Effect of elevated temperatures on growth and defence of Vachellia 1 sieberiana seedlings grown with or without grass.
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Warming is suggested to increase globally in the next few decades. Warming, soil nutrients, 2 water, CO2 and sunlight are the most important environmental factors for plant life. Elevated 3 temperatures can highly impact plant metabolism, photosynthesis and consequently woody 4 plant performance and fecundity. An increase in minimum temperatures will possibly result in 5 the transformation of grasslands to savannas. The effects of rising temperatures on growth and 6 defence of woody plant seedlings and trade-offs between growth and physical defence 7 particularly thorn length of woody plants that potentially invade grasslands have been studied 8 utilising open-top warming chambers (OTCs). Furthermore, it is important to study the effect 9 of warming on woody plant growth and defence when competing with grass. Field experiments 10 were conducted in three different seasons to determine the effect of warming on growth and 11 defence of Vachellia sieberiana seedlings growing with or without grass, and the effect of 12 warming on trade-offs between growth and defence of V. sieberiana seedlings growing with or 13 without grass. It was predicted that elevated temperatures would increase growth and thorn 14 length of V. sieberiana seedlings growing (1) with or without grass in the dormant season and 15 early growing season, but (2) in the later part of the growing season only when grass cover is 16 absent. It was also predicted that warming, with or without grass cover increases seedling 17 growth, but elevated temperatures would decrease investment in defences relative to growth of 18 V. sieberiana seedlings when grass cover is absent. To test these predictions, 120 seedlings 19 that were three weeks old were transplanted into 20 field plots. Ten plots were warmed, and the 20 other ten plots were not warmed. In both warmed and unwarmed plots the grass was either 21 cleared or not. OTCs raised air temperature by 1.0 to 2.5 °C. Stem length, plant height, stem 22 diameter, thorn length, and dry mass of shoot, leaf, and root were measured after 6 weeks for 23 the dormant season, and after 6 weeks and 12 weeks in the growing season for the first 24 experiments. For the second experiment, plant height, stem length, stem diameter and thorn 25 length were measured after 6 weeks. To determine effects of warming and grass cover on the 26 relationship between defence and growth, ratios of thorn length (a measure of defence) to plant 27 height, stem length and stem diameter (measures of growth) were calculated. The first 28 experiment revealed that warming increased growth of V. sieberiana seedlings in all seasons. 29 Defence was low in the presence of grass cover in the growing season, but not in the dormant 30 season. The second experiment revealed that warming and grass cover significantly and 31 independently reduced the thorn length: plant height ratio and thorn length: stem length ratio. There was a significant interaction effect between warming and grass cover on the thorn length: 1 stem diameter ratio. Lastly these results suggest that, with an increase of 1 – 2.5 °C regardless 2 of grass cover at the time of seedlings establishment, the rate of woody encroachment will 3 increase as temperature rises. The findings also suggest that there are trade-offs between the 4 growth of V. sieberiana seedlings and physical defence in terms of thorn length.