Abstract
A higher frequency of increasingly severe droughts highlights the need for short-term measures to adapt existing forests to climate change. The maintenance of reduced stand densities has been proposed as a promising silvicultural tool for mitigating drought stress. However, the relationship between stand density and tree drought susceptibility remains poorly understood, especially across ecological gradients. Here, we analysed the effect of reduced stand density on tree growth and growth sensitivity, as well as on short-term drought responses (resistance, recovery, and resilience) of Scots pine (Pinus sylvestris L.), sessile oak (Quercus petraea (Matt.) Liebl.), and ponderosa pine (Pinus ponderosa Douglas ex C. Lawson). Tree ring series from 409 trees, growing in stands of varying stand density, were analysed at sites with different water availability. For all species, mean tree growth was significantly higher under low compared with maximum stand density. Mean tree growth sensitivity of Scots pine was significantly higher under low compared with moderate and maximum stand density, while growth sensitivity of ponderosa pine peaked under maximum stand density. Recovery and resilience of Scots pine, as well as recovery of sessile oak and ponderosa pine, decreased with increasing stand density. In contrast, resistance and resilience of ponderosa pine significantly increased with increasing stand density. Higher site water availability was associated with significantly reduced drought response indices of Scots pine and sessile oak in general, except for resistance of oak. In ponderosa pine, higher site water availability significantly lessened recovery. Higher site water availability significantly moderated the positive effect of reduced stand density on drought responses. Stand age had a significantly positive effect on the resistance of Scots pine and a negative effect on recovery of sessile oak. We discuss potential causes for the observed response patterns, derive implications for adaptive forest management, and make recommendations for further research in this field.
Highlights
Climatic extremes, such as severe droughts, are expected to become more frequent and increase in intensity as a result of climate change [1]
To improve the state of knowledge, this study focuses on tree growth and tree drought reactions of Scots pine (Pinus sylvestris L.), sessile oak (Quercus petraea (Matt.) Liebl.), and ponderosa pine
In contrast to Scots pine, we found that mean tree growth sensitivity of ponderosa pine increased with stand density and was 35% higher in the untreated controls compared with low stand densities
Summary
Climatic extremes, such as severe droughts, are expected to become more frequent and increase in intensity as a result of climate change [1]. In the recent past, proven to be vulnerable to such drastic changes in growing conditions due to their slow natural adaptation rates, resulting in widespread tree mortality and decreased tree and forest growth [4,5]. Silvicultural strategies, such as growing more drought-resistant tree species and converting monocultures into mixed, uneven aged forests, are seen as promising long-term options for adaptation, to cope with the increasing threat of climate change to forest ecosystem health and functioning [6,7,8]. Reduced stand transpiration due to lower leaf area [18], as well as the formation of more extensive root systems [19], have been reported as contributing factors that may increase tree water availability under reduced competition
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