Ecohydrological consequences of drought‐ and infestation‐ triggered tree die‐off: insights and hypotheses

Abstract

Widespread, rapid, drought-, and infestation-triggered tree mortality is emerging as a phenomenon affecting forests globally and may be linked to increasing temperatures and drought frequency and severity. The ecohydrological consequences of forest dieoff have been little studied and remain highly uncertain. To explore this knowledge gap, we apply the extensive literature on the ecohydrological effects of tree harvest in combination with the limited existing die-off ecohydrology research to develop new, relevant hypotheses. Tree mortality results in loss of canopy cover, which directly alters evaporation, transpiration, and canopy interception and indirectly alters other watershed hydrologic processes, including infiltration, runoff, groundwater recharge, and streamflow. Both die-off and harvest research suggest that for most forests, water yield can be expected to increase following substantial loss of tree cover by die-off. We hypothesize that where annual precipitation exceeds ¾500 mm or water yield is dominated by snowmelt, watersheds will experience significantly decreased evapotranspiration and increased flows if absolute canopy cover loss from die-off exceeds 20%. However, recent observations suggest that water yield following die-off can potentially decrease rather than increase in drier forests. To reliably predict die-off responses, more research is needed to test these hypotheses, including observations of multiple water budget components and the persistence of ecohydrological effects with the post-die-off successional dynamics of tree recruitment, understorey growth, and interactions with additional disturbances. With die-off, mitigation and restoration options are limited and costly, necessitating societal adaptation; therefore, die-off ecohydrology should be a high priority for future research. Published in 2011. This article is a US Government work and is in the public domain in the USA.

Ecosystem CO2/H2O fluxes are explained by hydraulically limited gas exchange during tree mortality from spruce bark beetles

“…Disturbances can have a profound impact on ecosystem fluxes of water [Adams et al, 2012] and carbon [Amiro et al, 2010]. Though a bark beetle epidemic has similarities to disturbance caused by harvest or fire where overstory is dramatically removed or destroyed, it is fundamentally different because some of the canopy along with the understory can survive relatively unharmed and then utilize the resources relinquished by the dying trees [Veblen et al, 1991].…”

Climate-driven disturbances in the San Juan River sub-basin of the Colorado River

“…Water yields and peak streamflows in North American river basins are anticipated to either increase, decrease, or show no response to changing forest cover (see Table 2 in Adams et al, 2012;Schnorbus et al, 2010;Guardiola-Claramonte et al, 2011;Somor, 2010;McDowell et al, 2018). Causes of the reported changes have been related to topography (Schnorbus et al, 2010) and climate variability .…”

“…As fire, pests, and drought modify forests in response to climate change, a number of important energy and water fluxes become altered (Adams et al, 2012). Land covers with a low canopy profile and a small crown cover, e.g., shrubs or bare ground, partition water and energy in different manners compared to forests that have a large crown cover.…”

“…In addition, mountain-pine-beetle infestations change canopy conditions but do not change soil properties or moisture states. Drought may affect the forest canopy and the ability of soils to infiltrate water as trees die and soils desiccate (Adams et al, 2012). Effects may also be impacted by the type of post-disturbance regrowth and extent of disturbance that occurs, such as increases or a difference in shrubs, grasses, forbs coverage, or differences in the composition of tree species that replace the forests (Buma and Livneh, 2017).…”

“…In addition, we investigated the aridity effect upon water availability under forest disturbances using a one-cell analysis. Some studies have suggested an aridity effect, whereby basins with less than 500 mm annual precipitation will see streamflow decrease and vice-versa (Guardiola-Claramonte et al, 2011;Adams et al, 2012), although this finding is not supported in all work (Brown et al, 2010;Caldwell et al, 2016). For the single cell, we considered climate change by adding +3 • C (warm) and +6 • C (hot) to the temperature time series and changing precipitation by 20 or −20 %.…”