Road width as the main factor behind various road effects on forests
Although our study sites cover different climatic zones, forest ages and road ages, the results indicated that three variables measured here changed significantly with increasing distance from the road to the forest interior along wide roads, including biomass (tree and herbaceous plant) and soil pH. The increase of biomass may be due to the fact that the roads have created open spaces that provide more sunlight and space for the growth of plants. For the narrow roads, none of the measured variables showed statistically significant differences between the edge and the interior. Narrow roads had no measurable effects on the biological-environmental conditions of the forest, and the differences between wide roads and narrow roads are caused by road width. Previous studies reported that the highest species richness and diversity were found in the plots closest to the trail (Queiroz et al. 2014), while other road segments (5–6 m wide) had no significant effects on plant species diversity (Tehrani et al. 2015). Even for non-native species spread, verges hosted but did not facilitate the spread of alien species, and it is necessary to highlight the importance of considering regional climatic gradients, landscape context and road-verge properties themselves (Kalwij et al. 2008). Especially for a well-preserved region, where road dependency is strong, and the proportion of uncommon non-native species is small (Liao et al. 2019). Therefore, these effects were mainly caused by road width, which is a comprehensive factor that integrates multiple factors (usage frequency, disturbance intensity, traffic volume, surfacing material, animal behavior and so on).
We propose a shape-dependent model to explain the road width effects. Two patches with the same area will produce completely different effects due to different shapes (Fig. 10). In all locations (middle, transition area or edge) in a glade, the differences in temperature and moisture were associated with glade shape (similar square, rectangle, or long-narrow). There were similar trends in the middle of the glade, in the transition zone between the glade and forest, and at the edge of forest. The temperature and moisture differences between the middle of the glade and the forest interior were largest in the similar square glade, followed by the rectangle and then the long-narrow glade. This result indicates that different shapes of forest patches will have different effects on forests. Previous research suggests that road permeability can be improved by maintaining canopy cover along short sections of roads (Chen and Koprowski 2016). That is, canopy shelter causes the path to not have a significant impact. Our model indicated that road effects are induced by large forest gaps. Because a road has a large open area and a substantial influence on the entire forest path, there is no interference from the canopy, so the impact is relatively small.
In fact, road width is the main factor of various road effects on forests. For example, compared with informal trails (4 m), formal trails (2 m) had a protective effect on vegetation cover (Huang et al. 2015). Various effects of roads on forests are essentially caused by the road width. Narrow and gated roads, unlike wider and ungated roads, did not appear to contribute to edge effects on terrestrial salamanders (Marsh 2007). Road width and traffic density are major determinants of the barrier effect, whereas road surface (asphalt or concrete versus gravel or soil) is generally a minor factor (Forman and Alexander 1998). Road width was a good predictor of the magnitude of the effects (Marsh 2007). Additionally, road width explained the magnitude of edge effects much better than the total break in the forest canopy (Marsh 2007). Forman (2005) proposed simple spatial models, which indicated that road effects are relatively minor around narrow corridors, and roads have the least impact on small patches (Haskell 2000). The most severe effect is where a highway crosses a small natural patch, and the smallest effect occurs where a small road passes alongside a large patch (Forman 2006). When the road is small, the road effect is practically nonexistent, even when the road has a high overall length and coverage over a large area. Long-narrow roads contribute to the exchange of adjacent habitats, and wider roads could hamper energy interactions. For example, wider roads tended to produce steeper declines in abundance and richness of macroinvertebrate fauna and leaf litter (Haskell 2000) when the road was 24 m wide and surrounded by uninterrupted tropical forest; these roads created a strong contrast in the microclimate (Kunert et al. 2015). In addition, the effects have proportionally more impacts on forest areas in small fragments or fragments with convoluted shapes, both of which have higher perimeter-to-area ratios (Ewers and Didham 2007), increasing the edge-to-area ratio, which corresponded to a decrease in diversity within glades (Smith et al. 2007). Wide roads have greater effects on square patches than patches of other shapes. The same effects can be found for animal behavior; for example, this phenomenon has been observed with ladybird beetles, who emigrate from rectangular patches faster than square patches. Much of the observed variation in emigration rates from a series of published studies could be explained by the scaled effect of patch size (Nams 2011).
