Effects of phenological patterns and microclimate on litterfall of five shade trees in cocoa agroforestry systems in dry and moist semi-deciduous ecological zones of Ghana

In the heart of Ghana's cocoa country, the landscape is a patchwork of small farms where farmers grow chocolate's key ingredient under the protective canopy of taller trees. These shade trees are more than just a backdrop; they are a vital adaptation strategy. As global temperatures rise and rainfall patterns become erratic, these trees help cool the cocoa plants, shielding them from heat that can stop them from growing. But the relationship between the trees and the climate goes deeper than just shade. The trees themselves are living indicators of the weather, dropping their leaves, flowers, and fruit in response to the seasons. This falling debris, known as litterfall, is not merely waste. It is the engine of the forest floor, a slow rain of organic matter that breaks down to feed the soil, recycle nutrients, and store carbon. Understanding how climate change alters this natural cycle is crucial, because if the timing or amount of this leaf drop changes, the entire health of the farm could shift.

Researchers from Ghana and Denmark set out to understand exactly how changing weather patterns influence this cycle. They focused on five specific shade tree species commonly found in cocoa farms: Milicia excelsa, Terminalia superba, Terminalia ivorensis, Persea americana, and Morinda lucida. To get a clear picture, they compared two distinct environments in Ghana's Ashanti region: a drier semi-deciduous zone near Offinso and a moister semi-deciduous zone near Adansi Asokwa. In each location, they selected six cocoa farms that already hosted these trees. The team did not just guess at the weather; they installed automatic weather stations to measure temperature, humidity, rainfall, and sunlight both above and below the tree canopies. On the ground, they placed sixty specialized traps—essentially open nets suspended one meter above the soil—to catch every leaf, twig, flower, and seed that fell from the trees over the course of a year.

The study revealed that the trees respond to their local climate with remarkable precision, though not all trees react the same way. Two species, the avocado tree (Persea americana) and Morinda lucida, remained evergreen, keeping their leaves year-round in both the dry and moist zones. However, the timing of new leaf growth for the Terminalia ivorensis tree depended heavily on the rainfall. In the wetter zone, this tree began sprouting new leaves in March, but in the drier zone, it waited until May, delaying its growth until the rains were more reliable. The flowering and fruiting patterns also showed a split personality. The Milicia excelsa tree, for instance, produced flowers and fruit in the moist zone during the early part of the year, but in the drier zone, it failed to flower or fruit at all. This suggests that the drier conditions simply did not provide enough resources for the tree to complete its reproductive cycle.

When the researchers measured the total amount of litterfall, a clear difference emerged between the two zones. The moist semi-deciduous zone produced significantly more falling material than the dry zone. The avocado tree and Morinda lucida were the most prolific contributors in the moist area, dropping nearly 47 tons and 45 tons of material per hectare each year, respectively. In contrast, the Milicia excelsa in the drier zone produced the least, at roughly 25 tons per hectare. The data showed a strong link between the weather and this leaf drop. When rainfall and humidity increased, the amount of litterfall decreased. Conversely, when temperatures rose, the trees dropped more leaves. This pattern was most visible during the dry seasons, where higher heat and lower moisture triggered a surge in leaf shedding, likely as a way for the trees to conserve water.

The findings paint a picture of a delicate balance. In the drier zones, the trees are more sensitive to the lack of water, often delaying their growth or skipping reproduction entirely. In the wetter zones, the abundance of moisture allows for a more consistent cycle of growth and shedding, resulting in a richer layer of organic matter on the forest floor. The researchers found that while temperature had a weak positive link to litterfall, the drop in rainfall and humidity was the strongest driver for reducing the amount of material falling to the ground. This means that as the climate shifts and becomes drier or more unpredictable, the rhythm of these farms could change. The trees might drop their leaves at different times or in different quantities, which would alter how nutrients are recycled in the soil. For the farmers who rely on these systems, understanding these subtle shifts is not just an academic exercise; it is essential for maintaining the fertility of their land and the productivity of their cocoa crops in a changing world.