Fire frequency, agricultural history and the multivariate control of pine savanna understorey plant diversity

Abstract

Question: Human-altered disturbance regimes and agricultural land uses are broadly associated with reduced plant species diversity in terrestrial ecosystems. In this study, we seek to understand how fire frequency and agricultural landuse history influence savanna understorey plant diversity through complex relationships (i.e. indirect effects) among multiple biophysical variables.Location: Fort Bragg, NC, US, Savannah River Site, SC, US and Fort Stewart, GA, US. Methods:We use structural equation modelling (SEM) to evaluate the relationships among six groups of predictor variables and their influence on localscale species richness in pine savannas at 256 sites from three locations in the southeastern USA. In the model, fire frequency and agricultural history are hypothesized to control richness through a combination of direct effects, and indirect effects mediated by resource availability, tree abundance, understorey plant abundance and the O horizon (litter and duff depth).Results: Frequent fires promote richness by limiting tree abundance, which increases understorey abundance and reduces the O horizon. Frequent fires also limit the O horizon independent of tree abundance. Of the total positive effect of fire on richness, 70% is attributable to reductions in the O horizon and 30% to reduced tree abundance. Agricultural history has a negative effect on richness through a positive correlation with tree abundance, which decreases understorey abundance and increases the O horizon. Agricultural history has a modest negative effect on richness by reducing resource availability as well as a strong direct negative effect (38% of the total effect) that is unrelated to other modelled variables.Conclusions: Through a multivariate framework and large-scale data set, this study unites and tests our understanding of the factors that control plant species diversity in a fire-dependent ecosystem. We show that the effects of fire frequency and agricultural history on richness are largely mediated through other ecosystem attributes, including vegetation structure (i.e. tree and understorey abundance), resource availability and the O horizon. Persistent, negative effects of agricultural history demonstrate the slow rates of savanna plant community recovery on post-agricultural land and highlight the conservation value of frequently burned savanna remnants.

Effects of fire frequency on litter decomposition as mediated by changes to litter chemistry and soil environmental conditions

“…In frequently-burned systems, combustion is important for preventing litter accumulation and maintaining plant diversity [49]. We found that fires in triennially-burned sites combusted a greater proportion of P .…”

Altered beta diversity in post‐agricultural woodlands: two hypotheses and the role of scale

“…This discrepancy between our study and others may have occurred because any establishment limitation slowing re‐assembly in post‐agricultural longleaf pine woodlands were overwhelmed by stronger community assembly constraints occurring uniformly in both community types. In particular, post‐agricultural woodlands and remnant woodlands experienced equally strong fire suppression and consequently support deep litter and duff layers (Brudvig et al ), which suppress species richness in longleaf pine woodlands (Hiers et al , Veldman et al ). Another explanation for differences between these results and others is that our post‐agricultural sites were all immediately adjacent to remnant sites.…”

Savannahs of Asia: antiquity, biogeography, and an uncertain future

“…The pine species are found over a huge rainfall range of 900 mm , MAP , 3200 mm ( [65]; www.gbif. org, accessed 27 March 2016) and are often associated with low-nutrient soils, similar to pine savannahs in North America [78,79]. This physiognomic similarity may indicate that Asian pine savannahs persist where growth rates are slow due to soil nutrient limitation and fire occurs regularly [79].…”

“…org, accessed 27 March 2016) and are often associated with low-nutrient soils, similar to pine savannahs in North America [78,79]. This physiognomic similarity may indicate that Asian pine savannahs persist where growth rates are slow due to soil nutrient limitation and fire occurs regularly [79]. The herbaceous layer is dominated by fire-adapted Andropogoneae grasses (Cymbopogon, Eulalia, Imperata, Themeda) [61].…”

“…Importantly, none of the models were able to predict the extensive pine savannahs in the Himalayas, China, Sumatra and the Philippines [3], suggesting that these savannahs may occupy a unique climate space that is not found in Africa, Australia or South America. Possibly this is because they occupy higher elevations and cooler climates than any of the southern continent savannahs and are analogous to pine savannahs in North America [79], which are also underlain by C 4 grasses [102]. Thus, as is the case for the southern continents, the climatic space of Asian savannahs is different from other continents [7].…”