Assembly dynamics of a forest bird community depend on disturbance intensity and foraging guild

Abstract

Summary Tree plantations occur globally and are often promoted as a strategy to supply wood products for an expanding human population while reducing pressure on natural forests. Herbicides can accelerate growth of crop trees by suppressing competing vegetation, but little information is available about potential trade‐offs with early seral biodiversity resulting from more intensive management. Using data collected over 5 years immediately following stand‐replacing disturbance (clear‐felling), we used a large‐scale experiment to test how environmental filtering influences dynamics of avian community assembly. We evaluated avian responses to three levels of plant cover reduction (light, moderate and intensive herbicide applications) in relation to a control without herbicide. Under the environmental filtering hypothesis, we expected reduced avian species richness and higher turnover as broadleaf plant cover decreases with increasing management intensity. We predicted that the most intensive treatments would exert strong negative effects on leaf‐gleaning insectivores, including several species of conservation concern due to long‐term population declines. Richness of leaf‐gleaning bird species was reduced by 23–52% over the 5 years on moderate (standard practice) vs. control treatments, but effects were substantially smaller for the non‐leaf‐gleaner guild in years 1–4 (8–25%) and disappeared by year 5 (0%). Both leaf‐gleaner and non‐leaf‐gleaner functional groups continued to colonize moderate and intensive treatments at higher probabilities than the control in Year 5 (range: 0·17–0·29), likely due to rapid vegetation recovery after herbicide applications ceased. Planted conifers were >35% taller and >70% larger in diameter in the two most intensive treatments, leading to substantially more wood volume produced per unit area than on stands without herbicide applications. Synthesis and applications. Under current management regimes, stand‐level trade‐offs between conservation of avian diversity and production of wood commodities may be less severe than previously recognized. However, in landscapes where biodiversity conservation rather than wood production is the primary goal, managers can adjust the trade‐off by making relatively small reductions in herbicide use that should have positive impacts on richness of leaf‐gleaning insectivorous birds.

No evidence for a link between forest herbicides and offspring sex ratio in a migratory songbird using high-throughput molecular sexing

“…It is important to note, however, that our study was not designed a priori to distinguish between direct and indirect effects, making us unable to rule out alternative hypotheses. Nevertheless, when considered with the finding that the intensive stands experienced greater reduction in the woody vegetation upon which sparrows forage during breeding ( Kroll et al , 2017 ), our study provides evidence that rules out both direct and indirect effects of herbicide application on offspring sex offspring. We note that species can diverge in their response to herbicides and that the number of studies on herbicide application intensity on wild, free-ranging animal populations is limited; therefore, this topics merits additional study ( Kohler and Triebskorn, 2013 ).…”

“…Despite this, we found no evidence that sparrow nestling sex ratio was influenced by experimental herbicide application intensity; indeed, we detected only a 3% difference in sex ratio estimates between the most extreme treatments when all broods were considered. This is noteworthy because theory predicts that offspring sex ratio becomes increasingly female-biased as habitat quality is reduced ( Trivers and Willard, 1973 ; Charnov, 1982 ), and herbicide application led to pronounced and measurable effects on the composition of vegetation on our study sites ( Kroll et al , 2017 ). Why experimental treatments strongly altered sparrow habitat yet did not influence offspring sex ratio is unclear, but one explanation is that female sparrows were unable to control offspring sex ( Pike and Petrie, 2003 ).…”

“…Stands in the intensive treatment received herbicide applications during both years of our study (i.e. herbaceous spray in 2013, and broadleaf spray in 2014), whereas stands representing other treatments did not ( Kroll et al , 2017 ). Intensive treatment stands were sprayed in late April and early May and therefore coincided with the timing of territory establishment by sparrows on those sites (J. Rivers, unpublished data).…”

“…Finally, the intensive herbicide treatment consisted of a site preparation broadleaf spray (2010), a herbaceous spray in 2011, 2012 and 2013, and a broadleaf release spray in 2012 and 2014. We note that the timing, amount, and type of chemicals used were consistent across all study blocks within each treatment; such details are provided elsewhere ( Betts et al , 2013 ; Kroll et al , 2017 ).…”

“…To date there has been limited evidence for direct toxic effects of forest herbicides on free-ranging animal populations ( Tatum, 2004 ; McComb et al , 2008 ; but see Relyea, 2005 ), and this has been attributed to the short retention time of herbicides ( Newton et al , 1984 ) and their targeting of plant-specific physiological mechanisms ( Tatum, 2004 ; McComb et al , 2008 ). In contrast, the indirect effects of herbicides are thought to play a more important role in influencing animal populations, most commonly through herbicide-mediated changes to habitat ( Betts et al , 2013 ; Kroll et al , 2017 ).…”