Long‐term dynamics of liana seedlings suggest decelerating increases in liana relative abundance over time

Abstract

Over the past decades, tropical forests have experienced both compositional and structural changes. In the Neotropics, researchers at multiple sites have observed significant increases in the abundance and biomass of lianas (i.e. woody vines) relative to trees. However, the role of dynamics at early life stages in contributing to increasing liana abundance remains unclear. We took advantage of a unique dataset on seedling dynamics over 16 years in ~20 000 1‐m2 plots in a tropical forest in Panama to examine temporal and spatial trends in liana and tree seedling abundance. We found that the relative abundance of liana seedlings increased across the study period, from 0.18 in 2001 to 0.24 in 2017. However, increases in liana seedling relative abundance appear to have levelled off in more recent years. The observed increases in liana relative abundance appear to be the result of both higher survival and higher recruitment rates of liana seedlings compared to tree seedlings. Increasing liana abundance in the seedling layer was not explained by annual variation in dry season length, total rainfall or the proportion of area occupied by canopy gaps. In addition, liana seedlings did not exhibit a demographic advantage (i.e. higher recruitment or survival) over tree seedlings in dry habitats. Synthesis. Our results reveal that seedling communities experienced important compositional changes in the past, but liana seedling relative abundance may have stabilized in recent years. Longer‐term monitoring is needed to determine whether tropical forests will continue to experience compositional changes that may alter forest structure and ecosystem function.

The response of lianas to 20 yr of nutrient addition in a Panamanian forest

“…Therefore, in Old World tropical forests the loss of frugivores due to poaching is unlikely to affect the relative success of lianas over trees, which is consistent with the lack of evidence for increasing lianas in most Old World tropical forests (but see Pandian and Parthasarathy 2016). The hunting hypothesis cannot, however, explain recent observation on the Barro Colorado Island, Panama (BCI) 50‐ha plot, where hunters have been excluded since 1987, yet liana stem density and basal area increased substantially from 2007 to 2017 (S. A. Schnitzer, unpublished data ; see also Wright et al 2004, Umaña et al 2020).…”

“…There are now more than a dozen studies demonstrating that lianas are increasing in density, biomass, or productivity in mature neotropical and subtropical forests (reviewed by Schnitzer and Bongers 2011, Schnitzer 2015). The increase in liana abundance or biomass, either relative to trees or in absolute terms, has been reported for forests from Argentina, the Bolivian and Brazilian Amazon, Costa Rica, French Guiana, India, Panama, and Puerto Rico, as well as the subtropical southeastern United States (Phillips et al 2002, Wright et al 2004, Allen et al 2007, Schnitzer et al 2012, Laurance et al 2014, Pandian and Parthasarathy 2016, Ceballos and Malizia 2017, Hogan et al 2017, Umaña et al 2020).…”

Whole-Plant Seedling Functional Traits Suggest Lianas Also Support “Fast-Slow” Plant Economics Spectrum

“…Yet, evidence for both the lianas' growth/survival advantage over trees and their relevant trait differences from trees in the juvenile life stage are mixed. For example, a 10-year demographic inventory in a Puerto Rican tropical forest implied dry conditions and disturbance promoted liana seedling survival and abundance [21], while seedling dynamics over 16 years in a tropical forest in Panama revealed liana seedlings decelerated increases in the relative abundance of lianas over time [22]. The resource acquisition between lianas and trees suggests lianas seedlings may also follow a "fast-slow" growth spectrum, defined by whether they are light-demanding or shade tolerant [18].…”

Local canopy disturbance as an explanation for long‐term increases in liana abundance

“…We examined whether liana density and basal area were increasing over a 10-year period (2007)(2008)(2009)(2010)(2011)(2012)(2013)(2014)(2015)(2016)(2017) in the Barro Colorado Island, Panama (BCI) 50-ha plot. Previous evidence from BCI suggests that lianas have increased in productivity (Wright et al, 2004), the proportion of trees that they occupy (Ingwell et al, 2010) and in seedling number (relative to trees; Umaña et al, 2020); however, no study has quantified the change in liana sapling and adult abundance in an old-growth forest using a single longitudinal study, nor has any study successfully linked the increase in lianas to disturbance or any other factor (Schnitzer et al, 2020a).…”