Quantifying Growth Responses of Trees to Drought—a Critique of Commonly Used Resilience Indices and Recommendations for Future Studies

Schwarz, Julia; Skiadaresis, Georgios; Köhler, M.; Kunz, Jörg; Schnabel, Florian; Vitali, Valentina; Bauhus, Jürgen

doi:10.1007/s40725-020-00119-2

Cited by 129 publications

(146 citation statements)

References 81 publications

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“…The index ‘recovery’ is calculated as the ratio of mean growth in a post-drought period divided by growth in the drought year, while ‘resilience’ represents the ratio of average post-drought tree growth divided by mean growth in the pre-drought period. To overcome limitations of a predefined reference period (pre- and post-drought) 88 we used a slightly modified approach from the one proposed by Lloret et al 24 for calculating pre- and post-drought means (hereafter referred to as normal periods). These were defined as the average raw value of each growth or vessel-related variable in all non-dry years (i.e., excluding values in moderate and severe dry years) occurring before, between and following the two identified extreme droughts.…”

Section: Methodsmentioning

confidence: 99%

Groundwater extraction reduces tree vitality, growth and xylem hydraulic capacity in Quercus robur during and after drought events

Skiadaresis

Schwarz

Stahl

et al. 2021

Sci Rep

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Climate change is expected to pose major direct and indirect threats to groundwater-dependent forest ecosystems. Forests that concurrently experience increased rates of water extraction may face unprecedented exposure to droughts. Here, we examined differences in stem growth and xylem hydraulic architecture of 216 oak trees from sites with contrasting groundwater availability, including sites where groundwater extraction has led to reduced water availability for trees over several decades. We expected reduced growth and xylem hydraulic capacity for trees at groundwater extraction sites both under normal and unfavourable growing conditions. Compared to sites without extraction, trees at sites with groundwater extraction showed reduced growth and hydraulic conductivity both during periods of moderate and extremely low soil water availability. Trees of low vigour, which were more frequent at sites with groundwater extraction, were not able to recover growth and hydraulic capacity following drought, pointing to prolonged drought effects. Long-term water deficit resulting in reduced CO2 assimilation and hydraulic capacity after drought are very likely responsible for observed reductions in tree vitality at extraction sites. Our results demonstrate that groundwater access maintains tree function and resilience to drought and is therefore important for tree health in the context of climate change.

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Section: Methodsmentioning

confidence: 99%

Groundwater extraction reduces tree vitality, growth and xylem hydraulic capacity in Quercus robur during and after drought events

Skiadaresis

Schwarz

Stahl

et al. 2021

Sci Rep

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“…Linking responses in growth resistance and Δδ¹³C to tree mortality and carbon ecosystem fluxes remains challenging 11 . Moreover, it remains unknown how the here observed responses will affect tree recovery after and resilience 15,16 to (future) droughts. The strong drought legacy effects we observe in the second drought year (2019), the reported persistence of legacy effects for years 9 and successive declines in drought resistance under consecutive drought 13,15 are worrisome.…”

Section: Discussionmentioning

confidence: 99%

“…The definition and identification of drought is central to the analysis of drought effects. Here, we define drought as period with water deficits compared to normal conditions, where 'normal' can be quantified as a percentile of the long-term mean of meteorological or hydrological variables 16,53 .…”

Section: Drought Year Identificationmentioning

confidence: 99%

“…Following suggestions by Schwarz et al 16 we selected drought years based on climatic and hydrological information alone without considering tree growth reductions to avoid a biased selection that could for example result in the exclusion of drought years without reduced growth.…”

Section: Drought Year Identificationmentioning

confidence: 99%

“…In dendroecology the annual growth of trees (i.e. the width of tree rings formed each year) is a principle indicator of drought effects, which can be analyzed through quantifying growth resistance to drought (hereafter ‘growth resistance’) 15,16 . Growth resistance quantifies the ability of a tree to withstand drought stress through comparing growth in drought years with mean growth in a reference period, i.e.…”

Section: Introductionmentioning

confidence: 99%

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Cumulative growth and stress responses to the 2018–2019 drought in a European floodplain forest

Schnabel

Purrucker

Schmitt

et al. 2021

Preprint

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Droughts increasingly threaten the worlds forests and their potential to mitigate climate change. In 2018-2019, Central European forests were hit by two consecutive hotter drought years, an unprecedented phenomenon that is likely to occur more frequently with climate change. Here, we examine trees growth resistance and physiological stress responses (increase in carbon isotope composition; Δδ13C) to this consecutive drought based on tree-rings of dominant tree species in a Central European floodplain forest. Tree growth was not reduced for most species in 2018, indicating that water supply in floodplain forests can partly buffer meteorological water deficits. Drought stress in 2018 was comparable to former single drought years, but the cumulative drought stress in 2019 induced drastic decreases in growth resistance and increases in Δδ13C across all species. Consecutive hotter droughts pose a novel threat to forests under climate change, even in forest ecosystems with high levels of water supply.

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Forest restoration treatments increased growth and did not change survival of ponderosa pines in severe drought, Arizona

Fulé

Meador

Moore

et al. 2022

Ecological Applications

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We report on survival and growth of ponderosa pines (Pinus ponderosa Douglas ex P. Lawson & C. Lawson) 2 decades after forest restoration treatments in the G. A. Pearson Natural Area, northern Arizona. Despite protection from harvest that conserved old trees, a dense forest susceptible to uncharacteristically severe disturbance had developed during more than a century of exclusion of the previous frequent surface-fire regime that ceased upon Euro-American settlement in approximately 1876. Trees were thinned in 1993 to emulate prefire-exclusion forest conditions, accumulated forest floor was removed, and surface fire was re-introduced at 4-years intervals (full restoration). There was also a partial restoration treatment consisting of thinning alone. Compared with untreated controls, mortality of old trees (mean age 243 years, maximum 462 years) differed by <1 tree ha À1 and old-tree survival was statistically indistinguishable between treatments (90.5% control, 92.3% full, 82.6% partial). Post-treatment growth as measured by basal area increment of both old (pre-1876) and young (post-1876) pines was significantly higher in both treatments than counterpart control trees for more than 2 decades following thinning. Drought meeting the definition of megadrought affected the region almost all the time since the onset of the experiment, including 3 years that were severely dry. Growth of all trees declined in the driest 3 years, but old and young treated trees had significantly less decline.Association of tree growth with temperature (negative correlation) and precipitation (positive correlation) was much weaker in treated trees, indicating that they may experience less growth decline from warmer, drier conditions predicted in future decades. Overall, tree responses after the first 2 decades following treatment suggest that forest restoration treatments have led to substantial, sustained improvement in the growth of old and young ponderosa pines without affecting old-tree survival, thereby improving resilience to a warming climate.

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Quantifying Growth Responses of Trees to Drought—a Critique of Commonly Used Resilience Indices and Recommendations for Future Studies

Cited by 129 publications

References 81 publications

Groundwater extraction reduces tree vitality, growth and xylem hydraulic capacity in Quercus robur during and after drought events

Groundwater extraction reduces tree vitality, growth and xylem hydraulic capacity in Quercus robur during and after drought events

Cumulative growth and stress responses to the 2018–2019 drought in a European floodplain forest

Forest restoration treatments increased growth and did not change survival of ponderosa pines in severe drought, Arizona

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