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disturbances

A variety of human activities are responsible for the clearing of primary forest. Included in these are mining; plantation development; cattle ranching; commercial agriculture; subsistence farming; selective and clear-cut logging; large scale hydroelectic projects; planned and unplanned settlements and the development of supporting infrastructure (Kirby et al., 2006; Rudel et al., 2009; Barona et al., 2010; Lenzen et al., 2013; Barber et al., 2014). These disturbances encompass a variety of clearance methods and subsequent land use varying in intensity and extent. Secondary forest forms as a consequence of abandonment of these lands impacted by humans. Typically, the first 15 years of growth development in SF are characterized by rapid biomass accumulation

up to 100 Mg ha 1 _{(Brown and Lugo, 1990); after 15 years forest stands may diverge}

in the amount of biomass accumulation. This has been related to the history of disturbance. For example, Kellman (1970) found that slightly disturbed sites had higher biomass than severely disturbed sites. Large areas of forest considered to be primary or mature may be late secondary forests generated after historic land use practices such as shifting cultivation (Richards, 1955; Budowski, 1970; G´omez-Pompa and Vazquez-Yanes, 1974).

CHAPTER 2. BACKGROUND 24

2.5.1

Selective logging

Between 1999 and 2002 the area of land subjected to selective logging rose from 12,075

to 19,823 km2 _{in the top 5 timber producing states of Brazil (Asner et al., 2005). The}

main disturbance associated with selective logging comes from peripheral activities e.g. access roads and the removal of timber. The practice increases fire risk and reduces leaf litter nutrient content and mean canopy height (Villela et al., 2006). Regeneration in a↵ected areas is analogous to that which occurs in natural gaps in the canopy (Toledo et al., 2011b). However, West et al. (2014) showed that after 16 years of regrowth, areas logged using these methods had only recovered 77% of their original AGB compared to reduced impact methods which recovered 100%, similar to natural gaps.

2.5.2

Regeneration associated with shifting agriculture

Cleared land parcels rapidly lose productivity as the land is depleted of nutrients (Neill et al., 2013). In shifting agriculture the cycle is repeated, i.e. the plot is recultivated, after 1 to 12 years (Scatena et al., 1996). The fallow period allows rapid build-up of SOM (Don et al., 2011) and valuable nutrients (Szott et al., 1999). However, Sirois et al. (1998) concluded that the nutrient content of the soil decreased over an 8-year fallow period.

Growing populations have increased demand on these areas to sustain productivity. This has resulted in a less time between rotations and a reduction in productive capacity (Laurance et al., 2014; Palm et al., 2013). This causes further implications as the utilisation of marginal lands, which may not have been previously considered, increases (Laurance and Balmford, 2013). Such land often has steep slopes; poor drainage or nutrient poor soil. Regeneration on these lands will be slower than in other secondary forests (Feldpausch et al., 2004).

Nutrient limitation is a feature of Amazonian secondary forests where fire has been used to clear and manage vegetation, e.g. ‘slash and burn’ agriculture. Fertility is greatly reduced as soil minerals (nitrogen, sulphur, phosphorus, potassium, calcium and magnesium) are volatilised during burning (Mackensen et al., 1996; Sommer et al., 2000; Davidson et al., 2004). Much of the soil organic matter, sometimes up to 25% (Whitmore, 1990) is lost when fire is used as a clearance and management method.

CHAPTER 2. BACKGROUND 25 In addition to nutrient loss during burning, productivity diminishes as soil acidity increases. This occurs as aluminium (Al) becomes more soluble as cations are depleted. Phosphorus (P) quickly combines with Al making the P unavailable for plant growth. Native woody species can often cope with these conditions owing to mycorrhizal associations with their root systems (De Grandcourt et al., 2004).

Studies of areas previously cleared by ‘slash and burn’ and cultivated under shifting agriculture have shown the e↵ect of varying land use intensities on the AGB accumulation in SF. Zarin et al. (2005) found carbon accumulation in the BLA to be reduced by half after five burns (an intensive fire regime) compared to sites where fire hadn’t been used as a clearance tool. At sites in central Amazonia which were subject to first-cycle ‘slash and burn’ and long-term land use, biomass accumulation was in the form of a saturation curve. This indicated rapid initial biomass accumulation followed by a slow down later in succession (Gehring et al., 2005a). Biomass accumulation was reduced by 5 - 35% at long-term use sites which had gone through a second cycle of ‘slash and burn’ or extended cultivation (Uhl, 1987).

Hughes et al. (1999) found that carbon pool sizes were negatively correlated with the duration of the land use prior to abandonment. Kau↵man et al. (2009) found similar relationships between biomass stocks and accumulation rates with the length of cultivation period. Eaton and Lawrence (2009) noted a similar relationships with the number of cultivation cycles. Along with the age and region it significantly a↵ected the carbon stocks in live AGB and coarse woody debris, the most dynamic and the second largest pool of carbon. Both of these carbon pools declined with the number of cultivation-fallow cycles (Eaton and Lawrence, 2009). However, Hughes et al. (2000) and Steininger (2000) found that two to four cycles of shifting cultivation in Brazil and Bolivia had no significant e↵ect on the live biomass carbon pool.

