Trait data of Primula elatior from common garden experiments and field campaigns

Climate change and the resulting increased drought frequencies pose considerable threats to forest herb populations, particularly where additional environmental challenges jeopardize responses to selection. Specifically, habitat fragmentation may impede climate adaptation through its impact on the distribution of adaptive genetic variation, and cause evolutionary shifts in mating systems. 
To assess how habitat fragmentation disrupts climate adaptation, we conducted a common garden experiment with Primula elatior offspring originating from 24 populations sampled along a latitudinal gradient with varying climate and landscape characteristics. We then quantified a range of vegetative, regulatory, and reproductive traits under distinct soil moisture regimes to evaluate imprints of local adaptation and phenotypic plasticity. Additionally, we conducted a more extensive field campaign in 60 populations along the same latitudinal gradient to evaluate the potential evolutionary breakdown of reciprocal herkogamy.
For large, connected populations, our results demonstrated an evolutionary shift from a strategy in southern populations that seems aligned with drought avoidance — where plants minimize their exposure to dry conditions and optimize photosynthesis — to a drought tolerance strategy in northern populations, where plants adapted to function despite water scarcity. However, habitat fragmentation disrupted climate clines and the adaptive responses to drought stress in key traits related to growth, biomass allocation and water regulation. Additionally, our findings indicate the onset of evolutionary breakdown in reciprocal herkogamy and divergence in other key flower traits. The disruption of climate clines, drought responses, and adaptations in mating systems contributed to a substantially diminished flowering investment across the distribution range, with the most pronounced effects observed in southern fragmented populations. 

Synthesis: We present novel empirical evidence of how habitat fragmentation disrupts climate adaptation and drought tolerance in a wide range of traits along the range of the forest herb Primula elatior. These findings emphasize the need to account for habitat fragmentation while designing effective conservation strategies in order to preserve and restore resilient meta-populations of forest herbs amidst ongoing global changes.

Methods
The trait dataset was generated through a comprehensive research design aimed at investigating the impacts of climate change and habitat fragmentation on Primula elatior, a European forest herb. Data collection was structured into two main components: field-based seed and flower collection, and greenhouse-based common garden experiments.

Field-Based Seed and Flower Collection
Seeds were collected from 24 distinct populations of P. elatior, spanning a latitudinal gradient from southern France to northern Denmark. Each pair among the 24 populations was designed to include one population from a highly fragmented forest patch and another from a well-connected forest area, resulting in 12 unique pairs. These pairs were selected based on specific criteria that included measures of habitat fragmentation and connectivity, such as the Mean Patch Proximity Index and Mean Nearest Neighbour distance, population size, and a maximum elevation of 200m. In addition to seed collection, herkogamy was assessed in 60 populations (comprising 30 pairs) with a total of 2116 evaluated individuals, to examine the potential evolutionary breakdown in mating systems.

Greenhouse-Based Common Garden Experiment
The collected seeds underwent an initial germination experiment in petri dishes with perlite and water, under two treatments: one with vernalisation and one without. These seeds were monitored for germination success and lag time under controlled climate conditions. Seedlings were then transitioned into a controlled common garden greenhouse experiment, where they were cultivated under two distinct soil moisture regimes to assess their drought stress responses.

Additional Data Collection for Response Variables
A comprehensive array of response variables was meticulously recorded to cover the plant's lifecycle, from seed to adult.

Germination Traits: Seeds in petri dishes were monitored for germination success and lag time under controlled conditions.
Juvenile Growth Rate: Measured at specific time points from seedling to juvenile stage, capturing metrics such as growth rate in mm/day.
Adult Plant Traits: Includes adult growth rate, root-shoot ratio, and specific leaf area, measured at designated time points.
Leaf Morphology: Both abaxial and adaxial stomatal density, as well as abaxial glandular trichome density, were evaluated using microscopic imaging and a custom convolutional neural network model.
Flower Phenology: Captured metrics related to the timing of flowering events, such as the lag time before flowering.

 Flowering Investment Metrics: A composite metric of flower traits was derived using PCA, including the total count of produced flowers, stalk height, and flower opening duration.
Flower Colour Metrics: Evaluated using macro-photography and image segmentation algorithms to assess visual pollinator signalling.
Self-Compatibility: Assessed through manual self-pollination techniques in a controlled environment.
Herkogamy: Stigma and anther heights were measured in the field using a digital caliper for 60 populations to assess potential evolutionary breakdown in mating systems.

The dataset has been subjected to rigorous quality assurance, including validation steps for the neural network models used for image classification. For detailed methodologies, including the procedures and tools used for each trait, please see the main manuscript and appendices.