This paper aims to provide a broad overview of dry forests, with particular focus on the tropical dry forests found in Latin America as these represent the most extensive remaining contiguous areas of this type of forest in the world (Miles et al, 2006). We will consider the effects which human disturbance and climate change may have on these forests in the future and potential management options to respond to these.
Dry forests make up 42% of all tropical forests and are found across the world (Figure 1). They consist of two main types (i) temperate evergreen forests (found in the southern USA, northern India and southern China, northern Australia, the Rift Valley of Africa and parts of South Africa) and (ii) tropical seasonal forests which are mostly deciduous (found in southern Asia, Africa, Madagascar and Central and South America).
Definition
(S. H. Bullock, H. A. Mooney & E. Medina (eds) 1995) described dry seasonal forests as those occurring in the tropical regions where there are several months of severe drought. (Van Bloem, Murphy et al. 2004)provided a more detailed definition of dry forests which clearly illustrates the range across with they are found; tropical dry forests occur in frost-free terrains from lowlands to lower montane regions where potential evapotranspiration exceeds precipitation on an annual basis, annual precipitation is 500-2000mm and the mean annual bio-temperature is >17oc (Figure 2).
Characteristics
Van Bloem,S J. 2004}Typically, dry forests have a closed canopy, although this may not be the case in the driest parts of their range or if disturbance is prevalent. Dry forests are generally smaller in structure and simpler in composition than wetter forests of a given region, but there is enormous geographic variation in most features due to differences in climate, soil, biogeographic factors and disturbance history. The only unifying characteristic of the dry forest climate is the strong seasonality of the rainfall distribution resulting in dry forests being seasonally stressed by drought (Murphy & Lugo, 1995).
Van Bloem,S J. 2004Tropical dry forests occur on substrates ranging from nutrient-rich alluvial soils to nutrient-poor rock outcrops. (Huxman et al, 2004; Kurc and Small, 2007 cited in (Perez-Ruiz, Garatuza-Payan et al. ). Shallow or infertile soils tend to support evergreen forests, while deciduous forests generally grow on better soils. Tropical dry forest vegetation is mainly water rather than nutrient limited (Van Bloem, Murphy et al. 2004). All biological activity in the underground system is strongly limited by water availability and interactions with carbon and nutrient availability. Synchronization between activity of decomposers and fine root production at the beginning of the rainy season seems to be essential for an efficient balance between nutrient uptake by the plants and nutrient immobilization by microbes. (Cardoso, Kuyper 2006) Microbial activity decreases greatly in dry seasons.
Tropical dry forests are generally less species-rich than moister forests in terms of plants and vertebrates (Gentry, 1995). They have more structure and physiological diversity in life forms. Diversity of life forms is both structural (wood specific gravity, plant habit) and physiological (photosynthetic types, water relations, growth seasonality) (Mooney et al, 1995 cited in (Kalacska, Sanchez-Azofeifa et al. 2004). The common phenological response of drought deciduousness is exhibited by many woody species, however, the number of species and individuals with this response in tropical dry forests varies with each stage of succession and topography. This mix of deciduous and evergreen species gives tropical dry forests a kind of phenological complexity not found in moister forests. (Burnam, 1997 cited in (Kalacska, Sanchez-Azofeifa et al. 2004). A distinguishing feature of tropical dry forests is that they buck the trend of species richness increasing as one gets closer to the Equator - the most diverse tropical dry forests are farthest from the equator, in Mexico and South America, with the most species-rich areas being among the driest (Gentry, 1995).
Disturbance
Disturbance is an important factor in tropical dry forests and understanding succession after disturbance is critical for the development of sustainable management and conservation strategies. The main disturbances/threats to dry forests in Latin America are wind, fire and human disturbance.
