Microdisturbance to forest seedlings in New Zealand due to litterfall and animals

Abstract

Natural and artificial seedlings were monitored for damage by litterfall and animals over two years in five temperate forests in the North Island, New Zealand. Mortality was also recorded for natural seedlings. Two of the forest sites were Agathis australis/mixed angiosperm forest (Huapai and Cascades), two sites were mixed podocarp/angiosperm forest (Pureora and Ohakune), and one site was Nothofagus forest (Rotokuru). Litterfall damage rates of natural seedlings were similar to those of nearby artificial seedlings, and site means for the two were highly correlated (R2=0.95, P<0.005). However, there was no relationship found between non-trophic animal damage (damage without consumption) to natural seedlings and animal damage to artificial seedlings. Nor was there any correlation between artificial seedling damage, due to animals or litterfall, with natural seedling mortality. Artificial seedlings should not therefore be used to estimate seedling mortality due to microdisturbance. Monitoring natural seedlings enabled non-trophic animal damage to be readily detected and provided additional information on animal-plant interactions. The value of using artificial seedlings is that they provide a measure of the litterfall disturbance potential that is independent of the patchy distribution of natural seedlings within safe sites, and independent of species specific resistance to damage. An important proportion of the natural seedlings damaged at Huapai was due to non-trophic damage by animals (21.4%/yr.). Most of these seedlings were bitten off at the base and left uneaten and the remainder were uprooted. Non-trophic damage to seedlings by animals was negligible at the other four sites. However, a high proportion of palatable species were damaged at the two sites managed as restoration areas (Pureora and Rotokuru) by the Department of Conservation. Litterfall was the cause of an important proportion of the seedling mortality at each forest site (11-18%) except Cascades (2.4%), and it was an important cause of seedling damage at all sites. Annual litterfall damage to artificial seedlings at Huapai was greater than at any other site, and similar to that reported for several tropical forests. Macro-litterfall (>30x1.5 cm) surface area and macro-leaf-fall weight were both correlated with litterfall damage to artificial seedlings (R2>0.96, P<0.005 for both), but total macro-litterfall weight (ie. including deadwood) was not. This suggests that the rate of macro-leaf-fall is the primary factor that determines the litterfall damage regime. Macro-leaf-fall is in turn influenced by climate, because both the abundance of large leaved species and their productivity are linked to climate. However, protective vegetation within 2 m of the ground can reduce the risk of seedling damage by up to 75%. At Huapai and Pureora the rate of damage to artificial seedlings under different species combinations were compared. At Huapai, the risk ranged from 39%/yr beneath canopies with both Agathis australis and Cyathea dealbata down to 2%/yr beneath canopies without A. australis, C. dealbata, or Rhopalostylis sapida. At each site 20 seedling pairs of the two most abundant seedling species were selected. One of each pair was experimentally damaged by pinning to the ground in order to simulate litterfall damage. Pinned seedlings had a higher mortality over two years than the unpinned control group. However, resilience to damage, in terms of survival and height regained, varied substantially between species. Unexpectedly, pinned Nothofagus menziesii and Hedycarya arborea seedlings grew much faster in response to damage than nearby unpinned seedlings of the same species. Heterogeneity in litterfall risk among microsites, and differential resilience to litterfall damage among seedling species, indicates that some species have a greater chance of surviving litterfal in particular microsites than others. The litterfall damage regime, therefore, has the potential to contribute to regeneration niche differentiation and may thereby contribute to plant species richness.