Invasive and native woody plant encroachment: Definitions and debates

In this short opinion piece, we discuss the appropriate use of the term ‘invasion’ for woody plant expansion and refer to the various ways in which the term is being used in the literature. We point out the present confusion and make suggestions for the use of a more appropriate term (i.e., ‘woody plant encroachment’). We continue with an overview of the various definitions of ‘woody plant encroachment in the literature, we mention associated alternative terms, and we explain the circumstances in which each of these are used. With this piece, we hope to provide more clarity on the use of correct terminology related to woody plant expansion research.

The term “invasive”, for plants, can have a different meaning depending on the context. From a conservationist point of view, plant invasion is strictly referred to as the expansion of non-native species [1]. However, from a landowner or land manager’s point of view, the term “invasion” is often used for the expansion of native woody species into non-preferred (for the landowner and manager) pieces of land. A recent review that looked at the definition of invasive species included both native and non-native species in its final definition [2]. As it seems, there is confusion in the use of this terminology.

The expansion of woody species is a worldwide issue and can include both native and non-native species [3,4]. When referring to this phenomenon, the term “invasive” has been used inconsistently, and other terms used have different meanings depending on the study. For example, scientists have used the term “woody plant invasion” to examine the expansion of native woody species [5,6] and ecologists tend to use the term “woody plant encroachment”, which can include native [7] or both native and non-native species [4].

In our view, the term “invasive” cannot be used for the expansion of native species, since they technically do not “invade” an area, but merely increase their cover from already existing stands through various pathways. For such circumstances, we prefer the use of the general term “encroachment”. We further support the views of Moutou and Pastoret [1] and Chaneton, et al. [8], in using the term “invasive” only for non-native species that have an expansive character in the ecosystem of study, since not all non-native species have encroached capabilities, and some might be invasive in one area, and not in another [9]. For instance, the genetic potential of a woody plant has a higher chance to be reached under different, more optimal environments (including climate, soil, and nutrients) than in its home range; leading to more vigorous growth [4]. Therefore, we believe that “woody plant encroachment” (WPE) should be used as a more general term, including both the encroachment of native and non-native species, as supported by Archer, et al. [4].

There exist various definitions of WPE in the literature (Table 1). Archer, et al. [4] and Heisler, et al. [10] consider both native and non-native woody species in their definition, whereas Van Auken [11] considers only native. Except for the term “woody plant encroachment”, the terms “woody plant invasion”, “woody thicketization”, “woody plant expansion”, “invasion of woody weed”, “xerification”, and “invasion of shrubs” are also used (Table 2). As mentioned before, the term “woody plant encroachment” seems to be the best way of describing the expansion of woody plants, since it can include both native and non-native woody species. On the other hand, the term “woody plant invasion” or any other term that includes “invasion” is more confusing, since it is being used in the literature for both native and non-native species with an invasive character, even though it is most appropriate to be used for the latter case. Examples in the literature that uses the term “woody plant invasion” for native species are many [12-16], while those addressing non-native species are more [17-25]. Moving further, “woody thicketization” is a term that is related to the densification of shrubs and low-stature trees, which tend to fill the gaps between them, in areas with rainfall higher than 400 mm [26]. Although less common, there are example studies that use this type of terminology [4,27-30]. “Woody plant expansion” is a general term that is a good alternative to use for “woody plant encroachment”. Lastly, “xerification” has been connected to the expansion of woody plants in arid environments (e.g., desert shrublands). There, water and nutrients concentrate below the woody canopy, degrading the spaces between them, and causing higher runoff and erosion; a phenomenon referred to as “islands of fertility” [31]. Example studies that use the term “xerification” are those of Archer, et al. [30] and Schreiner-McGraw [32]. Apart from the aforementioned terms, according to the review of Eldridge, et al. [29], other terms that are being used for WPE include “woody thickening”, “regrowth”, and “bush encroachment” (Table 2). “Woody thickening” is used interchangeably with “woody thicketization”, “regrowth” is more commonly used by rangeland managers, while “bush encroachment” is a term that is more popularly used in studies conducted in the African continent. All in all, it seems that “woody plant encroachment” is the overarching term, which can be separated into “thicketization” or “xerification”, depending on the precipitation gradient of a region [32].

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