How did a summer drought affect plants beneath the boreal forest canopy?

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Extreme droughts are increasing in frequency and severity globally. Forest drought research has mostly focused on trees, so less is known about effects on understory species and how the local environment moderates them.

Researchers used a transplant experiment with rainout shelters at 25 sites to assess a 45-day experimental summer drought in six boreal forest understory plants. They recorded growth, vitality and reproduction immediately, 2 months and 1 year after the simulated drought, and examined how differences in ambient soil moisture and canopy cover among sites influenced drought effects on each species.

Drought negatively affected the growth and/or vitality of all species, with stronger and more persistent effects in bryophytes than in vascular plants. Two species associated with older forests—the moss Hylocomiastrum umbratum and the orchid Goodyera repens—were affected more than the more generalist species in the experiment. Drought reduced reproductive output in the moss Hylocomium splendens in the next growing season, but increased reproduction in the graminoid Luzula pilosa.

Higher ambient soil moisture reduced some negative drought effects on vascular plants. Denser canopy cover and higher soil moisture both alleviated drought effects on bryophytes, likely by alleviating cellular damage.

The experiment shows that drought can adversely affect boreal understory species and that effects might be stronger for bryophytes and species associated with older forests. The results indicate that drought effects can vary over small spatial scales and that forest landscapes can be actively managed to alleviate effects on boreal forest biodiversity. Examples include managing the tree canopy and protecting hydrological networks.