Climate variation, reproductive frequency and acorn yield in English Oaks.

AIMS -: Annually variable, but synchronous production of large seed crops ('masting') is a widespread phenomenon in temperate trees. Mounting concerns about the impacts of anthropogenic climate change (ACC) on plant reproduction, gives urgency to our need to understand better the role of climate on tree reproduction, and in particular, mast events. Unlike our understanding of reproductive phenology however, there is little consensus regarding how climate affects plant reproductive effort, or indeed the actual environmental triggers that underpin masting behaviour. METHODS -: We used a 27-year record of acorn yield from a population of 12 Quercus robur trees located in southern England to compare masting frequency and post-dispersal acorn yield each year for each tree, with long-term weather data over the same period. We focussed on discrete or sequential climate cues (temperature, precipitation, and frost days) as likely predictors of oak reproduction. IMPORTANT FINDINGS -: Annual post-dispersal acorn crop varied greatly; i.e. no acorns in 14 of the 27 years, but there was no sequential pattern of crop versus non-crop years indicating that weather, rather than resource limitation alone, dictated the timing of reproduction. Crop years were instead most closely associated with relatively cool late summer conditions in the preceding year, followed by anomalous summer warmth within crop year. Acorn yield increased following dry April and above average May and June temperatures within crop year. Although our results support a general association between warm late spring and summer conditions, and crop frequency and yield respectively, the influence of cooler later summer conditions in the year prior to masting highlights how a combination of weather cues may dictate the occurrence of mast years. Consequently, our results corroborate not only the hypothesis that temperature differentials between consecutive years, not absolute temperatures, may be the better predictor of mast seeding events, but lend support also to the suggestion that reproductive failure and resource accumulation resulting from a climate-linked environmental veto, drives future reproductive synchronization in temperate tree species.

Differential responses of vertebrate and invertebrate herbivores to traits of New Zealand subalpine shrubs.

Plant traits are influenced by herbivore diet selection, but little is known about how traits are affected by different types of herbivores. We related eight traits of 27 subalpine shrub species in South Island, New Zealand, to damage of these shrubs by introduced red deer (Cervus elaphus) and native invertebrate herbivores using phylogenetically explicit modeling. Deer preferentially consumed species that grew quickly, were low in foliar tannins, or had high leaf area per unit mass. However, these traits did not trade off against each other; rather, they could be related to different multivariate defense strategies. Although the proportion of leaves damaged by leaf-chewing invertebrates also increased with stem growth, invertebrates did not damage the same fast growing species as those preferred by deer. Other traits may also be important in determining herbivore preferences, as suggested by the high proportion of variation in herbivory explained by phylogeny. Last, we found that the composition of invertebrate herbivore communities was more similar among closely related shrubs, and consequently, the range of invertebrate-plant associations may change if introduced deer shift plant composition toward slow-growing species. Overall, our results demonstrate the importance of herbivore type and coevolved interactions for the adaptive significance of plant traits.

An analytical model assessing the potential threat to natural habitats from insect resistance transgenes.

We examine the role of ecological interactions on effective gene flow from genetically manipulated plants to their wild relatives. We do so by constructing and applying to oilseed rape (OSR) an analytical model for interaction between plants with and without an insect resistance (IR) allele in natural communities, incorporating documented levels of herbivore variability. We find that with reasonable values of advantage to the IR allele, little concomitant disadvantage (physiological costs of the allele) restricts it to low proportions of the natural population for large numbers of generations. We conclude that OSR IR transgenes are unlikely to pose an immediate threat to natural communities. Our model identifies those factors best able to regulate particular transgenes at the population level, the most effective being impaired viability of seeds in the period between production and the following growing season, although other possibilities exist. Because solutions rely on ratios, limiting values of regulating factors are testable under controlled conditions, minimizing risk of release into the environment and offering significant advancement on existing testing programmes. Our model addresses folivory but is easily modified for herbivory damaging the seed or directly affecting seed production by infested plants, or for pathogens altering seed survival in the seedbank.