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Marked seedlings. Our dataset includes 1463 marked individual oak seedlings in 535 5m2 plots over the 20 transects. Overall, oak seedlings occurred in a third of the searched plots. Most plots had only one seedling (236 plots) and only 3% (16 plots) of the plots had 10 or more seedlings, with a maximum density of 3.4 seedlings m2. Cohort years 2011, 2015, 2017 and 2022 had a greater influx of seedlings, and our oldest seedling is estimated to be 30 years or older. For seedlings where initial height was measured in their year of germination (i.e., in 2011 or later) mean initial seedling height was 14.8 cm (SD = 6.51 cm) and there was a small but significant decline in initial seedling height with time (-0.23 cm/year, F1,1298 = 19.97, p=<0.0001). The size of first year seedlings as measured by both initial height (F1,760 = 25.2, p<0.001) and leaf number (F1,752 = 11.1, p=0.009) increased on average with distance into the valley. The mean distance of oak seedlings into the valley has not increased over the course of the study and remains centered around 800m from the east entrance. Plot environment. Study transects encompassed a full range of topographic positions in the lower valley. Hemlock dominance ranged from absent (5 transects) to dominant (greater than 50% of trees; 3 transects). Tree basal area around the plots ranged from 18.8 – 43.5 m2 ha-1. Light transmission was universally low (all ≤13% TT). Light transmission (F1,5916 = 139.3; r2: 0.157, p<0.0001), hemlock dominance (F1,5916 = 2024; r2: 0.255, p<0.0001), and oak seedling density (F1,5916 = 73.9; r2: 0.126, p<0.0001) all decreased with distance further west into the valley. These data were gathered as part of the Hubbard Brook Ecosystem Study (HBES). The HBES is a collaborative effort at the Hubbard Brook Experimental Forest, which is operated and maintained by the USDA Forest Service, Northern Research Station.
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Multi‐cohort survival of northern red oak seedlings at a northern hardwood forest transition
Abstract Global change has created less stable forest systems and given urgency to understanding limitations to the establishment of tree seedlings beyond current range boundaries. We quantified trends in 13 years of annual northern red oak (QURU) seedling survival data for 1733 marked individuals at a local species distribution boundary within the northern hardwood forest in New Hampshire, USA. Over the study period, the median distance of seedlings into the valley did not change, although there was a net gain of 89 plots (5 m2) occupied. For a subset of seedlings that were marked in their year of birth (N = 937), we examined relationships among terrain, vegetation community, and initial individual seedling traits, and evaluated their effects on time to seedling mortality using a parametric accelerated failure time model. The year of seedling germination had the largest effect on survival with increasing mortality rates for seedlings from more recent cohorts. Seedlings had longer survival times where oak seedling densities were lower, shrub cover was higher, and when the acorn remained attached. Additionally, survival time was increased in higher elevation plots, which were also located further into the valley. Interannual seedling survival (N = 1580) was strongly impacted by seedling condition in the previous year, particularly leaf number and amount of leaf damage. Most seedling deaths occurred over winter, and seedlings failed to break bud the following spring. Interannual variation in seasonal climate, particularly deep, heavy snowpack in 2019 followed by drought conditions in 2020, coincided with recent elevated mortality. Overall, the median survival time of 3–4 years and the rapid turnover of the oak seedling population currently limit ability for expansion, although the net gain of occupied plots and increase in survival at higher elevation plots with lower QURU densities present some mechanisms that could promote expansion if the current suboptimal understory conditions shift to favor QURU.
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- Award ID(s):
- 1637685
- PAR ID:
- 10531639
- Publisher / Repository:
- Wiley Blackwell (John Wiley & Sons)
- Date Published:
- Journal Name:
- Ecosphere
- Volume:
- 15
- Issue:
- 8
- ISSN:
- 2150-8925
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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