The main factors a plant consider are:
- Temperature (it is important the number of chill hours the plant receives by year).
- Luminosity (related with Milhankovic Cycles and climate of Pleistocene in general).
- Availability of soil and water (related with ice covered surface).
The main change between Pleistocene's glaciar/interglaciar for plants is there is new surface to colonize. As ice retreives, ecological succession starts. Non soil lands and permafrost is gradualy replaced by soil, so grass, other plants, and termophyllous trees start to colonize the new environments, while Coniferous and other Perennial Plants get restricted. Palinologists find this succession is relatively quick in geological time terms, taking only some decades or centuries (Roucoux, K. H. et al, 2001).
This is true for all intereglaciars. Thermophilus taxa as Gramineae or Artemixia, and grassland in general colonize new ecological niches (Branch, N. P. et al, 2015).
This is specialy true at Holocene, where the displacement of cold taxa trees by grassland and other thermophyllous phyllums could became a prelude to Holocene's antrophologic agriculture (Hillman, C., 1996).
Pleistocene $\small\sf{CO_2}$ levels are lower at Pleistocene than at most of Earth's History, as shown in this graph:

Source: Wikipedia. From Gradstein et al.,2005
This leads to think plants are adaptated to higger $\small\sf{CO_2}$ levels than Pleistocene has had and have. This is in fact true, $\small\sf{CO_2}$ levels are practically the lower known on Earth's History and plants have adaptated theirselves to Earth's environment with higger concentrations.
Increasing $\small\sf{CO_2}$ levels at Cannabaceae makes them
grow more. Plants are bigger and grow faster with the same amount of
ligth and nutrients. Some cultivators use
$\small\sf{CO_2}$ superlevels to make their indoor cultives more
efficient. Photosynthesis become more
efficient.
Cannabis Sativa endures until 1500 ppm., where $\small\sf{CO_2}$ become toxic:

Source: ilovegrowingmarijuana.com
Both graphs are related. What is shown is thermophyllous plants are adapted non to Pleistocene CO2 levels but for Mesozoic / Tertiary ones. It would be interesting to know the tolerance for perennial plants and common Mesozoic ferns.
So $\small\sf{CO_2}$ has increased by human factors, but as
temperature has not done it a lot yet and the Earth stays at the same
point on Milhankovic Cycles, the only change that happens sensu
lacto because of $\small\sf{CO_2}$ emissions on Kingdom Plantae is a bit more of efficency on
photosynthesis and a bit more of landmass to colonize close to North Pole.
Related link: Stratigraphic International Scale
Katherine H.Roucoux, Nicholas J.Shackleton, Lucia de Abreu, Joachim Schönfeld, Polychronis C.Tzedakis (2001). "Combined Marine Proxy and Pollen Analyses Reveal Rapid Iberian Vegetation Response to North Atlantic Millennial-Scale Climate Oscillations" Quaternary Research
Volume 56, Issue 1, July 2001, Pages 128-132.
Nicholas P. Branch, Lionello Morandi (2015). "Late Würm and Early-Middle Holocene Environmental Change and Human Activities in the Northern Apennines, Italy". Università di Macerata, Dipartimento di Scienze della formazione, dei beni culturali e del turismo, Sezione di Beni Culturali, piazzale Bertelli 1, 62100 Macerata, Italia
Hillman, C. (1996). "The origins and spread of agriculture and pastoralism in Eurasia". UCL Press. ISBN-10 1857285379, 1857285387
Gradstein, FM, JG Ogg and AG Smith (2005) "A geologic time scale 2004", Cambridge University Press ISBN 0521786738