Per tree production , basal area and biomass were similarly less than half of values reported for well-drained sites in Canada. ANPP was about one third of values reported for stands in Manitoba and one quarter of values reported for Larch forests in Central Siberia and Scots Pine forests in Finland . Differences in growth allometry between our Alaskan stands and those from Northern Manitoba provide some evidence that the low productivity of Alaskan stands may be due to moisture stress. Regional mean annual precipitation was 30% lower in our Alaskan sites than in the Manitoba sites, indicating that available soil moisture may be lower in our sites. Our Alaskan trees were significantly shorter and had less stem mass per unit increase in DBH than their Canadian relatives. In black and white spruce stands across Canada, reduced height and shoot growth has been linked to soil moisture deficits . In response to water stress, trees may grow more wood per unit height , apparently to decrease the potential for embolism in xylem during periods of moisture stress . Our observation of changes in allometry and its influence on biomass also agrees with the observation that black spruce in Alaska may allocate more C below ground where moisture appears to be more limiting . Moss biomass began to accumulate surprisingly early in succession as indicated by the large increases in Ceratodon spp. and Polytrichum spp. over the first 4 year of succession in the 1999 dry site. Composition shifted to feather moss dominance in both the mesic and dry mature sites.
Because feather moss lack water-conducting tissues, it was surprising that its production was similar between mesic and dry sites despite an order of magnitude difference in moss biomass pools . As a result, ANPP per unit biomass, or production efficiency,raspberry container size was drastically lower in the mesic site, which may indicate lower light or nutrient availability in this site where mosses are both densely packed and beneath a closed canopy. Alternatively, it may indicate that that there is more brown moss in the mesic stand. Due to cool soils and moist conditions, decomposition of senescent moss may be slower in the mesic stand than in the dry stand, resulting in more intact brown material. Our measurements of moss biomass pools in the mesic mature stand were on par with green plus brown biomass pools in a black spruce/feather moss community in Washington Creek, AK , and twice as large as estimates for a similar community in Canada where only the green biomass was sampled .The growth of the “critical zone” paradigm has added impetus to closer investigation of soil-plant atmosphere interactions in ecohydrology . This follows from work emphasizing the importance of vegetation in regulating the global terrestrial hydrological cycle, with transpiration being the dominant “green water” flux to the atmosphere compared to evaporation from soils and canopy interception in most environments . More locally, the role vegetation plays in partitioning precipitation into such “green water” fluxes and alternative “blue water” fluxes to groundwater and stream flow has increased interest in the feed backs between vegetation growth and soil development in different geographical environments . The emerging consequences of climatic warming to changes in vegetation characteristics and the implications of land use alterations add further momentum to the need to understand where plants get their water from, and how water is partitioned and recycled in soil-plant systems . Stable isotopes in soil water and plant stem water have been invaluable tools in elucidating ecohydrological interactions over the past decade . Earlier work by Ehleringer and Dawson explained the isotope content of xylem water in trees in terms of potential plant water sources. Building on that, Brooks et al. showed that the isotope characteristics of xylem water did not always correspond to bulk soil water sources as plant xylem water was fractionated and offset relative to the global meteoric water line compared to mobile soil water, groundwater and stream flow signatures.
This led to the “Two Water Worlds” hypothesis which speculated that plant water was drawn from a “pool” of water that was “ecohydrologically separated” from the sources of groundwater recharge and stream flow . Research at some sites has found similar patterns of ecohydrologic separation and suggested it may be a ubiquitous characteristic of plant-water systems . Others have found that differences between plant water and mobile water may be limited only to drier periods , or may be less evident in some soil-vegetation systems . Direct hypothesis testing of potential processes that may explain the difference between the isotopic composition of xylem water and that of potential water sources has been advanced by detailed experiments in controlled environments, often involving the use of Bayesian mixing models which assume all potential plant water sources have been sampled . However, as field data become increasingly available from critical zone studies, more exploratory, inferential approaches can be insightful in terms of quantifying the degree to which xylem water isotopes can or cannot be attributed to measured soil water sources . As this research field has progressed, it has become apparent that extraction of soil and plant waters for isotope analysis is beset with a number of methodological issues . Soil waters held under different tensions may have different isotopic characteristics: for example, freely moving water sampled by suction lysimeters often shows a much less marked evaporative fractionation signal than bulk soil waters dominated by less mobile storage extracted by cryogenic or equilibration methods . Such differences between extraction techniques may be exacerbated by soil characteristics, such as texture and organic content, which may in turn affect the degree to which water held under different tensions can mix . Similarly, sampling xylem and its resulting isotopic composition has been shown to be affected by methodology. It is usually assumed that methods such as cryogenic extraction isolate water held in xylem, when in fact water stored in other cells may be mobilized to “contaminate” the results .
