Rebecca Darbyshire

Impact of future warming on winter chilling in Australia

Increases in temperature as a result of anthropogenically generated greenhouse gas (GHG) emissions are likely to impact key aspects of horticultural production. The potential effect of higher temperatures on fruit and nut trees’ ability to break winter dormancy, which requires exposure to winter chilling temperatures, was considered. Three chill models (the 0–7.2°C, Modified Utah, and Dynamic models) were used to investigate changes in chill accumulation at 13 sites across Australia according to localised temperature change related to 1, 2 and 3°C increases in global average temperatures. This methodology avoids reliance on outcomes of future GHG emission pathways, which vary and are likely to change. Regional impacts and rates of decline in chilling differ among the chill models, with the 0–7.2°C model indicating the greatest reduction and the Dynamic model the slowest rate of decline. Elevated and high latitude eastern Australian sites were the least affected while the three more maritime, less elevated Western Australian locations were shown to bear the greatest impact from future warming.

Determinants of establishment success for introduced exotic mammals

We conducted comparisons for exotic mammal species introduced to New Zealand (28 successful, 4 failed), Australia (24, 17) and Britain (15, 16). Modelling of variables associated with establishment success was constrained by small sample sizes and phylogenetic dependence, so our results should be interpreted with caution. Successful species were subject to more release events, had higher climate matches between their overseas geographic range and their country of introduction, had larger overseas geographic range sizes and were more likely to have established an exotic population elsewhere than was the case for failed species. Of the mammals introduced to New Zealand, successful species also had larger areas of suitable habitat than did failed species. Our findings may guide risk assessments for the import of live mammals to reduce the rate new species establish in the wild.

Challenges in predicting climate change impacts on pome fruit phenology

Climate projection data were applied to two commonly used pome fruit flowering models to investigate potential differences in predicted full bloom timing. The two methods, fixed thermal time and sequential chill-growth, produced different results for seven apple and pear varieties at two Australian locations. The fixed thermal time model predicted incremental advancement of full bloom, while results were mixed from the sequential chill-growth model. To further investigate how the sequential chill-growth model reacts under climate perturbed conditions, four simulations were created to represent a wider range of species physiological requirements. These were applied to five Australian locations covering varied climates. Lengthening of the chill period and contraction of the growth period was common to most results. The relative dominance of the chill or growth component tended to predict whether full bloom advanced, remained similar or was delayed with climate warming. The simplistic structure of the fixed thermal time model and the exclusion of winter chill conditions in this method indicate it is unlikely to be suitable for projection analyses. The sequential chill-growth model includes greater complexity; however, reservations in using this model for impact analyses remain. The results demonstrate that appropriate representation of physiological processes is essential to adequately predict changes to full bloom under climate perturbed conditions with greater model development needed.

Sun damage risk of Royal Gala apple in fruit-growing districts in Australia

This study estimated minimum air temperatures for potential sun damage for sunburn browning (non-netted and netted) and sunburn necrosis (non-netted) for Royal Gala apple in Australia. The approach estimated when conditions may be conducive to the development of sun damage in some fruit. The approach provides a measure of potential damage. This allows for more flexible analyses of potential sun damage which current models are unable to produce due to data limitations. The air temperature thresholds determined were 34.1 and 38.7 °C, respectively, for browning and necrosis for non-netted fruit and 37.9 °C for browning under netting. These air temperature thresholds were applied across southern Australia from 1911–2013 demonstrating different risk profiles between sites, inter-annual variability and the benefit of installing netting via a reduction in potential damage days. The results can be further extended to estimate impacts from climate change and assess the benefit of installing netting to adapt to increasingly extreme hot weather.

Fruit surface temperature of red-blushed pear: threshold for sunburn damage

ABSTRACT Pear fruit are subject to sun damage from high fruit surface temperature (FST). Threshold FST for the occurrence of sunburn are currently unknown and basic relationships between FST of pears and weather parameters have not been reported. Such knowledge is needed to aid implementation of management practices and improve orchard design to minimise sunburn. This paper reports relationships between pear FST and weather conditions. Continuous monitoring of FST and observations of sunburn damage enabled determination of threshold FST for the development of sunburn. Threshold FST for sunburn damage on ‘ANP-0131’ (Deliza®) pears was estimated to be 47.1 °C. Use of FST thresholds to improve mitigation of sunburn is discussed.

