Soil Compaction and Productivity Evolution in a Harvested and Grazed Mediterranean Scots Pine (Pinus sylvestris L.) Forest
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1. Introduction
Soil compaction and its related effects have been widely reported in the literature as major causes of soil degradation that can affect soil productivity and, consequently, vegetation development [1]. Soil compaction reduces porosity (especially the large pore fraction) and leads to a rearrangement and closer packing of soil particles, increasing the cohesive forces between them and reducing the connectivity of the pore network [2]. All of these processes affect four soil physical parameters that ultimately determine vegetation development and root growth [3]: aeration, temperature, water availability, and mechanical resistance to root elongation [4]. Changes in these parameters beyond certain thresholds, which depend on the species [5,6] and climate [7], can lead to limitations in root structure and function such as root elongation, water and nutrient uptake, or respiration. If the limitations are too severe or long-lasting, aerial growth and site productivity may also be negatively affected [1].
Most of the detrimental effects of soil compaction on vegetation were initially studied and characterized in agricultural systems where machinery impacts are high in intensity, frequency, and density [8,9,10]. Since the 1950s, this concern about the detrimental effects of compaction has been extended to forestry [1,11,12,13,14,15]. Abundant research on forest soil compaction over the years has shown that (1) both machinery and livestock are capable of compacting forest soils and (2) the areal extent, intensity, and depth of impacted soil depend on both site (soil texture, structure, organic matter and moisture, topography, weather, and climate) and forest use characteristics (frequency of perturbation, spatial distribution, or load size) [11,15,16,17,18]. However, most of the literature has focused on assessing compaction in areas with heavy traffic, mainly machinery, in scenarios where compaction has a higher risk of negative effects (mainly intensively managed wet and cold forests) [19,20,21,22,23,24,25]. Regarding the effects of compaction on vegetation development, only the effect on seedling rooting has been clearly demonstrated in the literature [26,27,28,29,30]. In contrast, research on aerial part growth, particularly within mature stands, is much more scarce and inconclusive [13,31], especially at long-term and landscape scales [32,33]. In the case of livestock compaction, concerns about the effect on vegetation have mostly focused on the effect on pasture productivity particularly on farm areas with high stocking rates [34,35].
Moving from local effects on soils and seedling roots to stand-scale effects on growth and site quality is not trivial. This requires, among other things, consideration of (i) the overall compacted area as well as its spatial pattern at the forest scale, together with its expected long-term evolution resulting from cumulative effects (balance between impact recurrence and system resilience), and (ii) the resilience of plant communities according to species composition and stand characteristics (age, structure, density, etc.).
The complexity and long timeframes of forest systems result in a wide variety of potential scenarios that are not easy to assess, but, as noted by [33,36], the ultimate effects of compaction on vegetation at the stand scale need to be assessed in order to provide sound management advice to forest owners and managers. Building up a sufficient body of long-term studies in the literature on the effects of soil compaction on forest productivity under a wide range of scenarios, together with accurate descriptions of sites, vegetation, and forest uses, would be a crucial step towards real progress in the analysis of forest soil compaction [12,37]. In this regard, there are very few examples in the literature where scenarios involving both machinery and livestock are analyzed, and they are mostly related to farming [38,39]. Nevertheless, the combination of forest with mechanized harvesting and extensively managed livestock (along with wild ungulates) is quite common in Spain. Thus, more than 55% of the Spanish territory is classified as nonagricultural and nonurban, and almost 70% of this is forest or woodland [40]. Most of this area is managed extensively with rotations of more than 80 years and mechanized harvesting every 10–15 years. Extensive free-grazing cattle, traditionally present in Spanish forests, explain a large part of the landscape characteristics [41]. Therefore, it is particularly important to evaluate the effects of machinery and livestock compaction in extensively managed forests in Spain. Furthermore, it should be considered that warmer and drier Mediterranean climatic conditions can mitigate many of the negative consequences [42] commonly associated with compaction in cold and wet climates [1].
The aim of this research was to analyze soil compaction and forest productivity in one of these cattle-machinery scenarios: a Mediterranean mountain forest that has been extensively managed for more than 100 years, mainly for timber and livestock production. The specific objectives of this study were (i) to provide reference values for the levels of compaction reached in this context; (ii) to determine whether livestock and mechanized harvesting were increasing soil compaction in the forest; and (iii) to assess the evolution of forest productivity in the presence of both compacting agents.
The long period of mechanized full-tree logging with hand-felling, winching, and skidder yarding, combined with extensive cattle grazing in the studied forest, resulted in local soil penetration resistance values that are likely to limit the root growth of Scots pine (Pinus sylvestris L.) growing on sandy loam soils in a Mediterranean climate. However, despite the observed long-period disturbance and increases in penetration resistance, no signs of forest productivity decline were detected, indicating that high local compaction levels do not necessarily imply significant effects on vegetation growth at the forest scale.
5. Conclusions
Fifty years of mechanized full-tree logging with hand-felling, winching, and skidder yarding, combined with over a century of extensive cattle grazing, may result in soil penetration resistance levels that are likely to limit root growth of Scots pine (Pinus sylvestris L.) growing on sandy loam soils in a Mediterranean climate. However, high local compaction levels do not necessarily imply productivity losses at the forest scale. Under these site conditions, appropriate forest planning along with diligently performed forest uses can reconcile long exploitation periods with forest sustainability.
The results obtained further underline that the impact of forest uses cannot be thoroughly assessed on the basis of the level and depth of induced soil compaction: it is paramount to also consider the overall percentage of the compacted area and its spatial distribution at the forest scale. Climatic conditions are also relevant, as soil water conditions may improve with compaction, especially in sandy soils in arid climates such as the Mediterranean.
With regard to the studied forest, given its compaction status, it would be advisable to concentrate machinery and cattle traffic on landings, tracks, and pastures and to carry out long-term monitoring of the evolution of compaction extent in less trafficked areas.
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