PENSAMIENTO Y SENTIMIENTO

Phenological events which occur in the spring are most sensitive to climate fluctuations, and research carried out by Sparks and Smithers (2002) provides evidence that spring is getting earlier as a result of climate change. In astronomical terms, spring is defined strictly by the position of the sun over the equator, whereas the general public conceive the term ‘spring’ and its beginning in terms of biological events (Sparks and Smithers, 2002).

91

Annual mean CET (°C)

4.8.1 Temperature Trends and First Flowering Dates

Temperature is not the only influence on phenology, but phenological events are most sensitive to temperature (Fitter and Fitter, 2002). Although there is variability in the mean annual temperature of the UK, the temperature does appear to be on the increase, with there being a significant warming trend since the mid 1980s, as indicated in chapter 2. The year 2006 experienced the warmest year in the Central England Temperature (CET) series, which goes back 350 years (Sparks, 2012). Figure 4.2 shows the records over the last century (trend as determined by Lowess smoothing), with cold winters still being experienced like that of 2010. As data relating to phenological events is not always consistent, records of a minimum of 20 years are recommended for making assumptions between climate and such events. Correlations can then be made between the data. Recently, first flowering dates have advanced in line with noticeable increases in temperature, evident in the CET record from around 1975 (Fitter and Fitter, 2002).

Figure 4.2 Annual mean Central England Temperature (°C) over the last century (1912-2011)

The thick grey line represents the underlying trend (Sparks, 2012)

The recording of spring events for the UK date back to 1736 (Sparks and Carey, 1995), with phenology therefore being the longest written biological record (Sparks and Smithers, 2002). The change is most apparent in ‘early’ spring species. Figure 4.3 displays the relationship between wood anemone (Anemone nemorosa) flowering and March temperature (with historical Royal Meteorological Society data and current data identified separately). It exemplifies that flowering and leafing events have advanced by 6-8 days for every 1°C rise in temperature, and the trend implies that data of this kind can be used to predict future change of species that have been recorded historically (Sparks and Smithers, 2002).

92

Figure 4.3 National mean first flowering date of wood anemone relative to March Central England Temperature (CET, °C) (Sparks and Smithers, 2002). Open circles represent data from 58 years of the RMS

phenological reports, filled circles are data for 1998-2000 from the UK Phenology Network

Figure 4.4 shows trends in the flowering of garden snowdrop (Galanthus nivalis) from Northumberland and Norfolk. Environmental differences between the areas, as well as Northumberland being at a higher latitude, results in different rates of advance, but a similar trend is shown for both series over the last 50 years (Sparks and Smithers, 2002). Rates of advance inherently vary across spatial climes (Schwartz et al., 2006).

Figure 4.4 First flowering dates of snowdrop in Northumberland (upper) and Norfolk (lower);

smoothed lines superimposed (Sparks and Smithers, 2002)

Amano et al. (2010) looked at first flowering dates over a 250 year index for 405 plant species in the UK, and also found climate change is having an effect on multiple species at multiple sites. Current flowering dates for the most recent 25-year period being 2-13 days earlier than any other respective time period since 1760. Fitter and Fitter (2002) also illustrated that in the last decade of the 20^th century flowering dates have advanced for 385 British plant species on average by 4.5 days

93

compared to the 4 decades before it (1954-1990), with 16% of species flowering considerably earlier by 15 days. There is much variation between species, but it was observed that annuals flowered earlier than perennials, and insect-pollinated species more than wind-pollinated species. Globally, there has been evidence to show an advance in phenology across multiple species (Parmesan and Yohe, 2003), but changes are not always consistent and can lead to asynchrony between species (Sparks, 2012). This is discussed in the next section.

4.8.2 Effects of Phenological Changes

Changes in first flowering dates can have several knock on impacts including likeliness of pollination success if the pollinating insects are no longer in synchronisation with such dates, possible alteration of interactions between coexisting species, and increases in the probability of hybridisation. The latter is likely if flowering dates between species gets closer, as illustrated with 12 calcareous grassland species (Fitter and Fitter, 2002). Effects on pollination will also impact the animals that rely on pollen, nectar and seed as a resource.

