Bryophytes as Indicators of Forest Disturbance: a new toolset for conservation

Bryophytes, though relatively understudied, are an important and diverse component of ecosystems with around 20 000 extant species. Three plant phyla make up bryophytes: liverworts (Marchantiophyta), mosses (Bryophyta) and hornworts (Anthocerophyta). The bryophyte life- cycle is unique among land plants for having a dominant gametophyte generation, a characteristic possibly retained from the first plant land-colonisers. Because of bryophytes' small size, their ecophysiology is particular and different to most other land plants, with moisture availability being a limiting factor for many species. Included in this, is the mechanism of desiccation tolerance (DT), which is almost exclusively found in bryophytes. Desiccation tolerance together with a small size means that bryophytes can occupy harsh habitats and substrates that are not available to most plants as they have the ability to efficiently utilise water in the form of water vapour. Bryophytes are therefore highly dependent on microclimate and consequently, have a high affinity to particular microhabitats. In forests, bryophyte reliance on microclimate and microhabitats make bryophytes particularly susceptible to disturbances due to a decrease in humidity and increase in insolation often associated with forest degradation. Bryophytes also have varying degrees of desiccation tolerance which means bryophytes will respond differently to forest degradation.

Tropical humid forests are one of the richest ecosystems but also, historically, one of the least protected. Currently, it is estimated that more than 50% of all tropical habitats are degraded. Madagascar is highly regarded for being a 'biodiversity and endemism hotspot' but is also known for the significant human threats to its ecosystems. The level of threat makes conservation of biodiversity both necessary and urgent and so quick, cost-effective and reliable methods that measure biodiversity responses to forest degradation are vital; one such method is the use of indicators. Indicators can be taxa, groups of taxa or abiotic characteristics. This study investigates the potential of using bryophytes as indicators of forest degradation based on their morphological and life-history traits and how these traits affect their environmental preferences.

A bryophyte trait database was created for 1430 taxa, 51 morphological and reproduction traits, five environmental traits, 13 ecological and distribution traits and three conservation traits. It is the largest bryophyte trait database to date, and is also novel in that it includes Malagasy bryophytes. Portuguese bryophytes were also included to inform on Malagasy species, for which data is scarce. Studies have found that it is possible to extrapolate bryophyte data from one region to another due to the high dispersal ability of bryophytes resulting in species, genera and families common to both regions. In the specific case of Madagascar and Portugal, 34% of Malagasy genera and 64% of Malagasy families are found in Portugal.

Many traits were found to affect species' environmental preferences from large-scale traits such as life-form and plant size to cell shape and spore size. Importantly, analyses conducted on Malagasy and Portuguese species individually showed that their traits have comparable responses to environmental preferences thus confirming that results from Portuguese species can indeed be used to extrapolate to tropical ones. Two trait profiles that characterise species of dry and exposed habitats, and species of humid and sheltered habitats were identified and used to assign species an indicator value. This methodology allowed the inclusion of species with missing trait data, which was the majority of Malagasy species.

Species, genera and families were identified that indicate particular environmental conditions. Species that indicate humid and sheltered conditions are those that have open life-forms and are large. Most epiphytic species are indicators of drier and more exposed conditions. The indicator index created therefore reflects the different responses of bryophyte species. These findings were validated with sampling of bryophytes in a lowland humid forest, in southeastern Madagascar, along a gradient of degradation. Two metrics were used to quantify degradation: a categorical one of four classes of forest degradation and non-forest (cleared forest for shifting cultivation) and an index based on various disturbance variables. This showed that using a finer-scale of degradation provided greater insight into the response of bryophytes to varying degrees of degradation.

Bryophytes have potential as indicators, and the IV metric created here needs further refinements. An important finding was that certain bryophyte traits (e.g. life-form) respond predictably to environmental conditions and forest degradation. These traits could therefore be used as a quick, simple and cost-efficient measure of forest degradation.