Forest understory resilience and soil dynamic response to anthropogenic wildfire in dry semi-deciduous forest in Puerto Rico

Background/Question/Methods

Human disturbance stimulates changes in the composition of subtropical dry forests and reduces their capacity for resilience. Wildfire, when introduced as a novel disturbance, creates severe effects on vegetation structure and soil dynamics that can lead disturbed niches to be rapidly colonized by invasive grasses. There is limited understanding of how short-term responses in soil nutrient fluxes after fire in dry forests can influence colonization of native tree species or invasive grass species. The main objectives of our study were to determine whether: 1) altered soil conditions would favor the growth of invasive grasses over native plant species in the understory in the short-run; 2) wildfire and bulldozing impact soil nutrient dynamics, and if so, how quickly soil nutrient fluxes return to original parameters. To answer these questions, a total of 108 understory sampling plots (1x1m) for 5 treatments were located in two mature forest stands that had recently burned as a result of human activity. Information about fire intensity, vegetation quantitative samples, soil moisture data, PRS(tm) probes for nutrient flux data, soil bulk density/nutrient content samples, and extracellular enzyme samples where collected for each plot. 

Results/Conclusions

Preliminary results showed correlation between a fast understory (sprouts and seedlings) response in burn sites that concurrent to a large flux of cations and anions released on the soil after the fire. Nutrient concentrations (mg/cm2/burial-length) of nitrogen (NO₃-N + NH₄-N), PO₄^3- (P), SO₄^2- (S) and Mn²⁺ are increasing over time in dry season periods. K⁺ and Ca²⁺ concentrations in soil continue to intensify in burn sites, but appear to have lower concentrations in forest sites, old burned sites or sites colonized by native grasses (Uniola virgata). In understory vegetation plots, a low seedling mortality rate provided a positive input for the species pool.  Naturalized species such as Leucaena leucocephala and invasive grasses such as Melinis repens and Megathyrsus maximus are among the most aggressively colonizing species on the bulldozed-treatment transect (gradually closer to roads). Native species such as Corchurus hirsutus, Croton glabellus, Croton discolor, and Waltheria indica are among the most common plants colonizing the burned areas in the forest. These findings illustrate how vegetation dynamics and soil nutrient availability after wildfire can provide opportunity for native species recovery and less opportunity for initial stages of grass invasions if all the conditions allow rapid understory coverage. Further analysis of soil bulk nutrients and density will provide a better understanding of pool dimensions and changes.