Most ecological processes occur in patches or are distributed along gradients, and this non-stochastic distribution violates the assumptions of classical statistics (Robertson 1987; Legendre & Fortin 1989; Legendre 1993). Legendre (1993) suggests that spatial or temporal autocorrelation in ecosystem properties promotes diversity of species and functional traits, and that small scales will exhibit autocorrelation, whereas larger scales will exhibit trends. This property of temporal and spatial autocorrelation has been observed at practically all scales (Legendre 1993) from microbe (Franklin et al. 2002) to landscape (Bishop, Schroder & Colby 2003) and forms an important foundation of the structure of ecological communities (Reiners & Driese 2004). Recent work on tree transpiration suggests spatial properties are connected to drivers in time (Adelman, Ewers & Mackay 2008; Loranty et al. 2008) but has not tested how these connections may change with different stand types. This study compares the spatial dynamics in tree transpiration of two stands varying in species composition and structure.
Tree transpiration, because it can be estimated continuously via sapflux (Granier 1987), scaled between stems and leaves with allometrics (Wullschleger, Meinzer & Vertessy 1998), and has a relatively well-understood mechanistic basis (Sperry et al. 2002), serves as an excellent venue for quantifying connections between temporal and spatial controls over ecosystem patchiness. Transpiration varies over a diurnal cycle that is driven by vapour pressure deficit (D), due to tree hydraulic responses and photosynthetically active radiation (Q0) at a particular soil moisture content (Whitehead et al. 1996; Hinckley et al. 1998). Stomatal conductance responds to the rate of water loss at the leaf level and is limited by the product of hydraulic conductivity (k) and the water potential gradient (Ψ) between leaves and soil (Franks, Drake & Froend 2007). This limitation results in a saturating relationship between transpiration and D due to hydraulic stress with trees having a larger initial stomatal conductance declining faster (Oren et al. 1999; Ewers et al. 2005). Hydraulic stress in trees is further increased by coarser soil texture (Hacke et al. 2000), increased soil N (Ewers, Oren & Sperry 2000) and decreased light conditions in the canopy (Schäfer, Oren & Tenhunen 2000).
Recent work has shown that transpiration demonstrates spatial autocorrelation that is driven temporally by D (Adelman, Ewers & Mackay 2008; Loranty et al. 2008). The most relevant spatial parameters within each range of temporal environmental drivers (e.g. low D or high D) are the range that is the distance of spatial autocorrelation, sill that is the maximum spatial variation and the nugget the minimum spatial variation. As predicted from a plant hydraulics hypothesis of spatial variation in time driven by D (Adelman, Ewers & Mackay 2008; Loranty et al. 2008), when D is low, trees are not hydraulically stressed and most trees’ stomatal conductance is high, the range is relatively long and the sill and nugget are low. When D is high, most trees are hydraulically stressed, and so individual tree variability in stomatal response leads to a shorter range and larger sill and nugget. No studies have tried to expand the predictions of the hydraulic hypothesis of these temporal (response to D) and spatial (change in range, sill or nugget with increasing D) patterns to edaphic conditions other than soil moisture or have attempted to predict how changing patterns of trees size will influence the rate of change in the spatial patterns.
We compared two stands in northern Wisconsin to test the relationship between edaphic conditions and spatiotemporal patterns of transpiration across varying species composition and stand structure. One stand had a forested wetland dominated by alder (Alnus incana L. Moench) that transitioned into an upland with trembling aspen (Populus tremuloides Michx) whereas the other stand was a mix of much larger sugar maple (Acer saccharum Marsh) and red pine (Pinus resinosa Ait). We predict that the (i) stand with larger trees will a have longer range and greater sill and nugget at a given D; (ii) the range, sill and nugget will decline faster with increasing D in the stand with large trees; and (iii) soil moisture, texture and/or N levels will be correlated with transpiration within a stand.