I live in the northeastern United States, so this time of year I think about leaves a lot. The color changes are the most obvious reason to be thinking about leaves, of course. In the autumn, the trees slow down and then stop producing chlorophyll, the green pigment needed for photosynthesis, so the yellow pigments also in the leaf become more prominent.
But it’s probably more complicated (and more interesting) than that: yellow leaves are a sign of disease during most of the year, so some insects are attracted to plants with yellow leaves. If a tree is already ill, it might be more vulnerable as insect food. It appears that some trees actually produce red pigments to hide the yellow pigments since the green pigments are no longer doing so, and it’s all to protect themselves from insects.
And it’s probably really even more complicated than that, too.
(I love science: it’s always more complicated, and we’re always wrong, but it’s so much fun to try to figure it out.)
So there’s one thing to think about when world-building: does your local plant-equivalent have seasonal changes in the coloration of its leaf-equivalents? If so, why? Does that have any other effects that matter in the ecology of the world, like attracting or repelling other organisms?
For that matter, are they green to start with? Most planets with life will need some way to convert solar energy into biochemical energy, which is what photosynthesis does. There are organisms that use geothermal energy or non-biological chemical energy, but most energy comes from the sun through photosynthesis. It seems likely that other planets will have the same energetic basis (but there’s plenty of room for speculation here).
And what else do leaves do? They’re flat so they can maximize sun exposure, and they’re thin so they can maximize gas exchange.
Except where conditions are less than ideal.
Desert plants have all sorts of leaf adaptations to make sure that they don’t lose too much moisture from that flat surface. Waxy coatings hold moisture inside, and hairs cut down on air movement across the surface of the leaf, reducing evaporation. Smaller leaves help reduce water loss too.
(Leaves have to take in carbon dioxide for photosynthesis, but that creates a way that water can be lost. Grasses and cacti have evolved different physiologies to deal with that problem, but I want to stick to shape instead of biochemistry.)
Very cold can be a lot like very dry, and some of the same leaf adaptations show up in arctic climates: hairs, waxy coatings, and especially having small leaves such as like pine needles.
Leaves in wet climates tend to have fewer teeth on the edges, while leaves from cold areas have more teeth.
In places where it’s warm and wet year-round, there isn’t usually a season when most plants lose their leaves simultaneously. In temperate climates, it’s cheaper for the plant to shed them than it is to maintain them through the winter. In seasonally-dry areas, plants might drop their leaves for the dry season, again to save resources. But in very hard environments, especially if they are low in nutrients, plants often keep their leaves year-round, even through the winter, because resources might not be available to replace them.
These are broad generalizations, and there are lots of exceptions, but for the world-builder matching leaf shape to climate can provide a quick hook for a realistic world. We’re used to certain kinds of plants in certain places, even if we don’t think about it.
Putting spruce tree-equivalents in cold areas, plants with small hairy leaves in deserts and plants with large leaves in jungles makes those areas feel right to the reader, so you can save the weird stuff for where it matters. Getting the little subtle things correct keeps from jarring the reader out of your world, and makes the big things seem more believable.