In dry years there is often more burning (Mesquita et al., 2001) and less growth. Whilst

in wet years there is less and CO2 output to the atmosphere is lower and growth is

higher. This was noted north of Manaus where SF were subject to di↵erent clearance histories as a result of wet or dry years. This led to the formation of di↵ering species compositions or successional pathways (Lucas et al., 2002; Mesquita et al., 2001). E↵ects of the previous land use were manifested in structural di↵erences when compared

CHAPTER 2. BACKGROUND 26 to undisturbed mature forest. At first-cycle regrowth sites 60% of the biomass was found in single stem plants whilst only 31% of the biomass present in recovering long-term use sites was constituted of single stems (Gehring et al., 2005a). Long-term land use can result in a higher biomass share of lianas; lower biomass share of palms and very high biomass share of multiple-stem plants. The latter of which can be linked to an increase in vegetative resprouting (Mesquita et al., 2001, 2015).

2.5.3

Regeneration on clear felled logging areas

Clear fell logging is predominantly carried out for timber extraction. This land use has a low frequency of clearance as the large trees can only be felled once. Following clearing the subsequent land uses can vary. The cleared land is sometimes turned into cattle pasture which can result in further clearing and weed control through burning (Uhl et al., 1988). Crops are often planted and this can involve tilling of the land by heavy machinery. Generally the regeneration following abandonment of these lands is then commensurate with the successions described below. However additional compaction through the use of heavy logging machinery may further inhibit successful regrowth (Asner et al., 2004).

2.5.4

Regeneration on cattle pasture

Cattle ranching is sometimes unprofitable owing to external market forces (Mesquita, 2000; Sir´en, 2007) resulting in grazing land being abandoned after a few years. Natural vegetation re-colonises the area from any remaining seed trees (Whitmore, 1990). This regeneration is limited by soil water availability; nutrient deficiencies and denuded seed and seedling banks (Comita and Engelbrecht, 2009; Mesquita et al., 2015). ‘Slash and burn’ methods are often used in initial clearing the forest for cattle pasture and periodic weed control. Grazing can lead to an increase in SOM; soil carbon and pH (de Moraes et al., 1996; Neill et al., 1996). SOM is important for: soil structure and porosity which determine water infiltration rate; moisture holding capacity; diversity and biological activity of soil organisms; and plant nutrient availability (Bot and Benites, 2005). SOM is positively correlated with AGB accumulation following conversion (Laurance et al., 1999). The pH was found to be 1–2 units higher in pastures that were three to twenty years old compared to levels in primary forest (de Moraes et al., 1996). Typical pioneer

CHAPTER 2. BACKGROUND 27 genera such as Vismia and Ceiba are adapted to a pH that is up to 2.5 units less than that found in these pastures (Menyailo et al., 2003).

Soil condition and surface drainage decreases with pasture age. This is often the result of compaction which reduces air spaces within the soil and makes it more impermeable. This reduction of aeration and oxygen di↵usion decreases microbial activity and the turnover of carbon and nitrogen. Compaction can lead to increases in bulk density which in turn raises the soils resistance to root penetration (Martinez and Zinck, 2004). Soil erosion, linked to a reduction of the vegetation cover, is evident in cattle pasture leading to nutrient leaching during high rainfall (Nepstad et al., 1990, 1996). Despite these limitations BGB is recovered faster on abandoned cattle pasture than on sites subjected to shifting agriculture (Martin et al., 2014).

Some pioneer tree species, such as Cecropia are unable to regenerate vegetatively after cutting, e.g. grazing by cattle. Therefore it is dependant on the seed bank and dispersal from primary forest to re-establish. In contrast to Vismia and other resprouter species (Mesquita et al., 2001; Norden et al., 2011; Williamson et al., 2014). It could be argued therefore that this denudation of the seedling bank is be responsible for limiting species diversity in early successional stands. Secondary forests regenerating after repeated burning an mechanical clearance of tree roots before being turned into cattle pasture only accumulated 5% of that in sites that were abandoned almost immediately (Uhl et al., 1988).

Studies, such as those by Mesquita et al. (2001); Feldpausch et al. (2004, 2007); Norden et al. (2011); Williamson et al. (2014); Mesquita et al. (2015), have demonstrated that SF regenerating on cattle pasture accumulated AGB at a slower rate than forest regenerating on areas which had been clear cut or subject to ‘slash and burn’. The same was true for the recovery of other stand properties such as basal area and species diversity

2.5.5

Regeneration on croplands

It has long been a misconception that tropical rainforest soil fertility in the BLA would be sustainable for the cultivation of crops after removal of the forest. In some parts this has led to a shift towards large scale agriculture (Carvalho et al., 2002; Simon and

CHAPTER 2. BACKGROUND 28 Garagorry, 2005; Morton et al., 2006; Buys, 2007; Barona et al., 2010). A continuous cycling of nutrients into biomass above the ground gives the impression of a productive system. Removal of the original forest cover curtails the nutrient cycle between the soil and forest canopy. These are highly weathered soils which require continuous addition of nutrients to sustain their fertility for the production of crops as soil carbon is depleted (Steiner et al., 2007). This fall in productivity leads to abandonment (Silver et al., 2000a), allowing the regeneration of natural vegetation. However, the tree species diversity and AGB in the secondary vegetation is limited by cultivation practices, e.g. SF derived from abandoned co↵ee plantations, were less species rich than those derived from agriculture as only those species used as shade trees prevailed (Brown and Lugo, 1990). Few other studies have focused on the regeneration of forest cover after this land use.