ArticlePDF Available

Tree diversity, tree height and environmental harshness in eastern and western North America

Authors:
  • Mass Audubon

Abstract and Figures

Does variation in environmental harshness explain local and regional species diversity gradients? We hypothesise that for a given life form like trees, greater harshness leads to a smaller range of traits that are viable and thereby also to lower species diversity. On the basis of a strong dependence of maximum tree height on site productivity and other measures of site quality, we propose maximum tree height as an inverse measure of environmental harshness for trees. Our results show that tree species richness is strongly positively correlated with maximum tree height across multiple spatial scales in forests of both eastern and western North America. Maximum tree height co-varied with species richness along gradients from benign to harsh environmental conditions, which supports the hypothesis that harshness may be a general mechanism limiting local diversity and explaining diversity gradients within a biogeographic region.
Content may be subject to copyright.
LETTER Tree diversity, tree height and environmental harshness in
eastern and western North America
Christian O. Marks,
1
*
Helene C. Muller-Landau
2
and
David Tilman
3
1
The Nature Conservancys
Northampton, MA 01060, USA
2
Smithsonian Tropical Research
Institute, Panama City, Panama
3
Department of Ecology, University
of Minnesota, St. Paul, MN 55108,
USA
*Correspondence: E-mail:
cmarks@tnc.org
Abstract
Does variation in environmental harshness explain local and regional species diversity gradients?
We hypothesise that for a given life form like trees, greater harshness leads to a smaller range of
traits that are viable and thereby also to lower species diversity. On the basis of a strong depen-
dence of maximum tree height on site productivity and other measures of site quality, we propose
maximum tree height as an inverse measure of environmental harshness for trees. Our results
show that tree species richness is strongly positively correlated with maximum tree height across
multiple spatial scales in forests of both eastern and western North America. Maximum tree
height co-varied with species richness along gradients from benign to harsh environmental condi-
tions, which supports the hypothesis that harshness may be a general mechanism limiting local
diversity and explaining diversity gradients within a biogeographic region.
Keywords
Alpha diversity, diversity gradients, environmental favourability, gamma diversity, harshness
hypothesis, maximum tree height, site index, tree species richness.
Ecology Letters (2016)
INTRODUCTION
An improved understanding of diversity patterns is a primary
concern for ecologists, evolutionary biologists, biogeogra-
phers, palaeontologists and increasingly conservation scien-
tists. Historical factors have been shown to play major roles
in explaining variation in diversity among biogeographic
realms (Latham & Ricklefs 1993), but explanations for geo-
graphic variation in diversity within realms have been less
compelling. We suggest that within a biogeographic region,
geographic variation in diversity for any given functional
group may be best explained by spatial variation in the
strengths of environmental factors that constrain the growth
and survival of species in that functional group. We call the
total effect of all such constraining variables environmental
harshness, and hypothesise that it acts as an ecological and
evolutionary filter on diversity. Harshness is intuitively
appealing as an explanation because of its qualitative consis-
tency with latitudinal diversity gradients, and was one of the
first hypotheses proposed for such gradients (Whittaker 1965).
Yet harshness has frequently been rejected as a useful basis
for diversity theory because of difficulty in defining environ-
mental harshness in a way that is easily measureable and is
independent of species richness (Rohde 1992).
Many explanations for diversity gradients, in general, and
latitudinal gradients in species richness, in particular, have
been proposed (for a review, see e.g. Mittelbach et al. 2007).
These hypotheses are not all mutually exclusive, as multiple
mechanisms surely contribute, with different mechanisms
dominating at different scales. Historical factors, such as the
area a biome has had over evolutionary time, have been impli-
cated as a dominant factor in explaining differences in diver-
sity among biogeographic regions (e.g. Latham & Ricklefs
1993; Fine & Ree 2006). In contrast, geographic variation in
diversity within these regions is often strongly associated with
current climate. For example, evapotranspiration, a correlate
of productivity, is strongly correlated with tree species rich-
ness in North America (Currie & Paquin 1987). One climate-
based mechanism could be that warmer temperatures are
associated with, or perhaps cause higher speciation rates due
to, higher mutation or evolutionary rates at higher tempera-
tures (Rohde 1992; Allen et al. 2006). There have been few
empirical tests of this hypothesis to date (e.g. Wright et al.
2006; Bromham et al. 2015), and none that distinguish
whether higher evolutionary rates are caused directly by
higher temperature. The fact that some diversity gradients are
unrelated to temperature such as soil bacterial diversity which
responds primarily to pH (Fierer & Jackson 2006), implies
that the variation in species richness along contemporary envi-
ronmental gradients within regions may require a more gen-
eral explanation.
We argue that quantification of environmental harshness
may provide an overarching explanation for geographic varia-
tion in diversity within a region for any particular functionally
similar group of species, such as trees. Swenson et al. (2012)
and Stahl et al. (2014) have shown that environmental harsh-
ness limits the range of species traits that are viable in trees.
Kleidon & Mooney (2000) have proposed that harshness lim-
its diversity by limiting the range of species traits that are
viable. However, a test of this hypothesis requires an opera-
tional definition of harshness that allows it to be observed
and measured in a way that is independent of diversity. It is
well known that climate variables, such as the minimum tem-
perature occurring in an area, and evapotranspiration, as well
as soil fertility, and other environmental factors influence
growth and mortality rates. However, although each of these
factors contributes to harshness, no single environmental fac-
tor is a measure of harshness. We propose that harshness can
©2016 John Wiley & Sons Ltd/CNRS
Ecology Letters, (2016) doi: 10.1111/ele.12608
be defined operationally in terms of a measurable physiologi-
cal or morphological trait that represents the integrated
response of individuals to the full suite of the factors that
contribute to harshness. We pursued this approach for tree
species richness, and test its predictions for alpha diversity (lo-
cal, within-site), beta diversity (turnover among sites) and
gamma diversity (total diversity at larger spatial scales) in
both eastern and western North America.
We propose that the maximum tree height observed among
many mature adult individuals at a given site is a useful inte-
grative indicator of the environmental suitability of that site
for trees. Foresters call the maximum height attained by a
given tree species at a given age the ‘site index’, which they
use as a measure of site quality. A tree’s height is critically
important for its acquisition of light, and thus for its growth,
survival, reproduction, competitive ability and fitness (King
1990). The relative harshness of environmental conditions
places constraints on the maximum tree height (Givnish et al.
2014; Stahl et al. 2014). Higher salinity, high wind exposure,
colder temperatures, shorter growing season, drought, nutrient
deficiency and soil anoxia all reduce maximum tree stature.
Mangroves, for example, experience multiple challenges: main-
taining mechanical integrity on soft ground in the face of
wind and wave exposure, and tolerating both high salinity
and daily flooding. Mangrove forests are less species rich and
of lower stature than nearby freshwater swamps, which in
turn are less species rich and shorter than nearby upland for-
ests (Beard 1955). Indeed, there are no mangrove forests at all
in temperate climates with significant freezing stress (Keogh
et al. 1999). For the tree life form, maximum height integrates
the interacting effects of multiple environmental limiting fac-
tors into a single easily measured index of environmental
harshness.
The total niche space that allows persistence of the tree life
form can be conceived as a volume in multidimensional space,
one of whose dimensions is height, with different species hav-
ing different mature heights that could range from the small-
est height defined as a tree, to the maximal height observed in
an area (Westoby et al. 2002). Other axes of importance likely
include differences in seed mass, leaf life span, wood density
and so on (Westoby et al. 2002). Clearly, the larger the maxi-
mum tree height that is possible in an area, the larger would
be the potential niche breadth along the mature height axis.
