26The biotic and the abiotic environment play a major role in shaping plant 27 phenotypes and their geographic distributions. However, little is known about the 28 extent to which plant phenotypes match local patterns of herbivory across fine-29 grained habitat mosaics, despite the strong effect of herbivory on plant fitness. 30 Through a reciprocal transplant-common garden experiment with clonally 31 propagated rhizomes, we tested for local phenotypic differentiation in bittercress 32 (Brassicaceae: Cardamine cordifolia) plants collected across an ecotonal habitat 33 mosaic. We found that bittercress in sunny meadows (high herbivory) and shaded 34 understories (low herbivory) have diverged in heritable growth and herbivore 35 resistance phenotypes. The expression of these differences was habitat dependent, 36 mirroring patterns of adaptive divergence in phenotypic plasticity between plant 37 populations in meadow and understory habitats at broader geographic scales, and 38 showed no evidence for a constraint imposed by growth-defense tradeoffs. Most 39 notably, plants derived from shade habitats exhibited a weaker shade-induced 40 elongation response (i.e., shade avoidance syndrome, SAS) and reduced resistance 41 to herbivory, relative to plants derived from sun habitats, when both were grown in 42 shade common gardens. Greenhouse experiments revealed that divergent SAS 43 phenotypes in shade conditions were expressed in offspring grown from seed as well. 44Finally, we observed partially non-overlapping flowering phenology between 45 habitat-types in the field, which may be at least one factor that helps to reinforce 46 habitat-specific phenotypic divergence. Altogether, our study illuminates how a 47 48 grained habitat mosaic. 49 50 51 55 2005; Richardson et al. 2014). However, our knowledge regarding the conditions 56 under which these patterns arise and persist has been disproportionately shaped by 57 studies at course-grained rather than fine-grained spatial scales (Richardson et al.58 2014). This bias has likely been shaped by the expectation that gene flow among 59 interspersed habitat patches is generally a strong homogenizing force, preventing 60 the establishment of habitat-associated phenotypic and genotypic variation at fine-61 grained spatial scales (Haldane 1930; Lenormand 2002). Although there is growing 62 evidence that heritable phenotypes track habitat mosaics at fine-grained spatial 63 3 scales [i.e. microgeographic phenotypic divergence; (Richardson et al. 2014)] we 64 have a limited understanding of the molecular, ecological, and evolutionary 65 processes that facilitate and maintain this variation in nature. 66 67 Defoliation by insect herbivores exerts strong selection on plant phenotypes (Louda 68 1984; Prasad et al. 2012; Agrawal et al. 2012). In mustards (Brassicaceae), 69 polymorphisms in genes that modify defensive chemicals underlie adaptation to 70 local herbivore communities (Prasad et al. 2012; Zust et al. 2012), and the 71 magnitude of geographic divergence at such loci is extr...

Organismal phenotypes often co-vary with environmental variables across broad geographic ranges. Less is known about the extent to which phenotypes match local conditions when multiple biotic and abiotic stressors vary at fine spatial scales. Bittercress (Brassicaceae: Cardamine cordifolia), a perennial forb, grows across a microgeographic mosaic of two contrasting herbivory regimes: high herbivory in meadows (sun habitats) and low herbivory in deeply shaded forest understories (shade habitats). We tested for local phenotypic differentiation in plant size, leaf morphology, and anti-herbivore defense (realized resistance and defensive chemicals, i.e., glucosinolates) across this habitat mosaic through reciprocal transplant-common garden experiments with clonally propagated rhizomes. We found habitat-specific divergence in morphological and defensive phenotypes that manifested as contrasting responses to growth in shade common gardens: weak petiole elongation and attenuated defenses in populations from shade habitats, and strong petiole elongation and elevated defenses in populations from sun habitats. These divergent phenotypes are generally consistent with reciprocal local adaptation: plants from shade habitats that naturally experience low herbivory show reduced investment in defense and an attenuated shade avoidance response, owing to its ineffectiveness within forest understories. By contrast, plants from sun habitats with high herbivory show shade-induced elongation, but no evidence of attenuated defenses canonically associated with elongation in shade-intolerant plant species. Finally, we observed differences in flowering phenology between habitat types that could potentially contribute to inter-habitat divergence by reducing gene flow. This study illuminates how clonally heritable plant phenotypes track a fine-grained mosaic of herbivore pressure and light availability in a native plant.

Plant distributions can be limited by habitat-biased herbivory, but the proximate causes of such biases are rarely known. Distinguishing plant-centric from herbivore-centric mechanisms driving differential herbivory between habitats is difficult without experimental manipulation of both plants and herbivores. Here we tested alternative hypotheses driving habitat-biased herbivory in bittercress (Cardamine cordifolia), which is more abundant under shade of shrubs and trees (shade) than in nearby meadows (sun) where herbivory is intense from the specialist fly Scaptomyza nigrita. This system has served as a textbook example of habitat-biased herbivory driving a plant’s distribution across an ecotone, but the proximate mechanisms underlying differential herbivory are still unclear. First, we found that higher S. nigrita herbivory in sun habitats contrasts sharply with their preference to attack plants from shade habitats in laboratory choice experiments. Second, S. nigrita strongly preferred leaves in simulated sun over simulated shade habitats, regardless of plant source habitat. Thus, herbivore preference for brighter, warmer habitats overrides their preference for more palatable shade plants. This promotes the sun-biased herbivore pressure that drives the distribution of bittercress into shade habitats.

24Habitat distributions of plants are often driven by abiotic factors, but growing evidence suggests 25 an important role for consumers. A textbook example of a plant whose habitat distribution is 26 shaped by consumers is bittercress (Cardamine cordifolia). Bittercress is more abundant in shade 27 than in sun habitats, and this is thought to arise because herbivore pressure is lower in the shade. 28The bittercress case study remains incomplete, as we still do not understand why herbivory is 29 lower in the shade. Herbivores may avoid shaded bittercress because the plants are lower quality, 30 or because herbivores simply prefer brighter, warmer habitats. We tested these alternative 31 hypotheses through a series of herbivore choice experiments. Scaptomyza nigrita, a locally 32 abundant specialist and dominant herbivore of bittercress, strongly preferred feeding and laying 33 eggs on bittercress we collected from shade versus sun habitats. Thus, shaded bittercress are 34 more, not less, palatable to these herbivores. Separately, S. nigrita strongly preferred feeding and 35 laying eggs on leaves held in treatments that simulated sun rather than shade habitats-36 regardless of whether leaves came from sun or shade habitats originally. The underlying 37 mechanism for a consumer-driven plant distribution appears to be a simple behavioral preference 38 of herbivores for brighter, warmer habitats. 39