An empirical study on the interactions between plants and their animal pollinators with phenological shifts after a doubling of atmospheric CO2, led to reduced flora resources for 17-50% of all pollinators, reduced overlap between plants and pollinators and decreased diet variety of the pollinators. Extinction of both plants and pollinators is the expected outcome when there is a disparity in interactions (Memmott et al., 2007).

4.8.3 Trees Fruiting Earlier

Data recording carried out by the public for the Woodland Trust (2011) showed that native trees are fruiting earlier than they were a decade ago, and that this may be a potential response to recent warming. Compared to the period 2000-2012 acorns are ripening 13 days earlier, beech nuts 19 days earlier and rowan berries nearly one month earlier.

4.8.4 Onset of Summer

The onset of summer also appears to be advancing, with 60% of summer flowering plants blooming earlier in the 1990s than in the period 1954-1963 (Kirbyshire and Bigg, 2010). Pollen release also appears to be occurring earlier, in line with spring temperatures, as found in a European study looking at the Birch pollen season (Emberlin et al., 2002).

94

4.8.5 Phenological Differences between Differing Provenances

When comparing phenology processes between varying provenances of Hawthorn (Crataegus monogyna) in mid-Wales, Jones et al. (2001) found that non-local provenances encountered bud-burst in some cases up to 5 weeks before local provenances, illustrating that even genetic differences can have an effect on phenological events. Deans and Harvey (1995) also found that budburst dates varied by more than 3 weeks when assessing phenologies of 16 European provenances of Sessile Oak (Quercus petraea) at a site in Scotland. Correlations between budburst and altitude, and budburst and latitude, showed that those provenances of southerly latitudes and high altitudes burst bud earliest. This emphasises the sensitivity of phenological events across scales.

4.8.6 Flowering Phenology and Distribution Change

In a study by Hulme (2011), by looking at the phenology (first flowering date recorded between 1970-2000) of 347 species he found that those with earlier flowering responses to spring temperatures had changed their distributions over the same period across the British Isles, a link previously not observed.

The onset of phenological events marks the start of the reproductive phase of the plant’s life cycle, with the “reproductive success of a population each year, the growth and the survival probability of individuals” (Cleland et al., 2007) determined by such events. It is assumed then, that with an earlier flowering time in response to warming, certain plants will be at an advantage and their probability of occurrence greater; they will exploit the longer growing season, may have improved interactions with pollinators (Walther et al., 2009) and resultantly be more favoured to increase range.

Conversely, earlier flowering may lead to a greater risk of damage by late frosts and thus poorer reproductive output (Miller-Rushing and Weltzin, 2009).

Species that are later flowering may be at a disadvantage to those species that have flowered earlier and utilised the available resources, therefore being out-competed (Miller-Rushing and Weltzin, 2009) with likely poor plant performance and reproductive output (Hulme, 2009b). However, plants which flower earlier and perform better have experienced declines in their range due to other environmental factors – namely the effect of agricultural intensification on arable weeds (Hulme, 2009a). Plants may also flower earlier in comparison to others but not increase their range to the extent that those flowering later have, and this can also be down to factors affecting distribution other than climate, e.g. soil fertility, pH.

95 4.8.7 Apparent Climate Signal

Increase in native species’ ranges can be down to other environmental variables (as is discussed in section 4.9.2) allowing them to persist in the environment, or because of human intervention, but there is an apparent climate signal between climate change and earlier flowering dates, especially as those natives having later flowering responses declined in distribution (Hulme, 2009b). In 1999 phenological events were accepted by the UK government as indicators of climate change (Cannell et al., 1999) so monitoring of these events will increase the field data relating the two variables.

Phenological shifts illustrate the impact recent warming may be having on biodiversity and that there can be knock on effects on reproductive performance, interactions with other species and ability to track future climate change. Phenological changes along with displacement of species ranges owing to climate change will “alter population-level interactions, community dynamics and have profound ecosystem and evolutionary consequences” (Fitter and Fitter, 2002). Sensitive ecosystems will be more at risk and conservation effort should be focused here, but novel communities will more than likely emerge in the future.