Understanding potential limitations to tree regeneration is essential as rates of tree mortality increase in response to direct (extreme drought) and indirect (bark beetle outbreaks, wildfire) effects of a warming climate. Seed availability is increasingly recognized as an important limitation for tree regeneration. High variability in seed cone production is a trait common among many northern temperate conifers, but few studies examine the determinants of individual tree cone production and how they vary with stand structure. In subalpine forests in the southern Rocky Mountains, USA, we monitored >1600
Warming temperatures and rising moisture deficits are expected to increase the rates of background tree mortalityâlow amounts of tree mortality (~0.5%â2% yearâ1), characterizing the forest demographic processes in the absence of abrupt, coarseâscale disturbance events (e.g. fire). When compounded over multiple decades and large areas, even minor increases in background tree mortality (e.g. <0.5% yearâ1) can cause changes to forest communities and carbon storage potential that are comparable to or greater than those caused by disturbances. We examine how temporal variability in rates of background tree mortality for four subalpine conifers reflects variability in climate and climate teleconnections using observations of tree mortality from 1982 to 2019 at Niwot Ridge, Colorado, USA. Individually marked trees (initial population 5,043) in 13 permanent plotsâlocated across a range of site conditions, stand ages and species compositionsâwere censused for new mortality nine times over 37 years. Background tree mortality was primarily attributed to stress from unfavourable climate and competition (71.2%) and bark beetle activity (23.3%), whereas few trees died from wind (5.3%) and wildlife impacts (0.2%). Mean annualized tree mortality attributed to tree stress and bark beetles more than tripled across all stands between initial censuses (0.26% yearâ1, 1982â1993/1994) and recent censuses (0.82% yearâ1, 2008â2019). Higher rates of tree mortality were related to warmer maximum summer temperatures, greater summer moisture deficits, and negative anomalies in ENSO (La Niña), with greater effects of drought in some subpopulations (tree size, age and species). For example, in older stands (>250 years), larger and older trees were more likely to die than smaller and younger trees. Differences in tree mortality rates and sensitivity to climate among subpopulations that varied by stand type may lead to unexpected shifts in stand composition and structure.
- Award ID(s):
- 1637686
- NSF-PAR ID:
- 10452897
- Publisher / Repository:
- Wiley-Blackwell
- Date Published:
- Journal Name:
- Journal of Ecology
- Volume:
- 109
- Issue:
- 5
- ISSN:
- 0022-0477
- Page Range / eLocation ID:
- p. 2203-2218
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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