Topography may mitigate drought effects on vegetation along a hillslope gradient

Abstract

Topography may mitigate drought effects on vegetation along a hillslope gradient through redistribution of soil moisture. We examined the interaction of topography, climate, soil moisture, and transpiration in a low-elevation, mixed-hardwood forest in the southern Appalachian Mountains.The effects of meteorological variation (wet and dry years) and topographic position (upslope and cove) were tested on daily soil moisture amplitude and recession and plot and species-specific transpiration. Trees in the cove plot were 17% taller and had 45% greater sapwood area than those in the upslope plot. Lower rates of soil moisture recession following rainfall events were observed at the cove plot compared to the upper plot. Greater daily soil moisture amplitude and plot transpiration, even in dry years, suggest that lower slope positions may have been buffered against moderate drought. We also observed similar transpiration in Quercus spp., Carya spp., and Liriodendron tulipifera in the cove plot between dry and wet years. Plot transpiration was reduced by 51% in the dry year in the upslope plot only, and transpiration by individual species in the plot reflected this pattern, suggesting water stress in dry years may be exacerbated by topography. With drought predicted to increase for these systems, the different drought responses of species, in addition to topographic effects, may lead to complex shifts in species composition.

Forests, shrubs, and terrain: top‐down and bottom‐up controls on forest structure

“…Canopy height may be limited hydraulically by resistances associated with the xylem conduits, gravity, and high vapor pressure deficits (D; Koch and Fredeen 2005). Higher summer temperatures, more frequent high D, and increased water stress have been observed at sideslope vs. cove locations in our study basin (Hawthorne and Miniat 2018). Higher summer temperatures, more frequent high D, and increased water stress have been observed at sideslope vs. cove locations in our study basin (Hawthorne and Miniat 2018).…”

“…8. As the growing season progresses, recharge, transpiration, and growth may be limited in ridge locations due to accumulating soil moisture deficits (Ford et al 2011, Thomsen et al 2013, Hawthorne and Miniat 2018. As the growing season progresses, recharge, transpiration, and growth may be limited in ridge locations due to accumulating soil moisture deficits (Ford et al 2011, Thomsen et al 2013, Hawthorne and Miniat 2018.…”

“…Terrain can also affect disturbance frequency and intensity, including drought mortality (Clinton et al 1993), and wind damage (Elliott et al 2002, Peterson 2007, Mitchell 2013, in turn affecting biotic structure and composition. Often affinities for a particular landscape location have been attributed to a spectrum of traits that suggest adaptation to moisture gradients from mesic coves to xeric ridges (Whittaker 1956, Bahari et al 1985, Day and Monk 1988, Elliott et al 1999, Denslow et al 2010, Hawthorne and Miniat 2018) and, to a lesser extent, temperature gradients from low to high elevations (Morin et al 2007, White andMillert 2008). Often affinities for a particular landscape location have been attributed to a spectrum of traits that suggest adaptation to moisture gradients from mesic coves to xeric ridges (Whittaker 1956, Bahari et al 1985, Day and Monk 1988, Elliott et al 1999, Denslow et al 2010, Hawthorne and Miniat 2018) and, to a lesser extent, temperature gradients from low to high elevations (Morin et al 2007, White andMillert 2008).…”

The Topographic Signature of Ecosystem Climate Sensitivity in the Western United States

“…Our results reflect the strong gradients in moisture associated with topographic positions that buffer ecosystems from regional climate dynamics (Hoylman et al, ; Hoylman et al, ; Ivanov et al, ; Swetnam et al, ). We posit that semiarid regions experience transient to prolonged shallow subsurface flow (e.g., Jencso et al, ) in convergent locations, thereby enhancing soil moisture and reducing ecosystem water limitations along downslope positions (Hawthorne & Miniat, ; Hwang et al, ; Maxwell & Condon, ). Atmospheric microclimates also occur in convergent landscape positions and contribute to feedbacks that reduce evaporative fluxes from the soil (Entekhabi et al, ; Hoylman et al, ) and prolong the buffering effect produced by soil moisture and groundwater movement in the near surface rooting zone.…”

Understanding and predicting frost‐induced tropical tree mortality patterns

“…Extensive tree mortality can trigger abrupt changes in forest composition, structure, and functioning since large amounts of biomass are hastily converted to necromass, altering forest carbon stocks (Anderegg et al, ; Frank et al, ; Lindroth et al, ) and successional pathways (Kreyling, Jentsch, & Beierkuhnlein, ). However, the effects of ECEs on forests can be modulated by topographic conditions, for example, by redistributing soil moisture during droughts (Hawthorne & Miniat, ), changing wind speed and direction during hurricanes (Lugo, ), or guiding the movement of air masses during cold waves (Snyder & de Melo‐Abreu, ). The strong influence of topography on the effects of ECEs is not usually analyzed in detail, which limits our capacity to develop robust models to understand and predict forest tree mortality patterns at local and landscape scales.…”