Furthermore, we suggest that for other niche axes as well, the
range of feasible possibilities in a site depends on the suitabil-
ity of the site for trees in general, which is indicated by maxi-
mum tree height. If as a first approximation, we assume that
a new species is more likely to form (speciate) within a region
if its niche differs by at least a certain minimal amount from
the niches of its competitors, then regions with less niche
space (from greater harshness) should equilibrate at a lower
tree diversity than those with greater niche space (Tilman
2004). This perspective assumes that multispecies coexistence
is promoted by tradeoffs among these life history strategies,
and predicts that tree species richness should be approxi-
mately proportional to niche volume in this multidimensional
space and thus scale with tree height (Loehle 2000). Tradeoffs
between competitiveness under benign conditions caused by
greater height growth rates vs. traits that permit survival
under harsh conditions might also explain why many stress
tolerant species are rare or absent from more benign habitats,
thereby contributing to beta diversity (Loehle 1998; Koehler
et al. 2012; Savage & Cavender-Bares 2013).
Apart from variation in the size of the tree niche space
along harshness gradients, there may be asymmetries in
extinction and colonisation if conditions vary over time, as
they do in glacial cycles for example (Slobodkin & Sanders
1969). Specifically, if environmental conditions become
harsher, species lacking the appropriate stress tolerance would
be more likely to go extinct locally (and globally if migration
to areas with appropriate climate was not possible), while
other species that had sufficient stress tolerance would likely
survive (Latham & Ricklefs 1993; Svenning 2003; Willis et al.
2007). When more benign conditions returned, the surviving
species would quickly colonise less harsh habitats from which
they are no longer excluded by competition from the now
extinct species. By contrast, if there were empty niches on the
harsh end of conditions, it would take substantial evolution-
ary change before species lacking stress tolerance could adapt
to the harsher conditions and colonise (Ricklefs et al. 2006;
Zanne et al. 2014). Indeed in trees, ancient lineages are associ-
ated with harsh environments (Page 2004) and few tropical
lineages have expanded into temperate parts of North Amer-
ica (Fine 2001). Moreover, despite several hundred millions of
years of evolution, no trees have evolved the ability to survive
and reproduce in the extremely harsh conditions of high alti-
tude and latitude, as evidenced by the existence of tree lines.
Tree lines demonstrate that no species of trees can survive
and reproduce beyond a certain coldness or harshness, condi-
tions under which century old outlier trees may be only a
metre tall (Wieser & Tausz 2007).
If these assumptions are a reasonable caricature of nature,
one would expect that, within a biogeographic region, the
maximum tree height at a site with a mature stand would be
a strong correlate of alpha species richness at that site. Like-
wise, the height of the tallest trees in a given forested sam-
pling site would be a reasonable predictor of alpha tree
species richness, even though on some plots the trees will not
yet be fully mature. Other measures of environmental harsh-
ness such as productivity, minimum temperatures and preva-
lence of shade, drought and soil water logging would also be
good predictors of alpha tree species richness. Furthermore,
maximum height and species richness would be expected to
show similar, but less robust, responses to these components
of harshness. Similarly, the maximum tree height occurring
across all plots in a region would be a correlate of total regio-
nal species richness. The relationship between maximum
height and richness should be stronger at the regional scale
because this would decrease the effect of plots in which the
tallest tree had not yet reached its full height. Finally, beta
diversity is expected to be positively related to environmental
heterogeneity among plots, as quantified by the range in mea-
sures of environmental harshness among plots. Our results
show that tree height is indeed a consistent index of environ-
mental harshness and a strong predictor of tree species rich-
ness in eastern and western North America, lending support
to the environmental harshness hypothesis as an explanation
of diversity gradients within regions.
©2016 John Wiley & Sons Ltd/CNRS
2C. O. Marks, H. C. Muller-Landau, and D. Tilman Letter
METHODS
We investigated the empirical relationships among species
richness, maximum tree height and multiple environmental
stress measures using the Forest Inventory and Analysis
(FIA) data gathered by the USDA Forest Service across a
network of monitoring plots covering the lower 48 states of
the USA (Forest Service U.S.D.A. 2008). We separately anal-
ysed data for two biogeographic regions: eastern North Amer-
ica and western North America (regions east and west of the
Prairies). For our purposes, we have defined biogeographic
regions as regions that have been sufficiently isolated from
each other for long enough to have separate species pools (i.e.
share few species), and that lack major internal barriers to
species dispersal that could have inhibited species from reach-
ing habitats where they could potentially survive and compete.
Eastern and western North America forests meet these criteria
for separate biogeographic regions (Tiffney & Manchester
2001; Donoghue & Smith 2004). Only 24 of the 348 species in
the FIA data we used were common to both regions several
of which are species introduced by people. We did not include
Alaska in the final analysis because Alaska was disjunct from
the rest of the region covered (preliminary analyses that
included Alaska produced qualitatively and quantitatively
similar results). We used the FIA data from the latest com-
pleted inventory cycle in each of the lower 48 conterminous
United States as of February 24, 2015. The FIA data may be
downloaded from the USDA Forest Service website (Forest
Service U.S.D.A. 2015).
Each FIA plot has a total area of 672 m
2
and consists of
four subplots arranged in a cluster, with one central plot and
three peripheral plots spaced 120°apart in a 36.5-m radius
circle around the central plot (Forest Service U.S.D.A. 2008).
We included only data from plots in natural stands with at
least eight trees per plot, for a total of 54 906 plots in eastern
North America and 27 788 plots in western North America.
Natural stands refer to plots that are forested (as defined by
the FIA manual), and lacking any evidence of artificial regen-
eration (i.e. trees were not planted). Trees were defined as
individuals which are 12.7 cm diameter or greater at breast
height (dbh) and alive at the time of census. For each plot, we
determined maximum tree height and (alpha) tree species rich-
ness. The FIA data also included plot productivity class (For-
est Service U.S.D.A. 2008).
We compiled information from multiple sources to obtain
various measures of environmental harshness for each plot.
From the FIA data themselves we used plot productivity
class, a measure of favourability (the opposite of harshness)
(Forest Service U.S.D.A. 2008). As a measure of cold stress,
we calculated plot-specific means of January minimum daily
temperatures between 1981 and 2010 (PRISM Climate Group
2012). Using tree species composition combined with species
tolerance values from a recent meta-analysis (Niinemets &
Valladares 2006), we calculated the abundance-weighted aver-
age shade tolerance, average drought tolerance and average
soil water logging tolerance for each plot. We note that each
of these quantities has limitations as a measure of harshness,
and should be taken as at best an imperfect indicator of one
or more stresses. For example, the species tolerance values are
imperfect measures of the underlying true species environmen-
tal tolerances, and plot averages of these are indirect and even
more imperfect measures of associated environmental stresses.
We analysed the among-plot relationships of alpha species
richness with each of the other variables by first fitting a cubic
spline for alpha species richness as a function of the indepen-
dent variable and then calculating Pearson correlation coeffi-
cients between fitted and observed plot-level values, with
separate analyses for eastern North America and western
North America. We similarly analysed the relationship of
maximum height with productivity and other measures
of harshness, and of productivity to the remaining measures
of harshness. All cubic splines were fitted using the ‘smooth.s-
pline’ function in the statistical software R using the default
settings for that function (number of knots =number of data
points^0.2; and knots are evenly spaced). More generally, all
statistical analyses and graphs were done in R, version 3.0.2
(R Core Team 2013).
