"If humanity hopes to realize its dreams of exploring the stars, we’re going to need to find ways to recreate life on Earth aboard a spaceship. Simply stockpiling enough vital supplies isn’t going to cut it, which is what led Julian Melchiorri, a student at the Royal College of Art, to create an artificial biological leaf that produces oxygen just like the ones on our home planet do."
leaf senescence begins with the advent of the summer solstice, as the days get shorter and sun becomes more distant. trees begin to reduce the production of chlorophyll — a green pigment critical to photosynthesis — and eventually begin to break down that which remains in the leaf in order to reabsorb its nitrogen.
as the green of leaves consequently begins to fade, other pigments in the leaf — carotenoids and flavonoids — also see reduced production in the leaf, but at a much slower rate than chlorophyll, which enables their yellow and orange colours to be expressed.
for some trees, colder temperatures trigger the synthesis of the flavonoid anthocyanin, causing those leaves with lower levels of other flavonoids or carotenoids to turn red. if other pigments are sufficiently present, the colours can blend into auburn.
it is believed that trees will produce anthocyanin to protect themselves from sap sucking insects that would otherwise be drawn to the yellow and orange colours of their leaves.
eventually, these non chlorophyll pigments fade themselves as the tree, in preparation for winter, denies its leaves of water (otherwise transported through some of the veins seen here). in an effort to also retain nutrients, a deciduous tree will then signal the abscission cells at the base of its leaves to swell.
this not only blocks the flow of nutrients but ultimately causes the leaves to tear away and fall to the ground, where the tree can then reabsorb any leaf nutrients through its roots before going into winter dormancy.
every autumn, tens of millions of monarch butterflies travel to their ancestral winter roosts in mexico’s mountain fir forests, coating the trunks of the trees in the orange of their wings, and causing the branches to droop under their collective weight.
surfing winds from southern canada and the northern united states, and taking directional cues from the sun and magnetic poles, they travel 4,500 kilometres over two months to reach their hibernation grounds — a feat that still remains a bit of a mystery, but which has been going on for millions of years.
interestingly, the autumn migration south is accomplished in one generation, which lives for about seven months, while the spring migration north is done over three generations, each living about six weeks.
last year’s migration, however, was the lowest on record, as excessive herbicide usage has reduced the supply of the milkweed plant which the monarch larvae rely on to feed, and which makes the monarch caterpillars toxic to predators. but the plant is now being destroyed from heavy use of roundup ready pesticides used in soy and corn crop production.
further complicating matters for the monarch is climate change, as drought along their migratory route has exacerbated milkweed decline, and colder spring temperatures has meant the temperature-sensitive cold-blooded butterflies are unable to begin their journey north.
and once they reach their hibernation sites in mexico, the butterflies, which rely on a thick forest canopy for protection from the cold and rain, encounter deteriorating forests from illegal logging.
experts, however, are hopeful that this year’s migration will double or triple, thanks in large part to the conservation efforts of the mexican government. nevertheless, this increase would still put monarch numbers at one tenth of their record high of one billion.