With increasing disturbance and degradation of tropical rainforests, estimates say that secondary forests now cover larger areas than old, undisturbed rainforest (Dent, DeWalt, & Deslow, 2013). Because tropical rainforests play an important role in regulating the carbon and water cycle (Davidson et al., 2013), the regrowth of secondary forests is essential to their performance of vital services to the global environment.

One cause of rainforest depletion is fire, both natural and human-induced to clear land for agricultural use. Although rainforests traditionally have high species diversity, most organisms have a low fire tolerance, save for large trees. As the forests disappear due to fire, as well as other causes, less vegetation survives to receive rainfall and return water into the atmosphere through transpiration. Instead, there is increased water runoff out of the remaining forest, carrying sediment containing important nutrients such as carbon and nitrogen. With less water reentering the atmosphere, droughts occur, and though other organisms can withstand water shortages, large trees cannot. Less rainfall also increases forest fire risk;drought, fire and depleting forests share a cyclic relationship. Rainforests are collectively the largest carbon sinks in the world, so as these ecosystems lose their ability to store carbon, the global climate feels the impact. (Davidson et al., 2013)

Speedy secondary forest regrowth is needed. Chiti, Grieco, Perugini, Rey, and Valentini (2013) tested carbon levels in the soil of undisturbed rainforest, secondary forest, single species tree plantations, and mixed species tree plantations (both constructed on previous rainforest land). The study found that although undisturbed forests had considerably higher carbon levels than any of the other sites, secondary forests reversed the soil carbon loss visible in plantations. This data shows that secondary forests start to inherit the same climate-regulating functions that undisturbed forests perform. Therefore, rainforest regrowth in depleted areas is beneficial to the global environment.

Taking fire into account, this growth could accelerate with the presence of organisms that respond well to fire. Pinus contorta, lodgepole pine trees, possess this special capability. These trees reproduce using serotinous pine cones, which typically only release their seeds after the waxy coating is exposed to extremely high temperatures, i.e. fire. Though native to North American boreal forests, these trees can survive and thrive well outside their native habitat, as commercial lumber plantations exist throughout the southern hemisphere (Richardson, Williams, & Hobbs, 1994). Would the presence of P. contorta cones along the edges of tropical rainforests influence the regrowth of secondary forests after burning events? Several possible outcomes exist. Circumstances such as shade intolerance and other unaccustomed circumstances could prevent the survival of this species. Conversely, they may flourish as the only fire- dependent organism in nearly desolate, burned forest and become invasive, thus reducing biodiversity. They may also positively contribute to forest regrowth, helping to replenish carbon while maintaining a high level of biodiversity. I will first consider P. contorta invadibility of various forest environments. As mentioned, biodiversity is essential to the authenticity of the forest; an invasive species would be detrimental. Chiti et al. (2013) found that biodiversity positively correlated to higher soil carbon levels. Undisturbed rainforests had the highest soil carbon levels, followed by secondary forests, then agricultural land on sites of past forest. On farmland, mixed species tree plantations had noticeably higher carbon levels than single species plantations. Were P. contorta to invade secondary forests, it would fail as a solution to encourage species composition recovery. Richardson et al. (1994) developed a widely accepted and cited model to predict pine invasions in various southern hemisphere environments. One pine examined in the research was a generic fire-dependent species with serotinous cones, equivalent to P. contorta. Environments modelled in the study were fire-prone shrubland, grassland, and temperate forest. Richardson et al. (1994) found that the fire-dependent species posed a high invasive potential in the shrubland and grassland environments, especially when fire served as a mechanism for reproduction. In temperate forests, however, this species had little success invading. Two important conclusions come from Richardson et al. (1994), along with other considerations. First, grassland and shrubland are low-diversity environments comparable to newly burned rainforest. P. contorta may invade preliminary secondary forests, inhibiting authentic forest regrowth. Secondly, in favor of the null effect, P. contorta seeds are unlikely invaders of dense forest. Sunlight is key for the success of P. contorta, so saplings that were unable to find a gap in the crowded forest, whose canopy received most of the sunlight, would die. This trend is similarly found in rainforests, where large tree survival depends on a sufficient opening in the forest canopy for sunlight to reach the floor. P. contorta pine cones existing within the abundance of life in an undisturbed forest may occasionally succeed, but overall will have no impact on community structure and diversity. Though grassland, shrubland, and temperate forest environments are relatable to certain stages of rainforest, they are not an exact replica. Richardson et al. (1994) noted that higher latitudes fare favorably when implicating pine invasions, so one must consider the likelihood of P. contorta surviving near the equator. While lodgepole pines have adapted to a wide range of temperatures, from -57oC in mountain regions to over 38oC on the California coast, the lowest known native habitat is still over 30oN. Though P. contorta is an opportunistic species, this still puts survivability into question. The next study will analyze an exotic tree invasive to tropical forests. I will consider the qualities that facilitated invasion. Brown, Scatena, and Gurevitch (2006) studied the prevalence of Syzygium jambos, a fruiting tree non- native to the tropics that has successfully invaded parts of Puerto Rican rainforests. Results found a negative correlation between forest age and S. jambos dominance. S. jambos were able to colonize in undisturbed forest, but were dominant in secondary forests. In one area with data dating back to 1936, when farmers abandoned the land, S. jambos established a dominant population that has significantly reduced plant diversity compared to nearby undisturbed forest. Though S. jambos successfully invaded secondary forests, it may have some essential adaptive characteristics that P. contorta does not. As a fruiting tree, S. jambos has a market for distribution with birds and other animal species (Brown et al., 2006). P. contorta can only use wind as a means of seed dispersal (Richardson et al., 1994). Furthermore, the ability of S. jambos to tolerate low levels of sunlight, combined with wide distribution, has allowed some survival in undisturbed forests (Brown et al., 2006). The Richardson et al. (1994) model predicted that serotinous cones in shaded, dense forests would die if they failed to find a gap in the canopy.