Evaluation of Phenotypic Plasticity in Reproductive Fitness Traits of Arabis Alpina L. (Brassicaceae) in the Afromontane Region of Mount Kenya

Abstract

The Afromontane region is widely recognized as a biodiversity hotspot, harboring rare, endemic, and threatened plant species. However, these species face multiple threats, including habitat fragmentation, deforestation, invasive species, overgrazing, agricultural expansion, and the impact of climate change. Climate change poses a significant environmental challenge in the region, resulting in rising temperatures and changing precipitation patterns. Consequently, the Afromontane flora has developed adaptations to cope with these fluctuating climatic conditions. These adaptations result in changes in species distribution, alterations in plant-animal interactions, increased vulnerability to pest and disease outbreaks, and variations in fitness parameters such as phenology, seed production, germination, and survival rates. Despite the ecological significance of this ecosystem and the potential impacts of climate change, our understanding of how its flora responds to these selective pressures remains limited. Additionally, there is a scarcity of studies investigating the influence of soil properties on the variability of reproductive fitness traits within, between, and among tropical Afromontane species. Although there have been studies exploring the effects of climate change on various aspects of plant biology, research specifically examining the impact of changing climatic factors on the germination and seedling survival rates of tropical Afromontane species is lacking. This study addresses these research gaps by assessing the phenotypic plasticity of reproductive fitness traits in Arabis alpina L. (Brassicaceae) in the Afromontane region of Mt. Kenya. It examines the impact of elevation and soil characteristics on the reproductive fitness traits of A. alpina, explores potential trade-offs among these traits, and investigates the influence of seed trait selection on its germination and survival rates. The populations of A. alpina were sampled using the belt transect method. Twenty-eight sample plots of A. alpina were identified, covering an altitude range of 3,763m to 4,600m. The onset and duration of flowering in A. alpina individuals were monitored visually. Seeds of the species were collected, processed, counted, weighed, and subjected to germination trials and seedling survival counts. Soil samples were randomly collected from the sample plots, and soil characteristics such as carbon, nitrogen, phosphorus, pH, and electrical conductivity were analyzed. Statistical analyses were performed using R version 4.3.1. A one-way analysis of variance was used to assess variations in the reproductive fitness traits of A. alpina across lower alpine, upper alpine, and nival vegetation zones. In addition, linear mixed-effects models were employed to determine variations in the reproductive fitness traits of A. alpina along the elevation gradient and establish the degree of association between these traits. A model convergence analysis was performed to evaluate the performance and accuracy of the models in estimating the response variables. The results indicate that soil nitrogen, carbon, phosphorus, and pH decrease while soil electrical conductivity increases with elevation. These variations in soil characteristics significantly contribute to the observed differences in the onset and duration of flowering, seed mass, seed number, germination, and seedling survival rates of A. alpina across vegetation zones and along the elevation gradient. Specifically, with every 100-meter increase in elevation, there is a 19-hour and 20-minute delay in the onset of flowering, a reduction of 1 day and 17 hours in the species' duration of flowering, a decrease of 0.4 mg in seed mass, an increase of 7 seeds, and an 8% decrease in germination and seedling survival rates. Trade-offs were observed among the reproductive fitness traits of A. alpina along the elevation gradient. As the elevation increases, a one-day delay in the species' onset of flowering results in a reduction of its duration of flowering by 1 hour and 41 minutes. Moreover, a decrease of 1 mg in the species' seed mass corresponds to an increase of 8 seeds. Furthermore, a 1% decrease in the species' germination rate leads to a 0.82% decrease in its seedling survival rate. Different seed resource allocation strategies have varying effects on the germination and survival rates of A. alpina. Particularly, a 1 mg increase in A. alpina seed mass leads to a 9.1% increase in its germination rate and an 8.5% increase in its seedling survival rate. In contrast, an increase of 1 seed results in a 0.88% decrease in the germination rate and a 0.78% decrease in the seedling survival rate of the species. These plastic responses of A. alpina in response to changing climatic conditions along the elevation gradient underscore the adaptive strategies employed by the species to thrive in diverse ecological niches within the Afromontane ecosystem of Mt. Kenya. This understanding will assist in predicting the future distribution and population dynamics of not only this species but also others in comparable environments. By identifying the underlying mechanisms of phenotypic plasticity and adaptive responses of Afromontane flora to environmental changes, conservation strategies can be developed to safeguard biodiversity and protect threatened species amidst the ongoing challenges posed by climate change.