2. Climate-change impacts on plants and plant pests
Geographical distribution and population dynamics of plant pests
Globally rising temperatures, extreme weather events and altered rainfall patterns are predicted effects of climate change on the environment. These and other related variables may have anticipated and unanticipated effects on plant biology, the distribution and abundance of plant and pest species, and natural enemies.
Different populations of pest species can respond in different ways to climate change, with the range of some shifting, some contracting, some expanding and others disappearing. Poleward expansion of many pests and pathogens has been noted since 1960 (Bebber, Ramotowski and Gurr, 2013). These asymmetric distribution changes can lead to new suites of pests in combination with host plants. The consequences of changes in pest distribution on future crop production and food security are considered to be hard to predict (IPCC, 2022b), although there are some valuable assessments of potential scenarios in Europe (EEA, 2019), China and the Americas (Ullah et al., 2023).
Ongoing rapid changes in community structure and species distributions directly linked to climate change have been widely documented both in natural communities and in invasive species. The expanding distribution of pine processionary moth (Thaumetopea pityocampa) in Europe has been linked to increased winter temperatures (Battisti et al., 2005). Numerous pests, such as the plant redroot pigweed (Amaranthus retroflexus), are also expected to expand their geographical range in association with climate change (Hyvönen, Luoto and Uotila, 2012). Examples of changing phenology include those described by Gordo and Sanz (2005), who showed how the phenology of Colorado potato beetle (Leptinotarsa decemlineata) and olive fly (Bactrocera oleae) had changed over the second half of the twentieth century as a result of climate change and described how these changes have the potential to influence the pest status of these pests. Changes in the phenology of pests or plants can lead to changes in the synchronization between the susceptible stage of the plant and the abundance of the feeding or infective stage of the pest. Such changes have the potential to increase or decrease the impact of the pest.
Evolution of plant pests
Some pests are thought to have evolved in response to climate change, leading to more virulent lineages. For example, Puccinia striiformis f. sp. tritici causes wheat yellow (stripe) rust and was previously found predominantly in cold areas. Since 2000, however, novel strains that are more aggressive and thermotolerant have been recorded spreading into new regions (Mboup, 2012).
Climate-change impacts on agriculture (crops or horticulture)
Temperature is one of the most important factors affecting the distribution and abundance of plants, because of the physiological limits of each species. It limits the geographical areas in which different crops can grow and also affects their development rate, growth rate and yields. Increased carbon dioxide levels are also likely to affect plant physiology by increasing photosynthetic activity, resulting in better growth and higher plant productivity. This in turn indirectly affects insects by changing both the quantity and quality of plants (Skendžić et al., 2021).
Although overall agricultural productivity has increased, climate change has slowed this growth over the past 50 years. Globally, related negative impacts have mainly been in mid- and low-latitude regions, but positive impacts have occurred in some high-latitude regions (IPCC, 2022a). The climate impacts for the past 20–50 years differ by crops and regions. Positive effects have been identified for wheat in northern Africa and northern Europe, rice in Australia and New Zealand, cereals in central Asia and maize, and soybean in northern America. The effects are mostly negative in sub-Saharan Africa, South America and the Caribbean, western and southern Asia, western and southern Europe, and at the overall global level (IPPC 2022c). Climate change has also impacted the productivity of vegetable and fruit crops in Nepal; for example, shifting climatic zones are reducing fruit production (Subedi, 2019).
Climate change is also opening new agricultural frontiers around the globe. Models suggest that the new frontiers will be most extensive in the northern hemisphere and in mountainous areas worldwide. Cold-tolerant temperate crops such as potatoes, wheat and corn have some of the greatest potential for expansion into these new areas. In addition, crops sown in existing agricultural areas are expected to shift their distribution in response to shifting climatic suitability. Shifting crop cultivation has the potential to cause major economic (e.g. food production) and environmental (e.g. biodiversity, ecosystem services) impacts. The environmental consequences of shifting crop production to new areas can include impacts on water, wildlife, pollinator interaction, carbon storage and nature conservation, on national to global scales (Hannah et al., 2020). Pests are likely to accompany their host crops into new areas unless appropriate risk mitigation is in place, and they have the potential to cause harmful impacts beyond those to the crops themselves.
Extreme events can damage crops and natural vegetation. These disturbances, in addition to the changing atmospheric conditions, provide ideal opportunities for invasive species to enter and spread. Some of the key features of invasive species give them the ability to colonize new disturbed areas (Orbán et al., 2021).
Climate-change impacts on trees, forests and the environment
Climate change will have positive and negative effects on forests, with varying regional and temporal patterns. Increasing productivity has been recorded in high-latitude forests such as those in Siberia, while in other regions negative impacts are already being observed, such as increasing tree mortality as a result of wildfires and droughts. Large pulses of tree mortality have been consistently linked to warmer and drierthan-average conditions for forests throughout the temperate and boreal biomes. Long-term data relating to tropical forests indicates that climate change has begun to increase tree mortality and alter regeneration. Climate-related dieback has also been observed, resulting from novel interactions between the life cycles of trees and pest species (IPPC, 2022c). For example, the incidence of sooty bark disease of sycamore trees (Acer pseudoplatanus) has been linked to drought conditions in Germany (Schlößer et al., 2023). Further information about the impacts of climate change on forest systems is provided in Chapter 5 of the IPCC Sixth Assessment Report (IPPC, 2022c): www.ipcc.ch/report/ar6/wg2/downloads/ report/IPCC_AR6_WGII_FullReport.pdf
