No abstract

Stemflow, a precipitation and solute supply to soils near tree stems, can play a wide array of roles in ecosystem functioning. However, stemflow’s ecohydrological functions have been primarily studied in forests with voluminous stemflow because resource subsidy is currently considered stemflow’s only impact on near-stem soils. This common assumption ignores controls that stemflow generation may exert via resource limitation (when stemflow < open rainfall and near-stem throughfall is negligible). We reviewed selected literature across numerous forests to evaluate the predominance of stemflow as a potential resource limitation to near-stem soils and characterized the concentrated, but meager, solute flux from low stemflow generators. Global observations of stemflow were highly skewed (skewness = 4.6) and leptokurtic (kurtosis = 28.8), where 69% of observations were ≤2% of rainfall. Stemflow ≤ 2% of rainfall is 10–100 times more chemically enriched than open rainfall, yet low volumes result in negligible solute fluxes (under 1 g m-2 y-1). Reduced stemflow may be the global and regional norm, creating persistently dry near-stem soils that receive infrequent, salty, and paltry precipitation flux if throughfall is also low. Ignoring stemflow because it results in scarcity likely limits our understanding of ecosystem functioning as resource limitations alter the fate of soil nutrients, energy flows, and spatial patterning of biogeochemical processes.

Short CommunicationLiving particulate fluxes in throughfall and stemflow during a pollen event.

Abstract. In vegetated landscapes, rain must pass through plant canopies and litter to enter soils. As a result, some rainwater is returned to the atmosphere (i.e., interception, I) and the remainder is partitioned into a canopy (and gap) drip flux (i.e., throughfall) or drained down the stem (i.e., stemflow). Current theoretical and numerical modeling frameworks for this process are almost exclusively based on data from woody overstory plants. However, herbaceous plants often populate the understory and are the primary cover for important ecosystems (e.g., grasslands and croplands). This study investigates how overstory throughfall (PT,o) is partitioned into understory I, throughfall (PT) and stemflow (PS) by a dominant forb in disturbed urban forests (as well as grasslands and pasturelands), Eupatorium capillifolium (Lam., dogfennel). Dogfennel density at the site was 56 770 stems ha−1, enabling water storage capacities for leaves and stems of 0.90±0.04 and 0.43±0.02 mm, respectively. As direct measurement of PT,o (using methods such as tipping buckets or bottles) would remove PT,o or disturb the understory partitioning of PT,o, overstory throughfall was modeled (PT,o′) using on-site observations of PT,o from a previous field campaign. Relying on modeled PT,o′, rather than on observations of PT,o directly above individual plants means that significant uncertainty remains with respect to (i) small-scale relative values of PT and PS and (ii) factors driving PS variability among individual dogfennel plants. Indeed, PS data from individual plants were highly skewed, where the mean PS:PT,o′ per plant was 36.8 %, but the median was 7.6 % (2.8 %–27.2 % interquartile range) and the total over the study period was 7.9 %. PS variability (n=30 plants) was high (CV > 200 %) and may hypothetically be explained by fine-scale spatiotemporal patterns in actual overstory throughfall (as no plant structural factors explained the variability). The total PT:PT,o′ was 71 % (median PT:PT,o′ per gauge was 72 %, with a 59 %–91 % interquartile range). Occult precipitation (mixed dew and light rain events) occurred during the study period, revealing that dogfennel can capture and drain dew to their stem base as PS. Dew-induced PS may help explain dogfennel's improved invasion efficacy during droughts (as it tends to be one of the most problematic weeds in the improved grazing systems in the southeastern US). Overall, dogfennel's precipitation partitioning differed markedly from the site's overstory trees (Pinus palustris), and a discussion of the limited literature suggests that these differences may exist across vegetated ecosystems. Thus, more research on herbaceous plant canopy interactions with precipitation is merited.

As watersheds are complex systems that are difficult to directly study, the streams that drain them are often sampled to search for watershed “signals.” These signals include the presence and/or abundance of isotopes, types of sediment, organisms (including pathogens), chemical compounds associated with ephemeral biogeochemical processes or anthropogenic impacts, and so on. Just like watersheds can send signals via the streams that drain from them, we present a conceptual analysis that suggests plant canopies (equally complex and hard-to-study systems) may send similar signals via the precipitation that drains down their stems (stemflow). For large, tall, hard-to-access tree canopies, this portion of precipitation may be modest, often &lt;2%; however, stemflow waters, like stream waters, scour a large drainage network which may allow stemflow to pick up various signals from various processes within and surrounding canopies. This paper discusses some of the signals that the canopy environment may impart to stemflow and their relevance to our understanding of vegetated ecosystems. Being a conceptual analysis, some examples have been observed; most are hypothetical. These include signals from on-canopy biogeochemical processes, seasonal epi-faunal activities, pathogenic impacts, and the physiological activities of the canopy itself. Given stemflow's currently limited empirical hydrological, ecological and biogeochemical relevance to date (mostly due to its modest fraction in most forest water cycles), future work on the possible “signals in stemflow” may also motivate more natural scientists and, perhaps some applied researchers, to rigorously monitor this oft-ignored water flux.