Changes in abiotic influences on seed plants and ferns during 18 years of primary succession on Puerto Rican landslides

Abstract

Summary1. Abiotic variables are critical drivers of succession in most primary seres, but how their influence on biota changes over time is rarely examined. Landslides provide good model systems for examining abiotic influences because they are spatially and temporally heterogeneous habitats with distinct abiotic and biotic gradients and post-landslide erosion. 2. In an 18-year study on 6 Puerto Rican landslides, we used structural equation models to interpret the changing effects of abiotic influences (landslide dimensions, slope, aspect, elevation, parent material and related soil properties) on seed plants (density and diversity), tree fern density, scrambling fern cover, canopy openness and soil development (nitrogen, soil organic matter, pH and cation exchange capacity). 3. Seven years after landslide formation, catchment size (the landslide area above the point of measurement) was the key abiotic factor influencing plants. The larger the catchment the greater was the diversity and density of seed plants. Conversely, the smaller the catchment the greater was the density of tree ferns and the cover of scrambling ferns. 4. Eighteen years after landslide formation, landslide slope was the key abiotic influence. The greater the slope, the lower was the density and diversity of seed plants and the greater was the scrambling fern cover. 5. Aspect, particularly east-facing slopes exposed to wind disturbances, positively influenced tree fern densities at both 7 and 18 years and negatively influenced seed plants and scrambling ferns at 18 years. Soils were least developed, that is, had lowest soil nitrogen and organic matter concentrations, after 18 years on steep slopes (like seed plants); soils were most developed near landslide edges, on hurricane-exposed slopes (like tree ferns) and where there were high soil potassium concentrations. 6. Synthesis. Abiotic variables have important influences on plant succession on landslides and the relative influence of different abiotic variables changes with time. Improved predictability of temporal dynamics will rely not only on understanding the effects of initial disturbances and subsequent biological responses but also on the different and changing influences exerted by each abiotic variable.

Long-term Entomological Research on Canopy Arthropods in a Tropical Rainforest in Puerto Rico

“…Prior to 1989, Puerto Rico had not experienced a major hurricane (≥ Category 3, >178 kph wind speed) for 33 years (Betsy in 1956), and a closed canopy with little understory characterized the forest. However, during the past 27 years, this site was struck directly by two major hurricanes, as well as by many minor hurricanes, tropical storms, landslides, and droughts (Brokaw and Grear 1991, Walker 1991, Heartsill-Scalley et al 2007, Walker et al 2013. Hurricane Hugo (1989, Category 4) caused >50% defoliation and reduced mean canopy height and plant biomass by >50% at the LEF (Brokaw and Grear 1991, Walker 1991, Scatena et al 1996, leaving severely damaged patches (30-60 m diameter) with extensive tree-fall (gaps) in a matrix of damaged standing trees (non-gaps) (Fig.…”

“…Hurricanes Marilyn (1995), Bertha and Hortense (1996), and Debby (2000) caused substantial defoliation and flooding but did not create appreciable gaps in the canopy. Hundreds of landslides occurred as a result of these hurricanes and rainstorms (Walker et al 2013). In addition, significant droughts (>20% reduction in annual rainfall) occurred during 1994 (the second driest year on record), when precipitation was only 41% of the long-term annual average, 1997 and 2002, and minor droughts (at least 15 consecutive days without rain) occurred in 1991(Heartsill-Scalley et al 2007, Scatena et al 2012.…”

Changes in spatial point patterns of pioneer woody plants across a large tropical landslide

“…In subzone E2, regular patterns persisted over the [2001][2002] period. This subzone presented a steep slope (Velazquez and Gomez-Sal, 2008), which has a highly negative influence on colonization of woody plants on landslides (Walker et al, 2013) by its association with post-landslide erosion (Walker and Shiels, 2008). Therefore, the continuation of regular spatial patterns in this subzone over the study period might be due to removal of early colonizing (Mian et al, 2008) per subzone were used under a null model of independence.…”

“…In the upper or erosional zones, where original soil and vegetation are entirely removed, germination, establishment and survival of woody pioneers are severely limited by patchy distributions of soil resources . Once established, however, early successional woody plants can facilitate the establishment of late successional ones by increasing soil nutrient and organic matter contents (Shiels et al, 2006;Walker et al, 2013), and decreasing the cover of forbs (Walker et al, 2010b). In the lower or depositional zones, which have a rich biological legacy consisting of the original soils and heterogeneous mixtures of broken plant parts, seeds, and rock fragments from the upper zones, there is often competition for light among pioneer trees and shrubs during early succession (Velazquez et al, 2009b).…”

Understory fern community structure, growth and spore production responses to a large-scale hurricane experiment in a Puerto Rico rainforest

“…landslides, hurricanes, Royo et al, 2011). Ferns can have a significant role in shaping the trajectory of forest development after a disturbance (Walker and Sharpe, 2010); thicket-forming fern species can increase soil organic matter (Walker, 1994) or physically inhibit forest recovery on landslides (Walker et al, , 2013. Such filtering effects, where fern species alter plant communities and/or ecosystem processes, have been reported widely: Polystichum montevidense and Blechnum penna-marina in Argentina (Enrico et al, 2004); Pteridium arachnoideum in Brazil (Silva Matos and Belinato, 2010); Blechnum discolor in New Zealand (Dearden and Wardle, 2008); and Sticherus bifidus, Gleicheniella pectinata and Cyathea arborea in Puerto Rico (Walker, 1994;.…”