Adaptive capacity in the foundation tree species Populus fremontii: implications for resilience to climate change and non-native species invasion in the American Southwest

Abstract

Populus fremontii (Fremont cottonwood) is recognized as one of the most important foundation tree species in the southwestern USA and northern Mexico because of its ability to structure communities across multiple trophic levels, drive ecosystem processes and influence biodiversity via genetic-based functional trait variation. However, the areal extent of P. fremontii cover has declined dramatically over the last century due to the effects of surface water diversions, non-native species invasions and more recently climate change. Consequently, P. fremontii gallery forests are considered amongst the most threatened forest types in North America. In this paper, we unify four conceptual areas of genes to ecosystems research related to P. fremontii’s capacity to survive or even thrive under current and future environmental conditions: (i) hydraulic function related to canopy thermal regulation during heat waves; (ii) mycorrhizal mutualists in relation to resiliency to climate change and invasion by the non-native tree/shrub, Tamarix; (iii) phenotypic plasticity as a mechanism for coping with rapid changes in climate; and (iv) hybridization between P. fremontii and other closely related Populus species where enhanced vigour of hybrids may preserve the foundational capacity of Populus in the face of environmental change. We also discuss opportunities to scale these conceptual areas from genes to the ecosystem level via remote sensing. We anticipate that the exploration of these conceptual areas of research will facilitate solutions to climate change with a foundation species that is recognized as being critically important for biodiversity conservation and could serve as a model for adaptive management of arid regions in the southwestern USA and around the world.

Adaptive trait syndromes along multiple economic spectra define cold and warm adapted ecotypes in a widely distributed foundation tree species

“…These extreme temperatures can simultaneously damage the electron transport capacity of Photosystem II and increase photorespiration (Allakhverdiev et al, 2008;Hultine, Froend, et al, 2020;O'Sullivan et al, 2017). Therefore, warm-adapted trees likely prioritize maintaining midday leaf conductance to evaporatively cool the canopy (Fauset et al, 2018;Hultine, Allan, et al, 2020;Hultine, Froend, et al, 2020;Michaletz et al, 2015). The combination of these leaf morpho-physiological features allows SD trees to achieve higher levels of transpirational cooling of their leaves while reducing heat gain from incident light radiation (Hultine, Allan, et al, 2020;Hultine, Froend, et al, 2020).…”

“…Therefore, warm-adapted trees likely prioritize maintaining midday leaf conductance to evaporatively cool the canopy (Fauset et al, 2018;Hultine, Allan, et al, 2020;Hultine, Froend, et al, 2020;Michaletz et al, 2015). The combination of these leaf morpho-physiological features allows SD trees to achieve higher levels of transpirational cooling of their leaves while reducing heat gain from incident light radiation (Hultine, Allan, et al, 2020;Hultine, Froend, et al, 2020). For example, another recent study using genotypes from the same common garden as the present study reported that the SD ecotype trees displayed 35% higher midday leaf transpiration rates and 4°C lower leaf temperature than the MR ecotype trees (Hultine, Allan, et al, 2020).…”

“…For example, the SD ecotype displayed greater capacity to supply water to their canopies to avoid thermal stress but potentially at the expense of being more susceptible to hydraulic dysfunction caused by maintaining lower minimum water potentials, having a higher theoretical maximum stomatal conductance, and having a larger mean hydraulic diameter in woody stems. Thus, increases in drought frequency can be expected to reduce the habitat niche of SD genotypes to locations where soil water availability remains stable independent of drought, such as along the margins of perennially flowing streams (Hultine, Allan, et al, 2020). The MR ecotype, on the other hand, displayed a set of morphological traits designed to increase sunlight exposure and maximize safety from freeze-thaw cavitation, but as a consequence may be unable to cool their canopies when exposed to episodic heatwaves that are likely…”

“…Consequently, P. fremontii and other similar tree species are rapidly becoming maladapted to their local environmental conditions (Grady et al, 2011;Hultine, Allan, et al, 2020;Hultine, Froend, et al, 2020;Merritt & Leroy Poff, 2010).…”

UAV thermal image detects genetic trait differences among populations and genotypes of Fremont cottonwood (Populus fremontii, Salicaceae)

“…Accurately estimated tree canopy temperature can be used to detect tree populations and genotypes with different rates of transpiration and carbon sequestration, which can help identify trees that can better respond to increasing temperature, episodic heat waves, and prolonged drought conditions (Ludovisi et al, 2017). Tree canopy temperature reflects critical plant physiological processes, particularly transpiration and leaf carbon budgets (Gates, 1968; Hultine et al, 2020a,b; Santini et al, 2019). For example, on the warm edge of its distribution, the groundwater‐dependent, riparian Fremont cottonwood is exposed to some of the warmest mean annual temperatures in North America, often approaching 50 °C in mid‐summer (Cooper et al, 2019).…”