Luquillo LTER

This experiment was designed to decouple the effects of canopy opening from those of increased detrital inputs on rates of detrital processing and resultant community and ecosystem processes. In a study initiated after massive inputs of organic matter from Hurricane Georges in 1998, the forest floor returned to prehurricane values very quickly, within 2-10 months (Ostertag et al. 2003). However, it was unclear to what extent this homeostasis was caused by increased rates of decomposition. Furthermore, if accelerated decomposition was implicated in rapid recovery, the relative contributions of environmental and resource changes wrought by canopy opening versus green leaf deposition on the forest floor were unclear because these factors are confounded in hurricane damage. A full factorial design was therefore used to tease apart the separate and combined effects of simulated storm damage on rates of mass loss in pre-weighed senesced and green litter cohorts inserted into litter decomposition baskets following application of canopy trimming and debris deposition treatments. Natural litter cohorts (i.e., organic forest floor material and subsequent natural litterfall separated into 3-month cohorts) were also weighed when replicate baskets were harvested at approximately 3-month intervals. In addition to obtaining mass and percent moisture of litter cohorts, the extent of fungal connections between litter cohorts was quantified. Fungal connections between partly decomposed and fresh leaf litter have been shown to be important in importation of phosphorus (the most limiting major nutrient in decomposition of tabonuco forest litter) into the freshly fallen leaves in order to rapidly build fungal biomass and associated acceleration of decomposition (Lodge 1993, 1996). The thickest of these fungal colonization & translocation organs (rhizomorphs, cords and hyphal strands) are primarily basidiomycete fungi, which have an almost unique capacity to cause white-rot by breaking down lignin in low-quality litter. A few white-rot basidiomycetes produce finer connections comprised of diffuse wefts of hyphae (e.g., Marasmius leoninus and related species), but the majority may represent ascomycetes and water molds that lack enzyme systems for breaking down lignin. White-rot basidiomycetes were shown in separate experiments to accelerate rates of decomposition of tabonuco leaves (Dacryodes excelsa) by 15% to 20% (Santana et al. 2005; Lodge et al. 2008), so any changes in fungal connectivity by basidiomycete fungi in response to the treatments should be related to nutrient exchanges between litter cohorts and changes in rates of mass loss.

Litterbaskets are used to study decomposition and nutrient cycling questions, and are often a better for understanding interactions between different litter cohorts than are leaf decomposition bags. We know from previous work here and elsewhere that: 1) basidiomycete fungi rapidly colonize freshly fallen litter (within the first 3 weeks of litterfall) from partly decomposed litter on the forest floor using rhizomorphs and cords (Lodge & Asbury 1988); 2) these fungal root-like structures transport nutrients from the old food base in order to build their biomass in the freshly fallen leaves, and are capable of tripling the phosphorus content in a senesced tabonuco leaf as indicated using radioactive phosphorus tracer in microcosm experiments (Lodge 1993; 1996); and 3) basidiomycete colonization accelerates leaf decomposition in the LEF (Lodge et al. 2008). Fungal translocation of nutrients is probably responsible for the increase in total CONTENT of N and P in leaf litter above 100% in El Verde (as in Zou et al.) and elsewhere in the tropics within 3-6 weeks of leaf fall (see Lodge 1993). In contrast, temperate forest floor litter is not usually colonized by basidiomycete fungi from the forest floor until 9-15 months after litterfall. Translocation of phosphorus into tropical litter with low phosphorus concentrations likely contributes to accelerated rates of leaf decomposition associated with basidiomycete colonization in tabonuco forest (Lodge et al. 2008), but the enzymatic capacity of basidiomycete to degrade lignin is a contributing factor (Santana et al. 2005).

Previous research in temperate forests shows a positive effect of increased litter depth on colonization by basidiomycete fungi. Unpublished data of Lodge & Asbury showed that drying of the litter layer reduced or eliminated basdiomycete colonization, while Lodge & Cantrell (1995) showed disappearance of some basidiomycete colonies in canopy gaps on ridges at El Verde after hurricane Hugo, or replacement of drought-sensitive strong nutrient translocators (i.e., Collybia johnstonii) with more drought tolerant species that translocated less P32. There were no previous data on effects of litter depth on basidiomycete fungi from tropical forests. We knew from studies after Hurricane Georges that 1. forest floor mass in secondary forest returned to pre-hurricane levels in about a year (Ostertag, Silver & Scatena?), but we did not know whether this was due to accelerated decomposition or reduced litter inputs after the storm. Thus, it was not really clear what mechanisms were involved in control of forest floor decomposition following hurricane disturbance.

This litterbasket decomposition experiment was designed to mimic as closely as possible post-hurricane conditions in order to follow mass loss and nutrient content of specific litter cohorts. To this end, a layer of SURFACE AIR-DRIED tabonuco leaves was placed between two screens on top of the existing forest floor layer in the litterbaskets (on the ground). In debris-addition plots, green leaves of Dacryodes, Manilkara and Sloanea IN HURRICANE AMOUNTS (as determined in Lodge et al. 1991) were added on top of the senesced litter layer screen after the canopy manipulations were completed in the CTE plots. Additional cohorts of litterfall were demarcated using screens added to remaining baskets when these were harvested ca. quarterly.

So far, we know that 1) canopy opening inhibited fungal connectivity between litter cohorts (mostly basidiomycete fungi, but the highest counts may be from diffuse hyphal connections by ascomycetes and water molds); 2) addition of green litter buffered the layers below from drying, mostly compensating for the effects of canopy opening; 3) fungal connectivity to the weighed layer of senesced tabonuco leaves was positively and significantly correlated with rates of leaf decomposition; 4) litter decomposition rates were higher than in dried leaf litter in a litterbag experiment in the CTE (González et al, unpublished), as in previous unpublished comparisons of dried versus undried litter; 5) but despite this, forest floor mass had not returned to pre-hurricane levels 1.5 years after CTE initiation. The data of Cantrell and Ortíz (lterdb165) on microbial composition in the litter cohorts from these baskets used methods that cannot distinguish basidiomycetes from other fungi. The results, however, suggest that colonization by basidiomycetes colonize accelerates the rate of early leaf decomposition and changes the trajectory of community succession. Nutrient analyses to determine if increases followed by decreases in inorganic nutrient pools, especially phosphorus, are associated with changes in patterns of fungal connectivity between the litter cohorts, and whether cohorts with low connectivity at the beginning have net losses rather than net gains in N & P stores.