Effects of canopy opening and debris deposition on fungal connectivity, phosphorus movement between litter cohorts and mass loss
Fungi are important for maintaining fast rates of decomposition in low quality tropical leaf litter via immobilization and translocation of limiting nutrients from sources to sinks and conserving nutrients after disturbance. Tropical trees often have low nutrient to carbon ratios. Disturbances such as hurricanes and logging transfer a large mass of green leaves with high nutrient concentrations to the forest floor, but the associated opening of the canopy dries the litter, inhibiting basidiomycete fungi that play critical roles in lignin degradation and nutrient conservation. We conducted a replicated block factorial experiment designed to disentangle the individual and interactive effects of canopy opening and green debris deposition on phosphorus (P) content, mass loss and fungal connectivity in decomposing leaf cohorts in subtropical wet forest in the Luquillo Mountains of Puerto Rico. Though green leaves had higher P concentrations they did not decompose significantly faster than senesced leaves. Mass loss differed among treatments after 14, 40.5 and 53 weeks decomposition. Mass loss at 7 weeks was predicted by P concentration at 7 weeks; mass loss in senesced leaves at 14 weeks was predicted by abundance of fungal connections between the senesced litter cohort and forest floor at 7 weeks. Fungal connectivity and P accumulation at 7 weeks and mass loss of senesced leaves beginning at 14 weeks were significantly different from and lower in plots with trimmed canopy and no debris than in the untrimmed plots with debris. Litter moisture was previously found to be significantly lower under open than closed canopy, and we found that moisture was a significant predictor of fungal connectivity in both senesced and green leaves. Deposition of green leaves ameliorated the inhibitory effect of canopy opening on fungal connectivity between litter cohorts by retaining moisture; consequently fungal connectivity and mass loss in senescent leaves did not differ between the Trim + Debris and the control treatments. Phosphorus content of senesced leaves increased significantly by 7 weeks in both trimmed and untrimmed plots with added green debris and in the control plots. Based on mass balance calculations, both the underlying forest floor and overlying green leaves likely contributed P to the decomposing senesced leaf cohort. Fungal translocation of P through hyphal connections between litter cohorts explains some of the changes in P content. Though fungi were important in conserving P, most of the P that was likely leached from green leaves was not retained in the litter layer.