Summary1. Anthropogenic global change compromises forest resilience, with profound impacts to ecosystem functions and services. This synthesis paper reflects on the current understanding of forest resilience and potential tipping points under environmental change and explores challenges to assessing responses using experiments, observations and models. 2. Forests are changing over a wide range of spatio-temporal scales, but it is often unclear whether these changes reduce resilience or represent a tipping point. Tipping points may arise from interactions across scales, as processes such as climate change, land-use change, invasive species or deforestation gradually erode resilience and increase vulnerability to extreme events. Studies covering interactions across different spatio-temporal scales are needed to further our understanding. 3. Combinations of experiments, observations and process-based models could improve our ability to project forest resilience and tipping points under global change. We discuss uncertainties in changing CO 2 concentration and quantifying tree mortality as examples. 4. Synthesis. As forests change at various scales, it is increasingly important to understand whether and how such changes lead to reduced resilience and potential tipping points. Understanding the mechanisms underlying forest resilience and tipping points would help in assessing risks to ecosystems and presents opportunities for ecosystem restoration and sustainable forest management.

The impact of drought on the hydraulic functioning of important African tree species, like Maesopsis eminii Engl., is poorly understood. To map the hydraulic response to drought-induced cavitation, sole reliance on the water potential at which 50% loss of xylem hydraulic conductivity (ψ50) occurs might be limiting and at times misleading as the value alone does not give a comprehensive overview of strategies evoked by M. eminii to cope with drought. This article therefore uses a methodological framework to study the different aspects of drought-induced cavitation and water relations in M. eminii. Hydraulic functioning of whole-branch segments was investigated during bench-top dehydration. Cumulative acoustic emissions and continuous weight measurements were used to quantify M. eminii's vulnerability to drought-induced cavitation and hydraulic capacitance. Wood structural traits, including wood density, vessel area, diameter and wall thickness, vessel grouping index, solitary vessel index and vessel wall reinforcement, were used to underpin observed physiological responses. On average, M. eminii's ψ50 (±SE) was -1.9 ± 0.1 MPa, portraying its xylem as drought vulnerable, just as one would expect for a common tropical pioneer. However, M. eminii additionally employed an interesting desiccation delay strategy, fuelled by internal relocation of leaf water, hydraulic capacitance and the presence of parenchyma around the xylem vessels. Our findings suggest that exclusive dependence on ψ50 would have misdirected our assessments of M. eminii's drought stress vulnerability. Hydraulic capacitance linked to anatomy and leaf-water relocation behaviour was equally important to better understand M. eminii's drought survival strategies. Because our study was conducted on branches of 3-year-old greenhouse-grown M. eminii seedlings, the findings cannot be simply extrapolated to adult M. eminii trees or their mature wood, because structural and physiological plant properties change with age. The techniques and methodological framework used in this study are, however, transferable to other species regardless of age.

Parasites of the larvae of the eucalypt-defoliating chrysomelid Paropsis atomaria Ol. in the Australian Capital Territory, Australia, in 1975-77 included the hymenopteran primary parasite Eadya paropsidis Huddleston & Short, the hyperparasites Perilampus tasmanicus Cam. and Mesochorus sp., and the tachinids Anagonia anguliventris (Mall.) (Froggattimyia anguliventris), F. tillyardi Mall. and Paropsivora sp. For the December-January 1st-generation larvae of Paropsis atomaria in 1976-77, E. paropsidis achieved an average 49% parasitism, and the tachinids 19%; the two kinds of parasites occurred together in 18% of parasitised larvae. Planidia of Perilampus tasmanicus attacked an average of 68% of the tachinid larvae and 70% of the larvae of E. paropsidis. Mesochorus occurred only in tachinid larvae, with a mean parasitism of 35% over the season, but it was not found after early January; multiparasitism with P. tasmanicus was found in 24% of parasitised tachinids. The number of progeny of the primary parasites deposited in one chrysomelid larva rarely exceeded 5 for tachinids and 6 for E. paropsidis. This occurred whether the parasites were intra- or interspecifically competing. The exceptionally high reproductive capacity of P. tasmanicus was related to poor survival: up to 50 planidia were found inside a single chrysomelid larva, in a phoretic relationship awaiting primary parasite larvae.

No abstract

Generally, tree species load photoassimilates passively into the phloem, while herbaceous species load actively. These phloem loading strategies have implications for phloem sugar concentration and growth potential. Whereas, in previous research, phloem loading identification was performed with 14 C-autoradiography, we suggest 11 C-autoradiography, because of its compatibility with plant-PET (positron emission tomography) scans. Because 11 C-autoradiography has been hardly used in plant sciences so far, it was tested in contrasting plant species: one temperate tree species, Populus tremula L., three tropical tree species, Erythrophleum suaveolens (Guill. & Perr.) Brenan, E. ivorense A. Chev., and Maesopsis eminii Engl., and two herbaceous crop species Solanum lycopersicum L. and S. tuberosum L. Our results confirmed that P. tremula is a passive loader, and Solanum spp. are active loaders. Erythrophleum spp. and young leaves of M. eminii showed the expected passive loading strategy, but the mature leaves of M. eminii showed an uncommon pattern. Images corrected for leaf tissue thickness supported that mature leaves of M. eminii used active phloem loading, which is linked to continuous investment in growth and new leaves, supporting the lower carbon storage levels often observed in tropical tree species. With this study, we demonstrate that 11 C-autoradiography is a powerful tool to acquire detailed tracer distribution in leaves to typify phloem loading strategies in plant species.