COS 115-6 - Adaptation to the Urban Heat Island in a Weedy Plant Species

Abstract: Weedy plant species are incredibly successful to life in urban environments, with adaptations in response to temperature and habitat fragmentation. The urban heat island (UHI) can act as a strong selective force, bringing them past their thermal limitations. However, the strength of the UHI is variable within and between cities and it is unclear whether plant responses are due to adaptation or phenotypic plasticity. We investigated this in a greenhouse temperature experiment using maternal lines of Lactuca serriola (prickley lettuce) from across four US cities: Baltimore, MD; Minneapolis-St. Paul, MO; Phoenix, AZ; and Los Angeles, CA. Seeds were collected along a gradient of UHI strength in summer 2018 and selfed for one generation (F1) in order to eliminate maternal effects (n=359). Plants were subjected to experimental conditions one month after growth, with a cool treatment (20C) and heated treatment (28C). Land surface temperature (LST) was from Landsat-8 satellite imagery was used to assess origin temperatures from each site. Morphological, ecophysiological, and phenological traits were measured over a 5-month period, including specific leaf area (SLA), leaf dry matter content (LDMC), above- and belowground biomass, maximum photosynthetic rate (A_max), and day of first flower.

The LST from all field sites ranged from 8.90 ºC to 76.20 ºC. Overall, L. serriola in the heated treatment were much smaller and performed significantly more poorly, with clear signs of heat stress (e.g., reddened leaves, short stature, and sun-leaf morphology). The heat treatment had significant impacts on both aboveground biomass, belowground biomass, and height overall (p_LME< 0.05, F_ABiomass=342.68, F_BBiomass=579.78, F_H=71.44). Leaf-level traits such as SLA and LDMC did not differ significantly with respect to the treatment, LST, or city. Interestingly, only L. serriola originating from LA and PX flowered during this period with significant differences between heated and unheated treatments (p< 0.05, F_1stFlower=12.39), however, there was no significance attributed to LST or city of origin. Post-hoc analyses suggest significant interactions with treatment for plants originating from PX. Together, the trait and phenological responses suggest a faster reproductive period in plants originating from the hottest city in this study.