Damage to artificial seedlings across a disturbed Afromontane forest landscape

Abstract

Seedling mortality is an important demographic bottleneck for forest regeneration, yet the factors influencing recruitment are often poorly characterized across space and time. In African highlands, where extensive patches of previously disturbed forests remain in persistent shrub-dominated states, patterns of damage to seedlings have not been examined in detail. We used artificial seedlings to determine how (nonherbivory-related) damage varies across a fragmented forest landscape in the Bwindi Impenetrable National Park, Uganda. A total of 848 artificial seedlings were established in 106 plots, each with 8 seedlings, across a gradient of environmental factors, including habitat type, topographic position, distance to water, and distance to human activity. We divided damage into that due to vertebrates, plant debris, and unknown agents. From our 16,960 monthly stem assessments, 1,289 damage events were recorded. The mean "seedling" damage per year (± 1 SE) was 59.5 ± 2.3%. Damage was significantly more frequent in sites dominated by short herbs (66.1 ± 3.0) than in sites dominated by shrubs (60.1 ± 2.6) and ferns (47.9 ± 2.7). The mean percentage of damage by each class of agents was 45.8 ± 2.1% for vertebrates, 21.4 ± 2.1% for plant debris, and 3.8 ± 0.6% for unknown agents. These rates surpass those reported in other montane forests, likely reflecting the density of large vertebrates in our site.Our results indicate that most damage to artificial seedlings arises from larger vertebrates but that most spatial variation in this damage reflects the impacts of falling plant debris. These damage processes appear sufficient to constrain the regeneration of tree species in areas where forest cover is patchy and fragmented.

Delayed effects of climate on vital rates lead to demographic divergence in Amazonian forest fragments

“…For instance, the increased cloud cover associated with elevated precipitation could lead to reduced photosynthetic activity and growth; plants in the resulting saturated soils could also have lower growth and elevated mortality (Parent et al, 2008). The wind storms accompanying extreme precipitation events (Espírito‐Santo et al, 2010; Negrón‐Juárez et al, 2018) could increase the likelihood of tree‐, branch‐, and litterfall, all of which are sources of mortality for understory plants (Scariot, 2000; Ssali et al, 2019). Finally, cool temperatures associated with unusually wet conditions may decrease flowering (Pau et al, 2013).…”

Sex ratios, damage and distribution ofMyrianthus holstiiEngl.: a dioecious afromontane forest tree

“…This is unlikely to be an artefact of our method as we would expect more damaged trees (being incomplete and thus less visible) to be less detected than less damaged stems. Studies of 'artificial seedlings' in Bwindi (an area near Ruhija and overlapping our Landscape C) show that mean yearly damage per 'seedling' was 59.5 ± SE 2.3% with most, 45.8 ± 2.1%, due to vertebrates (Ssali et al 2019). While we cannot identify the various sources of damage to larger trees with certainty, our observations suggest that large mammals are a major cause, unlike the case with 'artificial seedlings', the animals may be attracted by these palatable food plants for both their foliage and their fruits.…”

“…Terrain is generally steep with elevation ranging from 1190 m to 2607 m asl. The climate is equatorial with annual rainfall averaging between 1,130 and 2,390 mm with two rainfall peaks from March to May and September to November (Ssali et al 2019).…”

Delayed effects of climate on vital rates lead to demographic divergence in Amazonian forest fragments

“…For instance, the increased cloud cover associated with elevated precipitation could lead to reduced photosynthetic activity and growth; plants in the resulting saturated soils could also have lower growth and elevated mortality (Parent et al, 2008). The wind storms accompanying extreme precipitation events (Espírito-Santo et al, 2010;Negrón-Juárez et al, 2018) could increase the likelihood of tree-, branch-, and litter-fall, all of which are sources of mortality for understory plants (Scariot, 2000;Ssali et al, 2019). Finally, cool temperatures associated with unusually wet conditions may decrease flowering (Pau et al, 2013).…”