To analyse gamma (regional) scale values, we aggregated
data into grid cells by converting plot latitude and longitude
to an Albers equal area conic projection with the standard
parallels used by the USGS (Snyder 1982), and assigned plots
to grid cells with dimensions of 100 9100 km
2
. For each grid
cell, we calculated gamma species richness and average alpha
species richness, as well as minimum, mean, maximum and
range in the explanatory variables from plot data. To avoid
bias due to regional differences in sampling intensity in the
FIA data, grid cell values were calculated by taking the mean
of 1000 repeat samples of 100 plots from each grid cell,
including only cells with over 100 plots (230 grid cells in east-
ern North America, 116 in western North America). Sampling
was done without replacement (i.e. each sample of 100 plots
did not include any plots more than once). Within each
region, we analysed the bivariate relationships of grid cell
mean alpha species richness with grid cell means of maximum
height and other measures of harshness like minimum January
temperature by fitting separate cubic splines for each case,
and then calculating Pearson correlation coefficients (rvalues)
between the fitted and observed richness values. We similarly
analysed the relationships of gamma species richness with grid
cell extreme favourable values of harshness measures (i.e.
maxima of max height, productivity class and minimum Jan-
uary temperature, and minima of shade tolerance, drought
tolerance and water logging tolerance).
For each grid cell, we calculated beta species richness by
dividing gamma by average alpha species richness (Whittaker
1965). We then analysed the relationship of beta species rich-
ness with the grid cell ranges of maximum height, productivity
and other measures of harshness, using cubic splines and
Pearson correlation coefficients as above. Note that range and
maxima are highly correlated for most of these variables.
We tested various alternative methods to assess the robust-
ness of our results. For example, we tried including all plots
in grid cells, or including all grid cells regardless of number of
plots, or using a different minimum tree number per plot for
plot inclusion in the analyses. These variations led to only
slight changes in rvalues, leaving the qualitative results
entirely the same, and thus we report only the main results
here.
©2016 John Wiley & Sons Ltd/CNRS
Letter Tree diversity and tree height 3
RESULTS
Within each region, alpha (plot level) tree species richness was
highly significantly associated with plot maximum tree height
(Fig. 1). Consistent with the harshness hypothesis, productiv-
ity, January mean of minimum daily temperatures as well as
average plot shade, drought and water logging tolerance also
were correlated with alpha species richness (Fig. 1). These
relationships become stronger when using the 100 9100 km
2
grid cell mean of plot species richness and environmental vari-
ables (Fig. 2). Species richness was a monotonically increasing
function of maximum height and productivity, with a stronger
relationship for height than for productivity. In contrast,
alpha species richness sometimes showed a hump-shaped
response to minimum January temperature, mean shade toler-
ance, mean drought tolerance and mean water logging toler-
ance (Figs 1 and 2). In general, maximum tree height and tree
richness were similarly related to environmental variables in
qualitatively similar ways as richness (Fig. 1). In eastern
North America, maximum height had the highest explanatory
power for richness of any variable examined; in western
North America, unimodal relationships with shade tolerance
and water logging tolerance were slightly better predictors. If
analyses allowed only linear or monotonic relationships, then
maximum height would always be the strongest predictor of
alpha tree species richness.
In both regions, gamma species richness increased strongly
and almost linearly with maximum tree height (Fig. 3). Grid
cells with harsher environments as indicated by lower produc-
tivity, colder winter temperatures, shade, drought or water
logging had lower gamma species richness (Fig. 3). Grid cell
maximum height showed very similar responses to environ-
mental variation as did gamma species richness (Fig. 3). In
both regions, the maximum of maximum height was the sec-
ond-best predictor of gamma species richness, after the maxi-
mum of January minimum temperature (Fig. 3).
Beta diversity was strongly related to ranges of various
harshness measures. It was especially well predicted by range
in drought tolerance and range in maximum tree height
(Fig. 4). Overall, species richness was greater in eastern North
America than in the drier western North America (Table 1),
as is also evident in all figures comparing the two.
DISCUSSION
The results for both regions support our hypotheses that,
within biogeographic regions, benign environments have
greater species richness than harsher environments, and that
maximum tree height serves as an indicator of environmental
favourability for trees. Maximum tree height consistently
explains the variation in both alpha and gamma diversity as
indicated by the monotonic relationships in Figs 2 and 3.
Figure 1 Variation in alpha tree species richness among forest inventory plots in both eastern North America (red solid) and western North America (black
dotted) is explained in part by measures of environmental harshness: maximum tree height, productivity class, maximum tree diameter, mean of the
January daily minimum temperature, mean shade tolerance, mean drought tolerance and mean soil water-logging tolerance of the trees. Alpha species
richness and maximum tree height show qualitatively similar relationships with the other measures of environmental harshness, relationships shared with a
lesser degree by productivity. Lines are cubic splines with associated Pearson correlation coefficients, r. FIA productivity classes estimate the potential for
sites to produce wood in m
3
ha
1
year
1
based on mean basal area increment, and are as follows: 1 =01.39, 2 =1.403.50, 3 =3.505.94, 4 =5.958.39,
5=8.4011.54, 6 =11.5515.73, 7 15.74. See FIA manual for details (Forest Service U.S.D.A. 2008). Dots for individual plots not shown because of the
large number of field plots.
©2016 John Wiley & Sons Ltd/CNRS
4C. O. Marks, H. C. Muller-Landau, and D. Tilman Letter
Indeed, the relationships between maximum height and aver-
age alpha and gamma diversity shown in Figs 2 and 3 are as
tight as any reported in the diversity gradients literature (e.g.
Currie 1991), although correlation coefficients are not directly
comparable because of differences between studies in the geo-
graphic scale of analysis. The correlations are less strong at
the plot scale (Fig. 1), as would be expected given that not all
plots have mature trees to provide an accurate estimate of the
potential maximum tree height. This measurement error is
reduced when looking at patterns at the grid cell scale, which
are therefore much stronger (Figs 2 and 3).
Overall when examining results at local and landscape
scales, tree height was the best predictor of tree species rich-
ness, integrating effects of multiple stressors whose relative
importance varied with scale. At the local plot scale, shade
tolerance and water logging tolerance were slightly better pre-
dictors in western North America, but only because our
spline-based analysis allowed hump-shaped (i.e. unimodal)
relationships. If we had restricted the analysis to monotonic
relationships, maximum tree height would have been the
strongest predictor by far. Tree height remained the better
predictor in eastern North America and at grid cell scales. At
the grid cell scale, minimum January temperature was a
slightly better (and monotonic) predictor of richness than was
height, consistent with the importance of species cold hardi-
ness in determining range limits (Loehle 1998; Koehler et al.
2012), but it had clearly lower explanatory power at local
scales. At both plot and grid cell scales, the response of spe-
cies richness to individual measures of harshness took the
same form as the response of maximum height to these mea-
sures, even as the relationships changed with scale. In
particular, height echoes the hump-shaped responses of rich-
ness to shade, drought and waterlogged soils at the alpha
scale. These hump-shaped relationships are to be expected
given that droughty sites tend to have little water logging and
vice versa, and similarly dry sites are unable to support suffi-
cient foliage to create deep shade. The strength of investigat-
ing tree species richness gradients through the lens of
maximum tree height lies in this consistency for height to
respond in parallel to richness on multiple environmental gra-
dients including at the more difficult to predict smaller spatial
scales.
