To Plant, or Not to Plant? - A Reforestation and Afforestation Dilemma?

There's a great post covering deforestation over at Harmonising Humans & Nature, so I won't bore you with the facts. There's no way we can deny that deforestation is unsustainable and having a vastly negative impact upon the Earth's ecosystems; 70% of the worlds land organisms live in forests, yet deforestation destroys the habitats of these plants and animals (National Geographic). With all this said, it seems that right thing to do would be to stop cutting down the trees and re-plant them; either where they once stood, or elsewhere.

Sadly, things are never as straight forward. Planting trees or letting natural succession take place until areas once wooded long ago are reforested may, in some areas, impact negatively on biodiversity. Planting forests where there were once no forests (afforestation), to replace those lost elsewhere, can also have detrimental effects - often more so than reforestation.

We humans have been shaping and leaving our footprints upon this planet for tens of thousands of years, and animals have partly evolved to coexist with us. Landscapes have been anthropogenically modified in Western Europe, through deforestation and agriculture, for at least 4000 years; in Asia this modification is likely to have been going on for even longer (Anson Mackay, 2013). This causes a dilemma, as though many argue for returning habitats to their 'pre-human' state, the animals now living in these 'new' habitats may have adapted to them, or may not be the original organisms that previously lived in the area.

 Rannoch Moor, Scotland; A natural peat-land, though many bogs are present due to human modification and management of the landscape in the northern hemisphere  - image by Martin Sojka on Flickr.

Reforestation and afforestation are generally beneficial; both can increase biodiversity, have economic benefits (such as providing timber), can reduce soil erosion and run-off, and reforestation can help restore natural habitats (Reino et al, 2010). However, both may also have negative impacts on plant and animal species that have adapted to a woodland-free environment. 

After the abandonment of traditional mountain agriculture in many regions, natural reforestation can lead to a loss of open spaces which has been linked to a decline in grassland plant species that are vulnerable to landscape fragmentation (Sitzia et al, 2010). Though reforestation is beneficial for alpine birds whose species often experience population increases, Sitzia et al, (2010) noted that landscape fragmentation can act negatively upon species unable to live in small, patchy habitats.

Two studies into the effects of anthropogenic afforestation of Mediterranean farmland in the Iberian Steppes of Spain show the negative effects that afforestation may have in certain regions, including the direct loss of habitat for species of conservation concern, and increase in the abundance of predators (Reino et al, 2009)(Reino et al, 2010).

Whilst the planting of forests may lead to an overall increase in bird abundance and diversity in farmlands adjacent to forest patches, the overall increase is at the expense of grassland specialists (Reino et al, 2009). Steppe birds show strong negative edge effects, with the abundances of Calandra Larks and Short-toed Larks in particular depleted in grasslands directly adjacent to forest plantations. The abundances of these species increased further away from the edges of the plantations, and the highest species abundance and richness were observed in large arable patches. Though both species were listed as 'Least Concern' in the latest IUCN Red List report, their populations are in decline (BirdLife, 2013 - Calandra Lark)(BirdLife, 2013 - Lesser Short-toed Lark); habitat loss due to afforestation, which has already been accredited to regional declines of Lesser Short-toed Larks, may lead to greater pressure upon both species.

As the number of forested patches increased, Reino et al (2010) also documented an increase in the abundances of generalist predators, such as foxes and corvids (crows, ravens, magpies...); the increases of which have been associated elsewhere to population declines of ground-nesting birds (Fletcher et al, 2010).

Increased nest predation by predators such as foxes may lead to significant population declines of ground nesting birds, such as Eurasian Curlews in Scotland and Northern England (Douglas et al, 2013) - image by Jans Canon on Flickr.

A recent paper by Douglas et al (2013) on upland land use and declining waders populations highlights another case where afforestation and the fragmentation of moorland has been linked to increased predation rates, this time leading to a decline in breeding populations of the Eurasian Curlew (Numenius arquata), a bird listed as 'Near Threatened' in the latest IUCN Red List due to a 20-30% global population decline in the last 15 years (BirdLife - Eurasian Curlew, 2013). Both Douglas et al (2013) and Fletcher et al, (2010) also noted that predator control can help prevent population declines in species of conservation concern; for Lapwings, Curlews, Red Grouse and Golden Plovers, predator control lead to an observed population increase of ≥14% per annum, whilst a lack of predator control saw populations decrease by ≥17% per annum (Fletcher et al, 2010). 

The Eurasian Curlew, Numenius arquata - image by Davis Kwan on Flickr.

Though in the study by Reino et al (2010) it was concluded that nest predation rates (on artificial nests) showed no relation to predator abundances in the Iberian Steppes, they suggest that landscape composition and configuration may instead have a stronger impact due to edge density effects.With this, they recommend that afforestation in the Mediterranean -and other areas inhabited by ground-nesting birds of conservation concern- be avoided where possible. Where afforestation is unavoidable, they insist that populations be monitored closely for signs of declining numbers due to predation. They also suggest aiming to reduce habitat fragmentation and edge effects by afforesting a smaller number of large patches, rather than planting many relatively small patches (Reino et al, 2009).

References:

• Biodiversity and Landscape Change (Lecture) - Anson Mackay, 2013