Relationships between groundwater use, water table, and recovery of willow on Yellowstone's northern range

Abstract

Abstract. Excessive levels of herbivory, incision of stream channels, and climate warming are believed to be responsible for the decline of woody deciduous plants in riparian zones in western North America, declines that are likely to be associated with diminished biological diversity. In the northern elk wintering range of Yellowstone National Park, USA, overbrowsing by elk (Cervus elaphus), lowered water tables resulting from stream incision, and loss of activity by beaver (Castor canadensis) have been implicated in the decline of willow (Salix sp.) communities. Reducing elk browsing appears sufficient for willow recovery in some areas, but where water table changes have been dramatic, recovery may be slow or absent. The importance of water table changes is disputed because experimental results demonstrate water table limitations, but water table depth has failed to explain variation in willow height at landscape scales. One explanation for this apparent discrepancy is that willows that have survived intensive browsing by elk have maintained access to groundwater despite declining water tables. Using stable isotopes of water, we examined the relationships between groundwater use, water table depth, and height of heavily browsed Salix geyeriana. Salix geyeriana groundwater use varied from 30% to 80%, and was higher later in the growing season, when soil water was less available and shoot water potentials were lower. Late season groundwater use explained 26% of the variation in total height of willows (P ¼ 0.002), with taller plants using more groundwater. Water table depth explained only 8% of the variability in total height (P ¼ 0.051), with shorter willows having deeper water table depths. Groundwater use and water table depth were uncorrelated. Height recovery following a winter of heavy browsing was related to groundwater use, but not groundwater depth. We suggest that access to deeper water sources alleviates late season water stress, allowing for more rapid height recovery and higher total plant height. Variability in groundwater access may account for variability in height recovery at landscape scales.

Stream hydrology limits recovery of riparian ecosystems after wolf reintroduction

“…However, increased availability of water offers a plausible mechanism for these responses for two reasons. Photosynthetic rates of willows on undammed plots with deep water tables were limited by water availability [49], and isotopic analysis of water use by willows adjacent to our plots showed clear positive correlations between height and access to groundwater [24]. Tercek et al [50] observed that tall willows had twice rspb.royalsocietypublishing.org Proc R Soc B 280: 20122977…”

“…), by a population of elk (Cervus elaphus) released from control by predators [16][17][18][19][20]. However, it remains unclear if restoration of wolves has restored the riparian zone via a trophic cascade [21][22][23][24].…”

Interactions of sapsuckers andCytosporacanker can facilitate decline of riparian willows

“…Willows use soil water when it is available early in the growing season and deeper ground water later in the summer (Johnston et al 2011). Groundwater use by willows in Yellowstone National Park was not correlated with water table depth, and plants consistently used primarily ground water at sites with a water table deeper than 140 cm (Johnston et al 2011).…”

“…A fundamental difference in riparian willow and upland forest tree species decline is due to willows being phreatophytes whose primary water source is shallow ground water (Johnston et al 2011). Willows are moderately susceptible to drought stress; however, when they have access to a reliable source of ground water <3 m from the ground surface, they rarely experience hy- Note: 2011 models had identical model weights when compared with 2010 models.…”

“…forests throughout the Rocky Mountains in the United States and Canada (Anderegg et al 2012). Species and communities in upland areas have been more severely impacted than in riparian areas (Choat et al 2012), likely because upland plants rely on soil moisture recharged by precipitation, whereas riparian plants are phreatophytes that use available shallow ground water (Johnston et al 2011). However, riparian plant diebacks have also been documented.…”