A transcontinental experiment elucidates (mal)adaptation of a cosmopolitan plant to climate in space and time

Climate change and the global spread of non-native species are two of the
most significant threats to biodiversity and ecosystem function. Both
these phenomena subject populations to novel conditions, either in space
(species introductions) or in time (climate change), yet the role of
adaptation in how populations respond to these rapid environmental shifts
is poorly understood.  We conducted a large-scale
trans-continental common garden experiment using white clover (Trifolium
repens, Fabaceae) to test whether adaptive evolution to spatiotemporal
variation in climate could contribute to the ecological success of one of
the most widespread plant species in the world. Individuals from 96
populations of Trifolium repens (white clover) from both its native
(Europe) and introduced (North America) ranges were planted into four
experimental common gardens located in northern (Uppsala, Sweden) and
southern (Montpellier, France) Europe, and northern (Mississauga, Canada)
and southern (Lafayette, USA) North America. We recorded plant sexual and
clonal fitness in each common garden and assessed whether the strength of
local adaptation differed between the native and introduced ranges and
whether populations are rapidly adapting to climate change. Results show
that local adaptation was only evident when populations were transplanted
into common gardens located in the same range (native or introduced) from
which they originated and was driven by stronger selection (due to
climatic factors rather than herbivory) at lower latitudes in both ranges.
Our results indicate rapid local adaptation across a large latitudinal
gradient in introduced T. repens populations, along with an associated
adaptation cost when transplanted back into the native range. We also find
evidence of an adaptation lag in the northern common garden in the
introduced range, with plants from historically warmer climates exhibiting
the greatest fitness. These findings support two major conclusions: 1)
white clover can rapidly adapt to spatial variation in climate in its
introduced range as well as the native range, and 2) despite rapid
adaptation to novel environments, introduced white clover populations are
not keeping pace with rapid climate change. Overall, our results provide
insight into the role of adaptation in facilitating the ecological success
of non-native species in a rapidly changing world.