Giant Powerhouses: Secrets of the Himalayan Oak Forests

The banj oak forest can be talked of as a cloud forest of sorts. At the mid-elevational altitudes, precipitation is triggered easily by the warm moisture-laden winds hitting cool forest surfaces in the Himalayas. Rajesh Thadani, an ecologist who has worked extensively on the banj oak believes that these trees, being deep-rooted, pull water from deeper soil layers and transpire even when it is relatively dry all around, say in a drought. This is believed to vastly improve the moisture status of the soil and humidity inside the forest. Every local farmer in these parts swears by the banj oak forests. They say that when banj oak disappears, so do the streams, springs, and waterfalls. The dense vegetation and leaf litter of a protected oak forest clearly plays a critical role in enhancing percolation of rainwater down into the soil, recharging subsurface springs and streams.

The leaves of the banj oak are also favoured by the locals for composting and fodder. Banj leaf litter makes a very good fertiliser because much less nitrogen and phosphorus is withdrawn by the tree at the time of leaf-fall, in a process called “nutrient retranslocation”, in comparison to other local species. This makes the fallen leaves of banj oak rich in nutrients. The concentrated fall of the old leaves late in the spring makes them easy to gather, and during this period, local villagers spend much of their time gathering head loads of banj oak leaves for composting — something that has kept the terraced fields of the region productive for centuries. It is interesting that langurs seem to love feeding and resting more in the banj oak forests rather than the adjacent pine forests, that are far more extensive. The quality of the leaves obviously has a lot to do with it. The oaks of the Western Himalaya are mostly evergreen, maintaining a dense canopy through the year as a strong flush of new leaves comes in before the old leaves are shed.

Unlike the tropical rainforest, which is celebrated much more, tree species per unit area are few — a maximum of twenty co-dominants across its range. But there are a large numbers of epiphytes, orchids, shrubs, saplings, mosses, lichens, which create a highly complex vegetation structure. Complex vegetation structure leads to high diversity of fauna due to the expanded niches available for them for nesting, roosting, and feeding. So many more insects can live beneath the bark of the hospitable banj tree to survive out the winter. There are so many more places for birds to nest. And diversity begets diversity: there are as many as eight species of woodpeckers that occur here, each creating the cavities that provide homes for secondary cavity-nesters such as the Asian barred owl, verditer flycatcher, slaty-headed parakeet, the white-tailed nuthatch, the large brown flying squirrel and numerous other forest denizens. Maybe this is why, as many as 104 bird species (both year-round residents and summer visitors) breed in this narrow slice of the Himalayan forests during the summer; and this approximation is from a single site in Kumaon, the Mukteshwar-Maheshkhan area. Incredibly, numerous bird species are known to persist through winter, testifying to the ability of the banj oak forest to support insect fauna and other resources even during the coldest part of the year. Part of the answer may be the staggered fruiting of various shrubs and trees through the winter. So little is known of the extent to which animals, birds, and insects depend on the oak acorn. But when you see Eurasian jays and white-throated thrushes feasting on this bonanza in winter, you can see how important those resources are when little else is flowering or fruiting in the harsh weather.

There are also leopards and bears here. The bears apparently come for the feast of acorns in early winter, and the leopard comes for the sparse numbers of sambar and barking deer that still exist in these parts. Up to the early twentieth century, the presence of the tiger in these parts was testified to by Jim Corbett’s famous book Temple Tiger of Mukteshwar, possibly that was the upper limit of the tiger’s range in Kumaon even at that time. Our camera traps have revealed a number of small carnivores such as the Himalayan weasel, leopard cat, yellow-throated marten, and the masked palm civet.

Banj oak is often compared with its competitor, the chir pine, at the same altitude — a tree species that is rapidly becoming much more common in recent times than it ever was. Pine is known to be a pioneer species that easily colonises landslides and other open areas, but gives way to diverse hardwood stands over time. But here in the Western Himalaya, things are working in quite the opposite way. Both scientists and locals believe that pine is actually taking over oak forests over time as over-extraction of oak leaves and wood, frequent fires, and climate change are creating conditions that favour pine over banj oak. Local people, however, value the banj oak ecosystem far more than pine, for the multifaceted benefits it brings them over time, a true keystone species. Chir pine brings nothing but resin and timber, but the oaks bring fuel wood, fodder for their cattle, fertiliser for their fields, good water supply, and innumerable forest products.

