The Editor’s Choice paper for this issue is Woodland resilience to regional drought: Dominant controls on tree regeneration following overstorey mortality by Miranda Redmond and colleagues. Associate Editor Peter Bellingham has taken a closer look at the paper and shares his thoughts below…
All but one of the warmest years since 1880, based on global combined land and ocean annually averaged temperature, have been recorded since 2000. 2017 continued that trend; it was the third warmest year on record. Warmer temperatures are therefore interacting with current disturbance regimes to produce new “megadisturbances” to natural ecosystems (Millar & Stephenson, 2015).
Droughts are part of the disturbance regime of most natural ecosystems, but they are increasing in extent and severity as part of current climate change (Dai, 2013). Unprecedented interactions between droughts and warmer temperatures (“hotter droughts”) are likely to feature more in the 21st century, and it is important to understand and predict the consequences for natural ecosystems and the services they provide (Anderegg et al, 2015a).
Hotter droughts than have been experienced previously are affecting forest and woodland ecosystems in many parts of the world, leading to widespread tree mortality. Sometimes, as in western North America, insect attacks (bark beetles and defoliators) also contribute to tree mortality in these circumstances (Anderegg et al, 2015b). A key question, therefore, is whether forest ecosystems are resilient to this kind of widespread tree mortality; that is, can regeneration of seedlings ultimately replace the dying overstorey?
Miranda Redmond and colleagues addressed these questions in their paper recently published in Journal of Ecology. They worked in piñon (Pinus edulis) – juniper (Juniperus osteosperma and J. monosperma) woodlands across their range in the southwestern United States in the aftermath of a “hot drought” that caused widespread and severe mortality of adult trees. They investigated how overstorey trees, understorey vegetation, and local climatic and edaphic conditions interact to influence seedling regeneration of piñon and junipers along gradients of elevation and soil moisture in sites across Colorado, Arizona, and New Mexico.
They found that seedling recruitment of piñon and the junipers was greatest close to established trees, but this was most pronounced in the case of piñon. Piñon, much more than the junipers, depends on the presence of an established advance regeneration for its recruitment after the death of adult trees. High mortality of adult piñon trees had strong effects on its recruitment. Not only was seedling recruitment of piñon strongly reduced if mortality of adult piñon trees was high, but also, where there was high mortality of adult piñon trees, grass cover proliferated at the expense of piñon seedlings. The proliferation of grass cover at the expense of piñon regeneration (and to a lesser extent of juniper regeneration) was most pronounced in Arizona.
For both piñon and the junipers, seedling recruitment was least in sites in all three States where soil moisture was lowest and where the local climate was hottest and driest. Again, low soil moisture and hotter local temperatures had more detrimental effects on the recruitment of piñon than on the junipers.
The developing disturbance regime of hot droughts that cause death of canopy trees points towards a lack of resilience of piñon since its adults are killed and, where this happens, its recruitment is often low. Since junipers are less affected, the authors speculate that, over the parts of piñon’s range where it is most susceptible, there could be a shift towards juniper dominance (Mueller et al., 2005). This in turn would have ecosystem-level consequences, since piñon seeds are a key food resource for wildlife (Ligon, 1978) and people.
This article makes an important contribution to understanding the consequences of a new disturbance regime of hotter droughts: in this case the possibility of a contraction of piñon populations. Since hotter droughts are developing worldwide, it is important to determine whether the effects on some ecosystems are more severe than others. In Western Australia, for example, hotter droughts killed adult trees in eucalyptus forests, and caused structural changes, but composition changed little because, in contrast to the study by Redmond and colleagues, regeneration proceeded largely by resprouting from the belowground reserves of the adults rather than by seedlings (Matusick et al., 2016).
Peter Bellingham, Landcare Research, Lincoln, New Zealand
Read the full paper: Woodland resilience to regional drought: Dominant controls on tree regeneration following overstorey mortality and the accompanying press release from Colorado State University.