Human impacts:deforestation 4.The reproduction hypothesis Pollination,pollen tube growth. seed development,seed dispersal,germination and seedling establishment Seed weight with altitude SE() : 0.10-n m 5.Growth limitation 6.Carbon limitation Source limitation hypothesis: Tree growth is considered source limited Growth when carbon assimilation through photo- Photosynthesis Growth Sink limitation hypothesis: "Sink"driven "Source"driven Supply driver owth,but ental conditions. 6 6 Human impacts: deforestation Cleared woodland for settlements (Avers valley, 2000m, Grisons, Switzerland). Pollination, pollen tube growth, seed development, seed 4.The reproduction hypothesis dispersal, germination and seedling establishment Tranquillini, W. 1979. Physiological Ecology of the Alpine Timberline Seed weight with altitude Baker (1972) argued that alpine plants have smaller seeds, because they can devote fewer resources to sexual reproduction than lowland plants due to the short growing season. Decreasing weight per seed with increasing altitude in 14 populations of Saxifraga oppositifolia (left) and eleven populations of Scabiosa lucida (right) (Plüss & Stöcklin). Krummholz Seedlings Dead individuals Tree species line Adults Camarero & Gutierrez (2002) Plant ecology 162: 247 Timberline 5. Growth 5. Growth limitation limitation Ph t th i Growth Photosynthesis Growth 6. Carbon limitation limitation Photosynthesis “Sink“ driven “Demand“ driven Growth “Source“ driven “Supply“ driven Körner Ch (1998) Oecologia 115:445 Li MH et al. 2008. Tree Physiology 28, 1287-1296 Li MH et al. 2008. Plant, Cell & Environment 31, 1377-1387 Source limitation hypothesis: Tree growth is considered source limited when carbon assimilation through photo￾synthesis is insufficient to meet growth requirements. Sink limitation hypothesis: Trees are considered carbon sink limited when there is an abundant supply of the resources necessary to support growth, but growth itself is directly limited by environ￾mental conditions