Long-lasting effects of unplanned logging on the seed rain of mixed conifer-hardwood forests in southern South America

Abstract

Understanding the effects of disturbance on seed rain is critical to predict changes in forest species composition and diversity. Logging effects on seed rain in a mixed conifer-hardwood forest complex in southern Brazil were evaluated. One year of seed rain data were collected from a large-scale observational experiment in logged and protected forests and quantity and average seed size weighted by species abundance (CWM) were compared between old-growth and logged stands 55 years after logging activities. Using these data, variations in frequency of functional groups of species in the seed rain were examined to see if they could be attributed to logging. Results show that the number of seeds per trap was highly right-skewed, ranging from 13 to 12,788 seeds per trap in one year. Seed rain was affected by logging history, with seed traps in old-growth plots receiving significantly less seeds than traps in logged plots. All species included mean seed size weighted by species abundance were significantly smaller in logged than in old-growth forests. This difference persisted after the exclusion of Araucaria angustifolia, a large-seeded pioneer which was intensively logged, although the difference of seed size between the two forest classes was greatly reduced. Species abundance in the seed rain differed significantly from the established tree community, between logged and old-growth stands. The composition of the seed rain was much more variable than the composition of the established tree community and its points more scattered over the ordination space than the points corresponding to the protected forests. The number of collected seeds across different functional groups of species significantly differed between logged and old-growth plots. The seed rain of logged forests reflects their arrested succession as indicated by reduced abundance of functional groups such as pioneers, large seeded pioneers and Araucaria, as well as reduced functional diversity. Seed rain differences between logged and old- growth stands reflect the intensive logging of A. angustifolia.