Heavy and frequent thinning promotes drought adaptation in Pinus sylvestris forests
Abstract
Droughts and their negative effects on forest ecosystems are projected to increase under climate change for many regions. It has been suggested that intensive thinning could reduce drought impacts on established forests in the short‐term. Most previous studies on the effect of thinning on drought impacts, however, have been confined to single forest sites. It is therefore still unclear how general and persisting the benefits of thinning are. This study assesses the potential of thinning to increase drought tolerance of the wide spread Scots pine (Pinus sylvestris) in Central Europe. We hypothesized (1) that increasing thinning intensity benefits the maintenance of radial growth of crop trees during drought (resistance) and its recovery following drought, (2) that those benefits to growth decrease with time elapsed since the last thinning and with stand age, and (3) that they may depend on drought severity as well as water limitations in pre‐ and post‐drought periods. To test these hypotheses, we assessed the effects of thinning regime, stand age, and drought severity on radial growth of 129 Scots pine trees during and after drought events in four long‐term thinning experiments in Germany. We found that thinning improved the recovery of radial growth following drought and to a lesser extent the growth resistance during a drought event. Growth recovery following drought was highest after the first thinning intervention and in recently and heavily thinned stands. With time since the last thinning, however, this effect decreased and could even become negative when compared to unthinned stands. Further, thinning helped to avoid an age‐related decline in growth resistance (and recovery) following drought. The recovery following drought, but not the resistance during drought, was related to water limitations in the drought period. This is the first study that analyzed drought‐related radial growth in trees of one species across several stands of different age. The interaction between thinning intensity and time since the last thinning underline the importance to distinguish between short‐ and long‐term effects of thinning. According to our analysis, only thinning regimes, with relatively heavy and frequent thinning interventions would increase drought tolerance in pine stands.
What the paper shows and why it matters
(AI-generated)
Droughts are expected to increase in many regions, and thinning (removing some trees so the rest have more room and water) has been proposed to reduce their impact on forests. Earlier evidence comes mostly from single sites, one stand age or one thinning regime, and it is inconsistent on whether thinning helps trees to keep growing during a drought, to regain growth afterwards, or both. How general and lasting a thinning benefit is, and whether it depends on the thinning regime and the time since the last thinning, was an open question; the paper addresses it.
The study argues that thinning mainly improves how well Scots pine regains growth after a drought and only weakly how well it keeps growing during one, and that this recovery benefit fades with time since the last thinning, so that only heavy and frequent thinning would raise drought tolerance. It tested this with growth rings of 129 trees in four long-term thinning experiments in Germany, using statistical models that include thinning intensity, time since the last thinning, stand age and drought severity. The models, fitted to drought years selected by climate alone, predict that recovery after drought is higher than in unthinned stands one year after a heavy thinning, but lower than in unthinned stands 15 years after it. The effect on growth during drought was weak, not significant for heavy thinning against unthinned stands, and appeared in only one of the two ways of selecting drought years that the study compared, so the study urges caution.
The study reads these results as follows: the thinning benefit is short-lived, and thinning cannot prevent growth loss in severe drought. For the whole production cycle, it advises shorter thinning intervals, because recovery declines with time since the last intervention. It notes that the heavy thinning in the experiments may be considered extreme compared with usual practice in Germany. In thinned stands no negative effect of stand age appeared, whereas recovery declined with age in unthinned stands; the study notes that the trees were young (on average 27 years old at the time of the droughts, with the climate-only selection of drought years).