Reproduction and abundance of the wood frog (Lithobates sylvaticus) in post-harvest variable retention forests

  • Author / Creator
    Robinson, Matthew P
  • Many aquatic-breeding amphibians require freshwater habitat for reproduction and terrestrial habitat for refuge, foraging, and overwintering. Variable retention harvesting is a technique where live trees and other forest features are retained during timber harvesting in patterns that emulate natural disturbance. Wood frogs (Lithobates sylvaticus) are often associated with closed-canopy deciduous forests and utilize ephemeral (temporary) wetlands for reproduction. The objectives of this thesis were to (1) investigate the factors influencing upland abundance of wood frogs in post-harvest variable retention forest stands and (2) investigate the effects of wetland size and forest canopy on hydroperiod and tadpole performance in ephemeral wetlands of Alberta’s boreal mixedwood. To address these objectives, I used pitfall traps to live capture wood frogs across 4 levels of retention harvest (clearcut [0%], 20%, 50%, and unharvested control [100%]), and 2 forest types (deciduous and conifer), in 17-year post harvest forests at the EMEND experiment in northwest Alberta. I mapped breeding sites to account for breeding site proximity and used a LiDAR-based terrain moisture index (Depth-to-Water) to account for soil moisture. I also monitored 15 small ephemeral breeding wetlands from May to August 2015, at which I documented drying dates to assess relationships between wetland size (surface area, maximum depth), forest canopy cover, and hydroperiod. In 12 of the 15 wetlands, I measured tadpole performance by sampling tadpoles over repeated sampling sessions until tadpoles completed metamorphosis or until wetlands dried. I also measured physiochemical parameters (pH, conductivity, and temperature) and primary productivity (periphyton growth) to compare conditions between open- and closed-canopy wetlands. Abundance of adult wood frogs varied by season, with most captures occurring during early spring and summer months (May and June). Harvest retention level alone had no effect on abundance, but in late season (July and August), there was a significant interaction between retention and forest type where abundance decreased with retention level in deciduous sites, and increased with retention level in conifer sites. The interaction effect, however, was weak, and differences in capture rates between retention levels were small. During late season capture rates were higher in conifer forests relative to deciduous forests, with soil moisture (lower Depth-to-Water) significantly and positively related to capture rates. These results suggest early regeneration of aspen and availability of moist microhabitats create suitable upland habitat for wood frogs in early seral stage mixedwood forests. Among breeding wetlands, hydroperiod was related to wetland size, but not forest canopy cover. Depth was most related to hydroperiod and may therefore serve as a useful criterion for prioritizing protection of ephemeral breeding wetlands during forest harvesting. Growth and development of wood frog tadpoles were faster in wetlands with less surrounding canopy cover. Water temperature was higher in open-canopy wetlands relative to closed-canopy wetlands which may help explain observed differences in performance. There was, however, were no significant difference in primary productivity between open- and closed-canopy wetlands. I conclude that timber retention level has a weak effect on relative abundance of adult wood frogs in 17-year post harvest stands. Natural regeneration of deciduous species post-harvest may help provide suitable upland habitat for wood frogs in both deciduous and conifer forests. Protection of ephemeral wetlands with adequate depth and hydroperiod will help maintain local populations of wood frogs and other amphibians in managed forests in the boreal mixedwood.

  • Subjects / Keywords
  • Graduation date
    Fall 2017
  • Type of Item
  • Degree
    Master of Science
  • DOI
  • License
    This thesis is made available by the University of Alberta Libraries with permission of the copyright owner solely for non-commercial purposes. This thesis, or any portion thereof, may not otherwise be copied or reproduced without the written consent of the copyright owner, except to the extent permitted by Canadian copyright law.