Detection of edaphic discontinuities with ground-penetrating radar and electromagnetic induction

Quantification of edaphic properties which may regulate the spatial distribution of vegetation is often limited by the expense and labor associated with collecting and analyzing soil samples. Here we evaluate the utility of two technologies, ground-penetrating radar (GPR) and electromagnetic induction (EMI), for rapid, extensive and non-destructive mapping of diagnostic subsurface features and soil series map unit boundaries.Strong reflectance from fine-textured, near-surface soils obscured radar signal reflectance from deeper horizons at our field test site in the Rio Grande Plains of southern Texas, USA. As a result, ground-penetrating radar did not delineate known edaphic contrasts along catena gradients. In contrast, EMI consistently distinguished boundaries of soil map units. In several instances, gradients or contrasting inclusions within map units were also identified. In addition, the location and boundary of calcic or cambic-horizon inclusions embedded within a laterally co-extensive and well-developed argillic horizon were consistently predicted. Correlations between EMI assessments of apparent conductivity (ECa) and soil properties such as CEC, pH, particle size distribution and extractable bases were low (i.e., explained<6% of the variance), or non-significant. As a result, EMI has a high prospecting utility, but cannot necessarily be used to explain the basis for edaphic contrasts.Results suggest EMI can be a cost-effective tool for soil survey and exploration applications in plant ecology. As such, it is potentially useful for rapidly locating and mapping subsurface discontinuities, thereby reducing the number of ground truth soil samples needed for accurate mapping of soil map unit boundaries. An application, addressing hypotheses proposed to explain the role of edaphic heterogeneity in regulating woody plant distribution in a savanna parkland landscape, is presented.     

Deer prevent western redcedar (Thuya plicata) regeneration in old-growth forests of Haida Gwaii: Is there a potential for recovery?

The current increase in deer populations in many forests has fostered a growing concern about their impact on forest ecology. Sitka black-tailed deer (Odocoileus hemionus sitchensis) were introduced to Haida Gwaii (British Columbia, Canada) in the late 19th century, and they have dramatically affected the regeneration of woody species in both old- and second-growth forests since then. The lack of recruitment in western redcedar (Thuja plicata) in old-growth forests has been attributed to deer. The objectives of this study were to (1) experimentally confirm that deer browsing causes a lack of western redcedar recruitment and (2) assess the potential for and speed of recovery after a prolonged exclusion of deer. We installed a set of 20 enclosures and monitored them over a period of 8 years from 1997 to 2005. We compared temporal changes in redcedar cover and in the survival and growth of marked seedlings in plots that were or were not accessible to deer. Redcedar cover in the vegetation layer accessible to deer was generally low (from 3 to 5%) but higher inside the enclosures (an average difference of 2.3%). Protected seedlings survived better, were higher, presented more leafed shoots, and had less stems than unprotected individuals, features that suggest that deer were key to the lack of regeneration. However, growth was very slow (protected marked seedlings grew 2.5 cm on average in 8 years) and, under the current conditions, the time required for a protected seedling to escape deer would probably take over two decades. This very slow growth rate under closed canopy conditions probably reflects a gap-phase regeneration strategy and/or sensitivity to competition with other woody species. The combination of a very slow growth with a high palatability and a lack of physical defences, in contrast to the other dominant conifers in this ecosystem, probably explain why redcedar regeneration can be eliminated from old-growth forest by abundant deer populations.