The relationships with minimum January temperature, in
particular, nicely illustrate the strengths of tree height as an
integrative measure. At the plot scale, species richness shows
an anomalous decline at the mildest temperatures in eastern
North America. Notably, maximum height also declines in
this temperature range in eastern North America, and thus
maintains a monotonic relationship with richness (Fig. 1). At
the grid cell scale, species richness and maximum height both
plateau with respect to minimum temperature in the same
temperature range in eastern North America, and again main-
tain a monotonic relationship with each other (Fig. 3a). An
examination of the data suggests that the anomalous decline
in both species richness and maximum tree height at the mild-
est winter temperatures in eastern North America is at least in
a part due to increases in water logging in southern Florida, a
landscape dominated by the vast Everglades wetlands (results
not shown). Flooding and soil water logging cause anoxia in
the root zone which inhibits tree growth and thus reduces
height (crown dieback is a common symptom of flood stressed
trees). Long duration soil anoxia can cause tree mortality,
Figure 2 Variation among 100 9100 km
2
grid cells in mean alpha tree species richness in both eastern North America (upper panels) and western North
America (lower panels) is strongly explained by environmental harshness, specifically grid cell means of the following: maximum tree height, productivity
class, January mean of the daily minimum temperature and plot mean shade tolerance, drought tolerance, and soil water-logging tolerance. Lines are cubic
splines with associated Pearson correlation coefficients, r. Each dot represents one grid cell. See Fig. 1 for productivity classes.
©2016 John Wiley & Sons Ltd/CNRS
Letter Tree diversity and tree height 5
especially in seedlings of species lacking appropriate adapta-
tions to cope with a lack of oxygen, and hence acts as an eco-
logical filter reducing species richness. The strong seasonality
of precipitation in Florida can bring additional stresses like
drought and fire. In other words although conditions are
becoming less harsh with respect to temperature in southern
Florida, they are becoming harsher with respect to other
conditions. Regardless of the explanation for this specific geo-
graphic pattern, it is clear that maximum height as a predictor
of tree species richness appears to be robust to the idiosyn-
crasies of the geography of a particular region.
As would be expected, beta diversity was well predicted by
the range in stress measures, especially the range in maximum
tree height and the range in drought tolerance (Fig. 3). This
Figure 3 Variation in total species richness among 100 9100 km
2
grid cells in eastern North America (A) and western North America (B) is strongly
explained by environmental harshness as quantified by the following: maxima of tree height, productivity class, and January mean of the daily minimum
temperature and minima of plot mean shade tolerance, drought tolerance and soil water-logging tolerance. Maximum tree height shows a qualitatively
similar response. Lines are cubic splines with associated Pearson correlation coefficients, r. Each dot represents one grid cell. See Fig. 2 for productivity
classes.
©2016 John Wiley & Sons Ltd/CNRS
6C. O. Marks, H. C. Muller-Landau, and D. Tilman Letter
result confirms the well-established importance of soil mois-
ture gradients to tree species distributions and thus also beta
diversity in both eastern North America (Curtis & McIntosh
1951; Whittaker 1956) and western North America (Waring &
Major 1964; Whittaker & Niering 1964). This turnover in tree
species along moisture gradients has been shown to be related
to species physiological traits in both temperate (Cavender-
Bares et al. 2004) and tropical forests (Engelbrecht et al.
2007) consistent with our hypothesis.
These data suggest a potential hierarchy of stress gradients.
Temperate tree species diversity at the largest scale examined
here (gamma) is most strongly related to minimum winter
temperatures, species turnover within regions (beta) is most
strongly related to soil moisture and richness at the local plot
scale (alpha) is most strongly related to maximum tree height,
which integrates the effects of multiple stresses. These results
are consistent with the intuitively appealing idea that, at
increasingly small spatial scales, many different types of envi-
ronmental stresses act simultaneously, causing local species
richness to depend on the cumulative effect of these multiple
ecological filters as embodied in maximum tree height.
Results in the two biogeographic regions were qualitatively
similar, but with some substantial quantitative differences:
eastern North America had a larger species pool and higher
species richness at all scales (Table 1), while western North
America had greater tree heights (Fig. 1). Given the strength
of these regional differences, the strong positive relationships
between height and tree species richness observed within
regions are obscured when data for the two regions are com-
bined. Differences in the size of the tree species pool among
biogeographic regions can be explained by geologic history
and its influences on past and present area under similar cli-
matic conditions (Latham & Ricklefs 1993; Fine & Ree 2006).
Western North America once supported species rich meso-
phytic deciduous broadleaf forests dominated by Angiosperm
tree species similar to the deciduous forest species that survive
in eastern North America and eastern Asia. These mesic and
hydric deciduous species disappeared from western North
America during the tertiary, a time of climatic drying and
orographic change in western North America (Tiffney &
Manchester 2001; Donoghue & Smith 2004). Despite this loss
of mesic species in western North America, its species richness
today is still greater in taller, more mesic forests. This pattern
suggests that within biogeographic regions there is a determin-
istic process of ecological sorting and/or evolutionary change
that creates species richness distributions which are positively
correlated with environmental favourability and maximum
tree height in spite of the vicissitudes of geologic history.
Figure 4 Variation among 100 9100 km
2
grid cells in beta species richness (species turnover among plots) in both eastern North America (upper panels)
and western North America (lower panels) is strongly related to ranges of environmental harshness variables within grid cells: specifically, ranges of
maximum tree height, productivity class, January mean of the daily minimum temperature and mean shade tolerance, drought tolerance and soil water-
logging tolerance of the trees on the plot. Lines are cubic splines, with associated Pearson correlation coefficients, r. Each dot represents one grid cell. See
Fig. 2 for productivity classes.
Table 1 Tree species richness statistics from the Forest Inventory Analysis
data for the eastern and western coterminous United States, here referred
to as eastern North America and western North America respectively
Tree species richness Eastern North America Western North America
Mean alpha 5.8 2.7
Mean beta 7.9 5.6
Mean gamma 46.9 15.8
Region total 237 135
©2016 John Wiley & Sons Ltd/CNRS
Letter Tree diversity and tree height 7
The strength of evidence supporting the environmental
harshness hypothesis of species diversity gradients in North
American trees suggests that it merits more research in future.
An obvious next step would be to investigate if there are con-
sistent relationships between tree height and richness, and
environmental harshness in other parts of the world. Simi-
larly, can analogues to height in trees as an integrated mea-
sure of harshness be found for other life forms? Given that
historical factors clearly play a role in explaining diversity dif-
ferences among biogeographic regions, it would be interesting
to see whether they sometimes are sufficiently important as to
overcome the expected harshness induced patterns within
regions, for example in Europe where recent glacial cycles
have caused more tree species extinctions than in East Asia or
North America (Svenning 2003). Another important future
direction is investigating the mechanisms that could contribute
to diversity variation on harshness gradients, in particular the
potential roles of ecological sorting, evolutionary change and
net diversification. We have suggested that more benign envi-
ronments permit a broader range in heights and other traits
resulting in more niches favouring coexistence of more species.
This hypothesis could be tested with forest stand simulation
models for different types of forests within a region. Investiga-
tion of life history tradeoffs related to mature tree height and
height growth rate could further elucidate the underlying
mechanisms. We also proposed that there is an asymmetry
with respect to colonisation. Specifically, stress tolerant lin-
eages can easily colonise unoccupied niches in benign environ-
ments, whereas lineages from benign environments would first
need to evolve traits permitting growth and survival under
stressful conditions before they could colonise harsh environ-
ments. Since adaptation to more stressful conditions is a slow
process, we expect diversification rates to be higher in benign
environments (as indicated by greater mature tree heights and
height growth rates), a hypothesis that could be investigated
with phylogenetic methods.
Our findings suggest that maximum tree height is an effec-
tive integrator of the effects of environmental harshness, and
that harshness limits diversity. Maximum height is a single
easily measured trait that is a surprisingly strong predictor of
tree species richness. As such it has the potential to help with
conservation planning which is increasingly based on models
because actual species data are often lacking at the spatial res-
olution of individual land parcels. Proxy data layers available
as model inputs in GIS (geographic information systems) such
as elevation, geology and climate have been used to insure
inclusion of beta diversity in conservation priority area maps
(e.g. Anderson & Ferree 2010), but complementary
approaches to identify individual land parcels with relatively
high alpha diversity within a region are not as well developed.
