SDTF
A high degree of structural degradation is reported for SDTF globally (Miles et al. 2006), particularly for forest tracts that grow in topographic, edaphic and seasonally optimal conditions for agriculture, including a large propor- tion of Neotropical SDTF (Sànchez-Azofeifa and Portillo-Quintero 2011). Quantification of the extent and degree of fragmentation is limited for this biome given that these systems often represent the first frontier of agricul- tural development and are usually afforded low conservation priority rel- ative to their moist counterparts (Sànchez-Azofeifa and Portillo-Quintero 2011). This is of concern to the future management of SDTF, as land con- version, fragmentation and degradation are considered to be key drivers of change in these systems (Miles et al. 2006). Fragmentation can impact on the resilience of these systems with respect to 1) remnant tracts of frag- mented forest, and 2) the regeneration capacity of moderately and highly degraded forests. These are discussed below.
Fragmentation of SDTF occurs from processes such as local land use con- version, resource extraction and alteration of natural disturbance regimes (particularly with regard to fire) (Pulla et al. 2015). One of the most sig- nificant impacts of fragmentation on remaining SDTF patches is the alter- ation of local and regional climatic patterns (Sternberg 2001). In Amazonia, for example, widespread conversion of forest to pasture and crop land has resulted in higher albedo, reduced evapotranspiration and ground humid- ity (Coe et al. 2013), increased seasonality (Costa and Pires 2010), and de- creased precipitation (Knox et al. 2011) at forest/cleared land boundaries. In the long term, these processes serve to reduce overall forest carbon stocks (Chaplin-Kramer et al. 2015).
Changes in localised microclimates in Neotropical SDTF due to forest degra- dation have been shown to have important implications on plant function- ing, including reduced leaf life span and changed timing of flowering (Que- sada et al. 2011). This latter phenomenon has been shown to have impor- tant implications for pollinators that are dependent on sequential flower- ing, hence potentially reducing reproductive output and genetic diversity in the long run (Quesada et al. 2009, 2011).
Fragmentation, therefore, may play an important role in both 1) encourag- ing a forest to savanna state shift (particularly when forest ecosystems have lowered internal resilience due to reduced ecosystem functioning and/or are already approaching a precipitation or seasonality threshold), and 2) re- inforcing this state change, especially at the forest fringe (Sternberg 2001, Mayle and Beerling 2004).
High-level degradation (i.e. near-complete clearance) is thought to have long-term impacts on the future ecological expression of land previously colonised with SDTF, even if left to regenerate. This is largely due to the fact that processes associated with intense clearance will not permit regrowth of secondary forest. These include depletion and alteration of soil nutrient pools (García-Oliva and Jaramillo 2011), reduction in soil moisture content via compaction, increased runoff and soil erosion, reduction of biodiversity
(Maass 1995) and alternation of forest/soil moisture feedbacks (Coe et al. 2013). Combined, it has been speculated that these factors should favour the persistence of open formations (grassland or shrubland) (Maass 1995, Menaut et al. 1995). However, a number of studies reviewed in Pulla et al. (2015) regarding the recovery potential of SDTF following a range of often persistent disturbances of various types and magnitudes (including swid- den agriculture, pastoral land conversion, logging and natural disasters) indicate that such drivers are commonly insufficient to cause a fundamen- tal state shift to savanna (or any other habitat types). The authors use this body of work to emphasise the role of reinforcing parameters (e.g. climate, edaphic conditions and ecological memory – noteably the capacity of early succession SDTF species to resprout) in maintaining the resilience of these systems. Given this, it is also important to note that the ecological charac- teristics of extant tracts of SDTF may, in themselves, be representing of past disturbances e.g. (Mayle et al. 2007).
SASDTF
SASDTF units growing in soils suitable for agriculture (typically secondary semi-evergreen and some mixed deciduous types) have long been subjected to traditional swidden farming practices that are, in turn, inherently linked to the fire dynamics of the forests (Heinimann et al. 2007). The capacity of land disturbed by this practice to regenerate depends on factors such as fallow length, intensity and frequency of disturbances such as fire, site conditions (including long-term impact on soil compaction and quality), ecological memory and presence of wildlife (Heinimann et al. 2007, Tee- galapalli et al. 2009).
