Martin G. Barker

Effect of liana cutting on water potential and growth of adult Senna multijuga (Caesalpinioideae) trees in a Bolivian tropical forest

Abstract Lianas, or woody climbing plants, are a major constituent of seasonally dry tropical forests, and are thought to impact negatively their host trees. In this study we evaluated whether liana presence was associated with reduced leaf water potentials and growth in adult Senna multijuga trees during the dry season in a lowland Bolivian forest. We used leaf water potentials in trees as a first approach to assess trees’ water status, under the assumption that leaf water potentials become more negative when water losses (via transpiration) exceed gains (by uptake). We measured relative growth in girth at 1.5 m height (gbh) to quantify tree growth. At the beginning of the 1996 dry season (early June), we selected 20 S. multijuga trees 10–20 cm dbh, and measured their gbh. We also recorded pre-dawn and mid-day leaf water potentials in these trees. In ten experimental trees all lianas were then cut, while the remaining trees were used as controls. Pre-dawn and mid-day water potentials were re-measured 1 day after liana-cutting, and then every week in all trees for 1 month and then at 3 and 5 months, until the beginning of the next rainy season (November); gbh was measured again in July 1997 to estimate relative growth rate. Liana removal was associated with less negative pre-dawn (–0.3 vs –0.4 MPa) and mid-day (–0.5 vs –0.7 MPa) water potentials in trees during the dry season. This difference appeared as early as 1 day after cutting, and disappeared once the rainy season began. Liana-cut trees grew more (0.4 mm/mm year) than liana-uncut trees (0.2 mm/mm year). These findings suggest that lianas may interfere with water availability to these trees during the dry season, and may also hinder tree growth.

Photosynthetic characteristics of dipterocarp seedlings in three tropical rain forest light environments: a basis for niche partitioning?

Abstract In three tropical rain forest light environments in Sabah, Malaysia, we compared photosynthesis in seedlings of ten climax tree species with putatively differing shade tolerances. The objectives of the study were (a) to characterise the range of photosynthetic responses in ten species of the Dipterocarpaceae and (b) to elucidate those photosynthetic characteristics that might provide a basis for niche partitioning. Seedlings were acclimated (c. 7 months) in three light environments; understorey, partial shade and a gap (140 m2). The light environments represented a gradation in median diurnal (0630–1830 hours) photon flux density (PFD) ranging from understorey (4.7 μmol m−2 s−1), through partial shade (21.2 μmol m−2 s−1) to gap (113.7 μmol m−2 s−1). Integrated diurnal PFD were in the sequence gap > partial shade > understorey (15.2, 4.7, 1.3 mol m−2 day−1, respectively). In gap-acclimated plants, species differed in the photosynthetic light-response variables apparent quantum yield, dark respiration rate, light compensation point, net saturated leaf assimilation rate (Asat), and in stomatal conductance (gs sat) when assimilation rate (A) was saturated. A light-demanding pioneer species (Macaranga hypoleuca) and a shade-demanding understorey species (Begonia sp.) had, respectively, higher and lower Asat and gs sat than the dipterocarp species. In high-light conditions Asat and gs sat were strongly positively correlated in dipterocarp species. Differing photosynthetic characteristics of gap-acclimated plants suggest that, in these dipterocarp species, different rates of carbon fixation may be an important factor contributing towards niche partitioning. Mean integrated diurnal A (Adiurnal) in the gap, partial shade and understory were, respectively, 122.9, 52.7, 20.5 mmol m−2 day−1. Differences occurred in Adiurnal of dipterocarp species between light environments. When Macaranga was included, differences in Adiurnal were evident in the gap and partial shade, and in both cases were attributed to the pioneer. For the variable Adiurnal, there was of a shift in the rank position of Macaranga among light environments, but a shift did not occur among the dipterocarp species. Results from this study are consistent with the idea that rates of carbon fixation per unit leaf area may contribute towards niche differentiation between the climax and single pioneer species, but not within the group of climax species. Other physiological and/or carbon allocation factors may be involved in any niche partitioning; dipterocarp species often have inherently different growth rates and susceptibility to herbivory. As an alternative to niche partitioning, dipterocarp species may co-exist in natural light environments as a result of habitat disequilibrium or purely stochastic processes.

Forest canopy research: sampling problems, and some solutions

Conducting research in forest canopies is accompanied by both opportunities and difficulties. Opportunities occur because the canopy is important in overall forest structure and function, for example in containing a high proportion of species richness, being the site of forest-atmosphere fluxes, and in strongly influencing understory microclimate. However, despite this importance, the canopy has been largely neglected by researchers, until recently. Many difficulties in forest canopy research apparently arise from problems related to access. In a survey of 112 canopy researchers, examples of problems cited include: bias in the selection of study species or sampling locations; difficulties in obtaining adequate replication; problems in collecting data in a three-dimensional, complex environment; logistical constraints in moving between canopy sampling points; and, disturbance, sometimes including interference of the object being studied (i.e., ‘demonic intrusion’) caused by access. Many of these problems have at least partial solutions. These include: using multiple access techniques to give greater sampling flexibility; identifying appropriate units of replication within the canopy; using ground-operated canopy sampling methods; and, using protocols for unbiased and systematic data collection in three-dimensional sampling space. Designing experimental protocols for work in the canopy requires at the outset the allocation of time and resources for anticipating and overcoming the particular sampling problems associated with the work.

Closing the Feedback Loop? Iterative Feedback between Tutor and Student in Coursework Assessments.

We evaluate the case for using feedback iteratively, to improve student engagement and learning. In this model, students were invited to respond to tutor feedback with students’ own responses. Among the three courses/modules (three tutors) studied, differences in feedback styles were evident from: (a) thematic analysis of tutor comments and, for one course, and (b) comments in a student focus group. Students were inconsistent in their evaluation of tutor feedback but, in a more detailed study of one of the courses, there was clear evidence for the benefits of iterative feedback. Lessons from the main study were then applied to a course that had not previously incorporated iterative feedback. Using this experience, we provide suggestions for applying iterative feedback in assessments.

Tutorials for Enhancing Skills Development in First Year Students Taking Biological Sciences

Abstract In order to increase engagement and to consolidate skills, a tutorial-based skills course (module) was introduced as a compulsory component of first-year in the School of Biological Sciences, University of Aberdeen. We evaluated whether students had attained certain ‘graduate attributes’ during the course, comprising: transferable and generic skills and intellectual curiosity. Student feedback from course evaluation forms, student diaries and focus groups indicated that the course increased students’ confidence in questioning accepted wisdom and helped students to develop generic skills. Feedback forms generally indicated a strong appreciation of interactions with tutors, and also of learning in small groups, interacting with peers within degree programmes, personalised learning, and developing generic skills. The quality of students’ experience varied, probably due to differences among students themselves as well as with their tutors. Students’ use of the course website indicated strategic use of materials to enhance the development of transferable/generic skills. Activity on the course website tended to be greatest among those students who went on to achieve higher overall course marks. Tutors were more cautious than students in claiming that students were more ready to question information having done the tutorial course. Intellectual curiosity among students was more apparent later in the course, suggesting that students had been learning critical thinking skills.