Implications for forest road management
An important question for the management and mitigation of road effects on forests is how wide the road should be. If narrow roads cause fewer edge effects than wider roads, newly built roads should not be wider than necessary (Marsh 2007). The necessary width discussed in this study can also be seen as a threshold. Roads have inseparable positive and negative effects on plant communities, but the negative effects are of greater concern (Berges et al. 2013). However, there are no negative effects if the road width is under a threshold value. In addition, the ability of the ecosystem to repair itself can mitigate the negative effects of roads (Tehrani et al. 2015). Specifically, the perception of forest road management should change in the following three ways.
First, paths have no impact on forests, which means that if roads are needed (for research, management, or recreation purposes), they should be established at their minimum required size to minimize the impacts. “Minimum required size” here is a threshold; that is, the construction of roads at certain thresholds does not affect forests. Road construction is forbidden in many areas; however, this view may not be correct. Since a path appears to create a very small level of interference, such interference does not lead to the destruction of the forest canopy and does not affect forests. Then, we can infer that paths can be a part of the management of nature reserves, as long as they are kept below the threshold of minimum impact. Therefore, the appropriate construction of roads can be permitted, rather than completely banned.
Second, roads have negative impact on forests, and it can be inferred that wider roads would have greater impacts on forests. If a road exceeds the threshold of maximum effects, it may cause a collapse of the forest ecosystems, which would cause disastrous effects. For example, some models show that the viability of a population is determined not only by road density but also by the sizes and shapes of patches (Borda-de-Agua et al. 2011). Rainforest fragments in central Amazonia were found to experience a dramatic loss of aboveground tree biomass. These losses were largest within 100 m of fragment edges, where tree mortality sharply increased. Permanent study plots within 100 m of edges lost up to 36% of their biomass in the first 10 to 17 years after fragmentation (Laurance et al. 1997). Importantly, if the reality is that wider roads likely impact forest ecosystems, then the management of road thresholds in nature reserves may be a vital way to ensure future viability. This should be taken into account when considering the environmental impacts of new roads (Angold 1997).
Furthermore, road networks can impact biodiversity partly independently and hence add to any effects of habitat loss (Ahmed et al. 2014), and it may be driving the global loss of biodiversity (Cuaron 2000). Especially for animals, road networks have negative effects on four of the five bird categories tested (Mammides et al. 2015). For example, the movement of small animals is known to be inhibited by roads, this is likely to increase population isolation among vegetation remnants (Ascensão et al. 2017), which should result in lower rates of immigration into and recolonization of habitats in landscapes with high road density (Rytwinski and Fahrig 2007). In particularly, new road construction could have detrimental effects on animal populations (Prokopenko et al. 2017). Like snakes, anurans, lizards and so on, most susceptible to negative road impacts (Brehme et al. 2018). Based on the independent roadkill data, more than 70% of roadkill events occurred within the top 30% priority segments (Lin et al. 2019). Furthermore, significant positive correlation can be found between the fractal dimension of forest stands and road density across all scales (Miller et al. 1996). As a result, road networks and road density are important factors when studying the effects of roads on forests.
Third, there is a shift in the perspective on forest road management if thresholds are included. There should be a threshold of the road width that affects forests, as an abrupt impact may occur with increasing road width. Since our study indicated that the effects of wide and narrow roads differed, we can infer that the impacts of roads on forests are dependent on the threshold of road width: an abrupt effect may appear when the road width is larger than the threshold. In other words, if there is a change in the size of the road, which can indicate the effects of the roads on forest ecosystems, ranging from no effect to a sustainable effect and from a sustainable effect to a destructive effect, this change can be depicted through ecological thresholds. In fact, an application of the threshold concept should consider where extrinsic factors constrain the structure and function of ecosystems. When the level or intensity of an extrinsic factor reaches a threshold, the structure of the ecosystem, the rate of an ecological process, or the level of ecosystem function/service that can be attained is altered (Groffman et al. 2006). In fact, all kinds of thresholds should be studied to conserve ecosystems. How much of something do we need to keep people safe and well (Oliver 2016)? In this case, a road is one kind of extrinsic factor, and this factor is associated with size. Previous research has indicated that roads should be a minimum of 15 m in width and glades should be a minimum of 625–900 m2 in area for open spaces to significantly contribute to the biodiversity of plantation forests (Smith et al. 2007). A template is needed for proactively zoning and prioritizing roads during the most explosive era of road expansion in human history (Laurance et al. 2014). Different forest roads have different threshold values of tolerance or habituation to human disturbance. However, so far, no study has quantified the threshold value for the width of a road. Road effect thresholds should be studied more in the future.