightly greater (P < 0.001) in both seasons in the communal land than the other two land management systems. The study concludes that the game reserve sequestered more TOC and TN (kg ha-1) in the total vegetation than the other two land management systems during both summer and winter. The game reserve also stored more soil TOC (kg ha-1) than the ranch and communal management systems. The study recommends more research to be conducted by including land management systems in different agro-ecologies, and for a prolonged period, to ascertain the impact of land use practices and seasonal dynamics of the measured variables.

The effects of rising temperature on grassland have been studied utilizing open top warming chambers (OTC), however little research has been done on the effect of warming on the performance of woody plants in savannas. The objective of the study was to test the effect of elevated temperatures on growth and defence of woody seedlings that have the potential to invade grassland and this was examined for Vachellia sieberiana seedlings at Ukulinga experimental farm of University of KwaZulu-Natal, Pietermaritzburgusing OTC. Seeds of V. sieberiana were collected in 2016 around Pietermaritzburg and 200 seeds were germinated using agar, and then grown in pots with sandy soil in a greenhouse for a month. At a mean height and stem diameter of 13.5 cm and 2.16 mm respectively, 120 seedlings were transplanted into field plots, that were either cleared of grass or not in October 2017. The remaining 80 seedlings were used to develop linear regression models of stem diameter and total dry mass (DM) to estimate initial biomass of seedlings. The experiment was a fully crossed randomized design with four treatments (warmed with grass, warmed without grass, not warmed with grass and not warmed without grass) replicated 5 times. Plots were 2 x 2 m with six seedlings in each plot. Open top warming chambers were used to raise air temperature by ~1 °C. Stem length, plant height, stem diameter, thorn length, leaf dry matter, shoot dry matter, root dry matter and total dry matter (DM) were measured in January 2018. Warming significantly (p < 0.05) increased plant height, stem length, and stem diameter 1,3 to 1,4 fold and on average, doubled leaf DM, shoot DM, root DM and total DM. The removal of grass cover significantly (p < 0.05) doubled shoot DM and total DM, on average, while warming in the absence of grass doubled thorn length compared to plots with grass. Temperature to a level expected in the next few decades clearly was more beneficial than grass removal for V. sieberiana seedlings growth. Our results suggest that, regardless of grass cover at the time of seedlings establishment, the rate of woody encroachment will increase as temperature rises.

Sorghum [Sorghum bicolor (L) Moench] is adapted to high temperatures and water stress, and plays a critical role as food in dry areas, with the stover also utilized as fodder. Temperatures above 30 °C promote faster emergence and better seedling establishment, but very high temperatures affect osmotic regulation. Optimum temperature for emergence is 20-30 oC. The objective of the study was to evaluate the effect of high temperatures experienced in the Limpopo region on the emergence of new perennial sorghum cultivars, and subsequently assess seedling and plant growth. Temperature effects were assessed at two sites in communal areas: Vhembe (Vh) district (Temperature Humidity index THI 72-83; mean maximum air temperatures of 38 oC, mean annual rainfall 410 mm) and Sekhukhune (SK) district (THI 75-87; 40oC and mean annual rainfall of 380 mm). Experimental plots were established in January 2018 at sites characterised by a sandy loam soil type. Eight quadrats were randomly selected for assessment of germination, root length and mass and shoot growth (plant density, plant height, leaf numbers and length and stem diameter). Germination was assessed daily over 14 days, with weekly measurements on seedling growth undertaken thereafter on a weekly basis until eight weeks post emergence. At eight weeks whole plants were harvested for forage quality inference based on stem diameter. Germination percentage and seedling emergence was higher (P< 0.05) at Vh (86% and 79%, respectively) than at Sk (72%; 46%). Plant density was higher (P< 0.001) in Vh, ranging between 90 and-144, while it was < 40 plants/quadrat in Sk. At 4 weeks mean leaf numbers in Vh and Sk were 7 and 9, respectively, while mean leaf width was similar (1.8 cm) and plant height varied (23 vs 11 cm). Lateral root length was within the range 19-26 cm (Vh) and 25-30 cm in Sk, although the latter had massive root system (40-60 lateral roots vs 38-46 in Vh). Positive, but non-linear correlations were noted between lateral root length and shoot growth: (R2 = 0.66 and 0.7; at Sk and Vh, respectively). Stem diameter ranged between 0.5-1.3 cm at four weeks, but did not differ at tasselling, averaging 2.3-cm. Better growth performance in Vh could be related to the greater initial rates of root and shoot growth, which allowed plants better adaptability to stress at temperatures below 40 oC. Sorghum sown at the warmer site (Sk) emerged, but plant density was lower, which affected yield. Thinner stems are less lignified and easily macerated during rumination; hence, the crop at Sk was of better quality irrespective of the crop yield being lower than at Vh. The perennial sorghum cultivar evaluated did not perform well in the warmer area, which is of great concern; given that, Sk has a high human and cattle population that is dependent on climate smart crops.