But why is the banj oak so sensitive to the takeover by pine? To understand this, it is necessary to look at oak regeneration and seedling/sapling growth at close quarters. Oak acorns are highly packed bundles of energy that are collected and consumed by mammals and birds, but large numbers are hidden away for future famine situations as well. As banj acorns lie dormant for several months, the site where they finally come to rest is important. Strong, direct sunlight for even a few days can desiccate the acorns and render them unviable. Acorns germinate only in soils that are wet enough and sites that are well-shaded and rich in nutrients.

Once germinated, oak seedlings can lie in wait for years to grow into tall saplings. Oak seedlings survive and grow best in moderate shade, which can be found at forest edges, under a high canopy, in a forest gap, or even in a lightly lopped forest. Sometimes an unusually long drought period or hot summer can kill this tiny seedling within the first year itself, unless it is protected by the moist cocoon of leaf litter and soil. Growth rates are low, so a six-inch oak sapling that you spot in a dense forest may be more than ten years old! Now if a passing cow or deer takes a fancy to it and takes a bite out of it, and if this happens a few times, the oak seedling will turn into a coppice, a stunted bush, as new branches sprout to take the place of the lost leading tip. However, if the coppice is protected, one leading tip will gradually emerge and the tree will start growing in height again. This is easily evidenced by the extensive regrowth of banj oak trees in protected community forests and on abandoned agricultural land, seen over the last two decades.

It is frightening to think of how climate change is going to affect this keystone tree species of the Himalayan mid-hills. Warming will likely reduce both soil moisture, and forest humidity…and give the pine seedlings an edge over oak seedlings, particularly at the forest edges. The frequency of forest fires also seems to be increasing in the middle and lower Himalayas, which only adds to the negative effects on soil, flora, and fauna.

The rapid pace of unplanned development in the Himalayas does not bode well for this keystone species. Three processes are proceeding in parallel that threaten forests. Most visible and pervasive is the suburban expansion of resorts, cottages, and educational institutions into forest boundaries. Another stressor is the degradation of forests due to overuse, increased tourist visitation, and road-widening. All these problems are only compounded by the effects of global warming, known to be more adverse in the Himalayan region than elsewhere. This has been observed to reduce oak expanse and increase pine forest over the last few decades. There also seems to be a vicious cycle of reducing moisture in the forest soils as more and more springs and streams are diverted for human use. Such water diversion has been observed to dry out younger trees and increase seedling death — effects that no one has studied so far.

The increased vulnerability of banj oak to all the changes around it lead to further questions about what happens to the animals that depend on it. Our research shows that there are at least forty-five species of birds that stick to the security of the oak forest, shunning the pines, the orchards, and the fields; at most popping out for a quick snack to fruiting trees on forest edges. This group includes the wedge-tailed green pigeon, the chestnut-crowned thrush, the brown wood owl, the greater yellownape (woodpecker), the black-faced warbler, and the rufous-bellied woodpecker, birds that you are unlikely to see unless you spend some time in the forest. These are the forest specialists we now have to look out for, as they are most vulnerable to forest conversion and modification at this altitude.

Birdwatchers believe that warming has led to upward movement of several bird species that were earlier restricted to lower altitudes. But the slow-growing, large-seeded banj oak does not or cannot migrate quite so fast, nor can its bird fauna. So the resident oak forest birds will likely face competition from recent arrivals from lower altitudes. What will be the outcome of such competition? Only time will tell.

We are only beginning to understand this fascinating forest ecosystem, all the food-webs it supports, and more importantly, the ways it is responding to climate changes and other stressors. Being the fount of immense ecosystem services, banj oak forests must be better studied and protected if we are to save the mosaic of biodiversity, livelihoods, and commerce, that continue to be tightly interconnected in the Western Himalaya.