Biodiversity and Conservation *: ** **, ****.

**** ****** ******** **********. ******* in the Netherlands.

Monitoring shifts in plant diversity in response to

climate change: a method for landscapes

THOMAS J. STOHLGREN1,, APRIL J. OWEN2 and MICHELLE LEE2

1 Midcontinent Ecological Science Center, US Geological Survey, Natural Resource Ecology Laboratory,

Colorado State University, Fort Collins, CO 80523-1499, USA; 2 Natural Resource Ecology Laboratory,

Colorado State University, Fort Collins, CO 80523-1499, USA; Author for correspondence

(fax: +1-970-***-****; e-mail: abo929@r.postjobfree.com)

Received 18 January 1999; accepted in revised form 10 May 1999

Abstract. Improved sampling designs are needed to detect, monitor, and predict plant migrations and

plant diversity changes caused by climate change and other human activities. We propose a methodology

based on multi-scale vegetation plots established across forest ecotones which provide baseline data on

patterns of plant diversity, invasions of exotic plant species, and plant migrations at landscape scales in

Rocky Mountain National Park, Colorado, USA. We established forty two 1000-m2 plots in relatively

homogeneous forest types and the ecotones between them on 14 vegetation transects. We found that 64%

of the variance in understory species distributions at landscape scales were described generally by gradients

of elevation and under-canopy solar radiation. Superimposed on broad-scale climatic gradients are small-

scale gradients characterized by patches of light, pockets of fertile soil, and zones of high soil moisture.

Eighteen of the 42 plots contained at least one exotic species; monitoring exotic plant invasions provides a

means to assess changes in native plant diversity and plant migrations. Plant species showed weak af nities

to overstory vegetation types, with 43% of the plant species found in three or more vegetation types.

Replicate transects along several environmental gradients may provide the means to monitor plant diversity

and species migrations at landscape scales because: (1) ecotones may play crucial roles in expanding the

geophysiological ranges of many plant species; (2) low af nities of understory species to overstory forest

types may predispose vegetation types to be resilient to rapid environmental change; and (3) ecotones may

help buffer plant species from extirpation and extinction.

Key words: exotic species, multi-scale vegetation sampling, plant diversity, species-environment rela-

tionships, vegetation ecotones

Introduction

Paleoecological evidence clearly shows that plant species migrate long distances in

response to climate change (Woods and Davis 1989; Sykes and Prentice 1996; Kull-

man 1996). What surprises many plant ecologists are the modeled rates of migration,

with the pollen record of some tree species presumably migrating up to a kilometer

per year coinciding with climate warming in the Holocene (Pitelka et al. 1997). In

recent decades, emphasis on plant migrations has shifted to the rate of spread of exotic

invasive plant species. Eurasian cheatgrass (Bromus tectorum L.), for example, spread

over 200,000 km2 in about 40 years (Mack 1986; Pitelka et al. 1997). Even more rapid

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plant migrations may be likely in the near future in response to accelerated climate

change (Foley et al. 1996; Grabherr et al. 1994) and land-use change (Stohlgren et al.

1998c).

Managers of national parks and other natural areas are justly concerned about

rapid plant migrations. First, plant species could migrate from designated protected

areas in preserves to less protected areas outside preserves (Peters and Darling 1985).

Second, some plant species may be squeezed into smaller or fewer habitats, such

as high elevation or wetland sites (Whitlock 1993). Third, more sedentary species

could be out-competed by rapidly invading species, such as exotic plants. Vegetation

monitoring in nature preserves must be capable of detecting, monitoring, and pre-

dicting changes in species composition and plant migrations caused by factors such

as the ubiquitous threats of habitat destruction, altered disturbance regimes, climate

change, and nitrogen deposition from air pollution.

Many vegetation sampling programs may be poorly designed to detect change.

Vegetation plots have often been too small (commonly

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