Table 2.1: Comparison of successional stages of tropical forests under several proposed classification schemes (Franklin, 2003).
Time since Budowski (1965) G´omez-Pompa and V´azquez-Yanes (1985) Finegan (1996) Oliver and Larson (1990);
disturbance (years) Chazdon (2008)
0–1 Pioneer Herbaceous phase Herbaceous/ shrub/climber stage Stand initiation stage
1–3 Shrub stage
3–15 Early secondary Pioneer tree stage Short-lived pioneer stage
20–50 Late secondary Secondary tree stage Long-lived pioneer stage Stem exclusion stage 30–80 Mature tree stage Recruitment of shade tolerant tree species Understory re-initiation stage 100–200 Climax
>200 Old-growth stage Old-growth stage
2.2
Importance of tropical rainforests
Quantifying the global carbon cycle (GCC) is a critical element in controlling and reducing the widely acknowledged changes occurring in the Earth’s climate through the
CHAPTER 2. BACKGROUND 15
Table 2.2: Processes of vegetation dynamics associated with stages of secondary succes- sion in tropical forests (Chazdon, 2008). Successional stages based on the framework of Oliver and Larson (1990).
Stand initiation Stem exclusion stage Understory re- initiation stage Old-growth stage Germination of seeds in seed bank
Canopy closure Mortality of canopy trees Mortality of pioneer cohort in canopy Resprouting of rem- nant trees High mortality of lianas and shrubs
Formation of small canopy gaps
Range of gap sizes Colonisation of short-
and long-lived pioneer trees
Recruitment of shade- tolerant seedlings, saplings, and trees
Canopy recruitment and reproductive maturity of early- colonising species
Recruitment of shade tolerant and gap re- quiring canopy species and emergents Rapid height and
diameter growth of woody species
Growth suppression of shade intolerant trees in understory and sub- canopy
Increased heterogene- ity in understory light availability
Spatial heterogeneity in biomass and micro- topography High mortality of herbaceous species High mortality of short-lived, pioneer trees
Seedlings and sapling establishmentof shade tolerant tree species
Large woody debris
High rates of seed pre- dation
Dominance of long- lived pioneer trees
Tree recruitment of early-establishing shade tolerant species
Maximum diversifica- tion of trees and epi- phytes
Seedling establishment of shade tolerant tree species
Development of canopy and under- story tree strata Seedling establishment of shade tolerant tree species
Figure 2.2: Sequence of tropical forest succession regenerating on abandoned land pasture in Amazonia (Lucas et al., 2004).
CHAPTER 2. BACKGROUND 16 2013). Plants, primarily forests, have the largest fluxes of carbon to and from the atmosphere (Table 2.3). Alongside land use change (e.g. deforestation), these elements of the GCC are arguably one of the easiest for us to influence. Therefore, projects such as the United Nations (UN) initiative for Reducing Emissions from Deforestation and Forest Degradation (REDD+) are aiming to create conditions to minimise emissions due to land use change and disturbance. In the BLA tropical forests represent 25 % of intact forest landscapes globally (Hansen et al., 2013) and therefore play an important role in the GCC.
Table 2.3: Summarised sources and sinks of carbon globally (Qu´er´e et al., 2015) Stored (GtC) Flux into (GtC yr 1) Flux out from (GtC yr 1) Di↵ (GtC yr 1)
Crust 10⇥107 - - -
Ocean 10⇥107 92 90 +2
Plants 560 122 60 (atmo) 61 (soil) +1
Soil 1500 61 60 +1
Land use - - 3 -1
Fossil Fuel - - 7 -7
Atmosphere 750 220 214 +6
These ecosystems support high levels of biodiversity, including 93,500 plant species in the tropical forests of South America alone (Primack and Corlett, 2005) and 40,000– 53,000 tree species across the tropics (Slik et al., 2015). In addition to these valuable assets, tropical rainforests provide a range of ecosystem goods and services. They refer to the supply of valuable products and materials (including agricultural, forest, mineral,and pharmaceutical commodities); the support and regulation of environmental conditions (through processes like pollination, flood control, and water purification) and the provision of cultural and aesthetic benefits (including ecotourism, heritage, and sense of place) by ecosystems (Powledge, 2006).
2.2.1
Importance of regrowth forests
Globally there has been a loss of 2.3 million square kilometres of forest cover from 2000 to 2012 (Hansen et al., 2013). Whilst there has been a gain of 0.8 million square kilometres of forest cover in the form of secondary forest and plantations for the same period. In Brazil, de Almeida et al. (2016) showed that in 2012 the BLA was comprised of 4 % secondary forest primarily due to practices such as shifting agriculture. This approximates to 200,000 ha of regrowth. Assuming an average SF age of 10 years
CHAPTER 2. BACKGROUND 17
and an average AGB accumulation rate of 6.1 Mg ha 1 yr 1 (Poorter et al., 2016)
this equates to 12.2 million Mg ha 1 or approximately 1.9 % of the total AGB of the
Amazon (⇡640 million Mg ha 1, Avitabile et al., 2015). If these regrowth forests attain
an AGB equivalent to mature forest median of the BLA (252 Mg ha 1, Avitabile et al.,
2015) these forests would account for 6.3 % of the Amazon’s AGB. This demonstrates secondary forests current and potential contribution to the carbon sink capacity of the BLA. In fact, forests regenerating in tree fall gaps, have already been shown to be a net carbon sink in the BLA (Esp´ırito-Santo et al., 2014).
In some studies (e.g. Uhl et al., 1988; Mesquita et al., 2001; Wandelli and Fearnside, 2015), the capacity of these forests to accumulate biomass was found to be highly dependent on the intensity of land use prior to abandonment and they had yet to attain the same AGB as primary forests. The land use intensity is therefore crucial to understanding the successional ecology of these forests for determining their AGB accumulation potential. By determining the area and age of secondary forests, the carbon sink potential for the deforested land area can be established. This is in relation to the ability of forests to accumulate biomass regardless of whether they currently support regrowth forest. Such information can provide a vital and highly valuable planning tool for restoring forests through a range of initiatives relating to sustainable land use; Convention on Biodiversity (CBD) and REDD+ (Campbell et al., 2009). In particular, Target 15 of the Aichi CBD aims through conservation and restoration to enhance ecosystem resilience and the contribution of biodiversity to carbon stocks. This includes restoration of at least 15% of degraded ecosystems, thereby contributing to climate change mitigation and adaptation (Aichi, 2011).
The presence of early secondary forest restores soil organic matter (SOM) pools (Aweto, 1981; Lugo et al., 1986) through above and below ground (root turnover) litter inputs. This makes conditions more favourable for the species which succeed the pioneer species. In addition, forest regrowth benefits many other ecosystem processes enabling recovery of other forest parameters besides tree growth and diversity (Table 2.4).
CHAPTER 2. BACKGROUND 18 Table 2.4: Benefits of forest regrowth to the forest ecosystem following deforestation and abandonment.
Characteristics Source
Allow recolonisation of mycorrhizae after agriculture Bini et al. (2013); Ewel (1986)
Restore soil organic matter (SOM) Bini et al. (2013); Don et al. (2011); Martin et al. (2014) Restore soil nutrient levels Bini et al. (2013); Gehring et al. (2005b)
Biomass accumulation by tree growth can counter losses from disturbances Esp´ırito-Santo et al. (2014) Facilitate persistence of forest species in human-modified landscapes Chazdon et al. (2009)