Testing root trenching and stem girdling as silvicultural treatments in abandoned oak coppices

Abstract

Widespread tree growth decline and absence of acorn production are observed in formerly coppiced Quercus pyrenaica stands. Attempts to convert oak coppices into high forests by thinning have failed to date, possibly due to the massive root system developed by trees after centuries of coppicing. We tested root trenching and stem girdling as silvicultural treatments to mitigate this hypothetical root-to-shoot imbalance and enhance the physiological performance of multi-stemmed clonal trees. We expected that severing the roots within a clone by trenching would reduce root respiratory carbon losses. Alternatively, stem girdling would increase carbon availability above the girdled zone, whilst the communal root system is maintained by non-girdled stems. After clonal structure delineation in a one-hectare experimental plot, root trenching and stem girdling were applied to stems belonging to different clones. Twig hydraulic traits, leaf water potential, non-structural carbohydrates (NSC) concentration in twigs, stems and roots, stem xylem [CO2] as an indicator of root respiration, stem growth, and acorn production were discretely monitored up to 12 years after treatment application for comparison with control stems. Stem girdling led to short-term enhancements in NSC pools above the girdled zone. However, hydraulic dysfunction symptoms and canopy dieback were observed four years after girdling, eventually causing mortality. Root trenching initially increased root respiration and reduced stem growth without an apparent decline in the hydraulic function and NSC pools above-ground. Ten years after trenching, stem [NSC] increased while stem growth limitations diminished. Although not statistically significant, slight enhancements in acorn production were observed 6 and 12 years after trenching. We discourage stem girdling as a silvicultural practice to mitigate a root-to-shoot imbalance in abandoned oak coppices, as long-term detrimental effects outweighed potential benefits. Root trenching enhanced tree NSC above ground. However, such improvement may not offset the potential costs of root trenching.