Functional characteristics of riparian plant communities on abandoned poplar plantations benefit from not harvesting

Aims

Across the world, riparian forest cover has decreased due to land use changes together with flow regulation altering natural hydrologic processes. Within Europe, a common type of land use change present within floodplains is conversion to Populus x euramericana (hybrid poplar) tree plantations. This study evaluates how the active management of these hybrid poplar tree plantations has altered the functional characteristics of riparian plant communities within a highly regulated river. We also evaluate whether abandonment of these plantations can be used as a cost-effective strategy for restoring desirable functional characteristics.

Location

Middle Garonne River, SW France.

Methods

We used multiple metrics of functional diversity, community weighted means, and plant functional trait guilds to compare differences among natural riparian forests, active hybrid poplar tree plantations, and tree plantations that either were simply abandoned or harvested prior to abandonment.

Results

Overall diversity was lower and trait composition was dissimilar in poplar plantations relative to natural riparian forests. Neither of the abandonment scenarios improved functional diversity and the overall functional composition of both site types remained dissimilar to natural pioneer riparian forests. However, sites that were abandoned, without harvesting the poplar timber, resulted in more similarities with natural forests in community weighted means (CWM) values and guild abundance than harvested sites. Additionally, the functional composition of these abandoned plantations was indicative of mature floodplain forests which have disappeared in this system.

Conclusions

Active hybrid poplar plantations had low functional diversity and distinct composition reflecting the novel disturbance regime (i.e. regular understory clearing via soil disking) and site disconnection from the river hydrology within these plantations. However, our results suggest that abandoning these plantations by leaving the overstory intact may be a cost effective and useful strategy for increasing desirable trait values of degraded floodplain communities in cases where no other restoration method is viable.