Effects of leaf hair points on dew deposition and rainfall evaporation rates in moss crusts dominated by Syntrichia caninervis, Gurbantunggut Desert, northwestern China

Abstract

In arid areas where rainfall is rare and solar radiation is strong, reducing water loss and efficient water use are extremely important for plant survival. Desiccation-tolerant mosses are the dominant components of biological soil crusts (BSCs) in desert ecosystems. These mosses play important roles in soil surface stability, fertility, and ecological restoration. Crust-forming mosses have presumably developed special morphological and anatomical features to cope with harsh desert conditions, but the nature of these remains largely unknown. Some special morphological features (such as convex or concave leaves, papilla, and thick marginal cell walls) have been observed; providing evidence of long-term adaptation to the desert environment. Leaf hair points (LHPs) are important morphological structures in many desiccation- tolerant mosses. These are elongations of leaf midribs; they are generally white, with many small spiny teeth on their surfaces. A few preliminary studies have indicated that LHPs in several kinds of cushion mosses can reflect sunshine, absorb moisture from the air, and reduce water loss. Precipitation is the main water source in desert areas. The rainfall in the center of the Gurbantunggut Desert is only 70 to 150 mm per annum, and most rainfall events (89. 8% in 1998-2007 data) are less than 5 mm. Even so, these events still enable desert mosses to reach and to maintain hydration for a time. In addition, dew and fog (occult precipitation) provide small amounts of water; not enough to hydrate mosses, but with a greater frequency than rainfall. Occult precipitation is thus another important water source for desert biology, and has a crucial role in these ecosystems. Research into the role of the LHPs of drought-resistant mosses is timely. We collected a dominant desert moss, Syntrichia caninervis, from the Gurbantunggut Desert of Xinjiang, China, to investigate the effect of LHPs on water use. The LHPs of S. caninervis are often broken in nature. Our method involved comparison of moss patches with LHPs and moss patches with LHPs manually removed. Characteristics of dew deposition and short-term rainfall evaporation (1 mm, 3 mm, 6 mm) of S. caninervis crusts with and without LHPs were studied using purpose-built micro- lysimeters (PVC, h = 3. 5 cm, d = 5. 7 cm). Results indicated that (1) Dew amounts on crusts with LHPs were greater than on crusts without LHPs. After sunrise, the dew on crusts with LHPs dried slightly faster. Daily dew amounts on crusts with LHPs were greater than on crusts without LHPs. The total dew amount for seven days on crusts with LHPs was 10. 26% greater than on crusts without LHPs. Thus LHPs can improve dew amounts on S. caninervis crusts by 10. 26%. (2) Rainfall evaporation rates from crusts with LHPs were lower than from crusts without LHPs at every time of evaporation; this was most obvious during the early period. LHPs can significantly delay and reduce water evaporation, thus extending the hydration time of S. caninervis crusts. These differences in water utilization are a result of two aspects. More efficient dew use is attributable to LHPs increasing the surface roughness of the crusts. By reflecting light, forming internal capillaries, and reducing gaps between individuals, LHPs are able to reduce rainfall evaporation. In conclusion, the existence of LHPs is favorable for the utilization of dew and rainfall by S. caninervis crusts, and increases the ability of these moss crusts to adapt to arid conditions.