Brock et al. recently published their new article “The hare, tortoise and crocodile revisited: Tree fern facilitation of conifer persistence and angiosperm growth in simulated forests” in Journal of Ecology.
Author James Brock discusses this research in more detail and explains how tree fern understories are important determinants of forest structure.
Establishing how conifers compete with their ‘upgraded’ flowering angiosperm cousins, to explain conifer persistence in modern landscapes, has been a preoccupation of plant ecologists. The underlying question is how do (massive generalisations here) slower growing, less productive species with inferior photosynthetic structures and plumbing (vascular tissue), remain competitive and present in a community with angiosperms that are in many ways, their better?
Analogies to Aesop’s Fable’s hare and tortoise have been used: conifers have escape mechanisms to evade the competitive effects of angiosperms (however, calling angiosperms arrogant might be overdoing the analogy). Two of these mechanisms are: 1) conifers establish early post-disturbance and get a head start on the angiosperms; and 2) conifers establish under the canopy and then persist in the shade until the canopy opens and permits them to capture space in the forest. For the second hypothesis, however, the shade tolerance of conifers has its limits. Where deep shade is formed by a dense understory, conifer regeneration is seriously reduced; without any large-scale disturbance to the forest, conifers would likely disappear locally from the forest.
Southern temperate rainforests are frequently composed of mixed angiosperm conifer communities in the canopy, with dense understories of bamboo or ferns. The lowland rainforests of New Zealand are no exception and frequently have a near-continuous sub-canopy of tree ferns, either katote Cyathea smithii or ponga/silver fern Cyathea dealbata of All Blacks fame; the sub-canopy can be so dense that half of the stems in the forest are tree fern! The wet forests of the deep south of New Zealand also have thick understories of piupiu, a short-trunked species of Blechnum, with stands of tree ferns (Dicksonia and Cyathea) that cast a heavy shade on the forest floor. The presence of these dense, long-lived (tree ferns can live for several centuries) tall ferns in the understorey has previously been portrayed as a long-term inhibitor of conifer persistence in the forest community: conifers would be shaded out; any saplings that were lucky enough to establish would then be out-competed by the more shade tolerant angiosperms.
The northern lowland forests of New Zealand are comparable to those in the south: mixed podocarp (conifer) and broadleaf canopies, generally species rich with understories of shrubs, palms, lianes, small trees and tree ferns. In 2017 Narkis Morales and my PhD co-supervisor George Perry published a model parameterised on these northern forests. The model described the physiognomies (height, diameter at breast height of trunk, age) and the abundance of 7 species: two angiosperm and one conifer species in the canopy, one small angiosperm tree and two shrubby species. Although the northern forests have dominant tree fern understories, this clade of plants was not represented in the original model. The model functioned well and produced outputs that accurately represented New Zealand mixed forest ecosystems; however, as with all NZ forest models, stem density of the canopies was higher, and the canopy lower, than that of actual forests.
The growth and competitive components of the model built by George and Narkis was, like many other models, based on the value of trunk diameter (at breast height). So when we came to add tree ferns to the model we hit a problem: tree ferns, like palms and other plants with a monocotyledonous growth form, don’t really increase their diameter as they grow; there is no secondary thickening. Although rhizoids on the trunk can occasionally form a cone at the base of the trunk, this doesn’t well represent the overall growth of the plant. Where tree ferns had previously been represented in New Zealand forest models using trunk diameter parameters, they had not persisted in models at representative densities, and would disappear from the simulated forest community. We chose to parameterise tree ferns using a height-based growth function: Bruce Burns and I, along with a team of assistants, obtained height-growth data from permanent vegetation plots across the Auckland and Waikato regions of the North Island. This data enabled us to derive a height-growth relationship, and parameterise a tree fern growth form for the model.
We ran experiments to see how tree ferns affected the long-term presence of conifers in our digital forest. We used simulations of forest growth that started with different abundances of tree ferns in the model, then compared the relative abundance, age, height, and trunk diameter of the conifers over a 2,500 year period. The outcome was fascinating and entirely surprising: instead of seeing a decline in the conifer population with an increasing tree fern understorey, the conifers lived longer and grew taller with more tree ferns in the simulated forest.
Think of a forest as a fixed area that can support a finite number of stems. If all of the stems are species of plant that can reach the canopy, then the canopy will be densely packed and individual plants will be under intense competition from their neighbours. If we remove half of those canopy plants (readily possible in a model, moderately less so in the real world) and replace them with short-stature, long-lived plants (e.g. tree ferns) then the competition in the canopy across all individuals is reduced considerably. This means that conifers are partly relieved of competition from neighbours and can persist longer, grow taller, and increase their chances of regenerating. Angiosperms also benefit as they are similarly relieved of high levels of competition, and will be able to grow larger (which we also observed in our model). So, far from suppressing conifer presence in forests, the tree fern understorey reduces canopy stem density and helps conifers persist in the landscape. We think this makes sense when you consider the hundreds of millions of years over which ferns and conifers have persisted and co-existed. Another great outcome of the tree fern addition to the model was that the canopy tree densities were reduced to levels that more precisely represented natural forests; likewise, canopy height increased by at least 5 metres to more closely represent that seen in real New Zealand native forest systems. The height-growth model approach to representing tree ferns digitally worked well also – with tree ferns persisting in the simulated forest at realistic densities.
The main take home here is that where mixed forests have dense understories, rather than focussing on the dynamics of the canopy alone, we should be thinking of them as three component systems: conifer – angiosperm – fern (or other understorey dominant).
James Brock, University of Auckland – Te Whare Wānanga o Tāmaki Makaurau, New Zealand