Forests with high alpha diversity are of conservation impor-
tance because remaining forest patches in anthropogenic land-
scapes in general and protected forests in particular are
disproportionately shorter stands located in harsher less pro-
ductive environments (Sanderson et al. 2015), which tend to
also have lower alpha species richness, as we showed. Tree
canopy height can be easily mapped over large areas at high
spatial resolution with LiDAR (e.g. Lefsky 2010), and could
help identify forest parcels with potentially high alpha species
richness. Descriptions of forests in other biogeographic
regions suggest that empirical studies within these regions will
also reveal a monotonic relationship between maximum tree
height and tree species richness (e.g. Beard 1955). The general-
ity of harshness gradients limiting both tree height and species
richness implies that an improved understanding of the physi-
ological and life history tradeoffs between coping with stress-
ful environments and maximising tree height could also make
important contributions to developing an improved under-
standing of how communities of forest trees are assembled
and gradients in tree species richness emerge.
ACKNOWLEDGEMENTS
We thank Joe Wright, two anonymous reviewers and editors
Tim Coulson and Richard Bardgett for comments on earlier
drafts, and Charles ‘Hobie’ Perry for help with the FIA data.
HM and DT thank the participants in the fall 2004 graduate
seminar on Temperate and Tropical Plant Community Diver-
sity at the University of Minnesota for stimulating discus-
sions. CM is also grateful to Martin J. Lechowicz for earlier
thought-provoking discussions. We gratefully acknowledge the
financial support of the University of Minnesota, the David
and Lucile Packard Foundation and Le Fonds Qu
eb
ecois de
la Recherche sur la Nature et les Technologies.
AUTHORSHIP
CM originated the idea for the study in discussion with HM
and DT. CM performed the analyses and wrote the first draft
of the manuscript, and all authors contributed substantially to
revisions.
REFERENCES
Allen, A.P., Gillooly, J.F., Savage, V.M. & Brown, J.H. (2006). Kinetic
effects of temperature on rates of genetic divergence and speciation.
Proc. Natl Acad. Sci. USA, 103, 91309135.
Anderson, M.G. & Ferree, C.E. (2010). Conserving the stage: climate
change and the geophysical underpinnings of species diversity. PLoS
ONE, 5, e11554.
Beard, J.S. (1955). The classification of tropical American vegetation-
types. Ecology, 36, 89100.
Bromham, L., Hua, X., Lanfear, R. & Cowman, P.F. (2015). Exploring
the relationships between mutation rates, life history, genome size,
environment, and species richness in flowering plants. Am. Nat., 185,
507524.
Cavender-Bares, J., Kitajima, K. & Bazzaz, F.A. (2004). Multiple trait
associations in relation to habitat differentiation among 17 Floridian
oak species. Ecol. Monogr., 74, 635662.
Currie, D.J. (1991). Energy and large-scale patterns of animal- and plant-
species richness. Am. Nat., 137, 2749.
Currie, D.J. & Paquin, V. (1987). Large-scale biogeographical patterns of
species richness of trees. Nature, 329, 326327.
Curtis, J.T. & McIntosh, R.P. (1951). An upland forest continuum in the
prairie-forest border region of Wisconsin. Ecology, 32, 476496.
Donoghue, M.J. & Smith, S.A. (2004). Patterns in the assembly of
temperate forests around the Northern Hemisphere. Philos. Trans.
Royal Soc. B-Biol. Sci., 359, 16331644.
Engelbrecht, B.M.J., Comita, L.S., Condit, R., Kursar, T.A., Tyree,
M.T., Turner, B.L. et al. (2007). Drought sensitivity shapes species
distribution patterns in tropical forests. Nature, 447, 8082.
©2016 John Wiley & Sons Ltd/CNRS
8C. O. Marks, H. C. Muller-Landau, and D. Tilman Letter
Fierer, N. & Jackson, R.B. (2006). The diversity and biogeography of soil
bacterial communities. Proc. Natl Acad. Sci. USA, 103, 626631.
Fine, P.V.A. (2001). An evaluation of the geographic area hypothesis
using the latitudinal gradient in North American tree diversity. Evol.
Ecol. Res., 3, 413428.
Fine, P.V.A. & Ree, R.H. (2006). Evidence for a time-integrated species-
area effect on the latitudinal gradient in tree diversity. Am. Nat., 168,
796804.
Forest Service U.S.D.A. (2008). The Forest Inventory and Analysis
Database: Database Description and Users Manual Version 3.0 for
Phase 2. p. 243.
Forest Service U.S.D.A. (2015). Forest Inventory and Analysis National
Program. Available at: http://www.fia.fs.fed.us/tools-data/. (Last
accessed 24 February 2015).
Givnish, T.J., Wong, S.C., Stuart-Williams, H., Holloway-Phillips, M. &
Farquhar, G.D. (2014). Determinants of maximum tree height in
Eucalyptus species along a rainfall gradient in Victoria, Australia.
Ecology, 95, 29913007.
Keogh, T.M., Keddy, P.A. & Fraser, L.H. (1999). Patterns of tree species
richness in forested wetlands. Wetlands, 19, 639647.
King, D.A. (1990). The adaptive significance of tree height. Am. Nat.,
135, 809827.
Kleidon, A. & Mooney, H.A. (2000). A global distribution of biodiversity
inferred from climatic constraints: results from a process-based
modelling study. Global Change Biol., 6, 507523.
Koehler, K., Center, A. & Cavender-Bares, J. (2012). Evidence for a
freezing tolerancegrowth rate trade-off in the live oaks (Quercus series
Virentes) across the tropicaltemperate divide. New Phytol., 193, 730
744.
Latham, R.E. & Ricklefs, R.E. (1993). Global patterns of tree species
richness in moist forests: energy-diversity theory does not account for
variation in species richness. Oikos, 000, 325333.
Lefsky, M.A. (2010). A global forest canopy height map from the
Moderate Resolution Imaging Spectroradiometer and the Geoscience
Laser Altimeter System. Geophys. Res. Lett., 37, 15.
Loehle, C. (1998). Height growth rate tradeoffs determine northern and
southern range limits for trees. J. Biogeogr., 25, 735742.
Loehle, C. (2000). Strategy space and the disturbance spectrum: a life-
history model for tree species coexistence. Am. Nat., 156, 1433.
Mittelbach, G.G., Schemske, D.W., Cornell, H.V., Allen, A.P., Brown,
J.K., Bush, M.B. et al. (2007). Evolution and the latitudinal diversity
gradient: speciation, extinction and biogeography. Ecol. Lett., 10, 315
331.
Niinemets,
U. & Valladares, F. (2006). Tolerance to shade, drought, and
waterlogging of temperate northern hemisphere trees and shrubs. Ecol.
Monogr., 76, 521547.
Page, C.N. (2004). Adaptive ancientness of vascular plants to exploitation
of low-nutrient substrates a neobotanical overview. In: The Evolution
of Plant Physiology from Whole Plants to Ecosystems (eds Hemsley,
A.R. & Poole, I.). Elsevier Academic Press, London, UK, pp. 447466.
PRISM Climate Group (2012). Climate Data for USA. Available at:
http://www.prismclimate.org. (Last accessed 12 September 2013).
R Core Team (2013). R: A Language and Environment for Statistical
Computing. R Foundation for Statistical Computing, Vienna, Austria.
Ricklefs, R.E., Schwarzbach, A.E. & Renner, S.S. (2006). Rate of lineage
origin explains the diversity anomaly in the world’s mangrove
vegetation. Am. Nat., 168, 805810.
Rohde, K. (1992). Latitudinal gradients in species diversity: the search for
the primary cause. Oikos, 65, 514527.
