Selective tree felling and fragile forests

Forests cover 31 per cent of our planet’s land and harbour about 80% of terrestrial plant and animal biodiversity. While deforestation is happening at an alarming pace, there is another innocuous form of degradation that often goes unnoticed—selective logging.

Selective logging is a practice where individual, commercially important trees are felled instead of an entire swath of forests. This practice is prevalent in the tropical and subtropical regions. The impacts of this are thought to be lower when compared to other forms of deforestation, such as clear felling.

However, there is increasing evidence that selective logging alters the composition of plants, the microclimate and the availability of resources for various creatures that live in these forests. Studies also indicate that the impact of selective logging in the tropics is nearly 20 times greater than outright clear felling of the forest itself.

Determining the extent of damage to forest ecosystems from selective logging is difficult because such activities go undetected in satellite imagery. Subtle changes to aspects of the forest ecosystem, such as microclimate, can ultimately affect the proportion of the environment available for an organism to use. These changes can accumulate over time and space to result in long-term effects on populations of these organisms.

The Himalayan example

The Himalayan region is a biodiversity hotspot, housing about 20 habitats ranging from broad-leaved forests to snowcapped mountains, with several hundreds of species found nowhere else on Earth. The forests here have been rapidly declining due to various human-centric activities, including expansion of agriculture, shifting agriculture, and development in the form of roads or construction of dams in ecologically fragile habitats.

The development has not spared even the biodiversity hotspot of the Eastern Himalayas. The Global Forest Watch’s recent assessment in 2019 estimates that nearly 11,400 km2 of forests were lost in this region.

In a recent study, a team of researchers explored if selective logging has affected birds in the forests of the Eastern Himalayas. Spending several months in the remote jungles of the Eaglenest Wildlife Sanctuary, Arunachal Pradesh, they began by documenting changes to microclimate and arthropods (insects, spiders and other invertebrates having a segmented body).

They also explored the behavioural responses among birds in selectively logged areas compared to intact forests. Birds consume a variety of arthropods apart from fruits. Since logging activities can change the forest structure, there will be changes to the arthropod abundance and diversity. The birds that feed on them might have to search more for food. Some birds may move away from selectively logged areas when they do not find food.

To measure the microclimate differences between unlogged and selectively logged forests, they placed automated data loggers to record temperature and relative humidity changes every half hour over a 24-hour cycle.

Changes to microclimate

The selectively logged forests were, on average, warmer than the unlogged forests, with the peak temperature being nearly 3°C higher around mid-day compared to the unlogged forest. However, the temperature fluctuation was more variable in the logged forests than in unlogged forests.

Warmer temperatures are typically inversely related to the relative humidity. In the forests of the Eastern Himalayas, the logged areas typically experienced 20% lower relative humidity. The authors examined how these microclimatic differences may also affect the arthropod density. They found that the arthropod density found on plants was higher in the unlogged forest compared to the logged areas.

No such difference was detected among arthropods that are found on the ground. They also noticed that the arthropods capable of flight were higher in the logged forest, possibly because of the gaps in the otherwise continuous canopy.

Foraging success of birds

Animals make conscious decisions while foraging for food. Foraging success is measured as a proportion of the number of attempts made and instances of successfully capturing prey.

“We observed 2106 individuals of 97 species of birds but used a subset of this data, selecting birds that were seen at least ten times, for comparing foraging success,” explains Kanika Aggarwal, a Master’s student from Banaras Hindu University and who led the study that was published in the Journal of Forest Ecology and Management.

“Contrary to our expectations, insectivorous birds spent nearly 6% more time foraging in logged forests than unlogged forests. Foraging success also depended on the species, body size, how and where they forage,” she explains. This study was supervised by Dr Umesh Srinivasan, an Assistant Professor at the Centre for Ecological Sciences, Indian Institute of Science (IISc).

The authors compared the success of foraging among the birds. Birds that fly off from a perch, catch an insect in flight, and return to the perch are called ‘salliers’; those that search among leaves and pick up insects are called ‘gleaners’.

“Larger-bodied salliers had greater success in logged forests than unlogged forests, whereas gleaners had lower success in logged forests,” explained Aggarwal.

“We had observed a greater number of flying insects in logged forests, and it makes sense that birds that sally have a greater success,” she added.

The study demonstrates how even cutting down a few specific trees can leave lasting impacts on an ecosystem. The ability to forage is an important attribute affecting an organism’s ability to feed and reproduce. A bird foraging for longer periods means more chances of mortality due to being less vigilant about predators.

The impacts of selective logging persist for several years after logging activities are stopped. Regulating logging activities and restoring degraded forests could be effective strategies to mitigate the unintended consequences of selective logging.

(The author is an ecologist and faculty at ATREE)