This study revealed a large variability of F. sylvatica treeline elevation among as well as within the 15 mountain groups analysed. Our survey revealed that treeline elevation is the results of both climatic factors and anthropogenic disturbance that play a different role across the Apennine mountain range. First, we recorded a remarkable depression of treeline elevation in all mountain groups compared with the expected climatic potential, suggesting a pervasive anthropogenic effect. Moreover, we found that treeline elevation was consistently lower on warmer, south-facing slopes of all mountain groups analysed. This pattern, although still not explained, could be associated to a synergic interaction between climatic constraints (e.g. summer drought) and human disturbance (e.g. logging and grazing pressure) that may leads to the loss F. sylvatica canopy viability and of their regeneration capability. According to our hypothesis, we found within mountain groups the coexistence of a very depressed treeline, in several cases with elevation of ~ 1000 m a.s.l., with very high treeline placed well above 2000 m a.s.l. The co-occurrences of such variability in treeline elevation is sites that are few kilometres apart suggest that factors other than climate control this pattern. In this context, we argue that high elevation treeline exist in very remote and inaccessible suspended valleys that protected it from past and present anthropogenic disturbance. On the other hands, very depressed treeline are located in accessible areas that was extensively exploited in the past centuries. Hereafter, we discuss the treeline patterns and the potential causative factors for each of the 15 mountain groups analysed.
Northern Apennines
Apuan Alps had the lowest average treeline in Italy (814 m a.s.l.), lower than the adjacent Tosco-Emiliano. Moreover, the Apuan Alps have a very high value of Δ Maximum TLE – Average TLE, which indicate that the treeline could potentially reach high elevations, but the average values are heavily depressed. The Apuan Alps and the Tosco-Emiliano group are situated at a similar latitude and share a similar climate, with very high rainfall and a limited summer drought (Fratianni and Acquaotta 2017). The most striking feature of the Apuan Alps which may explain the depressed treeline is the very high population density around this mountain group, with several large towns (e.g. Massa Carrara, Viareggio) situated a few kilometers from the main peaks. As early as 1861, the human population numbered ~ 83,000 and ~ 88,000 around Mt. Pisanino and Punta Carina, respectively, two of the most important peaks in this group. More importantly, the mountain group was intensively exploited in pre-Roman times (Bruschi et al. 2004) and in recent centuries for its marble deposits (Carmignani et al. 2007), causing a dramatic change in mountain morphology and hence the complete removal of forest cover on several peaks. PCA analysis confirmed that past and present human population density is the most important explanatory variable for the low treeline elevation in this mountain group.
The Tosco-Emiliano group has a higher treeline elevation than the Apuan Alps, despite the similar climate and latitude. Moreover, it has low values of both Δ average TLE north – average TLE south and of Δ maximum TLE – average TLE, suggesting that it has been subject to more limited human disturbance compared to the nearby Apuan Alps. Therefore, the highest treeline, recorded at 1811 m a.s.l. in the Monte Prado peak (2054 m a.s.l.), probably reflects climatic limitations (Pezzi et al. 2008), the group lying at the boundary of the Northern Apennines and thus experiencing much colder weather conditions compared to mountain groups in the Central and Southern Apennines. Finally, PCA analysis indicates that rainfall affects treeline elevation in this group, which is indeed the wettest in the Apennines. In this regard a weak but positive correlation between treeline elevations across the Apennines with the amount of summer precipitation was documented (Bonanomi et al. 2018). Here we speculate that high rainfall can alleviate summer drought, especially in southern aspects, contributing to the presence of a high elevation treeline also on the southern slopes of these mountains.
Central Apennines
The Apennines in central Italy comprise nine main mountain groups, namely the Sibillini, Laga, Gran Sasso, Terminillo, Majella, Velino-Sirente, Simbruini, Marsicani, and Matese. Blasi (2010) placed the potential elevation limit of F. sylvatica in central Italy at 1900 m a.s.l. However, we found eight sites where the treeline elevation exceeded 2000 m a.s.l. (five in the Majella and three in the Marsicani group). The existence of several sites located well above the threshold of 2000 m a.s.l. clearly demonstrate that the climatic potential of F. sylvatica is much higher than previously thought (Blasi 2010). Currently, we focusing on these study sites to better define the upper climatic limitation of F. sylvatica in areas subject to minimal anthropogenic disturbance. The discovery of very high treeline, most of which are located in remote and inaccessible valleys, highlight the general and widespread treeline depression of Central Apennines. In fact, in all mountain groups analysed the average treeline elevation occurs several hundred meters below its potential climatic limit.
