What was done
The authors simulated the spatial relationship between temperature change and potential vegetation cover for Iceland over the period of the Holocene, evaluating their results against palynological and geomorphological data.

What was learned
During the Holocene climatic optimum, the authors calculated that vegetation may have covered about 60% of the land.  When the Roman Warm Period began to wane about 2300 years ago, however, a long-term vegetative decline commenced.  This decline continued until the Medieval Warm Period actually reversed the vegetation loss for about 400 years.  The appearance of the Little Ice Age, however, resulted in "an unprecedented low potential for vegetation for the Holocene that lasted c. 600 years, i.e., between AD c. 1300 and 1900."

What it means
As Olafsdottir and Gudmundsson (2002) have more recently shown for Icelandic soil, so it is with Icelandic vegetation: warm is good, cold is bad, it's as simple as that.

Reference
Olafsdottir, R. and Gudmundsson, H.J.  2002.  Holocene land degradation and climatic change in northeastern Iceland.  The Holocene 12: 159-167.