Climatic changes in the Anthropocene can be witnessed by the increasing temperature, shrinking lengths of cold periods, fewer frost days and nights, reduced snow cover, earlier snowmelt, and changing precipitation patterns (IPCC 2022). These abiotic changes lead to changes in living organisms as well, among which the phenological shaifts are the most spectacular ones, and were extensively studied in the past decades (Inouye 2022). Advancing spring periods can result earlier start of early season (spring) phenophases in plants, but a prolonged vegetative period and delay in late season (autumn) phenophases was also reported (Collins et al. 2021; Inouye 2022). Beside the changes directly caused by the temperature, earlier start of flowering and altered duration of flowering can be caused also by changes in precipitation availability (Jentsch et al. 2009).
Early-spring geophyte species can especially be sensitive to climate change, as their phenology is strongly dependent on the weather parameters, especially on temperature (Eppich et al. 2009). Temperature in general was found to have major effect on the growth and development of the storage organs of geophytes, higher temperature resulting in smaller storage organs (Sunmonu and Kudo 2015). As geophytes usually do not produce flowers below a certain bulb size, increasing temperature that limits the bulb growth expresses its negative effects on reproduction success as well, and in a long run, on population dynamics of geophyte species (Dafni et al. 1981; Khodorova and Boitel-Conti 2013). Aside from the effect of the increased temperatures in limiting bulb growth, in some species low temperatures in late spring, i.e. late frosts, can also have negative effects by decreasing the reproduction success of flowering individuals (Khodorova and Boitel-Conti 2013). Other weather parameters, like the amount of precipitation, usually have minor effects compared to that of temperature, mainly because of the ability of geophytes to cope with drier conditions by storing water in their belowground storage organs (Eppich et al. 2009, Howard et al. 2019; Nam and Kim 2020). A third factor that may affect geophytes’ population dynamics is light availability. Jacquemyn et al. (2010) found that light availability affected the flowering dynamics of Orchis purpurea, by enabling frequent, year to year flowering at lower costs under bright conditions compared to shaded places. However, similar relation in other geophyte species (Gagea lutea) may not exist or may vary from year to year (Sunmonu and Kudo 2014, 2015). These contrasting results may be related to differences in temperature conditions, which can overwrite the weaker effects of other factors (Sunmonu and Kudo 2015). All these findings indicate that our knowledge about the response of geophytes to the effects of weather parameters and other abiotic factors is still incomplete and that more studies are needed to understand them.
Deeper knowledge on the factors affecting the population dynamics of geophytes could help to support the designation of evidence-based conservation plans. The conservation of geophytes is especially important, as they may be threatened not only by the changing abiotic conditions, but also by direct and indirect human activities. Geophytes have a long history of human use: they have long been part of human diet due to their storage organs rich in carbohydrates (Molnár V. et al. 2017; Gill et al. 2021; Botha et al. 2022), they were and still are used for medicinal purposes due to their chemical compounds (Groner et al. 2022; Bradfield et al. 2023), and are often used for ornamental purposes because of their decorative flowers (Moradi et al. 2023). All these means of utilization make them frequent targets of overexploitation and illegal trade (Botha et al. 2022; Groner et al. 2022; Moradi et al. 2023).
Population monitoring is the base of conservation, as it aims to gather information about the state and dynamics of populations, and to indicate the need of intervention for conservation. However, geophytes can be easily overlooked during vegetation surveys due to their specific life cycles: in general they are detectable only in early spring so their numbers are often underestimated during monitoring surveys or they can be missed entirely (Gauthier et al. 2017; Endress et al. 2022). Unfavourable conditions and inadequate management can lead to unnoticed changes of their populations. As they have crucial role in maintaining spring communities, for example providing nectar for pollinators (Kudo and Cooper 2019), changes in the timing of flowering and in the number of flowers may result in significant changes in the early-spring communities related to them. That is why knowledge on specific life cycles, and proper timing of monitoring are of great importance when studying geophytes. For accurate population surveys, long-term monitoring is inevitable, given that individuals can enter, and remain in dormant state for multiple years; and relying on counting only aboveground parts of individuals only in certain years may be misleading (Lesica and Crone 2007; Miller et al. 2012). The work of monitoring practitioners would also be more effective if they could estimate the part of the populations hidden belowground. For this, filling the knowledge gap about the biotic and abiotic parameters that trigger dormancy and prolonged dormancy of geophytes would be of great help.
In our study we aimed to gather information about the weather parameters affecting the population dynamics of the spring meadow saffron, Colchicum bulbocodium Ker-Gawl., an endangered and strictly protected geophyte (Király 2007). C. bulbocodium has synanthous leaves, i.e., flowers and leaves of the individuals appear in the same season. The individuals set flowers between February and March, when temperature in bulb-level (10 cm soil depth) reaches 7˚C (Kovács 1992). Flowering individuals have short leaves, which continue to grow after flowers withering. The leaves of vegetative, non-flowering individuals appear later, when most flowers withered. Assimilation period lasts until May, when leaves start withering (Nagy 1984). Green capsules start to develop along the withering of flowers and appear aboveground in early May (Nagyné 1984; Andrési 1999). Capsules development continues until late May/early June, when capsules mature and open up in their apex to release seeds. Seeds bear elaiosomes, are dispersed by ants and germinate in autumn or spring (Gulyás 2006). Bulbs are renewed annually, and the regeneration of the new bulb ends in July, when the bulbs enter into a dormant state.
C. bulbocodium is a suitable model species for studying the effects of weather parameters on the population dynamics of early spring geophytes. Due to its complex phenology and population dynamics, population monitoring of this species is often inaccurate which largely complicates the design of species conservation plans. We aimed to understand the effect of weather parameters on the population dynamics of C. bulbocodium by studying a large population in East-Hungary for six consecutive years. We selected this population as this is one of the biggest populations in the region that enabled us to follow the demography in a representative number of plots. In our study we considered both actual and lagged weather parameters to study their effects on population dynamics of C. bulbocodium. This is because studies involving geophytes mostly focus on their growth, reproduction and phenological changes in response to the weather parameters and light conditions of the actual year (Eppich et al. 2009; Khodorova and Boitel-Conti 2013; Sunmonu and Kudo 2015; Vangansbeke et al. 2022; Jahn et al. 2023). However, it is also important to study the effect of lagged weather effects on the population dynamics of species (Prather et al. 2022; Evers et al. 2023). As Jahn et al. (2023) states, considering weather parameters that affect growth of the storage organs in geophytes, i.e. the weather parameters of the year preceding observations, should be studied more deeply, as it has fundamental effect on the phenology next year. Besides, as C. bulbocodium is a weak competitor (Kulikova et al. 2019), the effects of the surrounding vegetation should also be considered, when population dynamic is studied. For this purpose, in our study we also aimed to gather information about the effects of the biotic factors characteristic of the study site.
We addressed the following questions: i) Do the number of individuals show fluctuations in different phenophases across years? ii) Which weather parameters have the largest influence on the number of individuals in certain phenophases? iii) How can monitoring programs be designed to account for the complex phenology and population dynamics of the species and also incorporate weather effects?