Sibillini had the lowest average treeline in central Italy (1202 m a.sl.), with also a relatively low absolute maximum (1828 m a.s.l.). This mountain group is characterized by very ample and characteristic altitudinal grasslands (Catorci et al. 2008; Costanzo et al. 2009; Allegrezza et al. 2013) with F. sylvatica often limited to very steep, rocky slopes (Additional file 1: Figure S1). Moreover, the southern section of the group is largely deprived of forest cover which, even today, is intensively exploited for agricultural purpose at altitudes up to 1500 m a.s.l., i.e. Piano di Castelluccio, Norcia. These multiple lines of evidence suggest a pervasive negative impact of human activities such as agriculture and intensive grazing even at high elevations which may well have lowered the treeline in this group. The comparison between the Sibillini and Laga, a mountain group located less than 10 km south, is very informative. In central Italy, Laga had the highest average treeline (1831 m a.s.l.), coupled with the lowest Δ maximum TLE – average TLE (192 m a.s.l.), showing a very limited variability of treeline elevation in this group (Additional file 1: Figure S1).
These results are the opposite of the Sibillini, although the two groups share similar climatic conditions. Our analysis suggests that the treeline position in the Laga group is explained by two factors: the bedrock type and the human population density around these mountains. Laga is the only group in central Italy with arenaceous-pelitic flysch substrata, all other groups having limestone bedrock. A flysch substrate is much less permeable to rainfall than limestone, hence being able to retain large amounts of water in the soil and thus reducing summer drought (Gisotti 1983; Adamoli et al. 2012). As a result, trees are likely less affected by recurrent summer drought even at the treeline and in the southern faces of mountain peaks. Moreover, Laga had a very low past and present human density that, likely, do not provide a strong pressure on forests and, so, preserves an ample and continuous forest cover.
Gran Sasso and Velino-Sirente are two limestone groups with several high elevations and prominent peaks. Gran Sasso and Velino-Sirente also share low average treeline elevation with high values of Δ average TLE north – average TLE south. Indeed, the southern slopes of these mountains groups are almost devoid of forest cover with, for instance, the Campo Imperatore plateau (the largest in central Italy covering more than 75 km2) completely covered by grassland from 1600 m up to 2300 m a.s.l. (Additional file 1: Figure S1). Our PCA revealed that past human population is the most important explanatory variable for the low treeline elevation in the Gran Sasso group: both the groups have very large (> 50,000 inhabitants) ancient cities dating back to Roman times, located just at the base of the southern face of the main peaks, i.e. L’Aquila for Gran Sasso and Avezzano for Velino-Sirente. Human exploitation of the latter area goes back to Roman times during which reclaimed a large plateau covered by a natural lake for agricultural purposes in Avezzano (Burri and Petitta 2004). Moreover, in these two groups the differences between mountain aspects become striking, with the north face often covered by F. sylvatica up to 1800–1900 m a.s.l., while south faces have treelines that in several cases lie below 1200 m a.s.l.
Bonanomi et al. (2018) first reported that F. sylvatica treeline elevation is lower on warmer, southern aspects compared to the colder north across the Apennines. This counterintuitive pattern, considering the current theory proposed to explain treeline elevation (Körner 2012), has been hypothesized to be the result of a combination between water shortage, and alteration of microclimate conditions when F. sylvatica cover is removed for pasturage. Allegrezza et al. (2016) reported that, at the treeline on south-facing mountain slopes, plants outside the canopy cover may experience very high soil and air temperatures (up to 38 °C) and intense summer drought which restricts F. sylvatica regeneration capability on open patches. In this regard, F. sylvatica itself can act as an ecosystem engineer by modifying forest microclimate thanks to its canopy cover and thus facilitating seedling establishment. Hence, we speculate that depression of treeline on southern mountain faces is due to the loss of engineering capability of F. sylvatica after canopy removal which, if intact, modulates a positive feedback on microclimate buffering temperature extremes, increasing soil and air moisture by limiting wind impact (Jones et al. 1997). Since self-shading buffers local microclimate, we hypothesize that this effect will be more important in southern than in northern aspects where air and soil temperature are already buffered by the reduced solar radiation. In this regards, intense anthropogenic disturbance could exacerbate the stressful impact of intense solar radiation, high daily temperature and summer drought on southerly exposed vegetation gaps, with negative effects on the capability of F. sylvatica to regenerate and recolonize the disturbed areas that rapidly become grassland. In this context, the Velino-Sirente group had better represent this stressful ecological conditions for F. sylvatica treeline because combine a very high human pressure with the driest climate because of its position in the interior of the Italian peninsula (Pesaresi et al. 2017). Future work is needed to test this multifaceted hypothesis, comparing gap and closed forest microclimates in connection with eco-physiological studies that focus on the F. sylvatica regeneration niche (Grubb 1977) in northern and southern slopes expositions.
