Contrasting effects of two Acacia species on understorey growth in a drylands environment: interplay of canopy shading and litter interference

Abstract

Questions Do tree species with similar canopy structures but distinct litter production volumes affect the understorey growth in dryland environments differently? How does rainfall impact the magnitude and sign of these effects? Location Woodland savanna, northern Negev drylands, Israel. Methods We determined understorey herbaceous productivity beneath the canopy of two N‐fixing tree species (Acacia victoriae and Acacia salicina) with comparable canopy structures (canopy height and area) but distinct litter production volumes. Biomass was measured beneath A. victoriae trees and in the open area between the trees for 7 yrs and beneath A. salicina and the nearby open area for 2 yrs. Light measurements, litter biomass, fine root density and nutrients and gravimetric water content in the top soil were assessed for both species. Annual rainfall and the coefficient of variation in daily rain were used to determine rainfall effects on tree–grass relations. Results Acacia victoriae and A. salicina had contrasting positive and negative effects on understorey growth, respectively. Both species had comparable fine root densities, soil moisture and nutrient content beneath their canopies, with a higher soil water content than the open area. The large amount of leaf litter produced by A. salicina appeared to suppress the understorey growth, likely via light interference at the soil surface, which outweighed the positive effect of moisture conditions. Acacia victoriae that produced significantly lesser amounts of litter facilitated the understorey growth via canopy shading (likely through reducing evaporative loss). Both the rainfall amount and its intra‐annual variability reduced the magnitude of the tree effect but did not change its sign. Conclusions Tree leaf litter production may have a contrasting effect on understorey growth, ranging from positive to negative even in the same environment. Changes in rainfall amount and intra‐annual rainfall variability affect the magnitude of the effect but not its sign.

Individual vital rates respond differently to local‐scale environmental variation and neighbour removal

“…Towers et al. (2020) conducted their study in a year that was wetter than average, which was considered a possible reason for weak species‐specific responses to factors that are associated with moisture (canopy and litter cover and the presence of coarse wood debris) (Helman et al., 2017). In the year we conducted our study, conditions were drier than average with watering treatments representing 30%–90% of average rainfall, and yet we also observed few main effects of watering on vital rates.…”

Trenched and diked contours for sustainable agricultural utilization in changing world; soil hydrological, geomorphological, and physical perspectives from over 70 years of implementation in the Northern Negev

“…The influence of arid land afforestation on ecosystem hydrology is under disagreement among scientists. On one side, the tree root system enables better water infiltration and the tree may serve as an essential ecosystem engineer in these environments (Helman et al, 2017). However, on the other side, the tree consumes its growth and transpires high amounts of water (Stavi, 2019).…”

Requirements for the spatial storage effect are weakly evident for common species in natural annual plant assemblages

“…Sparsely distributed woody plant species often play an important role in structuring understorey community composition in semiarid ecosystems (Facelli and Brock 2000, Prober and Wiehl 2012, Helman et al 2017, Tessema and Belay 2017). In the same way, when species in the present study responded significantly to the environment, they tended to respond to the combined effect of overstory and litter cover.…”

“…Finally, it is possible that weak species‐specific responses were the result of the benign, above‐average rainfall conditions that occurred throughout 2018 when this experiment was conducted. Environmental factors that can ameliorate drought stress, such as the presence of CWD and the amount of overstory and litter cover, may play a much smaller role in mediating species’ performances in wet years (Helman et al 2017). Given that the spatial storage effect is dependent on partially uncorrelated responses to patch environments, this raises an interesting avenue for future research: does the strength of the spatial storage effect vary systematically with climate through time as a spatiotemporal dynamic?…”