Abstract
While temperature and precipitation comprise important ecological filtering for native ranges of forest trees and are predisposing factors underlying forest ecosystem dynamics, the extent and severity of drought raises reasonable concerns for carbon storage and species diversity. Based on historical data from common garden experiments across the Pacific Northwest region, we developed non-linear niche models for height-growth trajectories of conifer trees at the sapling stage using annual or seasonal climatic variables. The correlations between virtual tree height for each locality and ecosystem functions were respectively assessed. Best-fitted models were composed of two distinct components: evapotranspiration and the degree-days disparity for temperature regimes between 5 °C and 18 °C (effective temperature sum and growth temperature, respectively). Tree height prediction for adaptive generalists (e.g., Pinus monticola, Thuja plicata) had smaller residuals than for specialists (e.g., Pinus contorta, Pseudotsuga menziesii), albeit a potential confounding factor – tree age. Discernably, there were linearly positive patterns between tree height growth and ecosystem functions (productivity, biomass and species diversity). Additionally, there was a minor effect of tree diversity on height growth in coniferous forests. This study uncovers the implication of key ecological filtering and increases our integrated understanding of how environmental cues affect tree stand growth, species dominance and ecosystem functions.
Highlights
As early as the 1800s, a nature explorer, Alexander von Humboldt (1769–1859), observed that climate was the primary determinant of global vegetation patterns
We sought to address three commonly invoked questions: (1) What are the essential environmental drivers for tree height growth? Among important environmental cues, we expect climatic variables reported in an ample body of literature, such as temperature[12,17,18] and evapotranspiration (i.e., Eref)[14,15] to weigh more heavily in explaining different height growth and ecosystem functioning at different habitats in the study region
Using growth potential as the response variable, loadings in partial least squares regression (PLSR) indicated that the most critical annual climatic variables were degree-days below 18 °C [DD_18] and degree-days above 5 °C [DD5] (Fig. S4); scores plot showed no apparent indication of grouping or outliers, suggestive of a good coverage of height ranges (Fig. S5)
Summary
As early as the 1800s, a nature explorer, Alexander von Humboldt (1769–1859), observed that climate was the primary determinant of global vegetation patterns. Water availability rather than net precipitation is a key selective force in the Pacific Northwest, which is, in a broad sense, congruent with environmental limitations to the global pattern in plant height[22] These important environmental conditions co-select for a complex assortment of conifer species, including fir (Abies spp.), spruce (Picea spp.), pine (Pinus spp.), and hemlock (Tsuga spp.)[20]. > Thuja spp.] do not cope with drought-induced embolism and have lower wood density[26] These features allow shade-tolerant species inhabiting dense forests to be taller, better compete for light and rapidly occupy disturbed sites (but not necessarily be early successional species)[26]. (2) Do adaptive generalists have stronger environment–trait associations than adaptive specialists? Since adaptive specialists have a high intraspecific variability and have increased abilities to adapt to prevailing ecological gradients[23,24], we predict that adaptive specialists have better environment–trait matching (i.e., more accurate tree height prediction by the environment) within their ecological breadth
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