Sanderson, E.W., Segan, D.B. & Watson, J.E.M. (2015). Global status of
and prospects for protection of terrestrial geophysical diversity.
Conserv. Biol., 29, 649656.
Savage, J.A. & Cavender-Bares, J. (2013). Phenological cues drive an
apparent trade-off between freezing tolerance and growth in the family
Salicaceae. Ecology, 94, 17081717.
Slobodkin, L.B. & Sanders, H.L. (1969). On the contribution of
environmental predictability to species diversity. Brookhaven Symp.
Biol., 22, 8295.
Snyder, J.P. (1982). Map projections used by the U.S. Geological Survey.
United States Government Printing Office, Washington, DC, USA.
Stahl, U., Reu, B. & Wirth, C. (2014). Predicting species’ range limits
from functional traits for the tree flora of North America. Proc. Natl
Acad. Sci. USA, 111, 1373913744.
Svenning, J.-C. (2003). Deterministic Plio-Pleistocene extinctions in the
European cool-temperate flora. Ecol. Lett., 6, 646653.
Swenson, N.G., Enquist, B.J., Pither, J., Kerkhoff, A.J., Boyle, B.,
Weiser, M.D. et al. (2012). The biogeography and filtering of woody
plant functional diversity in North and South America. Global Ecol.
Biogeogr., 21, 798808.
Tiffney, B.H. & Manchester, S.R. (2001). The use of geological and
paleontological evidence in evaluating plant phylogeographic
hypotheses in the Northern Hemisphere tertiary. Int. J. Plant Sci., 162,
S3S17.
Tilman, D. (2004). Niche tradeoffs, neutrality, and community structure:
a stochastic theory of resource competition, invasion, and community
assembly. Proc. Natl Acad. Sci. USA, 101, 1085410861.
Waring, R.H. & Major, J. (1964). Some vegetation of the California
coastal redwood region in relation to gradients of moisture, nutrients,
light, and temperature. Ecol. Monogr., 34, 167215.
Westoby, M., Falster, D.S., Moles, A.T., Vesk, P.A. & Wright, I.J.
(2002). Plant ecological strategies: some leading dimensions of variation
between species. Annu. Rev. Ecol. Syst., 33, 125159.
Whittaker, R.H. (1956). Vegetation of the great smoky mountains. Ecol.
Monogr., 26, 180.
Whittaker, R.H. (1965). Dominance and diversity in land plant
communities. Science, 147, 250260.
Whittaker, R.H. & Niering, W.A. (1964). Vegetation of the Santa
Catalina Mountains, Arizona. I. Ecological classification and
distribution of species. J. Arizona Acad. Sci.,3,934.
Wieser, G. & Tausz, M. (2007). Trees at Their Upper Limit: Treelife
Limitation at the Alpine Timberline. Springer, Dordrecht, The Netherlands.
Willis, K.J., Kleczkowski, A., New, M. & Whittaker, R.J. (2007). Testing
the impact of climate variability on European plant diversity: 320
000 years of water-energy dynamics and its long-term influence on
plant taxonomic richness. Ecol. Lett., 10, 673679.
Wright, S., Keeling, J. & Gillman, L. (2006). The road from Santa
Rosalia: a faster tempo of evolution in tropical climates. Proc. Natl
Acad. Sci. USA, 103, 77187722.
Zanne, A.E., Tank, D.C., Cornwell, W.K., Eastman, J.M., Smith, S.A.,
FitzJohn, R.G. et al. (2014). Three keys to the radiation of
angiosperms into freezing environments. Nature, 506, 8992.
Editor, Richard Bardgett
Manuscript received 4 February 2016
First decision made 6 March 2016
Manuscript accepted 17 March 2016
©2016 John Wiley & Sons Ltd/CNRS
Letter Tree diversity and tree height 9
... However, environmental conditions are correlated with both biological communities and forest structure, which makes it difficult to assess the relationship between forest structure and species diversity and composition separating the environmental contribution (Castilho et al. 2006;Marciente et al. 2015;Schietti et al. 2016). Besides, the available studies investigating this relationship were carried out in temperate forests (Marks et al. 2016;Zellweger et al. 2016;Penone et al. 2018) and little explored in tropical forests (Bohlman 2015;Draper et al. 2021). ...
... Different tree strategies allow co-occurrence along a vertical light gradient, including the range of light conditions in the understory (Denslow 1980;Chazdon and Fetcher 1984;Poorter and Arets 2003;Poorter et al. 2005). The relationship between forest canopy height and vascular plant diversity has already been reported in Europe and North America (Zellweger et al. 2016;Marks et al. 2016), and our results mirror these findings on the local scale of tropical forests. ...
Preprint
Full-text available
Forest structure plays an important role in determining habitat suitability for plants and animals, but these relationships are poorly characterized for different biological communities in tropical forests. We used ground-based lidar to quantify structural metrics and determine their contribution in predicting species diversity and compositional changes between plots for nine biological groups in an Amazonian forest. For each group, we calculated Fisher's alpha index and summarized community composition using Principal Coordinates Analysis. As biological organisms may also react directly to hydro-edaphic conditions, we carried out variation partitioning analysis using linear regressions to disentangle the relative contribution of structural metrics and hydro-edaphic variables. Forest structure was related to species diversity and composition of some groups, specifically for plants, anurans, and birds. Mean canopy height, leaf area height volume, and skewness explained more than one-third of species diversity of palms and trees, with higher values relating to higher species diversity. Hydro-edaphic variables were the most important predictors of the main compositional axis for plant groups, but some structural metrics explained more than 30% of the secondary compositional axis for ferns + lycophytes, trees, birds, and anurans. Vegetation height and variability, vegetation quantity, and vertical structure, but not canopy openness, were the main structural characteristics modulating species diversity and composition. Our findings reinforce the potential to estimate species diversity and compositional changes across structural gradients using lidar-derived metrics in a hyper-diverse forest. Understanding these relationships advances our ability to make community predictions useful for conservation and provides new avenues to investigate the mechanisms impacting diversity.
... Based on our results, we can conclude that the most productive landscapes show lower spatial heterogeneity in terms of community composition in comparison with those where environmental conditions are less benign (e.g., dryer environments). This suggests that beta diversity increases with increasing environmental harshness, presumably through changes in the relative importance of stochastic versus deterministic processes, an idea that, to our knowledge, has hardly been explored in the literature (but see e.g., Marks, Muller-Landau, and Tilman 2016;García-Navas et al. 2021). Regarding this, we consistently found that beta diversity was driven by species replacement rather than species loss in the three analysed transects. ...
Article
Full-text available
Aim Variation in community composition along environmental gradients provides crucial information for identifying zones where species turnover is rapid and to ascertain whether compositional changes occur gradually or rather abruptly. We examined changes in bird community composition along three bioclimatic transects in Australia to test whether drivers of species turnover are consistent, rather than spatially contingent, across biologically contrasting ecosystems. We also detected potential transition zones associated with environmental thresholds and determined whether certain abiotic conditions promote a higher rate of community compositional turnover. Location Mainland Australia. Taxon Terrestrial birds. Methods We applied multivariate community analysis, generalised dissimilarity modelling (GDM) and threshold indicator taxa analysis (TITAN). Results We observed that environmental variables are better predictors of community composition than spatial distance, which indicates that species sorting, rather than dispersal, plays a key role in structuring Australian avian communities. Annual precipitation constitutes a key driver of species turnover regardless of the analysed transect. The most humid landscapes and those with a higher tree canopy show lower spatial heterogeneity in community composition compared to those with less benign environmental conditions (e.g., dryer environments). TITAN detected significant transition points and supported the results obtained using GDM, which suggests that bird composition change along the gradients is not monotonic. Main Conclusions Our results suggest that avian beta diversity increases with increasing environmental harshness, presumably through changes in the relative importance of stochastic versus deterministic processes. The obtained findings show that open forests and woodlands are extremely important ecosystems on this continent and deserve special attention in terms of conservation due to their vulnerability to global change. Lastly, this study exemplifies the value of combining community‐ and taxon‐based analyses to identify and interpret community thresholds, which can serve to pinpoint targets for preserving biodiversity.