Majella is a large and compact massif composed of a high elevation plateau (average elevation > 2600 m a.s.l.), with steep and topographically complex external faces (Whitehead 1951). The Majella treeline shows large variability, with the highest absolute values in central Italy (2061 m a.s.l.) and five sites where the treeline exceeds 2000 m a.s.l. but, at the same time, several sites with a treeline below 1200 m a.s.l. (Additional file 1: Figure S1). The considerable variability observed in the treeline within the Majella cannot be explained by climatic factors but is likely the result of uneven past human activities, since large population inhabits Majella. Treelines appear especially depressed in close proximity to settlements and to external mountain faces that were more accessible for timber exploitation. In Majella, however, depressed treelines coexist with a very high treeline that only occurs in remote and almost inaccessible hanging valleys (Additional file 1: Figure S1), an observation that supports the human accessibility hypothesis. That said, the occurrence of several treelines at elevations > 2000 m a.s.l. on all mountain aspects, including the southern slopes, raises the ecological climatic potential of F. sylvatica.
The Marsicani mountain group has the most peaks (48 in our analysis), with many showing a large variability in treeline elevation. In three sites, absolute maximum treeline elevation exceeded 2000 m a.s.l. (i.e. Monte Greco, Terratta, and Monte Rotondo) and, in several cases, high and low treelines coexist within the same peaks (e.g. Monte Marsicano, La Meta). In this large and heterogeneous group, the treeline elevation is negatively associated with human population but positively with the minimum temperature of the coldest quarter of the year. Bonanomi et al. (2018) found that the winter climatic effect was stronger in high elevation peaks (> 1900 m a.s.l.), reporting for the first time that winter temperature affects F. sylvatica treeline elevation in addition to the well-known impact of MAT and growing season temperature (Körner and Paulsen 2004). In this regard, the Marsicani appears a complex group where climate mostly control treeline elevation in high elevation peak, while human disturbance seems more important in low elevation mountains. In addition, in the Marsicani group the past human pressure was likely uneven distributed also at low elevations because previous studies discovered the most ancient F. sylvatica old-growth forest of Europe in this geographical area (Piovesan et al. 2005).
Southern Apennines
The Picentini and Alburni-Cilento mountain groups in southern Italy have low elevation peaks, the highest point being Mt. Cervati (1899 m a.s.l.). In these groups, PCA indicates the importance of mountain peak elevation as a physical limiting factor for treeline elevation, given that the ecological potential of F. sylvatica is well above 2000 m a.s.l. at this latitude (Bonanomi et al. 2018). As a consequence, in several peaks F. sylvatica reaches the top of the mountain (e.g. Mts. Gelbison, Mercori, Panormo, Polveracchio), especially in the northern aspect. On the other hand, in these two groups with their low elevation peaks, the contribution of climatic factors to treeline elevation is of less importance.
Located at the southern limit of the carbonatic continental Apennines, the Pollino massif includes five peaks exceeding > 2150 m a.s.l. Average treeline elevation is high in Pollino, which comprises five sites with treelines higher than 2050 m a.s.l., including the highest site ever recorded for the continental distribution of F. sylvatica at 2141 m a.s.l. on Mt. Serra del Prete. Indeed, the latter site demonstrates that in southern Italy the climatic potential distribution is well above 2000 m a.s.l., probably lying around 2100 m a.s.l. Despite this, in many places treeline elevation in the Pollino group is very low (often lower than 1200 m a.s.l.), causing the large variability in treeline elevation observed in the group. Here, the occurrence of the highest treeline in a mountain group with relatively low elevation peaks could be explained by the co-existence of several sites that were little disturbed, if at all, by human activities and, on the other hand, by other peaks that were heavily exploited for timber and intensive grazing. Further studies are needed to explain the uneven treeline distribution in this mountain group.
Orsomarso, located a few kilometers to the west side of Pollino, shares a similar climate, being characterized by higher MAT and MTCQ than other groups located in the central and, especially, northern Apennines. In Orsomarso the treeline has a high average elevation but, unlike the nearby Pollino group, the lack of mountain peaks with elevation above 2000 m a.s.l. explains the absence of a very high treeline. By contrast, Orsomarso has very low values of Δ average TLE north – average TLE south and Δ maximum TLE – average TLE indexes, all proxies of limited anthropogenic disturbance. Unlike the Pollino massif, the Orsomarso group has a low population density and is physically isolated, with few mountain roads and several peaks that are quite difficult to reach even today. These factors support the poor accessibility hypothesis and may well explain the very extensive forest cover of this group as well as the poor variability of treeline elevation observed between contrasting mountain aspects.