Mass, nitrogen content, and decomposition of woody debris in forest stands affected by excreta deposited in nesting colonies of Great Cormorant

Abstract

Great Cormorant (Phalacrocorax carbo), a piscivorous bird, has established breeding colonies in a coniferous forest near Lake Biwa in central Japan. This study investigated the possible effects of the colony’s excreta on the mass, nitrogen (N) content, and decomposition of woody debris. Study plots were established in forest stands representing four stages from breeding colony establishment to post-abandonment. The mass of fallen branches (diameter 1–5 cm) and coarse woody debris (logs, snags, and stumps; diameter ≥10 cm) was greater in forest stands colonized by Cormorants than a control stand never colonized by Cormorants. This was primarily attributed to Cormorant activity that caused increased mortality of standing trees and by Cormorants breaking branches for nesting materials. Nitrogen content of branches and logs that had fallen to the forest floor was negatively correlated with the relative density of wood. Nitrogen content of branches was consistently higher (at a given value of relative density) in the colonized stands than in the control stand. The increase of branch N content was possibly caused by the incorporation of N into decomposing branches with excreta-derived N supplied as throughfall and/or soil solution. The mean value of 2-year mass loss of recently dead branches and logs was significantly greater for woody debris in the smallest diameter class but was not significantly different among the forest stands. This suggests that the excessive supply of excreta-derived N and concomitant enrichment of N in soil had negligible effects on the initial stages of decomposition of woody debris.