All trees renew their foliage – even ‘evergreen’ conifers drop their needles after a few years. Some, however, shed them dramatically in a very short time and for various reasons. The famous Northern autumn ‘leaf fall’ for native trees such as the Sugar Maple are guided by day length and explains why newer introductions, such as the Norway Maple, don’t drop their leaves until December (or when there is an early cold spell).
Here in the warmer climates of Costa Rica, the native Ceiba tree in front of the house will shed its leaves and, two weeks later, the entire canopy of foliage will have been replaced with new leaves.
Trees which do this in hotter climates primarily shed them in dry season so that any disease present is removed, and the new leaves emerge in dry conditions, which fungi, in particular, find difficult to live in. The new extension growth of the limbs also occurs at this time and has immediate access to the nutrient resources which were gathered and stored when shutting down the old leaves. This primes the new growth to rapidly emerge and establish, enabling defense systems in the new growth to mature quickly.
Something different has happened this year with the Ceiba’s timing of leaf drop. Last year, December was hot and dry, and the leaf fall was completed by the end of the month. This year has been much more temperate, with cloudy conditions on most days and even periodic light rains. Still now in February most of the leaves remain!
The exception, as this picture shows, are the leaves on last year’s new growth – they have all been released and the stems are bare. This is because the coming extension growth will come from these limbs and, in the absence of old leaves, two benefits occur: First, there is no source disease near the new leaves and second, the recycled nutrient resources are waiting within the extension buds.
My guess is that the tree has decided that, unlike the dry December of last year, this year it will retain the old leaves as it is still finding enough water to provide sufficient cooling through transpiration and thereby keep the mass of leaves productive. This tells us that the tree actually has the ability to respond to seasonal change and is not guided by day-length or programmed timing of leaf replacement. In its early years after planting, it prioritized creating an extensive root system and, as in all successful trees, this gave it the foundation needed to find water and thus be able to adapt to seasonal demands.
Each season that leaves fall, they become organic matter which feeds the soil. The world’s soils are always in need of organic matter and so, especially in city soils, it is vital that the leaves are mulched and placed on soil near the roots. This organic matter enables the mycorrhizae to thrive and further enhances the ability of the root system to obtain all that the tree needs – no matter what the seasonal demands and variations are.
And as the climate changes become more dramatic and unpredictable, so too the ability of the symbiotic relationship between the plant’s roots and the mycorrhizae will be the resource which enables nature to adapt to the extremes of city life. Leaves are the food for the soil and a principle means to achieve the 5% organic matter necessary for healthy soils.