Article Scientifique

Abdelwahab Bessaad & Nathalie Korboulewsky
How much does leaf leaching matter during the pre-drying period in a whole-tree harvesting system?
Forest Ecology and Management, 2020, Volume 477, 118492
In European temperate forests, whole-tree harvesting increases nutrient exports and could compromise soil fertility in long term, especially when leaves, nutrient-rich compartments (leaves, fine and small wood) are exported. Pre-drying felled trees may allow leaves, twigs and branches to fall down or break during skidding, thereby remaining in the stand. However, the recommended pre-drying time is often based on expert estimates, and currently ranges from two to three months.

In this study, we developed an experimental device to quantify nutrient leaching via rainfall (pH: 6.8 ± 0.4) from fully developed leaves (collected in summer period) of four broadleaf species. We first set up an outdoor experiment under natural rainfall conditions to monitor the kinetics of nutrient leaching over around two and a half months. Second, we set up two controlled experiments under simulated rainfall conditions to investigate the effect of rainfall intensity and frequency on nutrient leaching.

Foliar K was highly leached 60–79%, followed by Mg: 19–50%, P: 22–30% and only small proportions for Ca and N, <16%. Nutrient leaching was positively correlated with rainfall amounts of <30 mm but small rainfall amounts <4 mm were more effective in leaching per unit (mm) than heavier rainfalls. More nutrients were leached out when the same rainfall amount was fractioned into small rainy events over several days.

However, leaf leaching remains unsatisfactory because a large part of nutrients is still exported by foliage. Total nutrient exports by whole-tree harvest including foliage increased nutrient exports by 1.2–1.6 times compared to conventional harvesting. The exports by foliage are of equal importance as fine and small wood exports and thus leaving the foliage on the forest would increase significantly nutrient saving. We therefore recommend harvesting during the leafless period when possible and otherwise, letting all the leaves fall to the ground before skidding not only for nutrient returns but also because easily degradable organic matter is very important for soil biological activity.

Keywords: Leaf leaching; Whole-tree harvesting; Pre-drying; Nutrient saving; Rainfall amount; Rainfall frequency

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