If you look at a graph of world CO2 levels and temperature from the Cambrian era to the present, you see little to no correlation between the two. But there a strong trend downwards in CO2 levels starting from 7000ppm in the Cambrian and dropping down to 180ppm during the Quaternary glaciation, the most recent downward excursion being 20,000 years ago. This drop in CO2 levels is to be expected since plants are busy pulling CO2 out of the air with the resultant fixed carbon being deposited into the ground and on the sea floor. A portion of this sea floor carbon gets subducted deep into the earth's interior via plate tectonics, becoming forever lost to the biosphere except for what little bits of it get released via volcanoes and mid-ocean hydrothermal vents. So the general long term trend is for atmospheric CO2 levels to drop as carbon gets pulled out of the air and put into the ground.
Angiosperms first appeared during the Cretaceous when CO2 levels were in the 1700 to 1800ppm range and have had to adapt to steadily dropping CO2 levels from that time forward. C3 photosynthesis is the original chemical pathway that evolved in algae during the Precambrian era and is found in all sea plants and most land plants. But 7 million years ago some groups of monocots and dicots evolved C4 photosynthesis in response to the extremely low CO2 levels they were experiencing during the Quaternary period. A drawback of C4 is that it works best at hot tropical temperatures, limiting its use in the cooler parts of the globe, so 85% of the world's plants are still C3.
If you look at a curve of plant growth rate vs. CO2 levels, it rises sharply with increasing CO2 levels until CO2 levels reach 1200ppm where the growth rate starts to level off. This increased growth rate at higher than current atmospheric CO2 levels has long been known to commercial greenhouse growers, many of whom use CO2 generators to maintain a CO2 level in the greenhouse of between 1000 and 1500ppm. At these higher CO2 levels, plants grow faster, have higher yields, produce larger, thicker leaves, larger flowers, require less water, and are more resistant to environmental stresses. Leaves grown at these higher CO2 levels produce fewer stomata and have to open them less to obtain the required CO2, increasing their drought tolerance.
Photorespiration, an inefficiency in the photosynthetic process where the enzyme RuBisCO reacts with 02 rather than the desired CO2, disappears at 1200ppm, but gets worse and worse as CO2 levels drop below that point, and is apparently as artifact of low CO2 levels. C4 photosynthesis minimizes losses due to photorespiration.
At the other end of the scale, growth and seed production in C3 plants drops off rapidly at lower than current CO2 levels. An Arabidopsis growing at 180ppm, the lowest CO2 levels experienced during the last ice age, had only 8% of the growth rate of the same plant growing at current CO2 levels. Below 150ppm, C3 plants essentially stop growing, so the 180ppm lows during the Quaternary glaciation came within spitting distance of killing off many C3 plants.
CO2 levels were 180ppm st the peak of the ice age, slowly rose to 270ppm as we came out of the ice age, then continued rising as human activity added CO2 to the atmosphere and is over 400ppm now. This boost in CO2 levels has goosed plant growth rates and is part of the oomph behind the increase in crop yields helping to feed the growing human populations. Satellite images are showing an 11% increase in foliage cover in arid regions around the world from 1982 to 2010. The spread in the seasonal cycle in CO2 levels monitored at Moana Kea observatory increased from 14ppm in 1975 to 17ppm in 2013 as worldwide plant photosynthetic activity is ramping up with rising CO2 levels.
So it looks like plants are best adapted to growing in the mid-1000's ppm CO2 atmosphere they originally evolved in, are showing reduced growth rates/seed production and displaying metabolic derangements like photorespiration in our modern CO2 impoverished atmosphere. So while some humans might be concerned about rising CO2 levels, the plants are saying "bring it on, we're half starved out here".