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Global decline of large old trees from anthropogenic alteration has gained considerable interest globally and could resulted in losses of numerous wildlife species. In recent decades, about 10-40% of the global species are facing critical threat of extinction. Some believe that sixth mass extinction event is perhaps in progress. Ross et al. 20183 described anthropogenic-related habitat loss as the biggest cause of biodiversity loss on a global scale. According to Committee on the Status of Endangered Wildlife in Canada (COSEWIC), “488 species are classified as extinct, extirpated, endangered, threatened, or of special concern, ” with habitat destruction as the most significant factor threatening 84% vulnerable species in Canada. Due to its higher economic value, loss of large old growth tree populations from selective-logging practices is classified as major factor to habitat loss for several endangered species 5. Critical to reverse these losses is to recognize the significance of large old tree habitats, understanding drivers of their decline and its impact on wildlife species relying on them for nesting and feeding. This essay is an outcome of issues/controversy identified during the field visit to Haliburton forest and wildlife reserve and will review empirical literature on high value old-growth large trees and associated wildlife population. The essay will also look at different management policies for the protection of these remaining giant vertical habitats.
One of the oldest and largest living organisms on earth are known to be large old-growth trees. These giant keystone vertical structures play a key role in ecological functioning and provide range of ecosystem services such as providing habitats or protective cavities to approximately 30% vertebrates species in some environments, supply sufficient food to large number of animals in the shape of nectar, fruits, flower and foliage, assist in connecting ecosystems by drawing pollinator and seed dispersers and providing nesting to numerous wildlife. These giant trees also sequester massive amount of atmospheric carbon, provide micro-environments to various species and play a vital role in local hydrological functioning. The keystone function of large old trees continue for hundreds of years even after tree death and provide nesting services to numerous species. The mature tree begin to produce number of distinctive physical characteristics such as massive amount of coarse woody debris, organic matter, nesting, den and roosting cavities and dead branches. Formation of large old tree habitat structures require hundreds of years. For instance, it takes 120 to 220 years for the formation of cavities in Eucalyptus tree These cavities/hollows provides key facilities such as feeding, nesting, roosting, escaping from predators or hibernating to significant number of taxonomic groups including vertebrates, invertebrates and mammals.
Large old-growth trees are prone to multiple range of human induced threats such as forest cutting, infrastructure development (highways, roads, residential projects, pipelines construction), clear cutting and selective logging operation, farm land and grazing. Out of around 3. 04 trillion global number of trees, 15 billion trees are cut per year as per the recent estimates, indicating critical reduction of tree population of various genera including old-growth large trees. Interestingly, there could be key interactions between drivers threatening large old tree habitats. For instance, land-use changes like forest fragmentation, clear-cutting or selective-logging can interact to enhance the sensitivity of some habitats to deadly wildfire. Lindenmayer et al 201311, explained that in mid-boreal forest of Sweden, large trees with diameter at breast height (dbh) above 50 centimeters are all almost entirely disappeared despite of their large concentration in the forest a century ago. In North America, less than 1% distribution of large dead Douglas-fir (Pseudotsuga menziesii) with dbh of greater than 63. 5 cm is remaining in the logged areas. In Australia, abundance of large old trees stands in Mountain Ash forest is expected a massive drop in population from mean of 5. 1 of large old trees/hectare at 1998 levels to the mean of 0. 6/hectare in the next five decades. In Amazonian Brazil, studies predicted that 53% of large old trees with dbh greater than 60 cm have died in the past 30 years. In Canada, Red pine (Pinus resinosa) trees found in Wolf Lake forest reserves are the largest old-growth conifers trees in North America and key source of food for songbirds, chipmunks, mice and squirrels. However, pine trees are currently facing looming threat from industrial, mining as well as logging activities, with only 0. 6% of old-growth red and white pine trees are intact and remain in their range prior to the European settlements in North America.
York suggested that removal of large big trees will likely creates long-lasting negative impact especially on wildlife and biodiversity and further argued that in spite of historic exploitation focusing on removal of large trees and their possible long-term damaging effects, cutting of these giants is still widespread in United States. Vaillancourt et al. argued that in boreal conifer forest, present of large tree structures is being threatened by current even-aged silviculture system, hence wildlife dependent on these habitats is also likely to be affected by extensive logging operations. Furthermore, large old tree are threatened by various other natural and unnatural drivers such as wildfire, browsing by herbivores, drifting agricultural pesticides spray, removal of trees in urban environments due to concerns on human safety, several diseases and dieback syndrome.
The association of some wildlife species are so significant that presence of large old tree act as a cost-effective alternate for their existence and abundance. In Australia, 42% of the vertebrates and 28% of reptiles rely on large tree cavities. In USA, John et al. 2017, studied spotted owl (Strix occidentalis) population across territories which experienced massive logging of large trees against unlogged region, and found high extinction rates of owl population when owl territories contained less number of big trees (>61 centimeters dbh) and canopy cover (>70%), whereas increased concentration of big trees leads to decline in extinction rates. Bowman and Robitaille 199713 explained habitat loss as the reason of American martens (Martes americana) local extinction from historic territories in North America. Martens normally like large over mature conifer forests which are being extensively logged in several parts of North America. In Canada’s Vancouver Island, nearly all black bears (Ursus americanus) use dens in trees with mean dbh of >100 centimeters. Large trees such as Douglas-fir (Pseudotsuga menzieseii) are also being used by bats species for roosting. In northwest pacific region, species which use trees with dbh >50 centimeters including Black bear (Ursus americanus), long-eared myotis (Myotis evotis), American marten (Martes americana) and fisher (Pekania pennanti) are categorized as sensitive or at risk.
Old tree stands provide also microclimate and thermal benefits during the winter due to their dense and large crown span as well as multi-layered composition. In British Columbia, Armleder and Waterhouse 199415, studied winter habitat requirements by mule deer (Odocoileus hemionus hemionus), and found high concentration of deer activities in old Douglas-fir stands which provide them good snow protection as well as conifer foliage produced through litter-fall which cater 62-89% of the mule deer feeding requirements during the winter.
Large old trees are keystone vertical structures and one of the oldest living organism in the world. However, their massive population decline mainly due to anthropocentric activities such as logging, clearing, habitat fragmentation bring them at risk of short-term or even permanent extinction. There is an urgent need for new decision-making and management action to preserve existing large old trees. Lindenmayer et al suggested long-term management of large trees, preservation of the regions where they are most likely to evolve and promotion of recruitment should be included in new conservation policies. Long-term monitoring and documenting detrimental impacts of loss of large old trees and understanding their key ecological role is also critical.
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