Life after recovery: post-drought compensatory growth in mature trees

Author Tom Ovenden examined the impact of extreme drought on Scots pine trees in recently published article, Life after recovery: Increased resolution of forest resilience assessment sheds new light on post‐drought compensatory growth and recovery dynamic. We hope you enjoy the brilliant video animation created by Tom to summarise the key findings of this research – included below!

This paper is part of Journal of Ecology‘s joint Special Feature with Journal of Animal Ecology on Ecological Resilience, which will be published in full later this year.

Extreme drought events are now impacting forest growth globally. In many place, these events are expected to become more frequent and severe in the coming decades, as a result of climate change. This makes understanding how both individual trees and the wider forest respond to, and recover from such events crucially important if we are to ensure our forests are resilient to the challenges of a future climate. However, current approaches to quantifying resilience limit our understanding of forest response dynamics, recovery trajectories and drought legacies by constraining and simplifying the temporal scale and resolution of assessment.

We developed an alternative approach to measuring resilience which uses dynamic regression to capture each individual tree’s relationship between climate and growth before drought. This relationship was then combined with post-drought climate data to forecast tree growth annually for the drought year and nine subsequent years, representing a scenario where no drought had occurred. This approach allowed us to compare observed annual growth with forecasted growth to calculate tree-level recovery times, quantify drought impact on tree growth and understand the implications for the wider forest over time.

Growth deficit derived from the difference between observed and forecasted growth (BAI). Chronology level annual growth deficit summed over time, representing individual tree cumulative growth deficit at a given stem height (grey lines), stand annual deficit calculated by summing annual growth deficit for all chronologies at a given stem height in a given year (solid green line) and the cumulative stand growth deficit calculated annually by summing the annual stand deficit over time (dashed yellow line) in the high density (ρH) and low density (ρL) stands at 0.3m, 1.3m and 3.3m stem heights. Annual values were calculated for the drought year in 1984 (vertical dotted red line) and the subsequent 9 years (1985-1993).

We found that the rate of recovery from a commonly experienced drought was highly variable, despite trees being the same age, species and growing at the same location. On average tree growth recovery took four years, with most trees taking between 1-6 years but some still hadn’t recovered nine years later. Similarly, pre-drought tree size and growth rate made a significant difference to how resilient they were, with faster growing trees being more resilient, while larger trees were less resilient. However, we found evidence for a temporal dependency in the stage during recovery that these pre-drought tree attributes were associated with higher growth resilience.

By following tree and stand level growth relative to a no-drought scenario throughout recovery and into a post-recovery phase, we found evidence for significant compensatory growth in many, but not all trees. This compensatory growth was sufficient to reduced estimates of drought-induced losses of radial growth, meaning that the total impact of drought was less after 9 years than after 4 years (which is the post-drought period commonly considered).

Compensatory growth of this kind has been recorded in fish, moths and grasses, but has been widely unexplored in mature trees following extreme events such as drought. Documenting this pattern in adult trees encouraging, as it seems to indicate that a mechanism might exist that enables forests to recover some of the impact drought has on growth. However, we need to establish if compensatory growth mechanisms are more widespread in other species, regions and conditions before drawing wider conclusions as to their importance in mitigating drought impact. Similarly, even if found to be more widespread, as compensatory growth necessarily occurs post-recovery, increases in drought frequency and severity could overwhelm any benefits. Nevertheless, our results do suggest that, when drought does not cause widespread tree mortality, current short-term assessments of forest drought response may risk underestimating tree and stand resilience and overestimate losses in above-ground biomass. This in turn could have knock on implications for both forest management targeted at increasing resilience, carbon budgeting and our understanding of drought legacy.

Thomas Ovenden University of Stirling & Forest Research – Northern Research Station, Scotland, UK

Read the full article in Journal of Ecology: Life after recovery: Increased resolution of forest resilience assessment sheds new light on post‐drought compensatory growth and recovery dynamics by Ovenden et al.

You can also read the Press Release for this paper on the BES News site: New research reveals the resilience of Scots pine trees to drought

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