Quantifying growth responses of trees to drought - a critique of the Lloret-indicators and recommendations for future studies

Abstract

Quantifying the resilience of forest ecosystems in response to droughts is fundamentally important to assess their capacity to function under intensifying climate change with more extreme droughts. The concept proposed by Lloret et al. (2011) to quantify resilience components of tree growth through indices of resistance, recovery, and resilience has become highly popular in dendro-ecological analyses because it allows a simple, yet highly efficient assessment of short-term responses of trees to drought. Until today these Lloret-indices have been used in 82 studies on growth response to drought of 66 trees species in different climatic regions. Despite their widespread use, some important drawbacks and limitations related to the application of these indices may lead to spurious results or misinterpretation of observed patterns. One major reason for this problem is the inconsistency regarding the selection and characterization of drought events as well as the calculation of the indices. Therefore, this review aims at identifying and discussing shortcomings and pitfalls of Lloret’s indices and to develop recommendations for a more robust and standardized procedure. In addition, we propose a new analytical framework that integrates the different Lloret-indices by comparing observed relationships between resistance and recovery of tree species or treatments to a hypothetical ‘line of full resilience’, which describes the relationship between resistance and recovery that leads to full resilience of radial growth at any given value of resistance. The fit of observed relationships between resistance and recovery with the hypothetical ‘line of full resilience’ can be used for comparative drought-tolerance assessments such as rankings of different tree species or treatments. The suggested approach could be used to harmonize quantifications of tree growth resilience to drought and it may thus facilitate systematic reviews and development of the urgently needed evidence base to identify suitable tree species and management options under climatic change.

Life after recovery: Increased resolution of forest resilience assessment sheds new light on post‐drought compensatory growth and recovery dynamics

“…(2018). This change in resilience with the choice of pre‐ and post‐drought period is in keeping with other recent work that highlights the limitations of considering only a single post‐drought integration window (Schwarz et al., 2020). Instead, we advocate assessing resilience at an annual resolution (Anderegg et al., 2015; Huang et al., 2018; Kannenberg, Maxwell, et al, 2019; Martínez‐Vilalta et al., 2012) to retain important information regarding the temporal dynamics of forest drought response.…”

“…Similarly, this property makes the comparability of resilience values difficult across study systems where the same integration period has not been used to calculate pre-and post-drought growth averages, for example, Merlin et al (2015) and Serra-Maluquer et al (2018). This change in resilience with the choice of pre-and post-drought period is in keeping with other recent work that highlights the limitations of considering only a single post-drought integration window (Schwarz et al, 2020). Instead, we advocate assessing resilience at an annual resolution (Anderegg et al, 2015;Huang et al, 2018;Martínez-Vilalta et al, 2012) to retain important information regarding the temporal dynamics of forest drought response.…”

Tree-Ring Evidence of Forest Management Moderating Drought Responses: Implications for Dry, Coniferous Forests in the Southwestern United States

“…Tree-rings provide one well-studied way to critically evaluate the effectiveness of management treatments, in spite of some well-known limitations. Cross-study comparisons using treering drought response metrics (Figure 1) can be improved with standardization (Schwarz et al, 2019) or using alternate response indices (e.g., Gleason et al, 2017). Tree-ring data can be especially powerful when coupled with additional observations, such as stable isotopes, providing key insights as to how management can shape drought response.…”

“…Problematically, indices of drought resistance, recovery, and resilience appear to be correlated (Gazol et al, 2017a(Gazol et al, ,b, 2018. These indices are also sensitive to the measurements of growth that are used (e.g., radial versus basal area increment), and suffer from potential biases arising from length and growth conditions of the pre-drought reference period, lack of standardization in defining drought severity and duration, variation in post-drought conditions, and differential species responses (Schwarz et al, 2019). These shortcomings complicate comparisons of drought and thinning effects across studies.…”

“…Further treering-based assessments of hardwood responses to drought and how management may influence these responses are needed. Note that the studies using drought resistance, recovery, and resilience metrics (Figure 1) are subject to important biases outlined by Schwarz et al (2019), making it challenging to generalize across studies. Also note that many of these studies set in the southwestern United States occurred at similar sites in northern Arizona and focused on ponderosa pine ( Table 1).…”