... Ascertaining why maximum tree heights are generally lower in the Americas remains an intriguing question. Studies show that canopy level and tree heights are highest in areas with stable climates that have sufficient rainfall to offset transpiration (Banin et al., 2012;Givnish et al., 2014;Gorgens et al., 2020;Larjavaara, 2014;Mao et al., 2020;Marks et al., 2016;Venter et al., 2017), that is areas F I G U R E 2 Distribution of species maximum heights across the three biogeographical regions for all tree species (left) and the 10% tallest tree species (right). The middle line represents the median and the lower and upper boxes represent the first and third quartiles, respectively. ...
... These results are inconsistent with a previous study that highlighted that there was no significant correlation between mean height and tree species diversity (Chen and Niu 2020). In contrast, Marks et al. (2016) found a significant positive relationship between tree species diversity and mean height in North America, strengthening our findings (Fig. 4a). Additionally, mean height and canopy openness significantly affected microsite conditions (e.g., temperature, light availability, nutrients, and water content in soil) in the understory (Chen et al. 2024;Yin et al. 2024;Zhang et al. 2024), which were positively correlated with tree species diversity in our study (Figs. ...
Article
Full-text available
Although numerous studies have proposed explanations for the specific and relative effects of stand structure, plant diversity, and environmental conditions on carbon (C) storage in forest ecosystems, understanding how these factors collectively affect C storage in different community layers (trees, shrubs, and herbs) and forest types (mixed, broad-leaved (E), broad-leaved (M), and coniferous forest) continues to pose challenges. To address this, we used structural equation models to quantify the influence of biotic factors (mean DBH, mean height, maximum height, stem density, and basal area) and abiotic factors (elevation and canopy openness), as well as metrics of species diversity (Shannon-Wiener index, Simpson index, and Pielou's evenness) in various forest types. Our analysis revealed the critical roles of forest types and elevation in explaining a substantial portion of variability in C storage in the overstory layer, with a moderate influence of stand factors (mean DBH and basal area) and a slightly negative impact of tree species diversity (Shannon-Wiener index). Notably, forest height emerged as the primary predictor of C storage in the herb layer. Regression relationships further highlighted the significant contribution of tree species diversity to mean height, understory C storage, and branch biomass within the forest ecosystem. Our insights into tree species diversity, derived from structural equation modeling of C storage in the over-story, suggest that the effects of tree species diversity may be influenced by stem biomass in statistical reasoning within temperate forests. Further research should also integrate tree species diversity with tree components biomass, forest mean height, understory C, and canopy openness to understand complex relationships and maintain healthy and sustainable ecosystems in the face of global climate challenges.
... Ascertaining why maximum tree heights are generally lower in the Americas remains an intriguing question. Studies show that canopy level and tree heights are highest in areas with stable climates that have sufficient rainfall to offset transpiration (Banin et al., 2012;Givnish et al., 2014;Gorgens et al., 2020;Larjavaara, 2014;Mao et al., 2020;Marks et al., 2016;Venter et al., 2017), that is areas F I G U R E 2 Distribution of species maximum heights across the three biogeographical regions for all tree species (left) and the 10% tallest tree species (right). The middle line represents the median and the lower and upper boxes represent the first and third quartiles, respectively. ...
Article
Aim: We test the hypothesis that wind dispersal is more common among emergent tree species given that being tall increases the likelihood of effective seed dispersal. Location: Americas, Africa and the Asia-Pacific. Time period: 1970–2020. Major taxa studied: Gymnosperms and Angiosperms. Methods: We used a dataset consisting of tree inventories from 2821 plots across three biogeographic regions (Americas, Africa and Asia-Pacific), including dry and wet forests, to determine the maximum height and dispersal strategy of 5314 tree species. A web search was used to determine whether species were wind-dispersed. We compared differences in tree species maximum height between biogeographic regions and examined the relationship between species maximum height and wind dispersal using logistic regression. We also tested whether emergent tree species, that is species with at least one individual taller than the 95% height percentile in one or more plots, were disproportionally wind dispersed in dry and wet forests within each biogeographic region. Results: Our dataset provides maximum height values for 5314 tree species, of which more than half (2914) had no record of this trait in existing global databases. We found that, on average, tree species in the Americas have lower maximum heights compared to those in Africa and the Asia Pacific. The probability of wind dispersal increased significantly with tree species maximum height and was significantly higher among emergent than non-emergent tree species in both dry and wet forests in all three biogeographic regions. Main conclusion: Wind dispersal is more prevalent in tall, emergent tree species than in non-emergent species and may thus be an important factor in the evolution of tree species maximum height. By providing the most comprehensive dataset so far of tree species maximum height and wind dispersal strategies, this study paves the way for advancing our understanding of the eco-evolutionary drivers of tree size.
... Ascertaining why maximum tree heights are generally lower in the Americas remains an intriguing question. Studies show that canopy level and tree heights are highest in areas with stable climates that have sufficient rainfall to offset transpiration (Banin et al., 2012;Givnish et al., 2014;Gorgens et al., 2020;Larjavaara, 2014;Mao et al., 2020;Marks et al., 2016;Venter et al., 2017), that is areas F I G U R E 2 Distribution of species maximum heights across the three biogeographical regions for all tree species (left) and the 10% tallest tree species (right). The middle line represents the median and the lower and upper boxes represent the first and third quartiles, respectively. ...
Article
Aim: We test the hypothesis that wind dispersal is more common among emergent tree species given that being tall increases the likelihood of effective seed dispersal. Location: Americas, Africa and the Asia-Pacific. Time period: 1970–2020. Major taxa studied: Gymnosperms and Angiosperms. Methods: We used a dataset consisting of tree inventories from 2821 plots across three biogeographic regions (Americas, Africa and Asia-Pacific), including dry and wet forests, to determine the maximum height and dispersal strategy of 5314 tree species. A web search was used to determine whether species were wind-dispersed. We compared differences in tree species maximum height between biogeographic regions and examined the relationship between species maximum height and wind dispersal using logistic regression. We also tested whether emergent tree species, that is species with at least one individual taller than the 95% height percentile in one or more plots, were disproportionally wind dispersed in dry and wet forests within each biogeographic region. Results: Our dataset provides maximum height values for 5314 tree species, of which more than half (2914) had no record of this trait in existing global databases. We found that, on average, tree species in the Americas have lower maximum heights compared to those in Africa and the Asia Pacific. The probability of wind dispersal increased significantly with tree species maximum height and was significantly higher among emergent than non-emergent tree species in both dry and wet forests in all three biogeographic regions. Main conclusion: Wind dispersal is more prevalent in tall, emergent tree species than in non-emergent species and may thus be an important factor in the evolution of tree species maximum height. By providing the most comprehensive dataset so far of tree species maximum height and wind dispersal strategies, this study paves the way for advancing our understanding of the eco-evolutionary drivers of tree size.
... Ascertaining why maximum tree heights are generally lower in the Americas remains an intriguing question. Studies show that canopy level and tree heights are highest in areas with stable climates that have sufficient rainfall to offset transpiration (Banin et al., 2012;Givnish et al., 2014;Gorgens et al., 2020;Larjavaara, 2014;Mao et al., 2020;Marks et al., 2016;Venter et al., 2017), that is areas F I G U R E 2 Distribution of species maximum heights across the three biogeographical regions for all tree species (left) and the 10% tallest tree species (right). The middle line represents the median and the lower and upper boxes represent the first and third quartiles, respectively. ...
