Earth Science

PUBLICATIONS IN EARTH SCIENCES (from another life)

Annual variation in soil respiration and its component parts in two structurally contrasting woody savannas in Central Brazil (with P Meir, G Saiz, L Maracahipes, B Schwantes Marimon & J Grace) Plant and Soil , 2012; 352:129–142.

Due to the high spatial and temporal variation in soil CO₂ efflux, terrestrial carbon budgets rely on a detailed understanding of the drivers of soil respiration from a diverse range of ecosystems and climate zones. In this study we evaluate the independent influence of vegetation structure and climate on soil CO₂ efflux within cerrado ecosystems. We found no significant difference in the annual soil CO₂ efflux between the two stands (p = 0.53) despite a clear disparity in both LAI (p < 0.01) and leaf litterfall (p < 0.01). The mean annual loss of carbon from the soil was 17.32(±1.48) Mg C ha⁻¹ of which approximately 63% was accounted for by autotrophic respiration. The relative contribution of autotrophic respiration varied seasonally between 55% in the wet season to 79% of the total soil CO₂ efflux in the dry season. Furthermore, seasonal fluctuations of all the soil respiration components were strongly correlated with soil moisture (R ² = 0.79–0.90, p < 0.01). Across these two structurally distinct cerrado stands, seasonal variations in rainfall, was the main driver of soil CO₂ efflux and its components. Consequently, soil respiration within these ecosystems is likely to be highly sensitive to any changes in seasonal precipitation patterns.

Estimates and relationships between aboveground and belowground resource exchange surface areas in a Sitka spruce managed forest (with N Barbier, J Cermák, J Koller, C Thornily, C McEvoy, B Nicoll, M P Perks, J Grace, P Meir) Tree Physiol, 2010; 30(6):705-14.

Our knowledge of the nature of belowground competition for moisture and nutrients is limited. In this study, we used an earth impedance method to determine the root absorbing area of Sitka spruce (Picea sitchensis (Bong.) Carr.) trees, making measurements in stands of differing density (2-, 4- and 6-m inter-tree spacing). We compared absorbing root area index (RAI(absorbing); based on the impedance measure) with fine root area index (RAI(fine); based on estimates of total surface area of fine roots) and related these results to investment in conductive roots. Root absorbing area was a near-linear function of tree stem diameter at 1.3 m height. At the stand level, RAI(absorbing), which is analogous to and scaled with transpiring leaf area index (maximum stomatal pore area per unit ground area; LAI(transpiring)), increased proportionally with basal area across the three stands. In contrast, RAI(fine) was inversely propotional to basal area. The ratio of RAI(absorbing) to LAI(transpiring) ranged from 7.7 to 17.1, giving an estimate of the relative aboveground versus belowground resource exchange areas. RAI(absorbing) provides a way of characterizing ecosystem functioning as a physiologically meaningful index of belowground absorbing area.

Ecological implications of anti-pathogen effects of tropical fungal endophytes and mycorrhizae (with E Allen Herre , L C Mejía, D A Kyllo, E Rojas, Z Maynard, and S A Van Bael) Ecology, 2007; 88(3):550-8.

We discuss studies of foliar endophytic fungi (FEF) and arbuscular mycorrhizal fungi (AMF) associated with Theobroma cacao in Panama. Direct, experimentally controlled comparisons of endophyte free (E-) and endophyte containing (E+) plant tissues in T. cacao show that foliar endophytes (FEF) that commonly occur in healthy host leaves enhance host defenses against foliar damage due to the pathogen (Phytophthora palmivora). Similarly, root inoculations with commonly occurring AMF also reduce foliar damage due to the same pathogen. These results suggest that endophytic fungi can play a potentially important mutualistic role by augmenting host defensive responses against pathogens. There are two broad classes of potential mechanisms by which endophytes could contribute to host protection: (1) inducing or increasing the expression of intrinsic host defense mechanisms and (2) providing additional sources of defense, extrinsic to those of the host (e.g., endophyte-based chemical antibiosis). The degree to which either of these mechanisms predominates holds distinct consequences for the evolutionary ecology of host-endophyte-pathogen relationships. More generally, the growing recognition that plants are composed of a mosaic of plant and fungal tissues holds a series of implications for the study of plant defense, physiology, and genetics.