Seasonal temperature–moisture interactions limit seedling establishment at upper treeline in the Southern Rockies

Abstract

Over recent decades, sharply rising temperatures without an accompanying increase in precipitation have created widespread heat-induced drought stress, or hotter drought. Tree-ring reconstructions have discovered significant declines in seedling establishment from hotter drought across montane and subalpine forest belts during this time, yet comparable studies at upper treeline are non-existent. In this study, we reconstruct annual patterns of seedling establishment at upper treeline in the Southern Rocky Mountains to test the hypotheses that establishment is governed by temperature-moisture interactions and that slope aspect mediates the influence of hotter drought. To test these hypotheses, we destructively sampled seedlings along a network of six study sites to reconstruct annual patterns of establishment on opposite north-facing and south-facing slopes. Results from this research can be summarized into two main points with respect to the influence of climate on seedling establishment at upper treeline over approximately the last three decades . First, temperature-moisture interactions throughout the year play a crucial role in facilitating successful seedling establishment. Second, and perhaps most striking, is the complete lack of establishment at all sites over the past decade. This could signify that a threshold has been surpassed and conditions are now beyond the climatic optimum for successful seedling establishment above treeline moving forward. These results expand upon similar findings from forests at lower elevations, introducing the likelihood that seedling establishment along the entire mountain forest belt of the Southern Rocky Mountains is being impacted by hotter drought. This means that any declines in the rate of seedling establishment across montane and subalpine forests will not be offset by increased recruitment at treeline.

Climate Change in New Mexico Over the Next 50 Years: Impacts on Water Resources

“…support successful vegetation regeneration (the regeneration niche) is generally narrower than the broader climatic range in which adult plants can grow and persist. Due to warming-induced aridity, the regeneration niche is likely now shrinking for many plant species (e.g., Bailey et al, 2021).…”

“…More warm/dry-(xeric-) adapted plants from lower-elevation sites will shift their distributions upslope (Kelly and Goulden, 2008;Brusca et al, 2013); and 3. Less cold-tolerant plants from southerly portions of New Mexico will shift their distributions northward and perhaps upslope (although note the recent documentation of warming temperature and dryness constraints on alpine tree establishment in northern New Mexico by Bailey et al, 2021).…”

“…Forests and Woodlands-Future climate warming and increased precipitation variability are anticipated to directly depress regional woody-vegetation productivity (Williams et al, 2013;Munson et al, 2021) and promote Southwest forest die-offs from hotter droughts (McDowell et al, 2015;Goulden and Bales, 2019). In concert with the associated intensification of ecosystem disturbances, particularly high-severity wildfire (Bowman et al, 2020;Pausas and Keeley, 2021), ongoing warming in New Mexico montane forests and upland woodlands is expected to increasingly constrain tree regeneration (Davis et al, 2019;Rodman et al, 2020;Bailey et al, 2021;Nolan et al, 2021) and further amplify widespread vegetation type conversion from tree-dominated forests and woodlands to non-forest ecosystems (Allen, 2014;Guiterman et al, 2018;Coop et al, 2020;Davis et al, 2020). Drier, low-elevation distributions and ecotone margins of individual tree species and particular vegetation communities will tend to respond to growing drought and heat stress with early, rapid, and pronounced mortality-induced upslope range retraction (Allen and Breshears, 1998;Davis et al, 2019;Parks et al, 2019).…”

Complexity of Regeneration Dynamic at the Ecocline between Mixedwood and Coniferous Domains of the Southernmost Boreal Zone in Eastern North America

“…Regeneration dynamics are closely dependent on seed availability and recruitment potential, which may be limited by many factors [12,13]. Tree seedlings, the first stage of regeneration, are specially dependent on seed source (parental trees) and soil substrates that are suitable for their establishment [14].…”

Opposite Tree-Tree Interactions Jointly Drive the Natural Fir Treeline Population on the Southeastern Tibetan Plateau

“…However, ubiquitous treeline advance has not been detected under the warming background in the last century, with the global mean advance rate of treeline position lagging far behind the warming velocity [17]. By contrast, the warming-induced tree densification of the treeline ecotone has been found in Europe, North America, and Asia when the moisture conditions are not limiting [10,[18][19][20][21]. Such a mismatch between the two ecological processes (increasing tree recruitment vs. slow upward advances of treeline position) at treeline ecotones could lead to intraspecific interactions [5,9,22,23], even though microsite facilitation from neighboring plants is generally considered to be one of the major drivers of structuring population dynamics in stressful environments such as in alpine treelines [7,24,25].…”