Climate Change: Horticulture

The climate conditions for world horticultural regions are projected to change. Farmers will be required to cope with increasing temperatures, changes to water availability, lack of winter chilling and increased extreme weather exposure. Compared with other agricultural pursuits, horticulture remains particularly exposed due to higher infrastructure and set-up costs and a lack of temporal and spatial flexibility in farming practices. Some regions may benefit from these climatic changes and become better suited to growing horticultural crops whereas in sites where the climate is already at the hotter end of the growing spectrum, continuing production may be judged too risky.

A robust impact assessment that informs actionable climate change adaptation: future sunburn browning risk in apple

Climate change impact assessments are predominantly undertaken for the purpose of informing future adaptation decisions. Often, the complexity of the methodology hinders the actionable outcomes. The approach used here illustrates the importance of considering uncertainty in future climate projections, at the same time providing robust and simple to interpret information for decision-makers. By quantifying current and future exposure of Royal Gala apple to damaging temperature extremes across ten important pome fruit-growing locations in Australia, differences in impact to ripening fruit are highlighted, with, by the end of the twenty-first century, some locations maintaining no sunburn browning risk, while others potentially experiencing the risk for the majority of the January ripening period. Installation of over-tree netting can reduce the impact of sunburn browning. The benefits from employing this management option varied across the ten study locations. The two approaches explored to assist decision-makers assess this information (a) using sunburn browning risk analogues and (b) through identifying hypothetical sunburn browning risk thresholds, resulted in varying recommendations for introducing over-tree netting. These recommendations were location and future time period dependent with some sites showing no benefit for sunburn protection from nets even by the end of the twenty-first century and others already deriving benefits from employing this adaptation option. Potential best and worst cases of sunburn browning risk and its potential reduction through introduction of over-tree nets were explored. The range of results presented highlights the importance of addressing uncertainty in climate projections that result from different global climate models and possible future emission pathways.

Productivity definition of the chilling requirement reveals underestimation of the impact of climate change on winter chill accumulation

Evaluation of chilling requirements (CR) of cultivars of temperate fruit trees provides key information to assess regional suitability, according to winter chill, for both industry expansion and ongoing profitability as climate change continues. Traditional methods for calculating CR use climate controlled chambers and define CR using a fixed budburst percentage, usually close to 50% (CR-50%), without considering the productivity level associated to this percentage. This CR-50% definition may underestimate the real CR of tree crops for optimal productivity. This underestimation is particularly important to consider as winter chill accumulation is declining in many regions due to climate change. In this work we used sweet cherry to analyse the traditional method for calculating CR in many Rosaceae species (CR-50%) and compared the results with more a restrictive, productivity focused method, with CR defined with a 90% bud break level (90%, CRm-90%) close to the optimal budburst which assures productivity. Climate projections of winter chill suitability across Europe using CR-50% and CRm-90% were calculated. Regional suitability landscape was highly dependent on the method used to define CR and differences were found for a wide area of the European geography, both cold and mild winter areas. Our results suggest a need to use an optimal budburst level for the assessment of CR for sweet cherry. The use of traditional methods to determine CR can result in an underestimation of productivity CR with negative consequences for the fruit industry, particularly as climate change advances.

Effectiveness of netting in decreasing fruit surface temperature and sunburn damage of red-blushed pear

ABSTRACT Pear fruit are subject to sun damage from high fruit surface temperature (FST). Shade netting has been shown to decrease FST and sunburn damage of apples but also decreased fruit colour. The aim of this study was to investigate the effects of netting on FST, fruit sunburn and fruit colour of the red-blushed pear cultivar ‘ANP-0131’. FST was measured using fine-wire thermocouples inserted into 54 fruit in a netted block of trees and into 56 fruit in adjoining rows of non-netted trees. Netting reduced the maximum FST by 10%. FST under netting did not exceed the reported sunburn damage threshold of 47°C despite air temperature reaching 38.6°C. The maximum FST in the non-netted site regularly exceeded the sunburn damage threshold and reached a maximum of 50.3°C. Assessments of sunburn damage showed a clear decrease in sunburn (amount and severity) for fruit from the netted trees; however, netting negatively affected colour intensity and coverage.