What drives growth of Scots pine in continental Mediterranean climates: Drought, low temperatures or both?

Abstract

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.

Minimum wood density of conifers portrays changes in early season precipitation at dry and cold Eurasian regions

“…Under more continental and colder conditions, cambial resumption is delayed and summer precipitation and temperature become more relevant as drivers of growth in species as Scots pine (Sánchez-Salguero et al 2015). This explains why earlywood density may not be related to spring precipitation in some Mediterranean Scots pine stands (Olivar et al 2015).…”

“…This research could explicitly consider wood anatomy to disentangle if MN changes are mainly due to lumen modifications, as we assume, or to changes in cell-wall thickness. For instance, in drought-exposed Width and density variables of tree-rings provide redundant information (Kirdyanov et al 2007, Büntgen et al 2010, Vaganov and Kirdyanov 2010, Galván et al 2015. This limitation could apply to MN which is negatively related to EW and both variables show opposite associations with rainfall (Tables 4 and S1).…”

“…Such precipitation-density coupling, possibly mediated by a conductivity reduction, could also explain the decrease in earlywood production observed in response to water deficit in drought-prone sites from the forest-steppe and Mediterranean biomes (e.g., Dulamsuren et al 2010;Camarero et al 2015). According to ecophysiological studies, in these cool-dry regions photosynthesis rates decrease in winter and summer and growing-season drought stress, caused by elevated atmospheric vapour pressure deficit and low soil water availability, leads to radial-growth reduction (Dulamsuren et al 2009;Gimeno et al 2012).…”

“…Continental climate conditions and low precipitation values explain why tree growth is constrained by drought during the growing season and a short growing season defined by long and cold winters in all the study areas (Block et al 2004, Dulamsuren et al 2009, Velisevich and Kozlov 2006, Devi et al 2008, Knorre et al 2010, Camarero et al 2015. In Siberia and Mongolia we sampled larch (Larix sibirica Ledeb.)…”

“…The southernmost study region is located in the Sierra de Gúdar (southern Iberian Range, eastern Spain) and includes two Scots pine stands and two Black pine stands. In this area, the study sites were distributed along an elevational gradient because Scots pine dominates at cold Mediterranean sites situated at higher elevation, whereas Black pine is dominant at mid-elevation sites experiencing sub-Mediterranean transitional climates (Camarero et al 2015). We sampled one site (LR in Table 1) where the two species cooccurred to check for differences between species coexisting under similar climate and soil conditions.…”