According to the competitive exclusion principle, species with low competitive abilities should be excluded by more efficient competitors, and yet they generally remain as rare species. Here, we describe the positive and negative spatial association networks of 326 disparate assemblages, showing a general organization pattern that simultaneously supports the primacy of competition and the persistence of rare species. Abundant species monopolize negative associations in about 90% of the assemblages. Contrarily, rare species are mostly involved in positive associations, forming small network modules. Simulations suggest that positive interactions among rare species and microhabitat preferences are the most likely mechanisms underpinning this pattern and rare species persistence. The

46Scots pine forests subjected to continental Mediterranean climates undergo cold winter 47 temperatures and drought stress. Recent climatic trends towards warmer and drier 48 conditions across the Mediterranean Basin might render some of these pine populations 49 more vulnerable to drought-induced growth decline at the southernmost limit of the 50 species distribution. We investigated how cold winters and dry growing seasons drive 51 the radial growth of Scots pine subject to continental Mediterranean climates by relating 52 growth to climate variables at local (elevational gradient) and regional (latitudinal 53 gradient) scales. Local climate-growth relationships were quantified on different time 54 scales (5-, 10-and 15-days) to evaluate the relative role of elevation and specific site 55 characteristics. A negative water balance driven by high maximum temperatures in June 56 (low-elevation sites) and July (high-elevation sites) was the major constraint on growth, 57 particularly on a 5-to 10-day time scale. Warm nocturnal conditions in January were 58 associated with wider rings at the high-elevation sites. At the regional scale, Scots pine 59 growth mainly responded positively to July precipitation, with a stronger association at 60 lower elevations and higher latitudes. January minimum temperatures showed similar 61 patterns but played a secondary role as a driver of tree growth. The balance between 62 positive and negative effects of summer precipitation and winter temperature on radial 63 growth depends on elevation and latitude, with low-elevation populations being more 64 prone to suffer drought and heat stress, whereas high-elevation populations may be 65 favoured by warmer winter conditions. This negative impact of summer heat and 66 drought has increased during the past decades. This interaction between climate and site 67 conditions and local adaptations is therefore decisive for the future performance and 68 persistence of Scots pine populations in continental Mediterranean climates.

SignificanceClimate warming is impacting the cryosphere in high mountain ranges, thereby enhancing the probability for more and larger mass-wasting processes to occur. This tree-ring–based snow avalanche reconstruction in the Indian Himalayas shows an increase in avalanche occurrence and runout distances in recent decades. Statistical modeling suggests that this increase in avalanche activity is linked to contemporaneous climate warming. These findings contradict the intuitive assumption that warming results in less snow, and thus fewer snow avalanches in the region, with major implications for disaster risk management and risk mitigation in a region with steadily increasing human occupation.

Climate, stand structure and local site conditions are potentially important determinants of forest dynamics. Understanding the relative contributions of competition and climate to tree growth is critical to project likely stand development under different climate change scenarios. Further, current competition levels and stand structure may reflect legacies of past forest management. Here, we analyze the effects of climate, site conditions and competition on radial growth in three Scots pine plots located along an altitudinal gradient. These stands are subjected to Mediterranean climate with continental influence, i.e., growth is limited by low winter temperatures and dry summer conditions. Current stand structure and retrospective analyses of radial growth (basal area increment, BAI) were used to model changes in tree growth as a function of competition (CI) and climate at an annual resolution. Negative exponential functions characterized the CI-BAI associations, whereas linear mixed-effects models were used to model BAI and to quantify the growth response to climate of trees under low and high competition. Competition effects on growth were steady over time regardless of the elevation and tree age. High competition levels negatively affected growth, with a proportionally stronger influence in suppressed trees than in dominant trees. Sensitivity of tree growth to climate increased with decreasing competition. Altitudinal gradientrelated growth responses to climate were found only for temperature variables. Specifically, growth at high elevations was mainly limited by low winter temperatures, whilst warm spring enhanced growth at middle elevations and late summer temperatures did it at low elevations. Since growth and its sensitivity to climate is more pronounced in low competition trees, we argue that the past management of the forest overrides site conditions and climate effects through the legacies on stand structure and competition. Pro-active forest management practices should be adopted to reduce the vulnerability of previously managed Scots pine forests currently threatened by the predicted warmer and drier conditions.