Interpretation of plant-soil water relationships can also be complicated by processes in plants and soils that alter isotopic compositions independently. For example, the spatio-temporal isotopic composition of soil water can change dramatically in relation to precipitation inputs, evaporative losses, internal redistribution and phase changes between liquid and gaseous phases . Moreover, there is increasing evidence that plant physiological mechanisms may affect water cycling and the composition of xylem water . These include effects of mychorrizal interactions in plant roots that may result in exchange and fractionation of water entering the xylem stream . Research also indicates that as flow in xylem slows, diffusion and fractionation can occur , which may involve exchange with phloem cells . Finally, there is increasing evidence that water storage and release from non-xylem cells may sustain transpiration during dry periods or early in the day , also affecting xylem composition. Thus, there is a need to understand the different timescales involved in uptake processes in the rooting zone, residence times and mixing of water in different vegetation covers . There is also evidence of differences between how such factors affect water movement in angiosperms and gymnosperms, as well as species-specific differences . Clearly, these methodological issues will take some time to address; in the interim there is a need for cautious interpretation of emerging data from critical zone studies in order to improve our understanding.A striking feature of isotopic studies of soil-vegetation systems is a bias to lower and temperate latitudes,raspberry plant container with northern latitudes and cold environments being under-represented . Yet, northern environments present particular challenges and opportunities to further advance the growing body of knowledge about plant-soil water interactions. For example, the coupled seasonality of precipitation magnitude and vegetative water demand can be complicated by the seasonality of the precipitation phase. Cold season precipitation that accumulates as snow can replenish soil water in the spring and be available to plants months after deposition . Despite the lack of studies, these areas are experiencing some of the most rapid changes in climate and, as a result, vegetation . The effects of climatic warming on patterns of snow pack accumulation and melt can have particularly marked consequences for soil water replenishment and plant water availability, particularly at the start of the growing season . Despite the importance of northern environments, remoteness and harshness of environmental conditions result in logistical problems that constrain lengthy field studies and data collection . This study seeks to contribute to the growing body of knowledge about plant-soil water interactions by expanding the geographical representation of sites in cold northern environments. We report the findings of a coordinated project on xylem water isotopic data collection in the dominant soil – vegetation systems of five long-term experimental sites. Isotopic characteristics of soil water have previously been reported for all five sites; this used a comparative approach with, as far as possible, common sampling methods across the sites for a 12 month period . Here, we present xylem water isotopic composition data collected using common methods over the same time period encompassing the complete growing season, and then relate findings to soil water isotopic compositions. The study was conducted at five long-term experimental catchments across the boreal or mountainous regions of the northern latitudes .
The catchments were part of the VeWa project funded by the European Research Council investigating vegetation effects on water mixing and partitioning in high-latitude ecosystems . Previous inter-comparison work on this project has examined such issues as changing seasonality of vegetation-hydrology interactions , soil water storage and mixing , water ages and modelling the interactions between water storage, fluxes and ages .At each site, plants and surrounding soils were sampled concurrently for isotope analysis following a common sampling protocol . Depending on the nature of the soil cover, the maximum depth of sampling varied from -20 cm at BB to -70 cm at Dry Creek . Sampling took place at 5 cm intervals for Bruntlad Burn, Dorset, and Krycklan with two to five replicates for each depth. At Dry Creek, sampling was done at -10, -25, -45, and -70 cm with two to four replicates. Sampling depths at Wolf Creek varied between -2 and -40 cm with one to three replicates. Daily soil moisture data based on continuous soil moisture measurements at 10 or 15 cm soil depth were available for each soil water sampling location at Bruntland Burn, Dry CReek, Krycklan, and Wolf Creek. Only weekly manual soil moisture measurements were available for Dorset, and daily soil moisture data were derived from soil physical modelling . The volumetric soil moisture data were used to assess the hydrologic state on the sampling days. Plant samples from trees with a diameter > 30 cm were taken horizontally with increment borers at breast height . Retrieved plant xylem cores were directly placed in vials without bark and phloem. Shrub vegetation was sampled by clipping branches. These were immediately placed in vials after the bark was chipped off or left on . All vials were directly sealed with parafilm and immediately frozen until extraction was conducted at Boise State University, Boise, Idaho, USA. There were five replicates for each species and day at the sites in Bruntland Burn, Krycklan, Dorset. At Wolf Creek, the number of replicates varied between two and five and there were always four replicates for each sampling campaign at the Dry Creek sites. In total, 1160 xylem water samples were collected; 831 for angiosperms and 329 for gymnosperms . Dates of sample events varied at each site, but included the end of the growing season/senescence, pre-leaf out the following year, post leaf out, peak growing season and senescence . Precipitation was sampled daily or on an event basis at Bruntland Burn and Krycklan. Daily to fortnightly precipitation sampling was conducted at Dorset, Dry Creek, and Wolf Creek. Melt water was sampled from lysimeters at Krycklan, Dorset, Dry Creek and Wolf Creek during several snow melt events, while snowfall seldom occurred over the study year at Bruntland Burn . Various measures were taken to prevent evaporation of collected precipitation, including paraffin oil and water locks prior to transfer to the laboratory. The long-term groundwater signal was assessed at all sites, apart from Dorset, using several sampling campaigns of springs and wells tapping the saturated zone over the last few years . There were no nearby wells from which to sample the regional groundwater at Dorset, which is found well below the surface in the granitic gneiss and amphibolite bedrock.