Until recently, disturbance associated with traditional short-cultivation, long- fallow cycle slash and burn techniques have seemingly been able to coex- ist with SASDTF dry forest succession (Boyd and McGrath 2001, Maxwell 2001, 2004, Heinimann et al. 2007). This is potentially due to the fact that disturbance associated with traditional techniques was mild to moderate in intensity, frequency and patch-size, allowing secondary forest regeneration post-abandonment to draw on both internal and external memory. While the impact of disturbance on plant functional traits has not been well quan- tified in SASDTF, work within Indian SDTF that host some similar species to SASDTF may shed some light on species- and community-level plant functional traits that contribute to ecological resilience to disturbance. In general, disturbance regimes (in association with climate) play an impor- tant role in shaping diverse physiognomic forest structures (canopy cover and height) across an ecoregion (Sapkota et al. 2009, Ramesh et al. 2010). Additionally, post-disturbance succession has been shown to result in the procession from clumped to uniform population distributions (Sapkota et al. 2009). Clumping of key species within Thai SEDF has also been ob- served after what is postulated by to represent a large-scale, severe distur- bance event, though this is attributed to edaphic and topographic habitat specialisation (Bunyavejchewin et al. 2003).
Chapter 2. Threshold dynamics of seasonally dry tropical forests 29
Sapkota et al. (2009) demonstrate that mild to moderate level disturbance can increase species regeneration (vs. undisturbed or heavily disturbed sites), and is, in fact, considered important for supporting key SDTF species (Shorea robusta (Roth)). However, it is important to note that this does not apply to all forest species (notably Haldina cordifolia ((Roxb.) Ridsdale) and Terminalia tomentosa (Sapkota et al. 2009). Plant traits, including seed size, have been found to be important for determining regeneration following mild to moderate disturbance, with large seeded species more likely to re- generate following low level disturbance (Khurana et al. 2006). Some large seeded species discussed in this study that co-occur in SASDTF include Ter- minalia tormentosa, T. chebula (Retz.), Bauhina racemosa (Lam.) and Diospyros montana. The observation that T. tormentosa is likely to regenerate follow- ing low level disturbance is intriguing given the the observed low regen- eration potential of this tree across a disturbance gradient as discussed in Sapkota et al. (2009). Given that this species has the largest seed size of all the species discussed in Khurana et al. (2006), it is possible that T. to- mentosa regeneration following perturbation may be restricted to very mild events. These findings indicate that pulses of low to moderate level distur- bance events appear to be an an important driver of spatio-temporal vari- ability in forest structure and composition. This may contribute to the unit heterogeneity observed across SASDTF, encouraging beta-diversity, and al- lowing for the development of a diverse source of external ecological mem- ory. Consequently, low-level disturbance events may contribute to overall SASDTF resilience.
Anthropogenic disturbance in SASDTF has, in many cases, intensified in severity and periodicity due to increased population and competition with other commercial land uses (Heinimann et al. 2007). These include for- est conversion for monoculture plantations and selective logging for high value timber species (McKenny et al. 2004, Li et al. 2014, You et al. 2015). Again, a series of studies in Indian SDTF may elucidate the impact of high intensity, high frequency disturbance on SASDTF. Khurana et al. (2006) demonstrate that following extreme perturbations, small-seeded species are likely to be more abundant in the regeneration phase, presumably due to reliance on external memory at these sites (i.e. recruitment from wind dis- persion). Some of these species that co-occur in SASDTF include Haldina cordifolia (somewhat contradicting the above-described findings of Sapkota et al. (2009)), Phyllanthus emblica, Bombax ceiba (Linn.) and Holoptelea integri- folia ((Roxb.) Planch.) If disturbance is severe and extensive enough such that ecological memory can not be relied upon for regeneration, even if they are abandoned in the long term, persistent secondary savanna formation may form in place of secondary forest (Goldammer 2002). Consequently, the role of disturbance in contributing to, or reducing ecological resilience of SASDTF needs to be considered in the context of the spatial scale, sever- ity, and frequency of the event/s.