TRANSPIRATION AND HYDRAULIC STRATEGIES IN a PIñON–JUNIPER WOODLAND

Abstract

Anthropogenic climate change is likely to alter the patterns of moisture availability globally. The consequences of these changes on species distributions and ecosystem function are largely unknown, but possibly predictable based on key ecophysiological differences among currently coexisting species. In this study, we examined the environmental and biological controls on transpiration from a piñon-juniper (Pinus edulis-Juniperus osteosperma) woodland in southern Utah, USA. The potential for climate-change-associated shifts in moisture inputs could play a critical role in influencing the relative vulnerabilities of piñons and junipers to drought and affecting management decisions regarding the persistence of this dominant landscape type in the Intermountain West. We aimed to assess the sensitivity of this woodland to seasonal variations in moisture and to mechanistically explain the hydraulic strategies of P. edulis and J. osteosperma through the use of a hydraulic transport model. Transpiration from the woodland was highly sensitive to variations in seasonal moisture inputs. There were two distinct seasonal pulses of transpiration: a reliable spring pulse supplied by winter-derived precipitation, and a highly variable summer pulse supplied by monsoonal precipitation. Transpiration of P. edulis and J. osteosperma was well predicted by a mechanistic hydraulic transport model (R2 = 0.83 and 0.92, respectively). Our hydraulic model indicated that isohydric regulation of water potential in P. edulis minimized xylem cavitation during drought, which facilitated drought recovery (94% of pre-drought water uptake) but came at the cost of cessation of gas exchange for potentially extended periods. In contrast, the anisohydric J. osteosperma was able to maintain gas exchange at lower water potentials than P. edulis but experienced greater cavitation over the drought and showed a lesser degree of post-drought recovery (55% of pre-drought uptake). As a result, these species should be differentially affected by shifts in the frequency, duration, and intensity of drought. Our results highlight the sensitivity of this woodland type to potential climate-change-associated shifts in seasonal moisture patterns and demonstrate the utility of mechanistic hydraulic models in explaining differential responses of coexisting species to drought.

Predicting plant vulnerability to drought in biodiverse regions using functional traits

“…Our interpretation is consistent with previous observations showing that species with higher maximum g s displayed greater sensitivity and an increased rate of stomatal closure in response to changes in vapor pressure deficit (53). It is also supported by observations that isohydric species are able to recover gas exchange rates more rapidly following a drought compared with anisohydric species (18,54).…”

Temperature sensitivity of drought-induced tree mortality portends increased regional die-off under global-change-type drought

“…Therefore, we suggest that higher respiratory loads associated with warmer temperatures incited differences in mortality, reflecting carbon starvation, not sudden hydraulic failure as the causal mechanism required to predict tree mortality differences in a future warmer world. Such results are consistent with inferences from recent observational and modeling assessments (30,31,35).…”

“…Yet experimental assessment of whether warmer temperatures associated with drought exacerbate tree mortality is lacking for any tree species, and therefore tree mortality has only been predicted in response to a simple metric of accumulated dry conditions (28,29). The sensitivity of tree mortality to temperature is dependent on which of 2 non-mutually-exclusive mechanisms predominates: (i) carbon starvation, whereby trees close stomata to keep safe levels of xylem pressure, stopping most photosynthesis, and rely on stored carbohydrates to support the metabolic costs of maintaining tissue; or (ii) catastrophic hydraulic failure, whereby trees maintain stomatal conductance during drought to continue photosynthesizing, but run the risk of xylem pressures suddenly exceeding cavitation thresholds beyond which air bubbles block transport of stem water (30)(31)(32). Both hypotheses are interrelated with biotic agents, such as bark beetles and associated fungi, and carbon starvation would preclude production of the photosynthate necessary for tree defense, thereby increasing susceptibility to biotic agents (9,33).…”

“…We experimentally investigated the temperature sensitivity of drought-induced mortality in Pinus edulis, a piñon pine tree that has exhibited regional-scale die-off in response to recent drought (3,4) and which has been evaluated in observational and modeling studies (30,31,35). We transplanted small, reproductively mature piñon pines into the environmentally controlled Biosphere 2 facility to grow them in either near-ambient or warmer (ϩ4.3°C) temperatures, and then imposed a drought treatment that completely excluded water from selected trees until all ''drought'' trees in both temperature treatments died.…”

Acclimatation of three co-occurring tree species to water stress and their role as site indicators in mixed pine-oak forests in the Sierra Madre Oriental, Mexico

“…Hence, P. pseudostrobus presented a typical pattern of isohydric water regulation, while J. flaccida was identified as an anisohydric species that maintain higher stomata conductance and allow W wmd to decline with decreasing soil water content (Tardieu and Simonneau 1998;McDowell et al 2008). Also other authors mention pine species (Pinus edulis) as good examples for isohydric regulation (Lajtha and Barnes 1991;Williams and Ehleringer 2000;West et al 2008;Breshears et al 2009) and identified juniper (Juniperus monosperma) as an anisohydric species (Loewenstein and Pallardy 1998a, b;Tardieu and Simonneau 1998;Franks et al 2007;West et al 2008;Breshears et al 2009). The course of seasonal W wmd of Q. canbyi indicated rather anisohydric water status regulation, since fluctuations of W wmd values corresponded to soil water availability and were very similar to those of J. flaccida varying notably over time.…”