Wind
High winds from hurricanes result in defoliation and breakage of trees, with long term consequences for forest structure. Return intervals for hurricanes range from 5 to 100 years. Mature dry forests in the hurricane-prone West Indies tend to have shorter canopies, higher stem densities, and a greater proportion of multiple stemmed trees than dry forests outside of the hurricane belt. (Imbert, Portecop 2008) studied old-growth dry forest on the island of Grande-Terre, Guadeloupe in the Caribbean, assessing various aspects of forest recovery after the forest had been struck by Hurricane Hugo. They found that 9 years after the hurricane, neither stem density and basal area, nor girth increment, had returned to pre-hurricane values and noted that hurricane disturbance in dry forests appears to affect the forest structure on a longer time-scale rather than the functioning of the forest ecosystem
Fire
Fire is a difficult feature to interpret in dry forests. In most locations, even in Africa, fire is not a frequent or severe aspect of the ecosystem. Lightning occasionally starts fires in dry forests, but they are low intensity, small scale, and usually quelled by rains. The sparse understory and grass cover found in dry forests limit the intensity of the ground fires that burn leaf litter, returning some proportion of nutrients to the soil as ash, while losing others to the atmosphere.(Van Bloem, Murphy et al. 2004) The majority of fires in dry forests have been created by humans in order to clear land, hunt animals and burn fallow to encourage new growth for livestock. (Kennard, Gould et al. 2002) studied the effects of fire on regeneration among woody plants in tropical dry forest in Bolivia. They found that high and low intensity burns reduced densities of viable seeds by at least 50%. They produced similar results to Miller and Kauffman, 1998b (cited in Kennard et al 2002) who compared the size of tree seedlings and sprouts after slashing and burning of dry forest in Mexico and found sprouts dominated the available growing space. Kennard et al, suggest that sprouts may have an advantage over seedlings as the larger root system of sprouts would extend deeper into the soil than that of seedlings and would give them more surface area for water and nutrient uptake. This would be particularly advantageous in dry forests where water is seasonally limiting. However, they found that sprout regrowth depended on the intensity of the initial fire which caused the disturbance, possibly due to the depth of penetration into the soil of high intensity fires.
Human Disturbance
In 1986, Janzen stated that tropical dry forests are among the most threatened ecosystems in the world as a consequence of intensive anthropogenic disturbance. Dry forests support a large fraction of the human population in the tropics as environmental constraints on human development are low compared to other ecosystems. Their fertile soils and mild climates make them highly suitable for agriculture and livestock. The dry season provides respite from rain and humidity, and reduces populations of agricultural and human pests. Livestock survive well in dry forest areas, and the relatively small trees are easy to clear for agricultural fields. As a result these forests are under intense pressure with large population concentrations occurring in dry forest areas (Murphy and Lugo, 1986). . Hoekstra et al (2005) estimated that approximately 48.5% of tropical dry forests have already been converted to other land uses at the global level; in the Americas alone, 66% of the dry forests have been converted (Portillo-Quintero, Sánchez-Azofeifa ). North, Central and South American dry forests have suffered intense transformations from agricultural development and cattle ranching. The long-term effects of extraction depend on the amount and pattern of biomass removed and the degree of soil disturbance or compaction that accompanies tree harvests. Conversion of forests to agricultural, industrial, or residential areas completely removes forest cover and disrupts roots and soil structure. Traditionally, shifting agriculture has been practiced in dry forest regions in a sustainable manner, but increasingly short fallow periods prevent re-establishment of the forest. Grazing and agriculture increase nutrient loss from the system by erosion. Grazing reduces forest biomass because cattle reduce understory growth while compacting the soil and accelerating erosion.
Trees are cut for firewood or charcoal production in most regions. Many trees coppice - an advantage when used for firewood (Mostacedo, Putz et al. 2009). Firewood accounts for 80-90% of dry forest harvest in Africa. Unfortunately wood grown for fuel is not growing quickly enough to sustain demand in many areas of Africa.
Forest Management
Climate change section
List parts of LAC that will be affected (ref IPCC 2007 report)
Figure 13.3
Figure 3: Predicted 2000-2010 South American and Central American deforestation hotspots and diffuse deforestation areas
Declining trend in rainfall in Western Central America (where notably there is a lot of TDF) - IPCC
Increase of tropical forest conversion to dry and very dry forests and savannahs (up to 47% shift in Holdridge life zones) at expense of temperate oak and conifers in region.
May see migration of tropical forests to higher altitudes due to changes in climate.
TDF is severely threatened
TDF has the capacity to fix a larger quantity of CO2 than other seasonally dry ecosystems
Ecosystem CO2 and water vapour exchange in monsoon period were highly influenced by precipitation (thus as precipitation falls - as predicted by IPCC - how will this influence these two factors in TDF? A further release of stored CO2?)
Land use changes causes perturbation of the ecosystem and can influence carbon stocks and fluxes.
Conversion of forest to agricultural land use invariably results in the depletion of SOC stock by 20-50% (see Covington Curve below). IPCC predicts major change in land use of LAC forests for pasture and expanding livestock production.
SOC is affected by the slope and aspect of the landscape a forest is situated in (could this mean depletion of SOC if tropical forests are forced to migrate to higher ground as climate changes?).
Frequency of weather and climate extremes and hurricane frequency and intensity will increase in LAC region - severe natural disturbances such as these have an impact on SOC.
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