#50 - Jun 2021 - Garden Stuff - © - Sandy Lang - slang@xtra.co.nz
LEAVESJune: Winter. Gutters clogged with autumn leaves. Time to tidy up… Leaf fall: Leaves don’t just fall off. First, trees detect the shortening autumn days and signal (hormones) for the leaves to ‘senesce’ - a process in which they are scavenged for useful stuff (amino acids, sugars, minerals) which they transport back into the tree. We see this as yellowing. Next, an ‘abscission’ layer of fragile cells forms where the leafstalk joins the twig. This isolates the leaf from the tree, stretching, breaking and sealing-off the vascular connections, and creating a ‘plane of weakness’ so a puff of wind breaks the leaf away. Lifespan: Leaf lifespan varies from weeks (annuals) to months (deciduous trees) to many years (conifers). In general, short-lived leaves are flimsy things, built cheaply (minimal investment in leaf structure) so by 8 months (or less) they’re wind-torn, sun/wind-burned, insect chewed. The long-lived leaves are thicker, tougher things (high investment in leaf structure) and these better endure the rigours a tough world. So, it’s cheap, throwaway or quality, built to last. Try an experiment. Dry out a leaf from a Pohutukawa tree, from a deciduous tree, and from Wandering willie – the different structural investments are obvious. Strategies: All trees shed leaves, but some (evergreens) shed them one by one, anytime through the year, as they become surplus to requirements (shaded, damaged, stomata no longer open/close to control water loss), while others shed them all at once in autumn (deciduous). Satellite images in winter show there are some areas of the world where deciduous forests predominate, and some where evergreen forests predominate, and some with a mix. In most climates, and on most soils, there’s a mix of evergreen and deciduous, suggesting neither strategy is quite without its special advantages/disadvantages. If there were a standout, ‘best strategy’, it’s likely all trees would do the same, everywhere. Photosynthesis: It’s generally true the flimsy-leafed, deciduous trees have higher leaf areas and photosynthesise faster than evergreen trees. So, the net amount of photosynthesis by a deciduous tree in 8 months is about the same as that by an evergreen tree in 12 months. New Zealand: Most (96%) of our native trees are evergreen. This might be something to do with our soils and climate (maritime - mild temperatures, windy, uniform rainfall etc) but more likely with our geological past and isolation.___________________________________
LEAVESJune: Winter. Gutters clogged with autumn leaves. Time to tidy up… Leaf fall: Leaves don’t just fall off. First, trees detect the shortening autumn days and signal (hormones) for the leaves to ‘senesce’ - a process in which they are scavenged for useful stuff (amino acids, sugars, minerals) which they transport back into the tree. We see this as yellowing. Next, an ‘abscission’ layer of fragile cells forms where the leafstalk joins the twig. This isolates the leaf from the tree, stretching, breaking and sealing-off the vascular connections, and creating a ‘plane of weakness’ so a puff of wind breaks the leaf away. Lifespan: Leaf lifespan varies from weeks (annuals) to months (deciduous trees) to many years (conifers). In general, short-lived leaves are flimsy things, built cheaply (minimal investment in leaf structure) so by 8 months (or less) they’re wind-torn, sun/wind-burned, insect chewed. The long-lived leaves are thicker, tougher things (high investment in leaf structure) and these better endure the rigours a tough world. So, it’s cheap, throwaway or quality, built to last. Try an experiment. Dry out a leaf from a Pohutukawa tree, from a deciduous tree, and from Wandering willie – the different structural investments are obvious. Strategies: All trees shed leaves, but some (evergreens) shed them one by one, anytime through the year, as they become surplus to requirements (shaded, damaged, stomata no longer open/close to control water loss), while others shed them all at once in autumn (deciduous). Satellite images in winter show there are some areas of the world where deciduous forests predominate, and some where evergreen forests predominate, and some with a mix. In most climates, and on most soils, there’s a mix of evergreen and deciduous, suggesting neither strategy is quite without its special advantages/disadvantages. If there were a standout, ‘best strategy’, it’s likely all trees would do the same, everywhere. Photosynthesis: It’s generally true the flimsy-leafed, deciduous trees have higher leaf areas and photosynthesise faster than evergreen trees. So, the net amount of photosynthesis by a deciduous tree in 8 months is about the same as that by an evergreen tree in 12 months. New Zealand: Most (96%) of our native trees are evergreen. This might be something to do with our soils and climate (maritime - mild temperatures, windy, uniform rainfall etc) but more likely with our geological past and isolation.___________________________________