Carbon Limitation and Drought Sensitivity at Contrasting Elevation and Competition of Abies pinsapo Forests. Does Experimental Thinning Enhance Water Supply and Carbohydrates?

Abstract

Stand-level competition and local climate influence tree responses to increased drought at the regional scale. To evaluate stand density and elevation effects on tree carbon and water balances, we monitored seasonal changes in sap-flow density (SFD), gas exchange, xylem water potential, secondary growth, and non-structural carbohydrates (NSCs) in Abies pinsapo. Trees were subjected to experimental thinning within a low-elevation stand (1200 m), and carbon and water balances were compared to control plots at low and high elevation (1700 m). The hydraulic conductivity and the resistance to cavitation were also characterized, showing relatively high values and no significant differences among treatments. Trees growing at higher elevations presented the highest SFD, photosynthetic rates, and secondary growth, mainly because their growing season was extended until summer. Trees growing at low elevation reduced SFD during late spring and summer while SFD and secondary growth were significantly higher in the thinned stands. Declining NSC concentrations in needles, branches, and sapwood suggest drought-induced control of the carbon supply status. Our results might indicate potential altitudinal shifts, as better performance occurs at higher elevations, while thinning may be suitable as adaptive management to mitigate drought effects in endangered Mediterranean trees.Water uptake and carbon metabolism are limited by drought according to the specific sensitivities of physiological processes, like hydraulic failure, carbon starvation, and phloem transport failure, to declining water availability [10][11][12]. Briefly, hydraulic failure appears to usually be related to a high xylem susceptibility to embolism [13][14][15], whereas carbon starvation is related to isohydric stomatal control [16,17]. Phloem transport failure is relative to carbon starvation and seems to be present when the drought-induced limitation of carbohydrate transport to growth exceeds the drought sensitivity of photosynthesis [12,18]. Consequently, the balance between water supply and demand, on the one hand, and sources and sinks of carbohydrates, on the other hand, characterize the inherent species' drought sensitivity and determine how water and carbon relationships affect tree survival [11,14,19]. Indeed, the timing of this sequential reduction, in functions, such as gas exchange and growth, has been largely related to recent events of forest decline and mortality worldwide [11,[16][17][18][19]. Nonetheless, the functional and structural dynamics of these critical processes in response to competition are still not well understood.We focused on the hydraulic characteristics and seasonal patterns of sap flow density (SFD), gas exchange, secondary growth, and non-structural carbohydrate (NSC) of the drought-sensitive fir Abies pinsapo Boiss [20][21][22][23]. We assumed that xylem vulnerability to embolism places a physical limit for A. pinsapo tolerance to water shortage while the dynamics of NSC would be related to drought sensitivity, for the extent o...

Tree-Ring Analysis Reveals Density-Dependent Vulnerability to Drought in Planted Mongolian Pines

“…A previous study of Norway spruce observed that, compared with healthy trees, unhealthy trees maintained only one-third of the sap flow. Sap flow density in the low-density stands (thinned) was significantly higher than the non-thinning stands [69]. In the current study, the sap flow velocity of the HDT group was~50% lower than the LDT and BT groups, revealing the poor hydraulic conductance of the HDT group and resulting in these trees growing more slowly [70].…”

Using ForeStereo and LIDAR data to assess fire and canopy structure-related risks in relict Abies pinsapo Boiss. forests

“…These findings stress the need for proactive adaptive management of A. pinsapo forests, including: (i) the creation of bare patches through shrub clearing, (ii) a reinforcement of the firewalls in the west part of the valley and (iii) promotion of grazing and trampling levels by wild ungulates (or domestic livestock if they were insufficient) to reduce shrub fuel load without compromising A. pinsapo . We also support the efficacy of thinning treatments for canopy structural diversity enhancement as an essential tool to avoid stand stagnation ( Linares, Camarero & Carreira, 2009 ; Linares et al, 2009 ; Lechuga et al, 2017 , 2019 ) and high CBD values, to reduce the probability of crown fires and thus increase resilience to wildfires ( Koontz et al, 2020 ), as well as to reduce climate change-induced tree mortality ( Linares, Camarero & Carreira, 2009 ).…”

Lean Pattern in an Altitude Range Shift of a Tree Species: Abies pinsapo Boiss.

“…This has resulted in populations that are suffering from structural disequilibrium, where mature individuals are prevailing and there is a lack of recruitment. Differences in growth and mortality in the populations of A. pinsapo throughout the altitudinal gradient have been attributed to variations in the water balance and the strong competition for water resources in the lower areas [48][49][50][51][52]. The bands of vegetation located in the lower areas constitute the trailing edge in the migratory dynamic and occupy the bottom 90-130 m of the altitudinal gradient, depending on the model.…”

“…However, colonisation above the upper limit for the species may be hindered by a multitude of factors (e.g., damage by frost and winter desiccation, heat flux or radiative warming of rooting zone) [53]. Even though the altitudinal displacement of populations of A. pinsapo has been proposed before as a consequence of a better energy balance at high altitudes [48], the speed of the altitudinal displacement of these populations has not yet been described in the manner that it has been carried out in other specific studies on species distribution displacement [5,7,11,[15][16][17]54].…”