Poor acclimation to drought in subalpine forest tree seedlings

Understanding how tree species will tolerate projected increases in drought is necessary to anticipate future forest dynamics and predict future forest productivity. Droughts linked to climate change have already caused widespread tree mortality across large areas of many forested regions, with adverse impacts on landscape structure and function. Whilst saplings and adult trees display strong synchronisation to their environment and relatively high tolerance to stress, conifer tree seedlings are more vulnerable to water stress given their shallow rooting depths and narrow carbon budgets. Despite this, there have been few studies investigating how drought affects natural populations of tree seedlings in the field, despite the critical roles they play in forest dynamics, range shifts and the overall resilience of forests to ongoing climate change.

In their new study published in AoBP, Goke and Martin subjected naturally established seedlings of co-dominant subalpine conifer species (subalpine fir, Abies lasiocarpa and Engelmann spruce, Picea engelmannii) in the southern Rocky Mountains, USA to 2 years of in situ summer precipitation exclusion. This exclusion treatment – set up using polycarbonate rain deflection shelters – simulated summer drought conditions similar to a failure of the North American monsoon. The authors compared the morphological and physiological responses of seedlings growing in drought vs. ambient conditions to assess the relative changes in drought tolerance traits as a function of seedling size.

Goke and Martin observed a striking lack of morphological and physiological acclimation to drought in the conifer seedlings, and a prioritisation of carbon gain traits at the expense of drought mitigation and tolerance. No morphological adjustments to drought mitigation traits were detected in either species, and both photosynthetic carbon gain and water use efficiency were greatly reduced reflecting poor whole-seedling acclimation to water stress, particularly for spruce. These results indicate canonisation of traits that, while useful for early seedling establishment, may indicate substantial vulnerability of seedling populations to prolonged or recurrent droughts. The authors conclude by stating that, “increased seedling mortality with climate change-induced drought is a likely outcome of these responses, which may in turn affect availability of seedlings for recruitment into larger tree size classes.”

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Goke, A. and Martin, P.H. (2022) “Poor acclimation to experimental field drought in subalpine forest tree seedlings,” AoB Plants, https://doi.org/10.1093/aobpla/plab077