Greenfall Links Groundwater to Aboveground Food Webs in Desert River Floodplains

Groundwater makes up nearly 99% of unfrozen freshwater worldwide and sustains riparian trees rooted in shallow aquifers, especially in arid and semiarid climates. The goal of this paper is to root animals in the regional water cycle by quantifying the significance of groundwater to riparian animals. We focused our efforts on the cricket, Gryllus alogus: a common primary consumer found in floodplain forests along the San Pedro River, in southeast Arizona, USA. Cottonwood trees make groundwater available to G. alogus as dislodged, groundwater-laden leaves (greenfall). We hypothesized that groundwater fluxes mediated by greenfall sustain G. allogus through the prolonged dry season and link these aboveground consumers to belowground aquifers. To test this hypothesis, we first characterized gradients in absolute humidity (air) and water stress in field-collected G. alogus. Absolute humidity declined with distance from river across wide stands of floodplain cottonwood forest during the dry season, but not during the rainy season. Similarly, G. alogus body water content declined along this gradient. Second, we measured evaporative water loss (EWL) by field-captured G. alogus in the laboratory at temperatures bracketing field conditions. EWL ranged from 0.056 0.009 g individual 1 d 1 to 0.13 6 0.03 g individual 1 d 1 (mean 6 SD, at 308 and 408C, respectively). These daily losses are high, but still less than the water content of a single cottonwood leaf (0.296 6 0.124 g H2O/leaf). Third, we designed field experiments to quantify the relative dependence ofG. alogus on greenfall. G. alogus more frequently consumed greenfall than various controls consisting of dried leaves. This preference occurred in distal habitats and during the dry season, but not proximal to the river or in the rainy season. Finally, we compared estimated daily water fluxes via greenfall to (1) estimates of water demand of the entire G. alogus population at our field site, and (2) reports of cottonwood transpiration and San Pedro River base flow from other authors. By our estimates, groundwater fluxes via greenfall sustain G. alogus populations despite their trivial magnitude compared to stream discharge and cottonwood transpiration. Primary consumers in turn provide dietary water to higher trophic levels (e.g., abundant and speciose birds in the region) through trophic pathways, thereby fueling secondary production from the bottom up. Thus, riparian trees root animals in the regional water cycle.