Disturbance history mediates climate change effects on subtropical forest biomass and dynamics

Abstract

The responses of forest communities to interacting anthropogenic disturbances like climate change and logging are poorly known. Subtropical forests have been heavily modified by humans and their response to climate change is poorly understood. We investigated the 9‐year change observed in a mixed conifer‐hardwood Atlantic forest mosaic that included both mature and selectively logged forest patches in subtropical South America. We used demographic monitoring data within 10 1 ha plots that were subjected to distinct management histories (plots logged until 1955, until 1987, and unlogged) to test the hypothesis that climate change affected forest structure and dynamics differentially depending on past disturbances. We determined the functional group of all species based on life‐history affinities as well as many functional traits like leaf size, specific leaf area, wood density, total height, stem slenderness, and seed size data for the 66 most abundant species. Analysis of climate data revealed that minimum temperatures and rainfall have been increasing in the last few decades of the 20th century. Floristic composition differed mainly with logging history categories, with only minor change over the nine annual census intervals. Aboveground biomass increased in all plots, but increases were higher in mature unlogged forests, which showed signs of forest growth associated with increased CO2, temperature, and rainfall/treefall gap disturbance at the same time. Logged forests showed arrested succession as indicated by reduced abundances of Pioneers and biomass‐accumulators like Large Seeded Pioneers and Araucaria, as well as reduced functional diversity. Management actions aimed at creating regeneration opportunities for long‐lived pioneers are needed to restore community functional diversity, and ecosystem services such as increased aboveground biomass accumulation. We conclude that the effects of climate drivers on the dynamics of Brazilian mixed Atlantic forests vary with land‐use legacies, and can differ importantly from the ones prevalent in better known tropical forests.

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

“…In the Sao Francisco de Paula National Forest, logging of the largest trees resulted in logged stands being shorter with greater densities of small and dead trees (Souza et al 2012). The logged stands are considered to be in arrested succession as indicated by Souza and Longhi (2019), and the present results indicate that this is reflected in the seed rain as well. This study has shown that logged forests in the study area showed signs of arrested succession as indicated by reduced abundances of functional groups like wind-dispersed large trees, large pioneers and Araucaria, as well as reduced functional diversity (De Avila et al 2011;Forgiarini et al 2015;Souza and Longhi 2019).…”

“…The logged stands are considered to be in arrested succession as indicated by Souza and Longhi (2019), and the present results indicate that this is reflected in the seed rain as well. This study has shown that logged forests in the study area showed signs of arrested succession as indicated by reduced abundances of functional groups like wind-dispersed large trees, large pioneers and Araucaria, as well as reduced functional diversity (De Avila et al 2011;Forgiarini et al 2015;Souza and Longhi 2019). It is possible that species in the functional groups large pioneers and wind-dispersed large trees have been responsible for the increased seed production in logged plots, while the lack of Araucaria in these plots was the main cause of their reduced overall seed size.…”

“…At present, basal area averages 43.3 m 2 ha -1 and does not differ between logged and unlogged stands, but tree density is higher in old-growth (average 42.5 trees ha -1 ) than in logged forests (average 28.8 trees ha -1 ), as is canopy height (average 13.2 m in old-growth and 11.7 m in logged stands) (Souza et al 2012). A total of 156 tree species were found in the study plots, which showed only minor abundance changes over nine annual census intervals, with recurrent regeneration failure of A. angustifolia and other long-lived pioneers in logged stands (Souza and Longhi 2019). Plots were permanent and the choice of their location was limited so as to avoid steep slopes and water courses, and were thus physiographically similar; their main differences were related to logging history (Souza et al 2012).…”

“…900 m and soils are derived from volcanic rocks. São Francisco de Paula has experienced increasing summer minimum temperatures and rainfall in the twentieth century (Souza and Longhi 2019). The region also experiences strong El Niño-Southern Oscillation effects, with excess rain and increased temperatures during El Niño years, and droughts and cooler temperatures during La Niña years (Garreaud et al 2009).…”

“…Both plant height and seed size are known to mediate the spatial extension of seed dispersal (Jackson 1981;Thomson et al 2011), which has disproportionally strong effects on dispersal limitation of dioecious species relative to monoecious ones (Heilbuth et al 2007). Although the long-term effects of logging on functional forest structure are poorly known, selective logging may alter the seed rain through changes in forest trait distribution (Both et al 2019), the collapse of logged populations (Ter Steege et al 2002), and arrested succession (Chazdon 2003;Souza et al 2012;Edwards et al 2014;Pyles et al 2018;Souza and Longhi 2019).…”