Vegetation on Earth’s surface plays a critical role in supporting a multitude of functions and services for humans. Vegetation provides us with food, energy and a place to relax. It can also provide a place of solitude, allowing us to connect with nature. Ongoing changes in climate and intensive human activities such as land clearing have resulted in huge changes in vegetation across the globe. One of the dramatic changes in vegetation is the rapid expansion of woody plants (trees and shrubs) across the globe, from ecosystems as broad as grasslands and deserts, to tropical and tundra environments. Fierce debate still rages over whether woody plants are a sign of degradation. Most of the animosity towards woody plants comes from the pastoral industry, mainly because woody plants compete with herbaceous species, which are the mainstay of the grazing industry.
Unencroached grassland (left) and encroached grasslands (right) in Inner Mongolia, China. In the encroached landscape, woody plants form patches, and grasses are sparser in the interspaces. Photos were credited to Jingyi Ding.
Despite a growing body of scientific knowledge demonstrating the importance of woody plants, extensive areas are still being removed, and removal programs are often heavily supported by governments. The aim is generally to restore encroached ecosystems to their original state so that they will support forage. Woody removal has focused on physical vegetation clearing, the use of herbicides, and fire, but there have been very few critical analyses of how effective these removal programs are. This begs the question; is woody removal and effective way to reverse encroachment and if so, under what conditions? This might seem a relatively simple question, but the lack of long-term empirical data on treatment effectiveness and changes mean that the answer is far from clear.
To address this, scientists from Beijing Normal University (China) and the University of New South Wales (Australia) conducted a meta-analysis of 524 studies across all continents except Antarctica, drawing on more than 12,000 independent pieces of information on the effects of woody removal and woody encroachment on a range of ecosystem functions.
The global distribution of 216 woody encroachment studies and 308 woody removal studies across the globe, with selected photos showing the encroached landscape.
We found that removing woody plants is unable to reverse the encroachment process and restore systems to their original state. Removing woody plants only successfully reversed ecosystem structure, and resulted in a 21% expansion in herbaceous cover. This compares with a 48% decline in vegetation structure under encroachment. This result means that removing woody plants reversed less than half of any reductions in herbaceous structure brought about by encroachment. Given that restoring herbaceous cover is generally the main objective of woody removal programs, removing woody plants would seem to be a poor strategy. Contrary to the general notion that encroachment of woody plants is symptomatic of a degraded landscape, we found that encroachment actually enhanced the average ecological response by 9%. For example, there was 31% greater carbon stocks, 18% greater soil fertility and 17% greater soil hydrology at encroached than unencroached sites. This clearly demonstrates an improvement in functional processes such as carbon sequestration and nutrient cycling following encroachment rather than claims that encroachment equates to a dysfunctional state. This work suggests that we can view encroachment as a form of natural regeneration in which woody plants forms fertile islands where they concentrate resources and provide refugia for plants and animals within a matrix of generally infertile soils. Conversely, the removal of encroaching woody plants had generally neutral effects on ecosystem functions, questioning the general perception that woody removal improves or restores ecosystem functions.
Physical removal fails to deter woody encroachment with woody plants re-encroaching the landscape after 5 years, the photo was taken in New South Wales, Australia by David Eldridge.
Removing woody plants cannot reverse encroachment, so why the mismatch between the two processes, encroachment and removal? A key factor here is the identity of the particular encroaching woody species. Our global meta-analysis indicated that plants with particular traits played different roles in the encroachment and removal processes. For example, deciduousness, plant height, and root type mainly regulated soil fertility and carbon stocks under encroachment, but were more closely associated with plant diversity and animal diversity under removal. Another linking factor is the stage of encroached woody plants being removed. For example, we found that removing woody plants at greater density and cover led to more rapid reductions in carbon stocks (-4% to -40%; removal compared to retention) and soil fertility (-15% to -53%), and expansion of bare soil (+37% to +84%) than removing encroached plants where cover is sparse or density is low. Thus, concentrating on encroached plants under more extensive encroachment (greater cover) would lead to greater reductions in more favourable functions than if removal targeted woody plants at lower levels of encroachment (less cover).
The importance of woody cover and woody plant traits was further explored using modelling approaches, and revealed that woody cover, and plants that were deciduous or had the ability to resprout, were the most important determinants of the effectiveness of removal on animal diversity. However, the type of removal treatment (e.g., physical, chemical or multiple removal methods) and time since treatment were the major determinants of the effectiveness of removal on woody and herbaceous cover.
The most widely encroached/removed woody plants genus.
In summary, although “chopping wood” can yield immediate results, it is not an effective way to reverse the impacts of encroachment on ecosystems. Rather, woody plants should be seen as providing important function in ecosystems by accumulating carbon and building up soil fertility. The message is clear; we need to think twice before ‘raising the axe”.
Ding, J., Eldridge, D. J. (2022). The success of woody plant removal depends on encroachment stage and plant traits. Nature Plants. Available from https://www.nature.com/articles/s41477-022-01307-7