Slope‐Aspect Induced Climate Differences Influence How Water Is Exchanged Between the Land and Atmosphere

Abstract

Plant transpiration is a major conduit for the transfer of water from the land to the atmosphere (Jasechko et al., 2013). Recent studies have advanced our understanding of how complex and sensitive the leaf-to-atmosphere link is to localized feedbacks, such as slope exposure and associated conditions (e.g., Amitrano et al., 2019;Harrison et al., 2020;Link et al., 2014). Microclimates have been highlighted in particular as a distinct control on long-term ecosystem vulnerability, separate from evolving macroclimate (De Frenne et al., 2013. The urgency of understanding the vulnerability of forest ecosystems to changing climate conditions in arid or Mediterranean climates has been underscored by mass tree die-offs in western North America (Asner et al., 2016;Fettig et al., 2019;D. J. N. Young et al., 2017), and worsening wildfire regimes around the world. Yet, the influence of natural microclimatic variations, such as those associated with topographic position, on plant-water relations and physiology is often omitted in models used to forecast ecosystem vulnerability, due to a lack of data (Mencuccini et al., 2019;Pappas et al., 2016).Our aim is to understand how microclimates caused by topographic complexity may feed back into spatial variations in tree physiology and ultimately variations in tree vulnerability to fire or mortality under stressful conditions. To do this, we embarked on a study at the University of California's Angelo Coast Range Reserve in Northern California, a site which is now NSF's Eel River Critical Zone Observatory. The focus is

Stressors Reveal Ecosystems' Hidden Characteristics

“…While primarily focused on determining the importance of soil type and geomorphology of the region, the study's most prominent finding is that shrubs growing on east and east north east facing slopes had significantly higher total plant volume and distance between neighbors than on any other slope face. Slopes in the region studied by Gearon and Young (2021) were much less steep, on average, than those studied by Bilir et al (2021). Thus, such an extreme gradient in light environment or available soil moisture would not expected.…”

“…Yet, during times of soil dry down (soil water content < 4.5%) latent heat flux (LE) as measured by identical above-canopy eddy covariance systems in each forest is up to 25% greater in the unaltered forest than in the disturbed (Figure 1). In an in silico "forcing condition swap" between the two sites, similar to that performed by (Bilir et al, 2021), it was shown that these differences were unable to be accounted for by the minor differences in temperature, light, vapor pressure deficit, or soil water conditions between the two forests (Matheny et al, 2014). The hydraulic strategies, defined as the distinct patterns of water acquisition, storage, and use, of the principle tree species comprising each plot were indicated as the most probable cause for the sustained differences in LE during periods of water limitation (Matheny et al, 2015;Matheny, Fiorella, et al, 2017).…”

“…Nonetheless, these results hold critical implications for ecosystem function and resilience and should be of interest to observationalists and modelers alike. Bilir et al (2021) attribute the madrones' differential responses to microclimate to be primarily due to the relative numbers of sun versus shade adapted leaves grown by trees on each slope. This morphological difference combined with the accompanying stomatal behavior (e.g., shade leaves are more likely to leave stomata open than sun leaves) will certainly lead to disparities in water use efficiency and carbon uptake.…”

“…This comment assesses the interplay of vegetation responses to microclimate-induced stressor variability in the context of steep slopes, water limitation, and moderate disturbance. Finally, the implications of these observational findings for advances in modeling ecosystem fluxes is examined.In their recent publication, Bilir et al (2021) analyzed water flux dynamics in a single population of Pacific madrones spread across two adjacent, steep hillslopes, one north-facing and one south-facing, near the Eel River Critical Zone Observatory in California. Leveraging heavily instrumented hillslopes, multiple historical cross-disciplinary collaborations, and a coupled model-observation study, the research team demonstrated that the north-and south-slope madrones exhibited distinct water relations throughout the course of the wet and dry seasons characteristic of a Mediterranean climate.…”

“…Traditional ecological scaling theory states that for an ecosystem at steady-state and receiving uniform insolation, the transpiration flux ought to be invariant (Enquist, 2002). The work of Bilir et al (2021) demonstrates to a large degree the importance of insolation adaptation for the regulation of transpiration fluxes but also the relative availability of water and its governance of each species' stomatal regulation strategy. Given the numerous potentially limiting controls on transpiration (e.g., insolation, soil moisture, VPD, wind speed, temperature, stomatal regulation stringency, hormonal signaling, etc.)…”