More than 5,000 measurements from 1,943 plant species were used to explore the scaling relationships among the foliar surface area and the dry, water, and nitrogen/phosphorus mass of mature individual leaves. Although they differed statistically, the exponents for the relationships among these variables were numerically similar among six species groups (ferns, graminoids, forbs, shrubs, trees, and vines) and within 19 individual species. In general, at least one among the many scaling exponents was <1.0, such that increases in one or more features influencing foliar function (e.g., surface area or living leaf mass) failed to keep pace with increases in mature leaf size. Thus, a general set of scaling relationships exists that negatively affects increases in leaf size. We argue that this set reflects a fundamental property of all plants and helps to explain why annual growth fails to keep pace with increases in total body mass across species.foliar traits ͉ plant allometry ͉ scaling relations S ize variations in foliar functional traits have received intense recent attention, because leaves are the principal photosynthetic organs of the majority of plant species, because the manner in which foliar traits change within or across species as a function of differences in leaf size can profoundly affect plant growth, reproduction, and ecosystem function, and because standing leaf mass is a critical component in empirical and theoretical plant allometry models (1)(2)(3)(4)(5)(6)(7)(8)(9)(10)(11)(12)(13)(14). Surprisingly, however, our knowledge about some very basic size-dependent (scaling) relationships is very incomplete, particularly in terms of how intra-and interspecific differences in mature leaf dry mass (M D ) correlate with foliar water mass (M W ), surface area (SA), and the nitrogen or phosphorus mass per leaf lamina (N L and P L , respectively), either within individual species or across taxonomically different species groups sharing the same life forms (and thus presumably similar foliar architectures and other functional traits).The importance of quantifying size-dependent variations among functional traits is evident from the general scaling relationship X ϭ ␤ M D ␣ , where X represents one among many functional traits influencing the physiological or mechanical functions of leaves (e.g., SA or M W ) and where ␤ and ␣ are, respectively, the elevation and slope of the log-transformed X vs. M D regression curve. Noting that the change in X with respect to differences in mature leaf M D (i.e., ѨX/ѨM D ) equals ␣ ␤ M D ␣Ϫ1 , the magnitude of X will be independent of intra-or interspecific differences in M D when ␣ ϭ 1.0; it will increase disproportionately with increasing M D when ␣ Ͼ 1.0; and it will fail to keep pace with intra-or interspecific increases in M D when ␣ Ͻ 1. Among these three possibilities, the first and second do not a priori result in negative consequences as mature leaf mass increases intra-or interspecifically. The first is size-independent and results in a ''break even'' relationship, w...

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At northern latitudes a rise in atmospheric humidity and precipitation is predicted as a consequence of global climate change. We studied several growth and functional traits of hybrid aspen (Populus tremula L.×P. tremuloides Michx.) in response to elevated atmospheric humidity (on average 7% over the ambient level) in a free air experimental facility during three growing seasons (2008–2010) in Estonia, which represents northern temperate climate (boreo-nemoral zone). Data were collected from three humidified (H) and three control (C) plots, and analysed using nested linear models. Elevated air humidity significantly reduced height, stem diameter and stem volume increments and transpiration of the trees whereas these effects remained highly significant also after considering the side effects from soil-related confounders within the 2.7 ha study area. Tree leaves were smaller, lighter and had lower leaf mass per area (LMA) in H plots. The magnitude and significance of the humidity treatment effect – inhibition of above-ground growth rate – was more pronounced in larger trees. The lower growth rate in the humidified plots can be partly explained by a decrease in transpiration-driven mass flow of NO3 − in soil, resulting in a significant reduction in the measured uptake of N to foliage in the H plots. The results suggest that the potential growth improvement of fast-growing trees like aspens, due to increasing temperature and atmospheric CO2 concentration, might be smaller than expected at high latitudes if a rise in atmospheric humidity simultaneously takes place.

A study was performed on saplings of silver birch (Betula pendula Roth) growing at the free air humidity manipulation site, which was established to investigate the effect of increased air humidity on tree performance and canopy functioning. The aim of the experiment was to simulate the impact of the increasing atmospheric humidity on forest ecosystems predicted for northern Europe. Artificially elevated relative humidity (RH), which causes transpirational flux to decrease, diminished nutrient supply to the foliage; leaf nitrogen content, phosphorus content and P:N ratio decreased. The changes in leaf nutritional status brought about a considerable decline in both photosynthetic capacity (Amax, Vcmax, Jmax) and tree growth rate. The manipulation induced diverse changes in tree hydraulic architecture and other functional traits. Different segments of the soil‐to‐leaf water transport pathway responded differently: leaf hydraulic conductance (KL) decreased, while hydraulic conductance of root systems (KR) and leaf‐specific conductivity of stem‐wood increased in response to elevated RH. Humidification caused the Huber values of stems to increase, thus reflecting changes in allocation patterns; relatively more resources were allocated to vascular tissue and less to foliage. The elevated RH induced substantial changes in specific leaf area (increased), branch‐ (decreased) and stem‐wood density (decreased). The observed responses suggest that the expected climate‐change‐induced increase in the growth rate of trees at northern latitudes (boreal areas) due to the earlier start of the growing season in spring or higher carbon assimilation rate could be smaller or null if temperature rise is accompanied by a rise in atmospheric absolute humidity.

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