Article
We test the hypothesis that wind dispersal is more common among emergent tree species given that being tall increases the likelihood of effective seed dispersal.
Article
Management and conservation programs require accessible scientific evidence to effectively plan and achieve their goals. Facing regional and species-specific threats, Gonopterodendron sarmientoi, an endangered tree native to the Gran Chaco region, requires scientific evidence to support management and conservation actions, notably in forest management and territorial planning. To bridge this crucial gap, we developed indices that integrate genetic, evolutionary, morphological, threat-related, and ethnobotanical data concerning G. sarmientoi at a landscape level. Across twenty-four localities of this species, we have created four distinct indices. The first index amalgamates data on genetics, functional traits, and paleoclimate information. The second index incorporates data on threats stemming from deforestation and climate change scenarios. The third index provides supplementary insights into the species’ utilization by indigenous groups. Finally, the fourth index combines all the aforementioned data to prioritize the most valuable and threatened localities. Through this comprehensive approach, we have identified specific in-situ and ex-situ management and conservation actions for G. sarmientoi. This approach considers broader-scale conservation efforts and considers large-scale variations and processes. The indices effectively link scientific data with actionable insights, providing decision-makers involved with G. sarmientoi with concise information that encapsulates the most pertinent aspects of species conservation.
Article
Full-text available
An important aim of plant ecology is to identify leading dimensions of ecological variation among species and to understand the basis for them. Dimensions that can readily be measured would be especially useful, because they might offer a path towards improved worldwide synthesis across the thousands of field experiments and ecophysiological studies that use just a few species each. Four dimensions are reviewed here. The leaf mass per area-leaf lifespan (LMA-LL) dimension expresses slow turnover of plant parts (at high LMA and long LL), long nutrient residence times, and slow response to favorable growth conditions. The seed mass-seed output (SM-SO) dimension is an important predictor of dispersal to establishment opportunities (seed output) and of establishment success in the face of hazards (seed mass). The LMA-LL and SM-SO dimensions are each underpinned by a single, comprehensible tradeoff, and their consequences are fairly well understood. The leaf size-twig size (LS-TS) spectrum has obvious consequences for the texture of canopies, but the costs and benefits of large versus small leaf and twig size are poorly understood. The height dimension has universally been seen as ecologically important and included in ecological strategy schemes. Nevertheless, height includes several tradeoffs and adaptive elements, which ideally should be treated separately. Each of these four dimensions varies at the scales of climate zones and of site types within landscapes. This variation can be interpreted as adaptation to the physical environment. Each dimension also varies widely among coexisting species. Most likely this within-site variation arises because the ecological opportunities for each species depend strongly on which other species are present, in other words, because the set of species at a site is a stable mixture of strategies.
Book
Emerging from decades of intensive research into alpine timberlines, Trees at their Upper Limit presents a complete modern synthesis of current knowledge on the ecophysiology of tree growth and survival on high mountains in Europe. Including chapters on soil properties and the role or mycorrhiza, carbon assimilation and allocation, phytopathogens, and the impact of global change on photooxidative stress, the book builds on Tranquillini’s landmark 1979 publication, Physiological Ecology of the Alpine Timberline. By combining new techniques and insights with existing core knowledge the authors explore a range of current hypotheses on tree life limitation to promote a greater understanding of the underlying mechanisms determining the upper timberline. Amid growing realization that high elevation forests have a crucial role to play in protection against natural hazards, this book represents a timely contribution to the current literature on timberline research. Drawing together more than 25 years of work, this unique book sets a new standard on the ecophysiology of trees growing at the alpine timberline. Edited by field leaders Gerhard Wieser and Michael Tausz, the book will appeal to researchers and advanced students in the fields of botany, ecology and plant ecophysiology, as well as to a wider audience interested in understanding the responses of the timberline ecotone to climatic and demographic change.
Article
One hypothesis to explain the latitudinal gradient in species diversity is the geographic area hypothesis. This hypothesis posits that the size of a biome has considerable influence on its species diversity. Since the tropics are so much larger than any other extra-tropical biome, one would predict the latitudinal gradient to resemble a step function if area and species richness were tightly correlated. When there is a smooth latitudinal gradient in species diversity, it must be because tropical species' ranges extend into extra-tropical areas, inflating the number of species in the extra-tropical areas nearest to the tropics. Here, using data for North American tree ranges, I test whether tropical species' ranges do extend into extra-tropical areas. In a second test, I expand my definition of a tropical species to include species from genera with tropical origins (speciation spillovers). This second test searches for the effect of spillover events over evolutionary time. Only a few tropical species also live in the extra-tropics and, therefore, the latitudinal gradient in tree diversity at large scales is a step function. Thus, spillover species do not contribute to the shape of the latitudinal gradient. However, speciation spillovers account for a quarter of subtropical areas' species richness, and past range expansion was probably important in generating today's North American tree diversity. The lack of tropical species expanding their ranges into North America may be a result of a trade-off between frost tolerance and growth rate.
Chapter
This chapter discusses the fundamental aspects of the edaphic adaptations of Ancient Living Vascular Plants (ALVP) such as clubmosses, horsetails, ferns, and conifers. These adaptations enable many ALVPs to continue to exist in sites in which they show exceptional abilities at "doing without," reflecting extremely efficient standards of nutrient management. These abilities enable the ALVPs to occupy diverse edaphically marginal habitats for which today most other modern plant competition is necessarily low. Among the total of nutrient-poor habitats arising, two broad regimes of tolerance types exist widely among ALVPs. These are tolerance of (1) regimes of low-nutrient levels per se, in which the great majority of mineral elements are either lacking or in very short supply, and (2) regimes of low-nutrient levels of essential mineral elements, but in which non-essential mineral elements may additionally occur in excess, some of which may be regarded as generally toxic. Wide arrays of ALVP species from diverse families are today specialist colonists of many low-nutrient sites under both temperate and tropical conditions. Not all ALVPs occur today in low-nutrient habitats, for many have adopted life-forms that enable them to avoid excesses of vegetative competition in other ways. Tolerance of low-nutrient regimes opens colonization opportunities for ALVPs, which are not necessarily available to competing species. Evidence from the living ALVPs suggests that low-nutrient toleration is likely to be a strategy that very probably similarly occurred and recurred through appreciable geological timescales, with cohorts of such species similarly existing wherever and whenever opportunity offered with at least as much efficiency and diversity as do their survivors today.
Article
Conservation of representative facets of geophysical diversity may help conserve biological diversity as the climate changes. We conducted a global classification of terrestrial geophysical diversity and analyzed how land protection varies across geophysical diversity types. Geophysical diversity was classified in terms of soil type, elevation, and biogeographic realm and then compared to the global distribution of protected areas in 2012. We found that 300 (45%) of 672 broad geophysical diversity types currently meet the Convention on Biological Diversity's Aichi Target 11 of 17% terrestrial areal protection, which suggested that efforts to implement geophysical diversity conservation have a substantive basis on which to build. However, current protected areas were heavily biased toward high elevation and low fertility soils. We assessed 3 scenarios of protected area expansion and found that protection focused on threatened species, if fully implemented, would also protect an additional 29% of geophysical diversity types, ecoregional-focused protection would protect an additional 24%, and a combined scenario would protect an additional 42%. Future efforts need to specifically target low-elevation sites with productive soils for protection and manage for connectivity among geophysical diversity types. These efforts may be hampered by the sheer number of geophysical diversity facets that the world contains, which makes clear target setting and prioritization an important next step. © 2015 Society for Conservation Biology.